Physics

Does anyone know the equations for the forces affecting a vehicle? I'm writing a program where a car drives around a track and I wanted to add physics to it. Eventually maybe I'll make a game out of it. -- Peter Vieth IGZ Handle: Fitek ICQ UIN: 3660410 Web page: http://www.netcom.com/~Fitek/index.html

Well, it depends on how fancy you want to get. Do you want true four-wheel suspension? Or are you willing to assume that the surface is perfectly flat and fudge things like body roll? Will each tire be modeled separately? The basic equation for the linear motion dynamics of any rigid body is, of course, the classic F=MA (it has a corresponding equation for angular motion). This is generally implemented as a set of differential equations: dV = F / M dP = V where V is velocity and P is position, expressed as 3-tuples. These are solved using some sort of numerical integration technique, such as Euler or Runga-Kutta. So the problem then becomes one of figuring out the forces acting on the vehicle. Not counting collisions and the like, the forces involved fall into one of two classes: vertical forces deriving from the suspension and the force of gravity; and horizontal forces deriving from the interaction of the tires with the road surface. For simplicities sake, most games have mostly ignored vertical forces up until now, since it GREATLY complicates things. Even dealing with the forces generated by the tires is complex enough that Nascar 1 and 2 (supposedly) really only model ONE tire, although of course they do keep track of separate tire temperatures. The problem further breaks down into longitudinal (acceleration/braking) and lateral (cornering) forces. Both usually involve modeling tire "slip" and the resulting friction; for longitudinal forces a simple function which returns a friction value of 0 for no slip, rising rapidly to the tires maximum coeff. of friction as slip increases and then trailing off asymptotically to constant value (representing a slipping tire's coeff. of friction) works pretty well. The force being generated is then proportional to the force pushing DOWN on the tire (usually the car's weight divided by the acceleration of gravity and divided again by the number of tires you are modeling) multiplied by the calculated coeff. of friction. Lateral forces are more complicated; one must take into account not only the amount of slip but the angle of slip as well, and that's all I can really say about that. I hope this has been helpful and that I didn't say too much that was obvious or already understood. I should say that it helps to have a thorough grounding in linear algebra and, to some extent, calculus (for the differential equations), although there is a lot of public-domain software around for doing the numerical integration stuff (I highly recommend "Numerical Recipes in C" if you don't already have it). Good luck! Todd >Does anyone know the equations for the forces affecting a vehicle? I'm >writing a program where a car drives around a track and I wanted to add

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>physics to it. Eventually maybe I'll make a game out of it. >-- >Peter Vieth >IGZ Handle: Fitek >ICQ UIN: 3660410 >Web page: http://www.netcom.com/~Fitek/index.html

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> Jesus,whats the matter with you people!!As extremely intelligent you people > seem to be,you cant figure out why a grooved tire has less grip then the > slicks????Its as easy as knowing that every *** molecule that is in > contact with the ground adds to the overall grip of the tire contact > patch.When you said that the friction remains the same regardless of the > surface size,I think your friend just interpreted the facts wrong and came to > a wrong conclusion.It CAN happen:)So dont let all this scientific mumbo jumbo > mess you up and just stick with common sense.

ah, but when you distribute the weight of the car over the larger smaller area of the contact patch on grooved tyres, there are less overall molecules to carry the weight of the car, thus the amount of downward pressure on each molecule is increased, providing more traction. That's why that chap said that it doesn't matter how much surface area is in contact, the friction size stays the same, because theoretically larger surface areas distribute the weight at less force per unit of area. But in reality not everything is so cut and dried. This model suits hard surfaces which don't deform at all. The nature of *** means that it's surface molds to fit the surface of the road (or anything else) that it touches, on a molecular level, giving extra sideways grip. It is this extra sideways grip which gives more traction with more contact area. Of course, I'm just theorising here, and so it's likely that this is not the right reason for *** gripping more:) Grant.

Huh? Please explain this physical impossibility......! > the weight of the car over the larger > smaller area of the contact patch -- Regards, Bruce. ---------- The GP Legends Historic Motor Racing Club is located at:- http://www.netspeed.com.au/brucek/legends/

Both of you could be right. It depends on the exact parameters in the problem: Any surface that is in contact with another surface has a certain maximum grip (Maximum friction-force). This maximum is some kind of function of the pressure on the surface, lets say Grip per unit area = (Pressure)^a, with a some exponential (this must be true, at least in first order approximation). When the surface decreases with an factor b (e.g. new surface = (old surface)/b, for b > 1), the presure increases with the same factor b In that case the Grip/area = (presure)^a = (old presure * b)^a = (old presure)^a * b^a = (Old Grip/area) * b^a The total grip becomes: (Grip/Area) * Area = (Old Grip/Area) * Area * b^a = (Old Grip/Area) * (Old Area)/b * b^a = (Old Grip) * b^(a-1) Conclusion: if the grip/area is proportional to the pressure (thus: a=1), then the grip wil be constant. If the grip/area is sublineair with the pressure (thus; a<1) the grip will increase when the area becomes less. Personally I think the latter case is most likely. Greetings, Han Monsees

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> > Jesus,whats the matter with you people!!As extremely intelligent you people > > seem to be,you cant figure out why a grooved tire has less grip then the > > slicks????Its as easy as knowing that every *** molecule that is in > > contact with the ground adds to the overall grip of the tire contact > > patch.When you said that the friction remains the same regardless of the > > surface size,I think your friend just interpreted the facts wrong and came to > > a wrong conclusion.It CAN happen:)So dont let all this scientific mumbo jumbo > > mess you up and just stick with common sense. > ah, but when you distribute the weight of the car over the larger > smaller area of the contact patch on grooved tyres, there are less > overall molecules to carry the weight of the car, thus the amount > of downward pressure on each molecule is increased, providing more > traction. That's why that chap said that it doesn't matter how much > surface area is in contact, the friction size stays the same, because > theoretically larger surface areas distribute the weight at less > force per unit of area. > But in reality not everything is so cut and dried. This model suits > hard surfaces which don't deform at all. The nature of *** means > that it's surface molds to fit the surface of the road (or anything > else) that it touches, on a molecular level, giving extra sideways > grip. It is this extra sideways grip which gives more traction with > more contact area. Of course, I'm just theorising here, and so it's > likely that this is not the right reason for *** gripping more:) > Grant.

---------------------------------------------------------------------------- "Het doel is de wind. En omdat het doel altijd blijft, blijft de wind ook altijd, dus zal het zeil nooit omvallen" Ronald Giphart in Rails ----------------------------------------------------------------------------

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> Jesus,whats the matter with you people!!As extremely intelligent you people >seem to be,you cant figure out why a grooved tire has less grip then the >slicks????Its as easy as knowing that every *** molecule that is in >contact with the ground adds to the overall grip of the tire contact >patch.When you said that the friction remains the same regardless of the >surface size,I think your friend just interpreted the facts wrong and came to >a wrong conclusion.It CAN happen:)So dont let all this scientific mumbo jumbo >mess you up and just stick with common sense.

Let's not forget, uh, chemical-mechanical adhesion, the kind that makes the marbles that you pick up after the race so sticky. I don't think rain tires have any of that- they don't need it- so it's safe to say that there's another reason that wets don't stick as well. Unless that was already mentioned, and I'm a stooge, or my newsreader sucks, or both :)

What we know: 1. Soft tyres has more grip than hard tyres. 2. Hard tyres last longer than soft tyres. 3. The friction force stays the same no matter what the contact size is. Do I have to continue, or have you figured it out already :o)? Suppose I add: 4. The more pressure per square centimeter there is on a tyre, the more the tyre wears. This leads to the conclusions that size dont matter on grip, but a larger size will give us a lower pressure per square centimeter, thus lower tyre wear, thus allowing us to have softer tyre, which gives us more grip :o). The grooves in the F1 tyres decrease the contact size, thus force the teams to have harder tyres to get the same tyre wear as before. /Christer, now I could be completely wrong, but I dont think so :o)

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> With that stated, how does the recent introduction of grooved tires (in > F1) have any bearing on the amount of grip which the tires provide? > Although the contact patch is reduced there would be no less friction at > work to keep the tires in the road. We were unable to figure this out. I > hope you can succinctly explain this in laymen's terms for me :) > Thanks in advance, > Simon > > Well, it depends on how fancy you want to get. Do you want true > > four-wheel suspension? Or are you willing to assume that the surface > > is perfectly flat and fudge things like body roll? Will each tire be > [snip] > > public-domain software around for doing the numerical integration > > stuff (I highly recommend "Numerical Recipes in C" if you don't > > already have it). Good luck! > > Todd

Of course, I didn't think this through enough in my post :o). With a smaller width tyre you have a smaller tyre to absorb the same energy (the weight of the car is the same), so it get's heated faster and probably has a higher running temperature. You will probably need a harder tyre just to cope with the heat. /Christer, I think I now understand why I want a tyre to have a contact area as large as possible :o).

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> From what I've understand, the most significant factor is heat > buildup, i.e. with the grooved (or smaller width) tires there is less > surface area available for heat dissipation and therefore heat builds > up faster. This obviously has an adverse affect on the tire's > performance and longevity. > >Todd - I read your reply with interest and I think I understood some > of > >it :) I have a another question though. A friend of mine, who is in > >science, has stated that the 'force' of friction (if I may call it > that) > >is the same regardless of surface area. That is, if you stood a brick > on > >end and slid it down an incline, the resistance generated by friction > >would be no less than if the brick were laying on its largest > surface. > >With that stated, how does the recent introduction of grooved tires > (in > >F1) have any bearing on the amount of grip which the tires provide? > >Although the contact patch is reduced there would be no less friction > at > >work to keep the tires in the road. We were unable to figure this > out. I > >hope you can succinctly explain this in laymen's terms for me :) > >Thanks in advance, > >Simon > >> Well, it depends on how fancy you want to get. Do you want true > >> four-wheel suspension? Or are you willing to assume that the > surface > >> is perfectly flat and fudge things like body roll? Will each tire > be > >[snip] > >> public-domain software around for doing the numerical integration > >> stuff (I highly recommend "Numerical Recipes in C" if you don't > >> already have it). Good luck! > >> Todd

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> What we know: > 1. Soft tyres has more grip than hard tyres. > 2. Hard tyres last longer than soft tyres. > 3. The friction force stays the same no matter what the contact size is. > Do I have to continue, or have you figured it out already :o)? Suppose I add: > 4. The more pressure per square centimeter there is on a tyre, the more

the tyre > wears. > This leads to the conclusions that size dont matter on grip, but a larger size > will give us a lower pressure per square centimeter, thus lower tyre wear, thus > allowing us to have softer tyre, which gives us more grip :o). > The grooves in the F1 tyres decrease the contact size, thus force the teams to > have harder tyres to get the same tyre wear as before. > /Christer, now I could be completely wrong, but I dont think so :o) Yes you are. Friction force increases as the contact patch increases. I know the simple physics equation for friction says otherwise, but it's not applicable to situations like *** on pavement. And to whoever says grooves affect heat dissipation is wrong also. I believe they said it was because slicks have more surface area to dissipate the heat. Think again on that one. Grooves add surface area. But the tread squirm (the true technical term) caused by the grooves generates heat, causing the tire to fail if pushed hard. One last comment. It pains me to think of all the people who believe the false answers they read on these newgroups and pass them on to other naive people. Everyone would benefit if unsure people would check up one the facts instead of just guessing. But then again maybe I'm the only one who cares about correct answers. Maybe it's my engineering degree, my work in a tribological lab, and my racing cars and motorcycles that lets me know the right answer to these questions. Not just listening to others guessing. Jer

>Maybe it's my engineering degree, my work in >a tribological lab, and my racing cars and motorcycles that lets me know >the right answer to these questions. Then again, maybe its your high-and-mighty, piss-poor attitude that causes people to rarely ask you these questions, knowing that you'll ream them out good if they even hazard a guess as to an answer to an amazingly complex question. Todd

> Maybe it's my engineering degree, my work in > a tribological lab, and my racing cars and motorcycles that lets me know > the right answer to these questions. > Not just listening to others guessing. Guessing is a pretty good word for an area where there is little theory and empiricism holds sway. How much does one have to increase the width to get a 10% increase in friction? (Double it? ;) - Matt

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> >Maybe it's my engineering degree, my work in > >a tribological lab, and my racing cars and motorcycles that lets me > know > >the right answer to these questions. > Then again, maybe its your high-and-mighty, piss-poor attitude that > causes people to rarely ask you these questions, knowing that you'll > ream them out good if they even hazard a guess as to an answer to an > amazingly complex question. > Todd

It's probably best not to head down this path, Todd. -Mike PS- Regarding N1 & N2, all 4 tires are modeled, not just one. Think of those cars as a brick with 4 wheels.(as opposed to a multiple rigid body system as in GPL)

I had second thoughts as soon as I hit the "Post" button. A little too late, unfortunately. I tried to qualify my statements as being my "what I'd heard," not any sort of last word. Thanks for setting me straight! Todd >It's probably best not to head down this path, Todd. >-Mike >PS- Regarding N1 & N2, all 4 tires are modeled, not just one. Think of >those cars as a brick with 4 wheels.(as opposed to a multiple rigid body >system as in GPL)

>And to whoever says grooves affect heat dissipation is wrong also. I >believe they said it was because slicks have more surface area to >dissipate the heat. Think again on that one. Grooves add surface area. But >the tread squirm (the true technical term) caused by the grooves generates >heat, causing the tire to fail if pushed hard. Well, I guess I violated Wolfgang's first rule of newsgroup etiquette (e.g. don't take flames personally!). I apologize for the outburst. Obviously you are correct about the surface area, since the grooves are just like fins on a heat sink. I still say that the grooves AFFECT heat dissipation, however, in that the resulting tire squirm generates more heat and the tire, even with the larger surface area, is unable to dissipate the additional heat effectively. Does that make sense? Todd

>With a smaller width tyre you have a smaller tyre to absorb the same energy (the >weight of the car is the same), so it get's heated faster and probably has a higher >running temperature. Just to make this stuff even more complicated... Since the tires are narrower, they'll also cool-off faster!!! Best, Edwin Solheim Check out The Paddock - a legendary site... for some cool Grand Prix Legends stuff and tips! (http://home.c2i.net/thepaddock)

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> Huh? > Please explain this physical impossibility......! > > the weight of the car over the larger > > smaller area of the contact patch

geez! what the hell was I trying to say??!?!?!:) I can't even figure it out myself, reading over my post just now! that's got to be a new one, totally confusing myself in addition to everyone else.

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> One last comment. It pains me to think of all the people who believe the > false answers they read on these newgroups and pass them on to other naive > people. Everyone would benefit if unsure people would check up one the > facts instead of just guessing. But then again maybe I'm the only one who > cares about correct answers. Maybe it's my engineering degree, my work in > a tribological lab, and my racing cars and motorcycles that lets me know > the right answer to these questions. Not just listening to others > guessing.

Have you considered that we might enjoy trying to figure things out ourselves? You may not have noticed that most of us append a little note to the end of our speculations pointing out that we're just guessing. Also, trying to find comprehensive documentation containing all the answers to our questions is not easy. I would look if i knew where to. Do you have any suggestions for us? Grant Reeve.

Wow!! So much for being modest. Jerry will now bend over so that we can bask in the sunshine that blazes from his fundamental orifice. Incidentally, what exactly is a 'tribological lab'? Is that a place where tribes are examined and studied? > Maybe it's my engineering degree, my work in > a tribological lab, and my racing cars and motorcycles that lets me know > the right answer to these questions. Not just listening to others > guessing. > Jer -- Regards, Bruce. ---------- The GP Legends Historic Motor Racing Club is located at:- http://www.netspeed.com.au/brucek/legends/

Oh...bummer, Grant! I was expecting some earth-shattering pronouncement....along the lines of Stephen Hawkings' theory, or, at the very least, Albert Einsteins'!! We could call it the "Geez Effect", perhaps?! :o)

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> > Huh? > > Please explain this physical impossibility......! > > > the weight of the car over the larger > > > smaller area of the contact patch > geez! what the hell was I trying to say??!?!?!:) > I can't even figure it out myself, reading over my post just now! > that's got to be a new one, totally confusing myself in addition > to everyone else.

-- Regards, Bruce. ---------- The GP Legends Historic Motor Racing Club is located at:- http://www.netspeed.com.au/brucek/legends/

Todd Heckel wrote Hehe, no problem Todd. You can check Mike Lescault's past posting on DejaNews, he clearly mastered the art of USENET posting. Just consider than on a typical day on r.a.s., at least 20% of the posts are irrevelant. But it leaves 80% of good still. You will never find a place or more scum and villany... Choose the right people to reply to, and choose your words. We all get caught in the bag. ;-) <learn to use the Force, like a Jedi master> - Fran?ois Mnard <ymenard> Good race at the Brickyard, (-o-) - Official Mentally retarded guy of r.a.s. - Member of the r.a.s. Ego-maniac club - Excuse me for my English (I'm French speaking) - Excuse me for being provocative (I'm dumb speaking) - "People think it must be fun to be a super genius, but they don't realise how hard it is to put up with all the idiots in the world."--

Tribology is the study of friction, lubrication, wear, contact of real surfaces, surface failures ............. I know this because of my engineering degree, but, unfortunately, I don't have sunshine that blazes from my fundamental orifice! (ha ha!!, but that would be a neat trick, wouldn't it?).

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>Wow!! >So much for being modest. >Jerry will now bend over so that we can bask in the sunshine that blazes >from his fundamental orifice. >Incidentally, what exactly is a 'tribological lab'? Is that a place >where tribes are examined and studied? >> Maybe it's my engineering degree, my work in >> a tribological lab, and my racing cars and motorcycles that lets me know >> the right answer to these questions. Not just listening to others >> guessing. >> Jer >-- >Regards, >Bruce. >---------- >The GP Legends Historic Motor Racing Club is located at:- >http://www.netspeed.com.au/brucek/legends/

I had some long explanations typed out and then the power went out. Damn Iowa weather. So here is the shorter, sweeter version. I agree with 1. Number 2 is also correct most of the time. The exception is when street tires are pushed to hard and they throw off large chunks of tread instead of wearing normally. Number 4 is probably correct other than the wording, but that is a nit picky point. (Pressure per square cm is redundant) Number 3 is the one I disagree on. I would have to refresh my memory on the exact answer to why the physics doesn't apply. I believe it has to do with the chemical or molecular interactions which sum up to add to the friction force. More surface area allows more bonds, which add to the total traction. Sort of like a material has a certain tensile strength, given in psi, but a larger piece of that material takes a larger force to break it. Probably a bad example but I think it serves the purpose. I'll do some researching tonight and see if I can find a documented answer that makes sense. Tread squirm is just what it sounds like. Tread blocks squirm around under the load exerted on them by cornering, etc. The flexing of the *** creates heat due to the internal friction, or whatever you want to call it. Excessive heat kills the tires. Remember some heat is good. Very important to remember when riding a motorcycle in freezing weather. Tires don't stick well when cold. brain too. > It's understanding that's important, not knowing, IMO. I believe it's Good vocabulary skills. I think I explained why wider tires have more grip - more molecular level bonds, and why grooves generate heat - tread squirm. So all that's left is a small apology. The out burst was probably uncalled for - blame it on a couple years of building frustration watching news groups give wrong answers to good questions. A smaller reason was the fact that people are more apt to reply to rude messages for some reason. As for being told 'correct answers', I'm happy to say you're mistaken. I've read every article, book, webpage, etc that I can get my hands on trying to understand the complex subject of vehicle dynamics. And explaining stuff to other people is a great way to test your own subject knowledge. Now I better post this before the power goes out again.

> I had some long explanations typed out and then the power went out. Damn > Iowa weather. So here is the shorter, sweeter version. We dont have that in Sweden, power outs, that is :o). You mean I could just write "The more pressure there is on the tyre surface, the more the tyre wears"? How about my conclusion that becasue we have a smaller pressure on a wider tyre, we can have a softer tyre? If so and if you're correct about 3 then this would just add to my conclusion, right? What you are saying is that traction increase if you make the same tyre a bit wider. You also say that this has it's reason in chemical or molecular interaction. This should mean that the surface can not be seperated in the middle, cause this would ruin the effect, right? So if we just added two more rear wheels to a car we wouldn't increase the total traction, cause there is no interaction between the tyre surfaces. If we make a groove in the middle of the tyre, then this would also ruin the effect, or have I misunderstood you? If we study a small area of the surface on a tyre, the same area will get gripier the wider the tyre is??? This totally blows me :o). I dont understand it. The width of a tyre compared to the size of the molecules seems to different to affect each other. This I can understand, though :o). I didn't understand the word "tread", I'm from Sweden, you know. I've looked it up now and I see how a groove can increase the squirmishness, thus increasing the heat. BTW, I dont think everybody who reads and posts to this NG comes from english speaking countries. It seems as if some of you forget this from time to time. As I understand it there has to be more bonds per area unit the wider the tyre is, and this doesn't make sense to me. Got it :o). I guess you have to learn to live with this, even in books of facts there are errors. I believe that everything you read or hear you should be a bit skeptical to, especially if it doesn't make sense. Since it's written and said by humans it will contain errors :o). I also believe one should have some skeptisism to ones own knowledge. I guess this gets more and more apparent as you get closer to the limit of human knowledge. The "experts" will always disagree. It's also a very good way to improve your understanding of the subject, if your pedagogic skills are any good, that is :o)./Christer, just love to use his human right to guess :o) "I dont trust air I cant see", Red October

What do you mean when you say more bonds add to the total traction? I am a little confused. (This sure is an interesting and enlightening topic though)

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> Todd - I read your reply with interest and I think I understood some of > it :) I have a another question though. A friend of mine, who is in > science, has stated that the 'force' of friction (if I may call it that) > is the same regardless of surface area. That is, if you stood a brick on > end and slid it down an incline, the resistance generated by friction > would be no less than if the brick were laying on its largest surface. > With that stated, how does the recent introduction of grooved tires (in > F1) have any bearing on the amount of grip which the tires provide? > Although the contact patch is reduced there would be no less friction at > work to keep the tires in the road. We were unable to figure this out. I > hope you can succinctly explain this in laymen's terms for me :)

Part of the answer is that the grooved tires use a harder *** compound than slick tires. The harder compound is required to combat "squirm", which has been discussed already. The harder compound is obviously going to provide less grip than a softer compound would, regardless of the width of the tire and size of the contact patch. Here is something else to consider in the narrower v. wider tire debate. Wider tires don't merely increase the tire contact patch, they also increase the track width of the vehicle. That is, the center-to-center distance between tires on opposite sides of the car is greater, which will tend to increase the mechanical grip produced (like that TV car commercial where they say "wider is better"). Also, the coefficient of friction is not constant in this real-world application, but I'm still trying to come up with a coherent explanation for it :) -- Pat Dotson B.S. Mechanical Engineering - Purdue University IMPACT Motorsports

All of this fascinating discussion raises an interesting point about the issue of developer involvement in this newsgroup: it's IN OUR OWN BEST INTEREST. We aren't the sole experts on the subject, we're just like the average (I think) sim racer with a huge interest (perhaps even a passion) in the subject and the great fortune to actually work in a field so closely related to our non-work interests. These discussions are wonderful because they give us access to people more knowledgeable than us in various fields - who are willing to share their knowledge for a pittance! Some (but not all) of this stuff can be found in many textbooks, but it can be rather dry, and of course the element of debate (and the ensuing enlightenment) is missing. The stuff about grip per unit area not being proportional to pressure was great; now I have a more theoretical basis for computing friction force based on the width of the tire. Okay, so lets talk about aerodynamic drag...o^) Todd

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>Part of the answer is that the grooved tires use a harder *** >compound than slick tires. The harder compound is required to >combat "squirm", which has been discussed already. The harder >compound is obviously going to provide less grip than a softer >compound would, regardless of the width of the tire and size of >the contact patch. >Here is something else to consider in the narrower v. wider >tire debate. Wider tires don't merely increase the tire >contact patch, they also increase the track width of the >vehicle. That is, the center-to-center distance between >tires on opposite sides of the car is greater, which will >tend to increase the mechanical grip produced (like that >TV car commercial where they say "wider is better"). >Also, the coefficient of friction is not constant in this >real-world application, but I'm still trying to come up with >a coherent explanation for it :) >-- >Pat Dotson >B.S. Mechanical Engineering - Purdue University >IMPACT Motorsports

>> Since the tires are narrower, they'll also cool-off faster!!! >Because??? The reason for me asking is that I dont understand why they >should. The heat doesn't travel very well within ***, so I guess :o) most >of the heat probably stays close to the contact surface and is cooled down by >the air flow and pavement. As I said, I dont understand this yet :o). You just answered that one yourself Christer.. Since theres less area or mass to cool-off, the process will go much faster. Another issue is the fact that a norrower tire will "flow" way much better thru the air, that a wide one - thus getting much more efficient cooling. Best, Edwin Solheim Check out The Paddock - a legendary site... for some cool Grand Prix Legends stuff and tips! (http://home.c2i.net/thepaddock

> Okay, so lets talk about aerodynamic drag...o^) I love your attitude, Todd :o), and I'm looking forward to this discussion :o). I'm having problem stopping myself from starting to guess on this one... I think the vacuums is the key here, find where the vacuums turn up around the car when at speed :o). /Christer

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> >> Since the tires are narrower, they'll also cool-off faster!!! > >Because??? The reason for me asking is that I dont understand why they > >should. The heat doesn't travel very well within ***, so I guess :o) > most > >of the heat probably stays close to the contact surface and is cooled down > by > >the air flow and pavement. As I said, I dont understand this yet :o). > You just answered that one yourself Christer.. > Since theres less area or mass to cool-off, the process will go much faster.

Less area is a bad thing if you want to cool something. The worst scenario is a sphere, cause regarding a sphere you have the worst relation between volume and surface area. You have to little surface area per volume. In a narrow tyre compared to a wide tyre this relation isn't very different, it's actually a little bit worse for the narrower tyre, cause it's closer to the circle shape, when intersected. What I'm actually is saying is that regarding heat dissipation through the contact surface, it's about the same no matter how wide or narrow the tyre is. This is of course on a tyre that is the same in any other way, but wider or narrower. /Christer

Heavens! Surely somebody with the overwhelming facilities of a tribology lab can keep the power on?!! Rub a few sticks together....that produces friction, therefore heat.....doesn't it? :o) > I had some long explanations typed out and then the power went out. Damn > Iowa weather. So here is the shorter, sweeter version. Regards, Bruce. ---------- The GP Legends Historic Motor Racing Club is located at:- http://www.netspeed.com.au/brucek/legends/

This thread is producing some really fascinating stuff! It's like having a university in my living-room, available 24 hours a day!! Of course, I don't understand a word of it but, Christ Almighty, did I IMPRESS people at work today!!! Thanks guys! > > Todd - I read your reply with interest and I think I understood some of > > it :) I have a another question though. A friend of mine, who is in > > science, has stated that the 'force' of friction (if I may call it that) > > is the same regardless of surface area. That is, if you stood a brick on.....................yada yada yada. -- Regards, Bruce. ----------

Of course, if it's raining, they cool off a lot quicker.

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> >> Since the tires are narrower, they'll also cool-off faster!!! > >Because??? The reason for me asking is that I dont understand why they > >should. The heat doesn't travel very well within ***, so I guess :o) > most > >of the heat probably stays close to the contact surface and is cooled down > by > >the air flow and pavement. As I said, I dont understand this yet :o). > You just answered that one yourself Christer.. > Since theres less area or mass to cool-off, the process will go much faster. > Another issue is the fact that a norrower tire will "flow" way much better > thru the air, that a wide one - thus getting much more efficient cooling. > Best, > Edwin Solheim > Check out The Paddock - a legendary site... > for some cool Grand Prix Legends stuff and tips! > (http://home.c2i.net/thepaddock

-- Regards, Bruce. ---------- The GP Legends Historic Motor Racing Club is located at:- http://www.netspeed.com.au/brucek/legends/

No, Christer. I drag my vaccuum around the house, cleaning behind the sofa like a good husband, and I have no friction problems at all....it glides along very smoothly. I'm starting to understand this stuff, I think :o)!!

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> > Okay, so lets talk about aerodynamic drag...o^) > I love your attitude, Todd :o), and I'm looking forward to this discussion :o). > I'm having problem stopping myself from starting to guess on this one... I think > the vacuums is the key here, find where the vacuums turn up around the car when > at speed :o). > /Christer

-- Regards, Bruce. ---------- The GP Legends Historic Motor Racing Club is located at:- http://www.netspeed.com.au/brucek/legends/

I'm not sure what you mean by "flow", but turbulent air flow will cool the tire more quickly than laminar (smooth) flow. Also, I may be misinterpreting what you said, but for a given mass of *** the one with more surface area will cool faster.

You're right, the molecules don't know what's goin on more than a couple microns away. Let me try to explain. If one square cm has 100 bonds, then ten sq cm will have 1000 bonds. Each individual bond adds to traction and the number of bonds is dependent on the surface area of the contact patch. So bigger tires have more bonds and therefor more traction. Got it? You're right, I didn't realize you were not from around here. I knew an exchange student from Sweden. Her name was Asa and she was the best looking girl I've seen in awhile. No, same number of bonds per unit area. More area then equals more bonds. Pedagogic huh? I know you're foreign now. No native english speaker would ever use a big word like that. Jer

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> Of course, if it's raining, they cool off a lot quicker. > > >> Since the tires are narrower, they'll also cool-off faster!!! > > >Because??? The reason for me asking is that I dont understand why they > > >should. The heat doesn't travel very well within ***, so I guess :o) > > most > > >of the heat probably stays close to the contact surface and is cooled down > > by > > >the air flow and pavement. As I said, I dont understand this yet :o). > > You just answered that one yourself Christer.. > > Since theres less area or mass to cool-off, the process will go much faster. > > Another issue is the fact that a norrower tire will "flow" way much better > > thru the air, that a wide one - thus getting much more efficient cooling.

I think the main way heat is removed from the tire is thru contact with the pavement. The air flow might play a role in open wheel cars, but even then I believe the car designers try to direct the air flow over the tires. The top of the tire is moving against the air flow and really disrupts things. A couple thoughts on heat traveling thru the ***. First, I think that the whole tire should be pretty uniform in temperature because there is no localized heating. Second, do race tires use steel belts? My guess is they don't but I'm not sure. But if they do, then the steel would conduct the heat pretty well. Jer

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>>Since theres less area or mass to cool-off, the process will go much >faster. >>Another issue is the fact that a norrower tire will "flow" way much better >>thru the air, that a wide one - thus getting much more efficient cooling. >I'm not sure what you mean by "flow", but turbulent air flow will cool the >tire more quickly than laminar (smooth) flow. Also, I may be >misinterpreting what you said, but for a given mass of *** the one with >more surface area will cool faster.

That should be: more surface at a given volume. Smaller tyres will have relative more surface. Remco

> No, Christer. > I drag my vaccuum around the house, cleaning behind the sofa like a good > husband, and I have no friction problems at all....it glides along very > smoothly. > I'm starting to understand this stuff, I think :o)!! What ever you do, DON'T vacuum in the garage when the car is on the road. (If you have a spoiler, the sudden loss of downforce from disrupting the starting vortex could lead to a *** accident.) ;) - Matt

Made me laugh out loud on that one, Bruce, thanks :o))). /Christer

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> Of course, I don't understand a word of it but, Christ Almighty, did I > IMPRESS people at work today!!! Thanks guys! > -- > Regards, > Bruce. > ----------

> You're right, the molecules don't know what's goin on more than a couple > microns away. Let me try to explain. If one square cm has 100 bonds, then > ten sq cm will have 1000 bonds. Each individual bond adds to traction and > the number of bonds is dependent on the surface area of the contact patch. > So bigger tires have more bonds and therefor more traction. Got it? :o))) Well, actually... no :o). Bare with me will you, please. If we have a tyre with a contact patch of one square cm and it has 100 bonds, then the pressure on one bond will be 1/100 of the pressure, right? If we increase the contact patch to ten square cm, it will have 1000 bonds. The pressure per bond will now only be 1/1000 of the pressure, or 1/10 of what it was on the smaller tyre, but you say the traction added from a bond will not have decreased with 1/10, but with less than 1/10, on the bigger tyre... Why? Not why you say it, but what causes it :o)? Well, it's my wife, and she doesn't remember you, but stay away anyhow :o). Then the traction from the bonds cant be proportional to the pressure. It seems as if the bonds get stickier the lower the pressure is??? /Christer, dont understand the magic bonds :o(

:o) Ur welkm!

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> Made me laugh out loud on that one, Bruce, thanks :o))). > /Christer > > Of course, I don't understand a word of it but, Christ Almighty, did I > > IMPRESS people at work today!!! Thanks guys! > > -- > > Regards, > > Bruce. > > ----------

-- Regards, Bruce. ---------- The GP Legends Historic Motor Racing Club is located at:- http://www.netspeed.com.au/brucek/legends/

> Okay, so lets talk about aerodynamic drag...o^) I know, I know, I know, I mean have some guesses :o))). When a car moves it leaves the air molecules that was behind it, so these molecules must be replaced, because vacuum is not acceptable within our atmosphere :o). This start of a vacuum behind the car will of course also pull on the car, thus drag. The principle is the same as in your vacuum cleaner, Bruce, it's as we had someone chasing us with a vacuum cleaner :o). With a wing it gets more complicated. Because the wing has forced the air upwards with speed it makes this air harder to suck in behind the car, hence the vacuum gets a little larger with a wing, and even larger with more wing :o). Now correct me :o). /Christer

Hi All, Understanding tires isn't easy. There is a branch of engineering that is called "tire mechanics" and I doubt that you will find anyone that is seriously involved in the business saying that they "know it all" <grin>. As well as _this_ thread on what happens between "***" (actually a complex brew of various polymers - long chain molecules, etc) and the road, there is the whole business of the rest of the tire and how different *** elements are loaded/stressed. For example, at the start of the footprint (of a rolling tire, not a sliding tire), the *** is just set on the road with no force at all - then various sliding happens as that "bit" of *** moves through the print. There is a summary of the basics in our book, "Race Car Vehicle Dynamics", available through the online bookstore at www.sae.org Sincerely, -- Doug Milliken Research Associates Inc.

Ha ha ha. Now you have me imagining what a tire 60 inches wide will do Christer. -- mark "A lot of people think racing is about going fast. But it's not. It's about going just slow enough to stay on the track." F1RS - http://www.nmia.com/~chaser/car/results.htm Remove us here and there to mail me. :o))) Well, actually... no :o). Bare with me will you, please. If we have a tyre with a contact patch of one square cm and it has 100 bonds, then the pressure on one bond will be 1/100 of the pressure, right? If we increase the contact patch to ten square cm, it will have 1000 bonds. The pressure per bond will now only be 1/1000 of the pressure, or 1/10 of what it was on the smaller tyre, but you say the traction added from a bond will not have decreased with 1/10, but with less than 1/10, on the bigger tyre... Why? Not why you say it, but what causes it :o)?

How's about acid rain???????????

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> Of course, if it's raining, they cool off a lot quicker. > > >> Since the tires are narrower, they'll also cool-off faster!!! > > >Because??? The reason for me asking is that I dont understand why they > > >should. The heat doesn't travel very well within ***, so I guess :o) > > most > > >of the heat probably stays close to the contact surface and is cooled down > > by > > >the air flow and pavement. As I said, I dont understand this yet :o). > > You just answered that one yourself Christer.. > > Since theres less area or mass to cool-off, the process will go much faster. > > Another issue is the fact that a norrower tire will "flow" way much better > > thru the air, that a wide one - thus getting much more efficient cooling. > > Best, > > Edwin Solheim > > Check out The Paddock - a legendary site... > > for some cool Grand Prix Legends stuff and tips! > > (http://home.c2i.net/thepaddock > -- > Regards, > Bruce. > ---------- > The GP Legends Historic Motor Racing Club is located at:- > http://www.netspeed.com.au/brucek/legends/

Now I am guessing. :-) Steel belts are pretty heavy and can cut a tire from the inside out. Under extreme heat, the steel separates from the *** fairly easily. Kevlar is a popular substitute and though not as strong, about 1/5 the weight of steel. Seeing that racing venues really dislike debris on the track, (especially small, hard to see, steel pokey things) and that racing tires frequently come apart, I would think Kevlar for tread and sidewall reinforcement along with other synthetics. The racetrack doesn't require the protection from puncture that would necessitate steel, and Kevlar will give stiffness where needed. -- mark "A lot of people think racing is about going fast. But it's not. It's about going just slow enough to stay on the track." F1RS - http://www.nmia.com/~chaser/car/results.htm Remove us here and there to mail me. I think the main way heat is removed from the tire is thru contact with the pavement. The air flow might play a role in open wheel cars, but even then I believe the car designers try to direct the air flow over the tires. The top of the tire is moving against the air flow and really disrupts things. A couple thoughts on heat traveling thru the ***. First, I think that the whole tire should be pretty uniform in temperature because there is no localized heating. Second, do race tires use steel belts? My guess is they don't but I'm not sure. But if they do, then the steel would conduct the heat pretty well. Jer

Shit, I finally made it to the end! :)) So I now know what Tribology is and have a new parameter to play with - chemical bonding. Now Jerry, let me throw a few educated ideas at you. 1/ I say that these chemical bonds are probably negligible under just about all circumstances compared to the friction effect. 2/ I say that 1/ is even more so at higher speeds where winged cars add massively to the downforce and consequently friction. Also, with or without wings, at high speeds this chemical bonding has less time to take effect and becomes even more negligable ie less time for chemical reaction to take effect. So, what does the one from the Tribology tribe say to this? And some accurate percentages would be much appreciated if you could ie the percentage ratio of the friction effect compared to this bonding stuff. (Come to think of it, how could one measure this? Perhaps by testing tyres of the same compound but different widths at the same speed/weight and calculating the difference in grip, etc, etc.....???????) I won't be surprised if you were nit picking in the first place and actually decieving this NG a little yourself. The laws of physics are the laws of physics (ie friction) and I doubt very much that a little extra chemical bonding will be a match for the mighty parameter of friction in this example. > You're right, the molecules don't know what's goin on more than a couple > microns away. Let me try to explain. If one square cm has 100 bonds, then > ten sq cm will have 1000 bonds. Each individual bond adds to traction and > the number of bonds is dependent on the surface area of the contact patch. > So bigger tires have more bonds and therefor more traction. Got it?

ROTFL!!!!!!!!!

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> Heavens! Surely somebody with the overwhelming facilities of a > tribology lab can keep the power on?!! Rub a few sticks > together....that produces friction, therefore heat.....doesn't it? :o) > > I had some long explanations typed out and then the power went out. Damn > > Iowa weather. So here is the shorter, sweeter version. > Regards, > Bruce. > ---------- > The GP Legends Historic Motor Racing Club is located at:- > http://www.netspeed.com.au/brucek/legends/

Bruce's wife - "Damn it Bruce, I told you to sweep and mop the lino, not vacumn it!"

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> No, Christer. > I drag my vaccuum around the house, cleaning behind the sofa like a good > husband, and I have no friction problems at all....it glides along very > smoothly. > I'm starting to understand this stuff, I think :o)!! > > > Okay, so lets talk about aerodynamic drag...o^) > > I love your attitude, Todd :o), and I'm looking forward to this discussion :o). > > I'm having problem stopping myself from starting to guess on this one... I think > > the vacuums is the key here, find where the vacuums turn up around the car when > > at speed :o). > > /Christer > -- > Regards, > Bruce. > ---------- > The GP Legends Historic Motor Racing Club is located at:- > http://www.netspeed.com.au/brucek/legends/

> Part of the answer is that the grooved tires use a harder *** > compound than slick tires. The harder compound is required to > combat "squirm", which has been discussed already. The harder > compound is obviously going to provide less grip than a softer > compound would, regardless of the width of the tire and size of > the contact patch. Still, the "squirm" factor is insignificant alongside the reduced contact patch and the resultant increased load per square inch of tyre. Much as friction is far pre*** over chemical adhesion :)) Not if the axels are shortened.

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> > Okay, so lets talk about aerodynamic drag...o^) > I know, I know, I know, I mean have some guesses :o))). When a car moves it > leaves the air molecules that was behind it, so these molecules must be > replaced, because vacuum is not acceptable within our atmosphere :o). This start > of a vacuum behind the car will of course also pull on the car, thus drag. The > principle is the same as in your vacuum cleaner, Bruce, it's as we had someone > chasing us with a vacuum cleaner :o). > With a wing it gets more complicated. Because the wing has forced the air > upwards with speed it makes this air harder to suck in behind the car, hence the > vacuum gets a little larger with a wing, and even larger with more wing :o). > Now correct me :o).

Sure will! :)) This vacumn only occurs at high speeds and is more accurately termed "eddy" (or was that Eddie?) or turbulance. Race cars and there wings are designed to be as aerodynamically efficient as possible. They will enjoy perfect slipstreaming (shearing effects close to the surface only) until the speed of air passing around them can no longer act in a perfectly slipstreamed fashion and then eddies begin to form which is less efficient. From memory, this is what this all means mathematically. Drag increases linearly whilst in perfect slipstreaming conditions. ie Drag = Coefficient of drag (C) * Speed the Coefficient of drag (C) in this example is related to the slipperiness of the surface (special paints, etc) and for the entire car would be some type of average of the sum of all the bits of the car ie tyres, wings, driver's helmet, body, etc, etc. As different bits of the car enter into non slipstreamed behavior as speed increases, then from this point the drag increases as the square of the speed. ie delta Drag = C * (delta Speed) squared (Not completely sure) So to use the front wing as an example, lets say it has perfect slipstreaming till speed "Y" and that the speed of the car is Y + Z Drag = [C * Y] + [C * (Z squared)] (Not completely sure) I am rusty on this but you get the general idea. What a sim developer quickly realizes here is that trying to do this to complete accuracy is virtually impossible since it would involve modelling every single outer part of the car and all their different C's and slipstream thresholds. So what could be done is to just give the entire car an average "C" and slipstream threshold (my guess is this is where most sims stand for now with only linear/overall adjustments for different wing settings) or take it a step or 2 further and just model the most important bits. Perhaps model front wheels and susp boxs as 1 piece, same for rear wheels/susp boxs, front and rear wings, and, of course, the body/driver/helmet. I would think we should expect the body/susp/tyres/driver/etc and wings to be modelled independantly in modern sims. ie adjusting ride height at the front and rear would increase/decrease drag/downforce and changing wing angles would adjust the drag/downforce for them and these 3 things should be calculated independantly and their sum be the overall areodynamic model of the car. Maybe Mike Lescault could enlighten us on any plans in this area for Cart Racing 2 and how it was done in ICR2 :))

This morning at work, when we were having our morning tea, there was a lull in the usual conversation about what we did on the weekend and who won the football and wasn't the World Cup soccer great and how are the kids and during this lull I just happened to say "I was at the start of the footprint (of a rolling tire, not a sliding tire)and the *** is actually a complex brew of various polymers - long chain molecules". Well! Did that put the kybosh on the conversation!! Talk about being at a loss for words....the others just mumbled something inconsequential and meandered off! Thanks to RAS I am now able to astound my co-workers, my family and my friends with my new-found vocabulary! Life is truly wonderful! > Hi All, > Understanding tires isn't easy.(etc) -- Regards, Bruce. ---------- The GP Legends Historic Motor Racing Club is located at:- http://www.netspeed.com.au/brucek/legends/

"Axels" as in Axel Foley, Beverly Hills cop, or "axles" as in they stop the wheels from running away from the car? :o)

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> > Part of the answer is that the grooved tires use a harder *** > > compound than slick tires. The harder compound is required to > > combat "squirm", which has been discussed already. The harder > > compound is obviously going to provide less grip than a softer > > compound would, regardless of the width of the tire and size of > > the contact patch. > Still, the "squirm" factor is insignificant alongside the reduced contact patch and the > resultant increased load per square inch of tyre. Much as friction is far pre*** over > chemical adhesion :)) > > Here is something else to consider in the narrower v. wider > > tire debate. Wider tires don't merely increase the tire > > contact patch, they also increase the track width of the > > vehicle. That is, the center-to-center distance between > > tires on opposite sides of the car is greater, which will > > tend to increase the mechanical grip produced (like that > > TV car commercial where they say "wider is better"). > Not if the axels are shortened.

-- Regards, Bruce. ---------- The GP Legends Historic Motor Racing Club is located at:- http://www.netspeed.com.au/brucek/legends/

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> With that stated, how does the recent introduction of grooved tires (in > F1) have any bearing on the amount of grip which the tires provide? > Although the contact patch is reduced there would be no less friction at > work to keep the tires in the road. We were unable to figure this out. I > hope you can succinctly explain this in laymen's terms for me :) > > Part of the answer is that the grooved tires use a harder *** > > compound than slick tires. The harder compound is required to > > combat "squirm", which has been discussed already. The harder > > compound is obviously going to provide less grip than a softer > > compound would, regardless of the width of the tire and size of > > the contact patch. > Still, the "squirm" factor is insignificant alongside the > reduced contact patch and the resultant increased load per > square inch of tyre. Much as friction is far pre*** over > chemical adhesion :))

If we are talking about grooved tires, then I think squirm _is_ probably a major factor in determining how hard the tire compound has to be. We are talking about individual small blocks of *** bearing the side load forces, rather than just a narrower slick tire. These smaller blocks on a grooved tire would be much more susceptible to "chunking" than a slick tire would. Even if total contact area is the same, a harder compound would be required for a grooved tire. I thought we were talking about tires? What if we lengthened the axles? -- Pat Dotson IMPACT Motorsports

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> Shit, I finally made it to the end! :)) So I now know what Tribology is and have a new > parameter to play with - chemical bonding. > Now Jerry, let me throw a few educated ideas at you. > 1/ I say that these chemical bonds are probably negligible under just about all > circumstances compared to the friction effect. > 2/ I say that 1/ is even more so at higher speeds where winged cars add massively to > the downforce and consequently friction. Also, with or without wings, at high speeds this > chemical bonding has less time to take effect and becomes even more negligable ie less > time for chemical reaction to take effect.

I think that the "chemical bond" or "chemical adhesion" that keeps getting mentioned is simply the "stickiness" property of *** as it heats. While I'd agree that this force is much smaller than the total friction force, it is hardly negligible. It is the difference in lap times between your first lap after coming out of the pits, and your second or third lap. A few tenths of a second gained by heating the tires is very significant. I keep telling myself this as I wildly swerve during my warm-up lap :) I don't belong to any tribe, but going back to the original question from Simon - the bulk of the difference between grooved tires and slicks can be boiled down to the harder *** compound required for grooved tires (the harder compound being required due to heat build-up caused by less contact patch surface area and associated squirm/chunking). In a static situation, slick tires vs. grooved tires (using the same *** compound) should have no difference in the maximum friction force they can generate. That would correspond to the equation for friction force, which ignores surface area. Further, I think that in a static situation, wider tires vs. narrower tires have no difference in the maximum friction force they can generate. At high speeds (and high temp), however, the larger contact patch of the wider tires will provide more "stickiness" adhesion, and they will be slightly faster. -- Pat Dotson B.S. Mechanical Engineering - Purdue University IMPACT Motorsports

> You're right, the molecules don't know what's goin on more than a couple > microns away. Let me try to explain. If one square cm has 100 bonds, then > ten sq cm will have 1000 bonds. Each individual bond adds to traction and > the number of bonds is dependent on the surface area of the contact patch. > So bigger tires have more bonds and therefor more traction. Got it? The bonds must be between the tyre and the pavement, of course, and not within the tyre as I thought. These bonds are not present when the tyre is cold, but when heated the tyre surface change to be a more glue-like surface and actually stick to the pavement as a kind of glue and increase the grip by quite a lot I would guess. Is this added grip proportional to the pressure on the tyre, when varying the pressure at the same temperature, or is it constant within reasonable sizes of car and tyre? It feels as if it is constant. Even a very light car would stick very much to the pavement, if we only could get the tyres hot enough. The effect of course varies with tyre temperature. Is this what you were trying to explain, Jer? /Christer, keep grasping more and more :o)

On Fri, 19 Jun 1998 11:37:00 GMT, Christer Andersson

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>Less area is a bad thing if you want to cool something. The worst scenario is a >sphere, cause regarding a sphere you have the worst relation between volume and >surface area. You have to little surface area per volume. In a narrow tyre >compared to a wide tyre this relation isn't very different, it's actually a >little bit worse for the narrower tyre, cause it's closer to the circle shape, >when intersected. What I'm actually is saying is that regarding heat dissipation >through the contact surface, it's about the same no matter how wide or narrow >the tyre is. This is of course on a tyre that is the same in any other way, but >wider or narrower.

I think you have this about exactly wrong. On a wider tire the contact patch is thin and wide relative to the airflow, while with a narrower tire the contact patch is more square and thus doesn't cool as well in the middle of the patch. - Nick

Oh no, formulas, please no formulas :o). Lets wait with the formulas until we have to calculate something and instead concentrate on how to understand this phenomena, okay :o)? I interpreted drag as somehing dragging the car, thus vacuum. It's of course not vacuum, but a lower air pressure than normal (if it gets to low, their will be rain). At the front of a moving car there is something trying to push the car backwards - air resistance. In front of the car there is of course air and to place the car where the air is, we have to move the air, and as we all know air has mass, so this will need some force :o). I guess we can do this in two way, either push the air in front of us (a flat front) or push it aside (a front formed as some kind of projectile). I wont try to understand why pushing the air is a bad idea, but we dont want a parachute effect, I guess. So we push the air aside; over, under and to the sides of the car. The amount of air pushed aside must be approximately the front area of the car (take a picture of the car from the front, and any part of the picture covered by the car is the front area) mulitplied with the length we have travelled. This mass of air being moved is of course moved faster the faster we go. The faster we have to move the air the greater force needed to do so, hence this force increase with speed. To calculate the force needed could be tricky, because the air doesn't only get moved it also compresses and the compression increases with speed. Now what we have created is locally higher pressure of air over, under and on the sides of the moving car, so when the now locally lower pressure (what I earlier unpedagogically called vacuum :o) ) of air behind the car gets in contact of this locally higher pressures, well, what I'm trying to say is that I dont think the air needs a lot of force to get down behind the car. Have you noticed how an F1 car narrows at the rear. The lower pressure sucking down the air behind the car is of course also pulling on the car itself. To recap, this lower pressure behind the moving car was because we travel away from the air behind the car, thus behind the car there will be locally lower air pressure. I dont think the lower pressure at the rear will help to suck the air from the front to the rear. Think of it as you're standing by the side of a track looking at the air in front of you over the track. An F1 car passes by. When it hits the air in front of you it will push that air upwards, when the car is straight in front of you the air above it is compressed and shortly after, because of being compressed and the lower pressure behind the car, pushed down behind the car. What's turbulence? /Christer, now out of air :o)

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> > > Okay, so lets talk about aerodynamic drag...o^) > > I know, I know, I know, I mean have some guesses :o))). When a car moves it > > leaves the air molecules that was behind it, so these molecules must be > > replaced, because vacuum is not acceptable within our atmosphere :o). This start > > of a vacuum behind the car will of course also pull on the car, thus drag. The > > principle is the same as in your vacuum cleaner, Bruce, it's as we had someone > > chasing us with a vacuum cleaner :o). > > With a wing it gets more complicated. Because the wing has forced the air > > upwards with speed it makes this air harder to suck in behind the car, hence the > > vacuum gets a little larger with a wing, and even larger with more wing :o). > > Now correct me :o). > Sure will! :)) > This vacumn only occurs at high speeds and is more accurately termed "eddy" (or was > that Eddie?) or turbulance. Race cars and there wings are designed to be as > aerodynamically efficient as possible. They will enjoy perfect slipstreaming (shearing > effects close to the surface only) until the speed of air passing around them can no > longer act in a perfectly slipstreamed fashion and then eddies begin to form which is less > efficient. > From memory, this is what this all means mathematically. > Drag increases linearly whilst in perfect slipstreaming conditions. ie Drag = > Coefficient of drag (C) * Speed > the Coefficient of drag (C) in this example is related to the slipperiness of the > surface (special paints, etc) and for the entire car would be some type of average of the > sum of all the bits of the car ie tyres, wings, driver's helmet, body, etc, etc. > As different bits of the car enter into non slipstreamed behavior as speed increases, > then from this point the drag increases as the square of the speed. > ie delta Drag = C * (delta Speed) squared (Not completely sure) > So to use the front wing as an example, lets say it has perfect slipstreaming till > speed "Y" and that the speed of the car is Y + Z > Drag = [C * Y] + [C * (Z squared)] (Not completely sure) > I am rusty on this but you get the general idea. > What a sim developer quickly realizes here is that trying to do this to complete > accuracy is virtually impossible since it would involve modelling every single outer part > of the car and all their different C's and slipstream thresholds. So what could be done is > to just give the entire car an average "C" and slipstream threshold (my guess is this is > where most sims stand for now with only linear/overall adjustments for different wing > settings) or take it a step or 2 further and just model the most important bits. Perhaps > model front wheels and susp boxs as 1 piece, same for rear wheels/susp boxs, front and > rear wings, and, of course, the body/driver/helmet. I would think we should expect the > body/susp/tyres/driver/etc and wings to be modelled independantly in modern sims. ie > adjusting ride height at the front and rear would increase/decrease drag/downforce and > changing wing angles would adjust the drag/downforce for them and these 3 things should be > calculated independantly and their sum be the overall areodynamic model of the car. Maybe > Mike Lescault could enlighten us on any plans in this area for Cart Racing 2 and how it > was done in ICR2 :))

If you would lay down on it while it's hot you'd stick to it and have great trouble getting up again :o). Would probably have to wait until it has cooled down. /Christer, in favor of the larger than 1000 ms pings multiplayer proposal, with collision detection on :o)

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> Ha ha ha. Now you have me imagining what a tire 60 inches wide will do > Christer. > -- > mark > "A lot of people think racing is about going fast. But it's not. It's about > going just slow enough to stay on the track." F1RS - > http://www.nmia.com/~chaser/car/results.htm > Remove us here and there to mail me. > :o). Bare with me will you, please. If we have a tyre > with a contact patch of one square cm and it has 100 bonds, then the > pressure on one > bond will be 1/100 of the pressure, right? If we increase the > contact patch to ten > square cm, it will have 1000 bonds. The pressure per bond will now > only be 1/1000 of > the pressure, or 1/10 of what it was on the smaller tyre, but you > say the traction > added from a bond will not have decreased with 1/10, but with less > than 1/10, on the > bigger tyre... Why? Not why you say it, but what causes it :o)?

I totally agree with everything you said, Pat, but would like to add that with a wider tyre you can have a softer compound and it would still last as long as the harder narrower tyre. /Christer Andersson M.S. Computer Science and Engineering, so I just know the basic mechanical physics :o)

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> I think that the "chemical bond" or "chemical adhesion" that keeps > getting mentioned > is simply the "stickiness" property of *** as it heats. While I'd > agree that this > force is much smaller than the total friction force, it is hardly > negligible. It is > the difference in lap times between your first lap after coming out of > the pits, and > your second or third lap. A few tenths of a second gained by heating > the tires is > very significant. I keep telling myself this as I wildly swerve during > my warm-up > lap :) > > So, what does the one from the Tribology tribe say to this? And some accurate > > percentages would be much appreciated if you could ie the percentage ratio of the friction > > effect compared to this bonding stuff. (Come to think of it, how could one measure this? > > Perhaps by testing tyres of the same compound but different widths at the same > > speed/weight and calculating the difference in grip, etc, etc.....???????) > I don't belong to any tribe, but going back to the original question > from Simon - the > bulk of the difference between grooved tires and slicks can be boiled > down to the harder > *** compound required for grooved tires (the harder compound being > required > due to heat build-up caused by less contact patch surface area and > associated > squirm/chunking). In a static situation, slick tires vs. grooved tires > (using the > same *** compound) should have no difference in the maximum friction > force they can > generate. That would correspond to the equation for friction force, > which ignores > surface area. > Further, I think that in a static situation, wider tires vs. narrower > tires > have no difference in the maximum friction force they can generate. At > high speeds > (and high temp), however, the larger contact patch of the wider tires > will provide > more "stickiness" adhesion, and they will be slightly faster. > -- > Pat Dotson > B.S. Mechanical Engineering - Purdue University > IMPACT Motorsports

> I totally agree with everything you said, Pat, but would like to add that with a wider tyre you > can have a softer compound and it would still last as long as the harder narrower tyre. Yep, that's true, just as it's true in the slick vs. grooved scenario. Unfortunately, there are so many variables involved in this discussion that something is bound to be left out. But, assuming that for each style of tire, you select a compound that will yield equal tire life, then the wider tire will have a softer compound (and more grip). The question is, will any of this help me tonight in my first official OSCAR Freedom Cup race? :) -- Pat Dotson IMPACT Motorsports

If you believe it will help you, it will do so :o). Anyway, good luck, Pat :o). /Christer

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> > I totally agree with everything you said, Pat, but would like to add that with a wider tyre you > > can have a softer compound and it would still last as long as the harder narrower tyre. > Yep, that's true, just as it's true in the slick > vs. grooved scenario. Unfortunately, there are so > many variables involved in this discussion that > something is bound to be left out. But, assuming > that for each style of tire, you select a compound > that will yield equal tire life, then the wider > tire will have a softer compound (and more grip). > The question is, will any of this help me tonight > in my first official OSCAR Freedom Cup race? :) > -- > Pat Dotson > IMPACT Motorsports

> Oh no, formulas, please no formulas :o). Lets wait with the formulas until we have to > calculate something and instead concentrate on how to understand this phenomena, okay :o)? I'd advise starting with basic formulas, actually. You can do an awful lot of vehicle dynamics/performance analysis with them. (This is only as deep as I go working in dynamics and performance, and I have a MS in aerodynamics. In order to go deeper, one hires a specialist in the field ;) Really "knowing why" some shape drags or lifts is complicated because you have to deal with the "boundary layer" between the car surface and the pretty-much-smoothly-flowing air. If you don't include these complicated effects (or cheat by throwing in a few experimental observations) you can't even do a theory that predicts lift or drag. ("Inviscid Flow") The air splits and flows around the vehicle. At the car surface, the velocity of the air is equal to the velocity of the surface - it's drug along with the vehicle. Even more air is drug along not-quite-so near the surface to some extent, just not fast enough to be as fast as the vehicle. Speeding up all this air decelerates the car somewhat (Newton's equal and opposite reactions, and all) - call it friction drag. The "static pressure" of the airflow (what would be measured by a pressure port at the surface and what creates the force that presses onto the surface) drops as the air molecules flow faster. They have to flow faster or slower to make changes in direction. Going up the windshield the molecules slow and the static pressure is higher. Going down the back of the car they speed up and the pressure drops. When it drops far enough, the air very near the surface actually is forced back forward by the static pressure gradient (rate of change). The boundary layer "Separates" and you get a lot of air roiling around behind the car. The pressure here is relatively low. So we have high pressures near the front of the car, and lower pressures at the back. Pressure times area is force, so the summary is we have a net force aft. Call it "Pressure Drag" or "Form Drag". When you use Lift to get more downforce what you are doing is forcing a lot of air upwards to get that downforce. The average force to change the direction of all that air slightly upwards has a component aft as well - call it "Induced Drag", for drag induced by lifting. Finally, if you get near supersonic speeds, you will add an important drag effect from shockwaves. "Wave Drag". Actually, if you don't include the effect of the complicated friction forces you are doing "Inviscid Flow" above and your theory will eventually find the air will part around the vehicle, the pressure will be different around different parts of the vehicle, but the overall net effect of all those differing pressures is zero lift, and zero drag. This is why I suggest starting with the basic equations useful for vehicle dynamics. You have just made a justification for why drag is commonly estimated as proportional to the area and also proportional to the "dynamic pressure". Density times velocity times velocity divided by 2 (just for fun.) (Unless you get near sonic speeds, the air can be considered "incompressible" (just like water) so the air really doesn't compress noticibly.) Hehe, I just glanced at "Fundamentals of Vehicle Dynamics," by Gillespie, S.A.E., Warrendale, PA, 1991, ISBN 1-56091-199-9 and find these are the FIRST and SECOND equations in 30 pages on aerodynamics ;) In practice, you use the equation: drag = drag_coefficient times area times dynamic_pressure for everything, including understanding that if you make the car twice as big, and go twice as fast then you will need six*** times the engine power portion alloted to fighting drag. If you want to know why the car gets less drag with the new wing airfoil in place of the old one ("what is the drag_coefficient") you ask your aerodynamicist ;) (Or learn enough aerodynamics to guess yourself) See Gilespie's aerodynamics chapter ;) - Matt

My God, what have I created???? :^) phenomena, okay :o)? course not vacuum, but a the car where the air is, will need some force :o). I (a flat front) or push it understand why pushing the we push the air aside; of the car (take a picture car is the front area) moved is of course moved greater force needed to do so, could be tricky, because increases with speed. under and on the sides of the unpedagogically called higher pressures, well, force to get down behind lower pressure sucking down To recap, this lower air behind the car, thus from the front to the at the air in front of you you it will push that it is compressed and behind the car, pushed down

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>behind the car. >What's turbulence? >/Christer, now out of air :o) >> > > Okay, so lets talk about aerodynamic drag...o^) >> > I know, I know, I know, I mean have some guesses :o))). When a

car moves it >> > leaves the air molecules that was behind it, so these molecules must be >> > replaced, because vacuum is not acceptable within our atmosphere :o). This start >> > of a vacuum behind the car will of course also pull on the car, thus drag. The >> > principle is the same as in your vacuum cleaner, Bruce, it's as we had someone >> > chasing us with a vacuum cleaner :o). >> > With a wing it gets more complicated. Because the wing has forced the air >> > upwards with speed it makes this air harder to suck in behind the car, hence the >> > vacuum gets a little larger with a wing, and even larger with more wing :o). >> > Now correct me :o). >> Sure will! :)) >> This vacumn only occurs at high speeds and is more accurately termed "eddy" (or was slipstreaming (shearing begin to form which is less conditions. ie Drag = the slipperiness of the type of average of the behavior as speed increases, every single outer part So what could be done is (my guess is this is important bits. Perhaps wheels/susp boxs, front and we should expect the these 3 things should be model of the car. Maybe

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> kids and during this lull I just happened to say "I was at the start of > the footprint (of a rolling tire, not a sliding tire)and the *** is > actually a complex brew of various polymers - long chain molecules". > Well! Did that put the kybosh on the conversation!! Talk about being at > a loss for words....the others just mumbled something inconsequential > and meandered off!

Gee Bruce, sorry to hear that you work in such a boring place <grin>. We often start the day with a comment like that, which leads into a long conversation...it's a wonder that we ever got that book written. Just in case you need a comment for tomorrow, here's a thought. The polymers that are used to make race tire "***" are quite different from those used in street tires. For example, don't try to use normal "racing ***" when it's cold - it has very little grip, much less than street tires at the same temperature. Remember the Indy 500 some years back when it was cold? On just about every re-start it seemed that someone lost it getting on the power coming out of turn 4, before their tires came up to temperature. -- Doug Milliken Research Associates Inc.

Great! That was fun. Thanks Matt. -- mark "A lot of people think racing is about going fast. But it's not. It's about going just slow enough to stay on the track." F1RS - http://www.nmia.com/~chaser/car/results.htm Remove us here and there to mail me. > > Oh no, formulas, please no formulas :o). Lets wait with the formulas until we have to > calculate something and instead concentrate on how to understand this phenomena, okay :o)? I'd advise starting with basic formulas, actually. You can do an awful lot of vehicle dynamics/performance analysis with them. <clip>

Wouldn't heat play the bigger part of the role? The compound that gets softer will have more surface area due to molding itself to the pavement. This appears to be true when going from an asphalt track to a concrete track on the same tires. Even with excessive sliding the concrete track offers much less grip than the rougher track does. The difference is astounding. If I use two different compounds on the same track in the same session it seems that the grip level of either compound is very close once they are heated. The harder compound only takes longer to get there. Also, while rocks and lizards and such will stick to the heated tire, heated tire chunks don't really stick to much else. -- mark Please excuse my English. I am American. F1RS - http://www.nmia.com/~chaser/car/results.htm Remove us here and there to mail me. > You're right, the molecules don't know what's goin on more than a couple > microns away. Let me try to explain. If one square cm has 100 bonds, then > ten sq cm will have 1000 bonds. Each individual bond adds to traction and > the number of bonds is dependent on the surface area of the contact patch. > So bigger tires have more bonds and therefor more traction. Got it? The bonds must be between the tyre and the pavement, of course, and not within the tyre as I thought. These bonds are not present when the tyre is cold, but when heated the tyre surface change to be a more glue-like surface and actually stick to the pavement as a kind of glue and increase the grip by quite a lot I would guess. Is this added grip proportional to the pressure on the tyre, when varying the pressure at the same temperature, or is it constant within reasonable sizes of car and tyre? It feels as if it is constant. Even a very light car would stick very much to the pavement, if we only could get the tyres hot enough. The effect of course varies with tyre temperature. Is this what you were trying to explain, Jer? /Christer, keep grasping more and more :o)

> There is a summary of the basics in our book, "Race Car Vehicle > Dynamics", available through the online bookstore at www.sae.org ah, now there's a book i would die to get my hands on :) i'm kinda into the physics of race cars myself and it had been recommended to me by certain leading sim developers, but i had gone and forgotten what the name of the book was. But when i just went and looked up your book the price scared me half to death! i think i'll wait until i get a good fulltime job before i can afford to buy that:) looking forward to getting it someday! Grant Reeve.

Yep, remember it well, Doug. Incidentally......just out of curiosity, is your organisation free of buffalos or do you provide buffalos with net access at no cost? Just curious. -- Regards, Bruce. ---------- The GP Legends Historic Motor Racing Club is located at:- http://www.netspeed.com.au/brucek/legends/

A monster, of course, and you should be proud of it :o))). /Christer, not afraid to confuse :o)

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> My God, what have I created???? :^) > >Oh no, formulas, please no formulas :o). Lets wait with the formulas > until we have to > >calculate something and instead concentrate on how to understand this > phenomena, okay :o)? > >I interpreted drag as somehing dragging the car, thus vacuum. It's of > course not vacuum, but a > >lower air pressure than normal (if it gets to low, their will be > rain). > >At the front of a moving car there is something trying to push the > car backwards - air > >resistance. In front of the car there is of course air and to place > the car where the air is, > >we have to move the air, and as we all know air has mass, so this > will need some force :o). I > >guess we can do this in two way, either push the air in front of us > (a flat front) or push it > >aside (a front formed as some kind of projectile). I wont try to > understand why pushing the > >air is a bad idea, but we dont want a parachute effect, I guess. So > we push the air aside; > >over, under and to the sides of the car. > >The amount of air pushed aside must be approximately the front area > of the car (take a picture > >of the car from the front, and any part of the picture covered by the > car is the front area) > >mulitplied with the length we have travelled. This mass of air being > moved is of course moved > >faster the faster we go. The faster we have to move the air the > greater force needed to do so, > >hence this force increase with speed. To calculate the force needed > could be tricky, because > >the air doesn't only get moved it also compresses and the compression > increases with speed. > >Now what we have created is locally higher pressure of air over, > under and on the sides of the > >moving car, so when the now locally lower pressure (what I earlier > unpedagogically called > >vacuum :o) ) of air behind the car gets in contact of this locally > higher pressures, well, > >what I'm trying to say is that I dont think the air needs a lot of > force to get down behind > >the car. Have you noticed how an F1 car narrows at the rear. The > lower pressure sucking down > >the air behind the car is of course also pulling on the car itself. > To recap, this lower > >pressure behind the moving car was because we travel away from the > air behind the car, thus > >behind the car there will be locally lower air pressure. > >I dont think the lower pressure at the rear will help to suck the air > from the front to the > >rear. Think of it as you're standing by the side of a track looking > at the air in front of you > >over the track. An F1 car passes by. When it hits the air in front of > you it will push that > >air upwards, when the car is straight in front of you the air above > it is compressed and > >shortly after, because of being compressed and the lower pressure > behind the car, pushed down > >behind the car. > >What's turbulence? > >/Christer, now out of air :o) > >> > > Okay, so lets talk about aerodynamic drag...o^) > >> > I know, I know, I know, I mean have some guesses :o))). When a > car moves it > >> > leaves the air molecules that was behind it, so these molecules > must be > >> > replaced, because vacuum is not acceptable within our atmosphere > :o). This start > >> > of a vacuum behind the car will of course also pull on the car, > thus drag. The > >> > principle is the same as in your vacuum cleaner, Bruce, it's as > we had someone > >> > chasing us with a vacuum cleaner :o). > >> > With a wing it gets more complicated. Because the wing has forced > the air > >> > upwards with speed it makes this air harder to suck in behind the > car, hence the > >> > vacuum gets a little larger with a wing, and even larger with > more wing :o). > >> > Now correct me :o). > >> Sure will! :)) > >> This vacumn only occurs at high speeds and is more accurately > termed "eddy" (or was > >> that Eddie?) or turbulance. Race cars and there wings are designed > to be as > >> aerodynamically efficient as possible. They will enjoy perfect > slipstreaming (shearing > >> effects close to the surface only) until the speed of air passing > around them can no > >> longer act in a perfectly slipstreamed fashion and then eddies > begin to form which is less > >> efficient. > >> From memory, this is what this all means mathematically. > >> Drag increases linearly whilst in perfect slipstreaming > conditions. ie Drag = > >> Coefficient of drag (C) * Speed > >> the Coefficient of drag (C) in this example is related to > the slipperiness of the > >> surface (special paints, etc) and for the entire car would be some > type of average of the > >> sum of all the bits of the car ie tyres, wings, driver's helmet, > body, etc, etc. > >> As different bits of the car enter into non slipstreamed > behavior as speed increases, > >> then from this point the drag increases as the square of the speed. > >> ie delta Drag = C * (delta Speed) squared (Not > completely sure) > >> So to use the front wing as an example, lets say it has perfect > slipstreaming till > >> speed "Y" and that the speed of the car is Y + Z > >> Drag = [C * Y] + [C * (Z squared)] (Not completely sure) > >> I am rusty on this but you get the general idea. > >> What a sim developer quickly realizes here is that trying to do > this to complete > >> accuracy is virtually impossible since it would involve modelling > every single outer part > >> of the car and all their different C's and slipstream thresholds. > So what could be done is > >> to just give the entire car an average "C" and slipstream threshold > (my guess is this is > >> where most sims stand for now with only linear/overall adjustments > for different wing > >> settings) or take it a step or 2 further and just model the most > important bits. Perhaps > >> model front wheels and susp boxs as 1 piece, same for rear > wheels/susp boxs, front and > >> rear wings, and, of course, the body/driver/helmet. I would think > we should expect the > >> body/susp/tyres/driver/etc and wings to be modelled independantly > in modern sims. ie > >> adjusting ride height at the front and rear would increase/decrease > drag/downforce and > >> changing wing angles would adjust the drag/downforce for them and > these 3 things should be > >> calculated independantly and their sum be the overall areodynamic > model of the car. Maybe > >> Mike Lescault could enlighten us on any plans in this area for Cart > Racing 2 and how it > >> was done in ICR2 :))

> I'd advise starting with basic formulas, actually. You must understand that most of us dont even understand the basic formulas, imaging what damage we could do if we started to use them without understanding them :o). Okay, friction drag :o). Okay, pressure drag :o). I've just did some calculations on an F1 car at around 200 kph and got this force, caused by the pressure drag, to around 3 kN. Am I way off here??? I at least hope it's closer to 3 kN than 0.3 kN or 30 kN. According to my simple and very approximative calculations it seems as this pressure drag is proportional to the speed squared, so if the speed doubles the force quadruples. Okay, induced drag :o). What do you mean zero drag? No pressure drag at all? In my simple world the air in front of the car has to be moved for the car to pass and that must take a force? Dont we have to understand them first :o)? Sounds as if I'm not way off on the pressure drag? Isn't that cheating :o)?/Christer, really enjoys this NG at times like this :o)

Grant -- Don't you have free public libraries? Around here, if I ask the local library for a book they go out and buy it (then I can take it out for 3 weeks at a time). Ah, well, anything for a sale... Also, I've seen our book advertized at VERY high prices in various magazines from Europe. It's often cheaper to order direct from the SAE. If you prepay with a credit card then they only add on USD 10.00 for international airmail -- which is a bargain, since the book weighs ~5 lbs (mass ~2.3 kilos). -- Doug > > There is a summary of the basics in our book, "Race Car Vehicle > > Dynamics", available through the online bookstore at www.sae.org > ah, now there's a book i would die to get my hands on :) i'm kinda > into the physics of race cars myself and it had been recommended > to me by certain leading sim developers, but i had gone and forgotten

> Incidentally......just out of curiosity, is your organisation free of > buffalos or do you provide buffalos with net access at no cost? > Just curious. Curiously enough, no one can seem to find any record of buffalos (bison) in this area -- the Western end of New York State. We do our best to buffalo the rest of the world to make up for it. ---------- The Buffalo Freenet is just that - free net access (text only), sponsored by the local university, the local library system, grants of old equipment (and etc) from a few companies, and user donations (yes, I do chip in). From what I hear, I get better service (uptime) and "help" than many friends that use commercial services.

Aha!! Clarification and the puzzle is solved! Good on you people for your efforts in providing a low-cost/free net access facility. An excellent idea!

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> Curiously enough, no one can seem to find any record of buffalos > (bison) in this area -- the Western end of New York State. > We do our best to buffalo the rest of the world to make up for it. > ---------- > The Buffalo Freenet is just that - free net access (text only), sponsored > by the local university, the local library system, grants of old equipment > (and etc) from a few companies, and user donations (yes, I do chip in). > From what I hear, I get better service (uptime) and "help" than many > friends that use commercial services.

-- Regards, Bruce. ---------- The GP Legends Historic Motor Racing Club is located at:- http://www.netspeed.com.au/brucek/legends/

Okay, when I said "let's talk about aerodynamics..." it was a JOKE! Ha ha, it is to laugh! Enough already! Mercy! Look, I'm the poor programmer who gets to implement all this stuff. My head hurts! ;<) Todd >> Oh no, formulas, please no formulas :o). Lets wait with the formulas until we have to this phenomena, okay :o)? the car where the air is, will need some force :o). I (a flat front) or push it understand why pushing the we push the air aside; of the car (take a picture the car is the front area) being moved is of course moved greater force needed to do so, could be tricky, because compression increases with speed.

If you stick your head out the window when doing 200 kph, you'll feel the drag :o). /Christer, this is no time for common sense

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> Okay, when I said "let's talk about aerodynamics..." it was a JOKE! > Ha ha, it is to laugh! Enough already! Mercy! Look, I'm the poor > programmer who gets to implement all this stuff. My head hurts! ;<) > Todd > >> Oh no, formulas, please no formulas :o). Lets wait with the > formulas until we have to > >> calculate something and instead concentrate on how to understand > this phenomena, okay :o)? > >I'd advise starting with basic formulas, actually. > >You can do an awful lot of vehicle dynamics/performance > >analysis with them. (This is only as deep as I go working > >in dynamics and performance, and I have a MS in aerodynamics. > >In order to go deeper, one hires a specialist in the field ;) > >Really "knowing why" some shape drags or lifts > >is complicated because you have to deal with the > >"boundary layer" between the car surface and the > >pretty-much-smoothly-flowing air. If you don't > >include these complicated effects (or cheat by throwing > >in a few experimental observations) you can't even > >do a theory that predicts lift or drag. ("Inviscid Flow") > >The air splits and flows around the vehicle. At the car > >surface, the velocity of the air is equal to the velocity > >of the surface - it's drug along with the vehicle. Even more > >air is drug along not-quite-so near the surface to some extent, > >just not fast enough to be as fast as the vehicle. > >Speeding up all this air decelerates the car somewhat > >(Newton's equal and opposite reactions, and all) - call > >it friction drag. > >The "static pressure" of the airflow > >(what would be measured by a pressure port at the surface > >and what creates the force that presses onto the surface) > >drops as the air molecules flow faster. They have to > >flow faster or slower to make changes in direction. > >Going up the windshield the molecules slow and the > >static pressure is higher. Going down the back of the car > >they speed up and the pressure drops. When it drops > >far enough, the air very near the surface actually is > >forced back forward by the static pressure gradient > >(rate of change). The boundary layer "Separates" and you > >get a lot of air roiling around behind the car. > >The pressure here is relatively low. So we have > >high pressures near the front of the car, and lower pressures > >at the back. Pressure times area is force, > >so the summary is we have a net force aft. Call > >it "Pressure Drag" or "Form Drag". > >When you use Lift to get more downforce > >what you are doing is forcing a lot of air upwards > >to get that downforce. The average force to > >change the direction of all that air slightly > >upwards has a component aft as well - call it > >"Induced Drag", for drag induced by lifting. > >Finally, if you get near supersonic speeds, > >you will add an important drag effect from shockwaves. > >"Wave Drag". > >> At the front of a moving car there is something trying to push the > car backwards - air > >> resistance. In front of the car there is of course air and to place > the car where the air is, > >> we have to move the air, and as we all know air has mass, so this > will need some force :o). I > >> guess we can do this in two way, either push the air in front of us > (a flat front) or push it > >> aside (a front formed as some kind of projectile). I wont try to > understand why pushing the > >> air is a bad idea, but we dont want a parachute effect, I guess. So > we push the air aside; > >> over, under and to the sides of the car. > >Actually, if you don't include the effect of the complicated friction > >forces you are doing "Inviscid Flow" above and your theory will > >eventually > >find the air will part around the vehicle, the pressure will be > >different around different parts of the vehicle, but the overall net > >effect of all those differing pressures is zero lift, and zero drag. > >> The amount of air pushed aside must be approximately the front area > of the car (take a picture > >> of the car from the front, and any part of the picture covered by > the car is the front area) > >> mulitplied with the length we have travelled. This mass of air > being moved is of course moved > >> faster the faster we go. The faster we have to move the air the > greater force needed to do so, > >> hence this force increase with speed. To calculate the force needed > could be tricky, because > >> the air doesn't only get moved it also compresses and the > compression increases with speed. > >This is why I suggest starting with the basic equations useful for > >vehicle dynamics. You have just made a justification for why > >drag is commonly estimated as proportional to the area > >and also proportional to the "dynamic pressure". > >Density times velocity times velocity divided by 2 (just for fun.) > >(Unless you get near sonic speeds, the air can be considered > >"incompressible" (just like water) so the air really > >doesn't compress noticibly.) > >Hehe, I just glanced at "Fundamentals of Vehicle Dynamics," > >by Gillespie, S.A.E., Warrendale, PA, 1991, ISBN 1-56091-199-9 > >and find these are the FIRST and SECOND equations in 30 pages > >on aerodynamics ;) > >In practice, you use the equation: > > drag = drag_coefficient times area times dynamic_pressure > >for everything, including understanding that if you make the > >car twice as big, and go twice as fast then you will need > >six*** times the engine power portion alloted to fighting drag. > >If you want to know why the car gets less drag with the > >new wing airfoil in place of the old one > >("what is the drag_coefficient") > >you ask your aerodynamicist ;) (Or learn enough > >aerodynamics to guess yourself) > >> What's turbulence? > >See Gilespie's aerodynamics chapter ;) > >- Matt

the drag 200 kph... that's like 20 miles per hour, right? Actually, I prefer to stick my hand out the window and pretend that it's a wing, angling it up and down and feeling the lift and downforce. I only stick my head out the window when I have my dog along for the ride. He hates it when I do that. Todd

Hi Doug, > Don't you have free public libraries? Around here, if I ask the > local library for a book they go out and buy it (then I can take > it out for 3 weeks at a time). Ah, well, anything for a sale... Yep we do have them here, but they never quite have the exact books I want to get out. It wouldn't feel right to me to go ask them to buy a book that I know I'm the only person probably in the whole area who is remotely interested in reading. It's a waste of their money. And I keep forgetting to hand library books back so eventually I could probably have bought the book from the fines.... :) I am moving to the States in a month or three so I'll just pick the book up from somewhere then. This is good for numerous reasons: a) i'll have a job and can afford it (and the company may even buy it for me) b) lower shipping costs and shipping time, c) it will mean i take 2.3kg less luggage with me on the plane:) On the other hand in our dangerous modern societies it would probably be a good idea to have a book that big at hand to defend myself against possible intruders and other nefarious individuals. Although it's debatable as to which method would drive away an attacker more promptly: swinging the book at him/her/it, or opening up the book and starting to read out a section on tyre tread deformation at high slip angle cornering.... Meanwhile i'll just stick to reading a couple of Carroll Smith's books, that i finally found at our librarying when i discovered all the larger size books were in a different section. thanks! Grant.

You aren't serious, are you? > 200 kph... that's like 20 miles per hour, right? -- Regards, Bruce. ---------- The GP Legends Historic Motor Racing Club is located at:- http://www.netspeed.com.au/brucek/legends/

40? 60? It's kind of like Canadian money, right? (I can say that, 'cause my wife is Canadian, as are my kids. Don't worry, I'll get mine.) Doesn't it fluctuate? Or is it ever decreasing? 8>)

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>You aren't serious, are you? >> 200 kph... that's like 20 miles per hour, right? >-- >Regards, >Bruce. >---------- >The GP Legends Historic Motor Racing Club is located at:- >http://www.netspeed.com.au/brucek/legends/

On Thu, 25 Jun 1998 07:55:54 -0600, "Todd Heckel" >40? 60? It's kind of like Canadian money, right? (I can say that, >'cause my wife is Canadian, as are my kids. Don't worry, I'll get >mine.) Doesn't it fluctuate? Or is it ever decreasing? 8>) Ouch...this is good stuff. Stop bashing my Loonie!! :) Mike

On Wed, 24 Jun 1998 15:48:09 -0600, "Todd Heckel" >200 kph... that's like 20 miles per hour, right? LOL!! Yes, I like to play airplane too...and I'm forty don't you know? :) Mike

Sounds like a funny dog, what kind is it? Admit going 200 sounds a lot more fun than going 20 :o). /Christer, promotes the "Global online racing, with pings larger than 1000 ms and collision detection"-proposal

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> >If you stick your head out the window when doing 200 kph, you'll feel > the drag > 200 kph... that's like 20 miles per hour, right? Actually, I prefer > to stick my hand out the window and pretend that it's a wing, angling > it up and down and feeling the lift and downforce. I only stick my > head out the window when I have my dog along for the ride. He hates > it when I do that. > Todd

Oh no, she's not funny at all. She's a Rottweiler. And she likes the window all to herself. Well, yeah. Kind of like how earning $100,000 Canadian sounds a lot more fun that $30,000 U.S. (Rim shot!) Sorry, just couldn't resist. Okay, I'll behave from now on. Todd

Actually, I like the Loonie. I just can't stand the fact that more good racecar drivers are coming out of Canada these days than just about anywhere else. So I have to retaliate somehow. The only alternative would be to start telling jokes about the Canadian military. Then there's all those Canadian actors...

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>On Thu, 25 Jun 1998 07:55:54 -0600, "Todd Heckel" >>40? 60? It's kind of like Canadian money, right? (I can say that, >>'cause my wife is Canadian, as are my kids. Don't worry, I'll get >>mine.) Doesn't it fluctuate? Or is it ever decreasing? 8>) >Ouch...this is good stuff. Stop bashing my Loonie!! :) >Mike

Thank Heavens....a sense of humour! :o) You had me a bit worried there for a sec!

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> 40? 60? It's kind of like Canadian money, right? (I can say that, > 'cause my wife is Canadian, as are my kids. Don't worry, I'll get > mine.) Doesn't it fluctuate? Or is it ever decreasing? 8>) > >You aren't serious, are you? > >> 200 kph... that's like 20 miles per hour, right? > >-- > >Regards, > >Bruce. > >---------- > >The GP Legends Historic Motor Racing Club is located at:- > >http://www.netspeed.com.au/brucek/legends/

-- Regards, Bruce. ---------- The GP Legends Historic Motor Racing Club is located at:- http://www.netspeed.com.au/brucek/legends/

On Thu, 25 Jun 1998 21:04:44 -0600, "Todd Heckel" >Actually, I like the Loonie. I just can't stand the fact that more >good racecar drivers are coming out of Canada these days than just >about anywhere else. This is where I get to gloat... And this is where you gloat :). Mike

Further to the debate on drag etc... The total drag factor on a car is the product of the frontal area and the drag co-efficient (this is from memory btw). An aerodynamic car such as a Vauxhall Calibra (sorry can't think of a US Example) has a co-eff of about 0.26 (currently lowest - the ads said) A modern family car such as a Ford Mondeo will be about 0.32 A boxy car - such as the old Audi Quattro will be over 0.4! Note that this figure is only affected by the ease of flow around the body NOT the frontal area. For example a Boeing 747 may well have a lower drag coefficient than an Audi Quattro, but the 747 will still have to overcome more drag! I would think an F1 or Indy car (on a road /street course) has a fairly high drag co-eff - simply because the designers trade of drag co-eff against creating more downforce by sticking wings in the airflow. Of course the frontal area is quite small - smaller now with F1's narrower cars and tyres. So by multiplying then together we have drag. This is the tricky bit.... Obviously, when the car is stationary we have no aerodynamic drag (Unless it's windy but thats just silly). I cant quite remember how this figure translates to how much power you need, but I do remember reading that for a normal car you need to have five times the power to do double the speed. Before you tell me that an F1 car could do over 110 MPH with 140 BHP remember - an F1 car or any other track car never reaches its absolute maximum speed - they are always geared for the longest straight - don't know if this applies to the Mulsanne at Le Mans though... Hope this help's - Btw the downforce on an F1 car dramatically increases rolling resistance as well as drag. Surely someone who reads this group is or knows an aerodynamacist.... -- Michael C Wallace