Locked Tyre & Turning Tyre

Why does a locked tyre give less grip than a turning tyre? -- Rob Swindells 3rd equal in Goldline Bearing FFord Championship (18 Points); 9 points behind Championship Leader (as of round 4 of 6) "If you're a great driver, you will get through. It's not harder than any time before." ~Martin Brundle "If we could do this every week, we would be more consistent..." ~Bryan Robson (I think)

Because the dynamic friction coefficient of vulcanised *** is less than the static one. Also, it takes a larger decrease in force to go from dynamic back to static friction (unlocking the tyre) than the increase in force nescessary to go from static to dynamic friction (locking the tyre). Jan./putting the engineering degree to work. ;-) ------ >Why does a locked tyre give less grip than a turning tyre? ><snip>

You need to know a bit about the relationship between tyre slip and the force it can produce. A modern tyre produces a peak of lateral force at between 7 and 10 degrees slip. This means that if you go over this value then you are actually losing available grip. This is where a good driver gains speed, by not sliding the car tooooo much. The same is true in the longitudinal direction (ie in braking and acceleration). The relationship between slip and tyre force is roughly linear at lower values, then peaks and drops off. A wheel that is locked, or that is partially locked (ie large slip) therefore goes over this peak value and produces less longitudinal force than an unlocked wheel. Hope that helps! Neil

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> Why does a locked tyre give less grip than a turning tyre? > -- > Rob Swindells > 3rd equal in Goldline Bearing FFord Championship (18 Points); > 9 points behind Championship Leader (as of round 4 of 6) > "If you're a great driver, you will get through. It's not harder than > any time before." ~Martin Brundle > "If we could do this every week, we would be more consistent..." ~Bryan > Robson (I think)

:>Why does a locked tyre give less grip than a turning tyre? :><snip> : Because the dynamic friction coefficient of vulcanised *** is less than : the static one. Isn't that just restating his question in a mathematical way? Surely what he's asking is "why is the dynamical coefficient less than the static one"? [Incidentally, could someone tell me approximately (percentage) how much less. I was discussing this on some other news group.] My _guess_ (and it is a guess) is that when the wheel is turning, the *** is pressed down into the road as it comes into contact whereas when the wheel is sliding it is not "digging in" to the surface in the same way but is skipping over the top. [Think of a cog wheel turning along a ridged surface versus a cog wheel skipping over the top of the ridged surface - in the first case the teeth of the cog have time to "dig in" whereas in the second they don't.] Also, I imagine that a sliding tyre heats a great deal - I understand that after a certain temperature, tyres move from being "sticky" to being greasy and therefore slide more easily. This may well have some effect. -- Richard G. Clegg Only the mind is waving Dept. of Mathematics (Network Control group) Uni. of York. www: http://manor.york.ac.uk/top.html

Ok think about it this way. When you go to push a stove or something heavy, it takes a lot of force to get it to move. Once you get the stove to actually start to move, it moves along quite easily. This is just like a tire. When it isn't locked, it reacts like the stove that hasn't broken the "bond" between it and the floor. Once a tire locks up, it is like it has broken the bond and begins to slide freely just like the stove. Now the reason for this I don't know. I could give you some theories. You would have to research the friction levels of objects that are sliding and objects that are at rest (compared to the ground, i.e. the contact patch of the tire is standing still compared to the road). Jesse Black

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>:>Why does a locked tyre give less grip than a turning tyre? >:><snip> >: Because the dynamic friction coefficient of vulcanised *** is less than >: the static one. > Isn't that just restating his question in a mathematical way? >Surely what he's asking is "why is the dynamical coefficient less than >the static one"?

(Good observation!) I think you're right on--at the small-scale level, bits of the tire are molding themselves around little bumps in the road. In fact, this is why tires violate the "law" that friction is independent of contact area--*** is soft enough that having a larger contact patch will provide more "handles" to grab onto the road--of course this tapers off to some extent. The reason that a tire will actually give peak traction in a partial slip condition (e.g., six degrees or so of slip angle, or 90 percent rotation speed for braking), is that parts of the tire are flexing and gripping, other parts are sliding, and as you crest the top of the slip angle curve, more and more of the bits are sliding. Contrary to popular belief, the a tire's sliding friction is not that much lower than peak--sort of a 90 percent-ish thing. This is why a lot of driving instructors tell people to lock the brakes up in a panic braking situation--for a non-professional driver, messing around with modulating brake pressure is not likely to decrease stopping distances, plus a car with both fronts locked up is an extremely stable, predictable object for other cars to avoid... of course a racing driver not only wants to eke out a few extra percent, but also save the tires. The "greasy" effect of temperature is not likely to affect the static/dynamic, as it takes a little while for the tire temperature to increase significantly. There was a thread earlier about the benefits of locking up the brakes sliding into the pits, but I can't remember what the "consensus" was. -Dave >-- >Richard G. Clegg Only the mind is waving >Dept. of Mathematics (Network Control group) Uni. of York. >www: http://manor.york.ac.uk/top.html

WOW! Thanks for the answers guys. I think I really knew why in my mind - it was just hard to explain to myself :) Mind if I ask another one?? :-)

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> Why does a locked tyre give less grip than a turning tyre? > -- > Rob Swindells > 3rd equal in Goldline Bearing FFord Championship (18 Points); > 9 points behind Championship Leader (as of round 4 of 6) > "If you're a great driver, you will get through. It's not harder than > any time before." ~Martin Brundle > "If we could do this every week, we would be more consistent..." ~Bryan > Robson (I think)

-- Rob Swindells 3rd equal in Goldline Bearing FFord Championship (18 Points); 9 points behind Championship Leader (as of round 4 of 6) "If you're a great driver, you will get through. It's not harder than any time before." ~Martin Brundle "If we could do this every week, we would be more consistent..." ~Bryan Robson (I think)

>:>Why does a locked tyre give less grip than a turning tyre? >:><snip> >: Because the dynamic friction coefficient of vulcanised *** is less than >: the static one. > Isn't that just restating his question in a mathematical way? >Surely what he's asking is "why is the dynamical coefficient less than >the static one"? Then why didn't he phrase it like that? -I'm an engineer, I need an exact question. Reading meaning into stuff is for mathematicians. ;-) As another poster remarked, 90%-ish sounds about right. I don't remember exactly. Right on the money there, although it takes some doing to cause melting. Jan. --

: Contrary to popular belief, the a tire's sliding friction is not that : much lower than peak--sort of a 90 percent-ish thing. Aha - thanks very much - I'd been wondering about that. : This is why a lot of driving instructors tell people to lock the : brakes up in a panic braking situation--for a non-professional driver, : messing around with modulating brake pressure is not likely to decrease : stopping distances, plus a car with both fronts locked up is an : extremely stable, predictable object for other cars to avoid... Of course it might be stably and predictably sliding off the road (like me trying to round Parabolica). I've also seen driving instuctors giving the advice to "pump" the brakes so that the front wheels lock and unlock rapidly (I presume no modern car has the brake bias set so that the rears will lock easily? Otherwise it could be a disaster). That would seem to provide a compromise between braking and being able to steer. I _used_ to brake in GPL like that (don't shout at me - I'm TRYING to do it properly now) - actually it seemed to have some advantages - when the nose bucked up and down the weight transfer seemed to have some good "stopping" effects - I guess that for some of the time you have a really large stopping force (as the weight goes temporarily to the front). Is it possible that by inducing a huge forward weight transfer you can get better braking temporarily - can this be taken advantage of or is it too temporary to make much difference. It sounds silly but if you could bounce the car between heavy front/brakes locked up/no steering and heavy rear/no brakes/lots of steering you might be able to get BETTER braking and control than the standard "not quite locked up" technique. If you could make it that you were only braking for 1/2 the time but in that time there was 3 times the weight on the front axle then you'd be a more efficient braker. [Hmm... maybe this won't work - there's only so much weight there to transfer.] Must just rush home to experiment with "stupidly springy car". -- Richard G. Clegg Only the mind is waving Dept. of Mathematics (Network Control group) Uni. of York. www: http://manor.york.ac.uk/top.html

Even in the wet ? I could very well be wrong, but it certainly FEELS a lot less than 90% in the wet. Regarding locking up the fronts: depending on the road surface and tyres (S.A. spelling!) some cars seem to have a habit of locking one side before the other, with the result that the car skews before going into a full on slide. Front wheel drives, especially, seem prone to "darting about" when attempting to brake at the adhesion threshold. Anyone else experienced this ? Ryan

Anyone heard of Anti lock braking? There are a lot of videos from the various car makers that show two cars side by side one with anti lock and one without. Mercedes being a good example. The Anti lock braking works every time. As soon as the tyre stops turning and starts skidding you have lost control! It is no good stopping sooner if something else gets in your way! An easy way to imagine it is driving on ice. Bang the anchors on and away you will go with no control what so ever. Keep the tyre rotating and you stand a chance.. There is another thread in this group about driving/racing sims and possible improvements to your everyday driving. One word - No. Until someone comes up with a driving simulator like a flight simulator (ie a hydraulic based replica like pilots practice on) it will still be just a coordination/reflex challenge (albeit a ***y difficult one) to beat the PC. Try karting for improving your driving.

:> sliding. Contrary to popular belief, the a tire's sliding friction is not : that :> much lower :> than peak--sort of a 90 percent-ish thing. : Even in the wet ? I could very well be wrong, but it certainly FEELS a lot : less than 90% in the wet. I suppose that, in that case, you have to be much much more careful about locking up - I imagine it's to do with "hydroplaning" - the locked up tyre scoots along on a thin film of water with virtually no friction. : Regarding locking up the fronts: depending on the road surface and tyres : (S.A. spelling!) some cars seem to have a habit of locking one side before : the other, with the result that the car skews before going into a full on : slide. Front wheel drives, especially, seem prone to "darting about" when : attempting to brake at the adhesion threshold. Anyone else experienced this : ? Well it's bound to happen that the inside tyre (English spelling!) will lock up first if you're turning at all. Otherwise I suppose it might happen if the road was cambered. -- Richard G. Clegg Only the mind is waving Dept. of Mathematics (Network Control group) Uni. of York. www: http://manor.york.ac.uk/top.html

More like 75%-ish Not where I come from. Instructors have always told me and anyone else I know that you should endeavour to maintain control of a vehicle at all times. When the wheels are locked you have no control whatsoever. If an instructor told me to do this, I'd have to question his qualifications. Dave Sent via Deja.com http://www.deja.com/ Share what you know. Learn what you don't.

>Not where I come from. Instructors have always told me and anyone else >I know that you should endeavour to maintain control of a vehicle at all >times. When the wheels are locked you have no control whatsoever. >If an instructor told me to do this, I'd have to question his >qualifications. From what I heard this was standard procedure in police training, although it sounds pretty bizarre to me. Perhaps it's just less likely to confuse someone if in a crisis situation you do what is natural (stand on the brakes with all your might) rather than think about locking up, modulating, steering etc. If you do that a lot (brake on the limit, even in a sim) it somewhat becomes second nature, but otherwise the split-second hesitation could cause more harm than the locking up. Cheers! John

On Thu, 26 Aug 1999 11:55:10 +0800, Brad Ashworth >There is another thread in this group about driving/racing sims and possible >improvements to your everyday driving. One word - No. One more word - wrong. Karting will improve your driving more than sim-racing, but sim-racing will improve your driving more than sitting in front of the TV with a beer. I haven't raced for years, but recently went on a "driving day" organised by a local BMW dealership. It was one of those "damp" days so common in the UK, and I was whizzing around the track in an M3 Evo. Coming out of a corner the car got _well_ sideways, with a ditch on one side and gravel on the other. All that really registered with me was a "squeak" noise to my left, I gathered the car up and booted it off down the track. A couple of seconds later I realised the "squeak" had been my passenger who was whimpering in shock, that the car had in fact snapped sideways and that I had corrected it and carried on without consciously registering the fact. Now I do that every day in GPL, and I would attribute that reaction far more to GPL than to the racing I did in the past. FWIW my passenger drove next, did the same thing at a different point and got a real tank-slapper going, locked up the brakes and ended up stopped inthe middle of the track facing backwards - he raced as much as I did in the past - <rhetoric> what could cause such things to be more pratised in my mind than his? </rhetoric> Cheers! John

The difference is the difference between static & sliding friction. Lets do a little experiment shall we? <G>...... Put your palm down on a table and push on it. It resists breaking 'traction', but when it starts to slide it slides quite easily. Another example is to go move your couch or refrigerator. Once your get it moving it requires less effort to keep it moving, once it stops it requires considerably more force/energy to start it moving again. NOTE: This has very little to do with momentum, so don't assume anything like that. Kinetic & potential energies are an entirely seperate subject. Got it? Sliding friction is always less that static friction. Sliding friction can be beneficial to a racer in many ways tho. A great illustration is a good Rally driver who slides the back end around in a corner to achieve a higher/quicker angle of rotation in a situation where apex speed & exit speed is less important than rotaional speed. They are able to start accelerating before they would be able to if they had just driven the corner 'at the edge of traction' like a typical road racer does. The situation & of course many other variables dictate the proper technique for a particular corner. Being a good racer requires an excellent understanding of phsyics IMO. "Never pat a burning dog~!!!"

In General you are correct. Being I deal with everyday in Pneumatics cylinders I had to design a cylinder that the static and dynamic frictions were as close as possible. It can be done, but requires some special stuff. For cars tho static and dynamic frictions will always be different values. Dave

: The difference is the difference between static & sliding friction. : Lets do a little experiment shall we? <G>...... : Put your palm down on a table and push on it. It resists breaking : 'traction', but when it starts to slide it slides quite easily. Poor experiment - it is wholly subjective - maybe it slides quite easily because you're not applying the same force? How can you tell "sliding easily" from "just sliding" (if I push my hand slowly enough it seems to be "just sliding" to me). If sliding friction is much less and you apply "the same force" then your hand would continue to accelerate - didn't notice mine doing that. :-) : Got it? Sliding friction is always less that static friction. I'm not sure if this general conclusion is correct. Certainly I'm willing to believe it is the case in racing situations and for a large variety of cases it is correct. I'm pretty sure, however, that we can find substances where sliding friction is greater. I posted an experiment about static/sliding friction on rec.autos.sports.f1.moderated this week which you might still be able to find describing an experiment using a ***ised mousemat a "pulley" and a weight showing that, for a *** mousemat at least, sliding friction actually is pretty similar to static friction. In brief: use paperclips and cotton to fasten the mousemat to a weight - use a pulley or some kind of roller to hang the weight over the side of the desk. Load the mousemat with "downforce" (small weights) until there is just enough weight to stop it sliding. Give the mousemat a shove. If the mousemat accelerates off the desk then the sliding friction was much less. My experience was that the mousemat would slow down and either continue to move very slowly or stop. Meaning that, in that case, the sliding friction wasn't much lower (if any) than the static friction. -- Richard G. Clegg Only the mind is waving Dept. of Mathematics (Network Control group) Uni. of York. www: http://manor.york.ac.uk/top.html

> From what I heard this was standard procedure in police training, > although it sounds pretty bizarre to me. Perhaps it's just less likely > to confuse someone if in a crisis situation you do what is natural > (stand on the brakes with all your might) rather than think about > locking up, modulating, steering etc. I agree with teaching ppl to lock up being bizarre and even dangerous. My reasons being... Yes, standing on the brakes is the natural reaction. Unfortunately, it often leads to sliding straight into an oncoming truck, a moose, a tree or something else that will certainly spoil your day. Generally, ppl panic brake (yup, that's the term - panic being BAD while driving) when they find themselves headed straight towards such an object. Thus, pumping the brakes and getting just a little steering (I really don't think most ppl can do modulated breaking efficiently in a crise situation) will be a better idea than locking them up. Whether or not you get the speed down as much as you possibly can is irrelevant when you don't hit... I have tried pump braking for real several times and it works. I have also tried locking up the brakes both when playing around and on a skid course (mandatory in driver training here). You do stop, of course, but you will not avoid any obstacles. Add to that the fact that most cars have the largest crash zones towards the front and most passenger safety devices are built to take up acceleration towards the front. With pump braking, the car tends to maintain it's direction. With locked brakes, it will spin like a top and chance will deside which direction it will hit. Add to THAT the natural reaction many people have when the car starts to spin. They release the brake and the car goes off to the side it happens to be pointing at that moment, again quite possibly hitting something or someone. Allowing people to follow their natural instincts in an unnatural situation (driving a car) isn't the thing I would do... Oh yeah, and most ppl will not need this before the braking distance is way too short anyway so they'd probably be better off steering around or into the ditch in the first place. I agree with my driving instructors here. Locking up probably is the worst thing to do, short of stepping on the accelerator (as many ppl will do when they panic and mash both feet down). Much better to have pump braking embedded somewhere in the spinal cord of all drivers. My $.02, /ft P.S. I read somewhere that in most car crashes, the driver only hits the brakes AFTER impact anyway...

Perhaps I expressed myself poorly--obviously maintaining control over the vehicle is always important--however, even with power brakes, most drivers don't realize how hard you have to press the pedal, and hence err on the side of not braking hard enough. Driving with the fronts locked up is fine if going straight is the best direction to be going--driving schools usually teach seperating swerving and braking, as combining the two often won't work very well for a non-racing driver. The other situation where it's advantagous to lock up the brakes is in a spin--hence the old saw, "in a spin, both feet in". Once you've lost control of a spin, if you don't lock up the brakes, the tires can "catch" while the car is at an odd angle, suddenly slamming it sideways--not good in a multi-car accident scenario.

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>Not where I come from. Instructors have always told me and anyone else >I know that you should endeavour to maintain control of a vehicle at all >times. When the wheels are locked you have no control whatsoever. >If an instructor told me to do this, I'd have to question his >qualifications. >Dave >Sent via Deja.com http://www.deja.com/ >Share what you know. Learn what you don't.

Why am I always so late in these lovely threads :o(. I bet someone has already explained why racing tyres seem to laugh at Newton's laws... Okay I'll read the rest of the posts... /Chrille >I think you're right on--at the small-scale level, bits of the tire are molding >themselves >around little bumps in the road. In fact, this is why tires violate the "law" that >friction >is independent of contact area--*** is soft enough that having a larger contact >patch will provide more "handles" to grab onto the road--of course this tapers off >to some extent.

Just make the cylinder rotate, that'll fix it ;o). /Chrille

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>> Got it? Sliding friction is always less that static friction. >In General you are correct. Being I deal with everyday in Pneumatics >cylinders I had to design a cylinder that the static and dynamic frictions >were as close as possible. It can be done, but requires some special >stuff. >For cars tho static and dynamic frictions will always be different values. >Dave

No? Dont mind the other thread, elaborate :o). If everyday driving means to and from work, then I might agree with you, but on the race track I think it by now would be obvious a racing sim would prepare you better than watching Telly with a beer. I've heard real race car drivers are more often involved in traffic accidents than non race car drivers. If it's true it could be because race car drivers need more speed to get day to day driving interesting... Then again if you see the day to day driving as some kind of endurance race where the oldest driver alive is the winner, you might use your simulator experience to your advantage. Follow traffic rythm, dont upset the other drivers, be aware of anything that moves or could be moving soon. Then again I could be very tired from working the whole week... /Chrille >There is another thread in this group about driving/racing sims and possible >improvements to your everyday driving. One word - No.

Okay, so noone noticed this one :o). I could be wrong but ordinary street tires follow the Newton laws very nicely. The grip being proportional to the contact area. The advantage of a bigger contact area is that the pressure per area gets smaller, so you can have a softer tire with a bigger contact area, thus getting more overall grip. The reason a racing tire breakes the Newton laws is because at a certain temperature interval it gets sticky. Place your hand on such a tire and you will have trouble removing it - I shit you not :o) (loved that expression in the movie "The Rock" :o)). So the larger contact area, the more glue effect. The glue effect dont care one bit about what the pressure per area is. /Chrille, and that's my principles, if you dont like them, I have others ;o) >I think you're right on--at the small-scale level, bits of the tire are molding >themselves around little bumps in the road. In fact, this is why tires violate the "law" that

A common problem is not to get into the pedal from the MOMENT braking is needed. I usually find it safer in a semi-emergency to brake at the threshold of adhesion from the moment I see the need to brake, and then tail off the braking if the situation subsides. Ryan

rob, swindells, at, netscape, online, dot, co, dot, uk (robswindells) Sim racing is far more enjoyable when you take it seriously, and it's too easy not to take it seriously. I think what's needed is a sim racing "reality chair", that has a spring loaded baseball bat that snaps around and smacks you in the ribs when ever you***up. It would add a little intensity to the driving. -- Don Scurlock Vancouver,B.C.

> <Show off>I was crusing down a road the other week when a dog ran right out in > front of me! I must have been doing over 50mph, but managed to brake sharply, > check for oncoming traffic, 'swerve' to the right and missed the dog by inches! I > heard just a hint of tyre squeal, but my heart rate shot up! :-) </Show off> Haha, good job. You are aware that swerving for animals on the road is bad driving behaviour too though, aren't you? Ppl who do that quite often manage to hit something (or *shudder* someone else) - or get the car sideways, if the speed is high enough. /ft, funspoiler who swerves for mooses, children and not much else (Yup, I once had the horrible experience of having a kid jump out from behind a parked car... Luckily i was doing 30 km/h at the time and could brake/steer around, but I did not feel like driving for weeks and months afterwards... And you'll find me the slowest driver ever when there are parked cars or other view obscuring objects along the road. Please try to out-slow me, everyone!)

Hope so! I've managed it in the wet, coming out of the carpark at work - just like on the movies :) > or get the car sideways, if the > speed is high enough. -- Rob Swindells 3rd equal in Goldline Bearing FFord Championship (18 Points); 9 points behind Championship Leader (as of round 4 of 6) "If you're a great driver, you will get through. It's not harder than any time before." ~Martin Brundle "If we could do this every week, we would be more consistent..." ~Bryan Robson (I think)