physics question

Why do rally drivers drive sideways on gravel or snow? No doubt, on gravel you get faster laps by driving sideways (e.g. Speedway - motorcycle racing on gravel ovals). We all know that driving sideways on tarmac is much slower (but more fun!). There must be a physical explanation for this 'friction phenomenon'. I know it's slightly off-topic, but I cannot find an answer to this question for quite a long time...

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>Why do rally drivers drive sideways on gravel or snow? No doubt, on gravel >you get faster laps by driving sideways (e.g. Speedway - motorcycle racing >on gravel ovals). We all know that driving sideways on tarmac is much slower >(but more fun!). There must be a physical explanation for this 'friction >phenomenon'. >I know it's slightly off-topic, but I cannot find an answer to this question >for quite a long time... >

Hi, i try to explain, but i don't know if its true. Here's the facts : 1)The more load you have on a tire the more grip you have 2)On loose surfaces as snow or dirt, drivers removes the sway bars ( antiroll bars ). to increase lateral load transfert. 3) The tighter the curve is more the driver drive sideway Here's the explanation: 1) dirvers drive sideway to increase the lateral load transfert. 2) When driving sideways longitudinal forces helps to counter act the centrifugal force. well, this is how i explain it, and feel it when driving ;o) regards, Sebastien TIXIER - Game Developer Dynamics and Car Physics http://www.eden-studios.fr GPLRank Normal:-44.24 Monster:-124.44

"Wojtek" <[email hidden]> wrote in message news:a9im69$3k4a7$[email hidden]... > Why do rally drivers drive sideways on gravel or snow? No doubt, ongravel > you get faster laps by driving sideways (e.g. Speedway - motorcycleracing > on gravel ovals). We all know that driving sideways on tarmac is muchslower > (but more fun!). There must be a physical explanation for this'friction > phenomenon'. As Mr. Tixier said, it has to do with "load transfer". In a 4WD car, all four wheels generate thrust. The idea is the generate the most amount of thrust IN THE DIRECTION OF TRAVEL (which may not be where the car is POINTING), AND carry enough speed INTO the turn so you carry more speed OUT of the turn. On gravel and snow, due to sliding, the direction of travel and direction facing would NOT be the same. If you slow to the point where they are the same, you lose speed. By forcing the tail loose you can carry more of the velocity INTO and OUT OF the corner. --KC

There are two reasons for this, one of which Sebastien explained already. The other reason is that loose surfaces act somewhat differently than, say, tarmac, in that the material gets moved/thrown by the tyre, and this gives an additional reaction force which makes sideways driving advantageous. The other reason that Sebastien explained is that in a 4wd vehicle the tradeoff betwen performance and control is the best ina sideways situation where you are for the most part quit close to the peak of performance and can control the behaviour well by the throttle, preventing understeer which can hamper performance more. -Gregor On Wed, 17 Apr 2002, Wojtek wrote:

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> Why do rally drivers drive sideways on gravel or snow? No doubt, on gravel > you get faster laps by driving sideways (e.g. Speedway - motorcycle racing > on gravel ovals). We all know that driving sideways on tarmac is much slower > (but more fun!). There must be a physical explanation for this 'friction > phenomenon'. > I know it's slightly off-topic, but I cannot find an answer to this question > for quite a long time... > > >

I have a different theory. I really doubt that the force from throwing all that material rearward is very significant in comparison to the weight of the vehicle. I think what is happening is kind of like viscous friction. When you spin the wheels on loose ground, you are shearing off a layer and moving it rearward at the velocity of the contact patch. At some point lower in the ground, the material is not moving. Between those two points you have a velocity gradient and loose material that is behaving like a fluid. Fluid or viscous friction increases with relative velocity. Spin the wheels faster and you get more force. Point the car more towards the turn center and the viscous friction force contributes to the centripetal acceleration of the vehicle. And this would be a higher force than normal cornering because you can generate a larger relative velocity difference from a spinning wheel than from one which is simply sliding sideways across the surface. "Gregor Veble" <[email hidden]> wrote in message news:[email hidden]... > There are two reasons for this, one of which Sebastien explainedalready. > The other reason is that loose surfaces act somewhat differently than, > say, tarmac, in that the material gets moved/thrown by the tyre, andthis > gives an additional reaction force which makes sideways driving > advantageous. The other reason that Sebastien explained is that in a4wd > vehicle the tradeoff betwen performance and control is the best ina > sideways situation where you are for the most part quit close to thepeak

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> of performance and can control the behaviour well by the throttle, > preventing understeer which can hamper performance more. > > -Gregor > > On Wed, 17 Apr 2002, Wojtek wrote: > > > Why do rally drivers drive sideways on gravel or snow? No doubt, on

gravel > > you get faster laps by driving sideways (e.g. Speedway - motorcycleracing > > on gravel ovals). We all know that driving sideways on tarmac ismuch slower > > (but more fun!). There must be a physical explanation for this'friction > > phenomenon'. > > I know it's slightly off-topic, but I cannot find an answer to thisquestion > > for quite a long time... > > > > > > >

I do believe you are onto something here. Though the physics of such materials is tricky at best, probably they do behave as fluids to a certain extent and this contribution is most likely really significant. Let's see how much the contribution from the material being thrown is; let's assume that 10kg/s (a high estimate) are being thrown at a velocity of 10 m/s (36km/h), this only ammounts to a force of 100N which indeed is almost insignificant as it's probably somewhere around 1% of total grip (10000N for a 1000kg car cornering at 1g). Indeed there must be other contributions. Luckily the resulting empirical equations from both of these contributions are that the force is proportional to the slip velocity, and since the proportionality constant is unknown, I believe we have all the info needed here for anyone attempting a gravel rally sim :). -Gregor On Thu, 18 Apr 2002, Haqsau wrote:

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> I have a different theory. I really doubt that the force from throwing > all that material rearward is very significant in comparison to the > weight of the vehicle. I think what is happening is kind of like > viscous friction. When you spin the wheels on loose ground, you are > shearing off a layer and moving it rearward at the velocity of the > contact patch. At some point lower in the ground, the material is not > moving. Between those two points you have a velocity gradient and loose > material that is behaving like a fluid. Fluid or viscous friction > increases with relative velocity. Spin the wheels faster and you get > more force. Point the car more towards the turn center and the viscous > friction force contributes to the centripetal acceleration of the > vehicle. And this would be a higher force than normal cornering because > you can generate a larger relative velocity difference from a spinning > wheel than from one which is simply sliding sideways across the surface. > > > > "Gregor Veble" <[email hidden]> wrote in message > news:[email hidden]... > > There are two reasons for this, one of which Sebastien explained > already. > > The other reason is that loose surfaces act somewhat differently than, > > say, tarmac, in that the material gets moved/thrown by the tyre, and > this > > gives an additional reaction force which makes sideways driving > > advantageous. The other reason that Sebastien explained is that in a > 4wd > > vehicle the tradeoff betwen performance and control is the best ina > > sideways situation where you are for the most part quit close to the > peak > > of performance and can control the behaviour well by the throttle, > > preventing understeer which can hamper performance more. > > > > -Gregor > > > > On Wed, 17 Apr 2002, Wojtek wrote: > > > > > Why do rally drivers drive sideways on gravel or snow? No doubt, on > gravel > > > you get faster laps by driving sideways (e.g. Speedway - motorcycle > racing > > > on gravel ovals). We all know that driving sideways on tarmac is > much slower > > > (but more fun!). There must be a physical explanation for this > 'friction > > > phenomenon'. > > > I know it's slightly off-topic, but I cannot find an answer to this > question > > > for quite a long time... > > > > > > > > > > > > >

One of the reasons I got to thinking along these lines is due to some conversations I have had with a friend who does a lot of four wheeling. He doesn't race, but he is in a club that goes on a lot of weekend trips, and apparently the challenge is to see who can find the deepest mudhole and still get out of it. ;o) He builds his own Jeeps from the ground up, and they actually end up a lot like a racing vehicle by the time he is done with them. Anyway, he had also made the comment to me about getting thrust from throwing material rearward, although he was not referring to cornering, he was referring to getting out of a mudhole. Either way it seems that we are only talking about an acceleration of around 0.01 G using your numbers, not significant at all. Another reason I am thinking along these lines, though, is that I believe he told me once that in really loose material it was actually better to have narrow tires, so that you would sink into the ground more instead of floating along over the top. This I think fits with my theory about getting a behavior from the tires that is like viscous friction - the farther you sink in, the more surface area of the tire is in contact with the dirt, and the more total friction force. Now that would be interesting to put in a sim. "Gregor Veble" <[email hidden]> wrote in message news:[email hidden]... > I do believe you are onto something here. Though the physics of such > materials is tricky at best, probably they do behave as fluids to a > certain extent and this contribution is most likely reallysignificant. > > Let's see how much the contribution from the material being thrown is; > let's assume that 10kg/s (a high estimate) are being thrown at avelocity > of 10 m/s (36km/h), this only ammounts to a force of 100N which indeedis > almost insignificant as it's probably somewhere around 1% of totalgrip > (10000N for a 1000kg car cornering at 1g). Indeed there must be other > contributions. > > Luckily the resulting empirical equations from both of thesecontributions > are that the force is proportional to the slip velocity, and since the > proportionality constant is unknown, I believe we have all the infoneeded > here for anyone attempting a gravel rally sim :). > > -Gregor >

I'm guessing the "more load = more grip" thing, if true, is nonlinear; if it were strictly true, than this would mean that the load sensitivity is inverse to asphalt, which would imply that a typical RWD sedan (e.g., 55 % front weight, same tires front and rear, more roll stiffness in front to make it "safely" understeer) would be oversteer on gravel, while an oversteer car would become understeer. In my experience, production cars still understeer in loose surfaces unless the handbrake or some sort of throttle technique (left-foot brake on FWD, or dipping the clutch on RWD) is used to degrade rear grip. Also, I wouldn't expect removing ARBs to change steady-state total lateral load transfer--isn't that merely a function of lateral force, CG height, and track width? I would, however, expect removing ARBs to reduce wheel-hop on bumpy surfaces. I think the biggest factor is that the friction peak on loose surfaces is at an extremely wide slip angle, probably due to a quasi-viscous behavior of dirt building up on the outside edge. I think there's also some threshhold where for very tight turns, it's faster to rotate the car by sliding than to actually take the turn "properly"--hence autocrossers use the handbrake even on asphalt. Whatever is true, there's no doubt that there isn't enough stuff written about offroad tire behavior!!! -Dave P. "S�bastien Tixier" <[email hidden]> wrote in message news:[email hidden]... > >Why do rally drivers drive sideways on gravel or snow? No doubt, ongravel > >you get faster laps by driving sideways (e.g. Speedway - motorcycleracing > >on gravel ovals). We all know that driving sideways on tarmac is muchslower > >(but more fun!). There must be a physical explanation for this 'friction > >phenomenon'. > >I know it's slightly off-topic, but I cannot find an answer to thisquestion

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> >for quite a long time... > > > Hi, > > i try to explain, but i don't know if its true. > > Here's the facts : > > 1)The more load you have on a tire the more grip you have > 2)On loose surfaces as snow or dirt, drivers removes the sway bars ( > antiroll bars ). to increase lateral load transfert. > 3) The tighter the curve is more the driver drive sideway > > Here's the explanation: > > 1) dirvers drive sideway to increase the lateral load transfert. > 2) When driving sideways longitudinal forces helps to counter act > the centrifugal force. > > well, this is how i explain it, and feel it when driving ;o) > > regards, > > Sebastien TIXIER - Game Developer > Dynamics and Car Physics > http://www.eden-studios.fr > GPLRank Normal:-44.24 Monster:-124.44

On Sat, 20 Apr 2002 17:00:39 -0500, "Dave Pollatsek" <[email hidden]> wrote: >I'm guessing the "more load = more grip" thing, if true, is nonlinear; if it >were strictly true, than this would mean that the load sensitivity is >inverse to asphalt, which would imply that a typical RWD sedan (e.g., 55 % >front weight, same tires front and rear, more roll stiffness in front to >make it "safely" understeer) would be oversteer on gravel, while an >oversteer car would become understeer. Well, no. In Asphalt you also have more lateral force with more load. Just look at the pacejka formula. However that's true that for very strong load the lateral force start to decrase. >In my experience, production cars >still understeer in loose surfaces unless the handbrake or some sort of >throttle technique (left-foot brake on FWD, or dipping the clutch on RWD) is >used to degrade rear grip. That's true, the car always loose lateral force on the driven wheels because those wheel tends to slip more. >Also, I wouldn't expect removing ARBs to change steady-state total lateral >load transfer--isn't that merely a function of lateral force, CG height, and >track width? I would, however, expect removing ARBs to reduce wheel-hop on >bumpy surfaces. Rally driver usually removes the ARB on wet tarmac because it's a really slippy surfaces. Nothing to do with bumps. It's, again, to increase the lateral load transfert.

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>I think the biggest factor is that the friction peak on loose surfaces is at >an extremely wide slip angle, probably due to a quasi-viscous behavior of >dirt building up on the outside edge. I think there's also some threshhold >where for very tight turns, it's faster to rotate the car by sliding than to >actually take the turn "properly"--hence autocrossers use the handbrake even >on asphalt. >Whatever is true, there's no doubt that there isn't enough stuff written >about offroad tire behavior!!!

I think the optimum slip angle on slippy surfaces is tighter, because the contact patch on loose surfaces has less friction so it's deformation is smaller on slippy surface. IMO, regards, Sebastien TIXIER - Game Developer Dynamics and Car Physics http://www.eden-studios.fr GPLRank Normal:-44.24 Monster:-124.44

On Sat, 20 Apr 2002 17:00:39 -0500, "Dave Pollatsek" >I'm guessing the "more load = more grip" thing, if true, is nonlinear; if it >were strictly true, than this would mean that the load sensitivity is >inverse to asphalt, which would imply that a typical RWD sedan (e.g., 55 % >front weight, same tires front and rear, more roll stiffness in front to >make it "safely" understeer) would be oversteer on gravel, while an >oversteer car would become understeer. Well, no. In Asphalt you also have more lateral force with more load. Just look at the pacejka formula. However that's true that for very strong load the lateral force start to decrase. That's true, the car always loose lateral force on the driven wheels because those wheel tends to slip more. Rally driver usually removes the ARB on wet tarmac because it's a really slippy surfaces. Nothing to do with bumps. It's, again, to increase the lateral load transfert. I think the optimum slip angle on slippy surfaces is tighter, because the contact patch on loose surfaces has less friction so it's deformation is smaller on slippy surface. IMO, regards, Sebastien TIXIER - Game Developer Dynamics and Car Physics http://www.eden-studios.fr GPLRank Normal:-44.24 Monster:-124.44

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>believe he told me once that in really loose material it was actually >better to have narrow tires, so that you would sink into the ground more >instead of floating along over the top. This I think fits with my >theory about getting a behavior from the tires that is like viscous >friction - the farther you sink in, the more surface area of the tire is >in contact with the dirt, and the more total friction force. Now that >would be interesting to put in a sim.

In Rally, on snow, they also use narrow tire, because those tire can go deep in then snow and reach for the ice / stiffer surface, so the nails can grip on those surface. Maybe its the same on mud. narrow tire can reach for the solid ground through the liquid mud ... Sebastien TIXIER - Game Developer Dynamics and Car Physics http://www.eden-studios.fr GPLRank Normal:-44.24 Monster:-124.44

S�bastien Tixier <[email hidden]> wrote in message news:[email hidden]...

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> On Sat, 20 Apr 2002 17:00:39 -0500, "Dave Pollatsek" > <[email hidden]> wrote: > >Also, I wouldn't expect removing ARBs to change steady-state total lateral > >load transfer--isn't that merely a function of lateral force, CG height, and > >track width? I would, however, expect removing ARBs to reduce wheel-hop on > >bumpy surfaces. > > Rally driver usually removes the ARB on wet tarmac because it's a > really slippy surfaces. Nothing to do with bumps. It's, again, to > increase the lateral load transfert.

Surely the only things affecting total LLT are CG height, track width and lateral acceleration? I was under the impression that disconnecting ARBs in low grip is done to improve suspension compliance, thus making the most of the grip, and because with less grip available the car doesn't need to be as stiff in roll to control the sprung mass. Jonny

Lateral weight transfer can be increased slightly by removing bars because the roll angle will increase, and rally cars have a fairly high CG, therefore the CG and its acceleration vector are going to point farther to the outside. But I can't imaging being able to take much advantage of this without playing hell with the steering geometry. More importantly I think is that by decreasing roll resistance the peak normal force experienced during roll will decrease, thereby decreasing the minimum effective traction coefficient as the car is rolling. In other words grip during transient maneuvering will improve. Also bars are just really nasty on uneven surfaces. My Jeeping buddy took his off because it decreased suspension compliance too much. Of course he has a slightly different requirement than a rally driver - where they see a large rock as something to avoid, he sees it as something to drive over. ;o) But they still face a similar problem due to the types of roads they drive on - lots of one-wheel deflections, which are really nasty if you are using ARB's. "Jonny Hodgson" <[email hidden]> wrote in message news:OiYw8.22246$[email hidden]...

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> I was under the impression that disconnecting ARBs in low grip is > done to improve suspension compliance, thus making the most of the > grip, and because with less grip available the car doesn't need to > be as stiff in roll to control the sprung mass. > > Jonny > >

Now that you mention it I think that is why off-roaders do that. Sinking deeper doesn't necessarily get you to dry dirt, but it does get you down to a more hard packed mud. "S�bastien Tixier" <[email hidden]> wrote in message news:[email hidden]...

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> > In Rally, on snow, they also use narrow tire, because those tire can > go deep in then snow and reach for the ice / stiffer surface, so the > nails can grip on those surface. Maybe its the same on mud. narrow > tire can reach for the solid ground through the liquid mud ... > > Sebastien TIXIER - Game Developer > Dynamics and Car Physics > http://www.eden-studios.fr > GPLRank Normal:-44.24 Monster:-124.44

Sorry, I wasn't being very clear... by more grip I meant higher coefficient of friction, not actual friction force. I guess what it boils down to is the behavior along an axle (let's assume a single-axle rally car for convenience on a flat road.) Let t be the fraction of lateral load transfer, then the normal forces at the wheels are mg*(1-t) and mg(1+t). For simplicity, let's normalize everything to mg, so this becomes (1-t) and (1+t) Let the coeff of friction be a linear function of load, f(x) = ax + b, which will be accurate for these purposes since we're just looking at the behavior at a given load +/- a small delta. then the max lateral grip should be: (1-t)*f(1-t) + (1+t)*f(1+t) = uhmmmm..... (really, I have a point here, bear with me.... ;) (1-t)*(a(1-t)+b) + (1+t)*(a(1+t)+b) = a-at+b - at + at^2 -bt + a + at + b + at + at^2 + bt = 2a + 2b + 2at^2 = 2(a + b + at^2) (whew, stuff cancelled out!) So this shows that when "a" is negative (which is true of tires on asphalt, or at least of every tire data I've ever seen), the maximum cornerering force is at t=0, or no load transfer. This, of course, explains why increasing front roll stiffness causes understeer and increasing rear roll stiffness causes oversteer: because the total grip at the stiffer end has more lateral load transfer, and thus loses more grip. So if on dirt, "a" would have to be positive to have the effect of lateral load transfer increasing grip, which would also have the effect of flipping the understeer/oversteer of a car, which is the behavior I'm having trouble with imagining or matching up to what I've seen. I think I can imagine this being true for really wet tarmac, as you say, because increasing the load would decrease hydroplaning. I suppose the same argument could be made for a tire on dirt making a "furrow" in the ground, this could cause a "digging in" effect, which could result in positive "a"... What we really need is a tire dyno for dirt. I'm thinking of an old school bus driving down a dirt road with a tire mounted on an arm sticking off the side bolted to some bathroom scales... throw in some C-clamps and a lot of duct tape for good measure. I'm still pretty sure that the peak is at a wide slip angle, although until one of us actually coughs up some real data, we're just ... (insert colorful aphorism of your choice here) -Dave P. PS. I'm actually only *mostly* joking about the bus thing. I just may try it some day... ;) "S�bastien Tixier" <[email hidden]> wrote in message news:[email hidden]... > On Sat, 20 Apr 2002 17:00:39 -0500, "Dave Pollatsek" > <[email hidden]> wrote: > > >I'm guessing the "more load = more grip" thing, if true, is nonlinear; ifit > >were strictly true, than this would mean that the load sensitivity is > >inverse to asphalt, which would imply that a typical RWD sedan (e.g., 55%

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> >front weight, same tires front and rear, more roll stiffness in front to > >make it "safely" understeer) would be oversteer on gravel, while an > >oversteer car would become understeer. > > Well, no. In Asphalt you also have more lateral force with more load. > Just look at the pacejka formula. However that's true that for very > strong load the lateral force start to decrase. > > >In my experience, production cars > >still understeer in loose surfaces unless the handbrake or some sort of > >throttle technique (left-foot brake on FWD, or dipping the clutch on RWD)

is > >used to degrade rear grip. > > That's true, the car always loose lateral force on the driven wheels > because those wheel tends to slip more. > > >Also, I wouldn't expect removing ARBs to change steady-state totallateral > >load transfer--isn't that merely a function of lateral force, CG height,and > >track width? I would, however, expect removing ARBs to reduce wheel-hopon

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> >bumpy surfaces. > > Rally driver usually removes the ARB on wet tarmac because it's a > really slippy surfaces. Nothing to do with bumps. It's, again, to > increase the lateral load transfert. > > >I think the biggest factor is that the friction peak on loose surfaces is

at > >an extremely wide slip angle, probably due to a quasi-viscous behavior of > >dirt building up on the outside edge. I think there's also somethreshhold > >where for very tight turns, it's faster to rotate the car by sliding thanto > >actually take the turn "properly"--hence autocrossers use the handbrakeeven

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> >on asphalt. > >Whatever is true, there's no doubt that there isn't enough stuff written > >about offroad tire behavior!!! > > > I think the optimum slip angle on slippy surfaces is tighter, because > the contact patch on loose surfaces has less friction > so it's deformation is smaller on slippy surface. > > IMO, regards, > > Sebastien TIXIER - Game Developer > Dynamics and Car Physics > http://www.eden-studios.fr > GPLRank Normal:-44.24 Monster:-124.44

Also, wouldn't removing anti-roll bars on a slippy surface also make sense because on a slippy surface, the lateral forces will be smaller in magnitude, so the suspension doesn't have to generate as strong of a roll reaction, and thus compliance could be increased? Sort of the same reason you move your brake bias rearwards in the wet? (Because less longitudinal load transfer) "S�bastien Tixier" <[email hidden]> wrote in message news:[email hidden]... > On Sat, 20 Apr 2002 17:00:39 -0500, "Dave Pollatsek" > <[email hidden]> wrote: > > >I'm guessing the "more load = more grip" thing, if true, is nonlinear; ifit > >were strictly true, than this would mean that the load sensitivity is > >inverse to asphalt, which would imply that a typical RWD sedan (e.g., 55%

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> >front weight, same tires front and rear, more roll stiffness in front to > >make it "safely" understeer) would be oversteer on gravel, while an > >oversteer car would become understeer. > > Well, no. In Asphalt you also have more lateral force with more load. > Just look at the pacejka formula. However that's true that for very > strong load the lateral force start to decrase. > > >In my experience, production cars > >still understeer in loose surfaces unless the handbrake or some sort of > >throttle technique (left-foot brake on FWD, or dipping the clutch on RWD)

is > >used to degrade rear grip. > > That's true, the car always loose lateral force on the driven wheels > because those wheel tends to slip more. > > >Also, I wouldn't expect removing ARBs to change steady-state totallateral > >load transfer--isn't that merely a function of lateral force, CG height,and > >track width? I would, however, expect removing ARBs to reduce wheel-hopon

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> >bumpy surfaces. > > Rally driver usually removes the ARB on wet tarmac because it's a > really slippy surfaces. Nothing to do with bumps. It's, again, to > increase the lateral load transfert. > > >I think the biggest factor is that the friction peak on loose surfaces is

at > >an extremely wide slip angle, probably due to a quasi-viscous behavior of > >dirt building up on the outside edge. I think there's also somethreshhold > >where for very tight turns, it's faster to rotate the car by sliding thanto > >actually take the turn "properly"--hence autocrossers use the handbrakeeven

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> >on asphalt. > >Whatever is true, there's no doubt that there isn't enough stuff written > >about offroad tire behavior!!! > > > I think the optimum slip angle on slippy surfaces is tighter, because > the contact patch on loose surfaces has less friction > so it's deformation is smaller on slippy surface. > > IMO, regards, > > Sebastien TIXIER - Game Developer > Dynamics and Car Physics > http://www.eden-studios.fr > GPLRank Normal:-44.24 Monster:-124.44

Waow, ;o) Your explanations are really interesting however i can't see form wich theoreme you get your equation ?

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>Sorry, I wasn't being very clear... by more grip I meant higher coefficient >of friction, not actual friction force. I guess what it boils down to is >the behavior along an axle (let's assume a single-axle rally car for >convenience on a flat road.) >Let t be the fraction of lateral load transfer, then the normal forces at >the wheels are >mg*(1-t) and mg(1+t). For simplicity, let's normalize everything to mg, so >this becomes >(1-t) and (1+t) >Let the coeff of friction be a linear function of load, f(x) = ax + b, which

you mean coefficient of fiction or friction force here ? i would said friction force. Fritcion force F = k . Load. If you mean the coeficient of friction, i didn't know that it was variable with the load , i thought it was constant at constant temperatures for a given couple material/surface. >will be accurate for these purposes since we're just looking at the behavior >at a given load +/- a small delta. >then the max lateral grip should be: >(1-t)*f(1-t) + (1+t)*f(1+t) = hmm, i don't see where did you take this equation from ? Could you explain me what's is 'a' in your equations.

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>uhmmmm..... (really, I have a point here, bear with me.... ;) >(1-t)*(a(1-t)+b) + (1+t)*(a(1+t)+b) = >a-at+b - at + at^2 -bt + a + at + b + at + at^2 + bt = >2a + 2b + 2at^2 = 2(a + b + at^2) (whew, stuff cancelled out!) >So this shows that when "a" is negative (which is true of tires on asphalt, >or at least of every tire data I've ever seen), the maximum cornerering >force is at t=0, or no load transfer. This, of course, explains why >increasing front roll stiffness causes understeer and increasing rear roll >stiffness causes oversteer: because the total grip at the stiffer end has >more lateral load transfer, and thus loses more grip.

Yes this is something all setup's gurus know ;o)

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>So if on dirt, "a" would have to be positive to have the effect of lateral >load transfer increasing grip, which would also have the effect of flipping >the understeer/oversteer of a car, which is the behavior I'm having trouble >with imagining or matching up to what I've seen. I think I can imagine this >being true for really wet tarmac, as you say, because increasing the load >would decrease hydroplaning. I suppose the same argument could be made for >a tire on dirt making a "furrow" in the ground, this could cause a "digging >in" effect, which could result in positive "a"...

Well, you have also to take account that when driving with lot of sideways the rear wheel slip at lot more that the front wheel, so they loose more lateral grip that the front making the car oversteer. After thinking about that i think now that the more important effect of driving sideway on slippy surface is to use the longitudinal forces to couter act the centrigal forces. regards, Sebastien TIXIER - Game Developer Dynamics and Car Physics http://www.eden-studios.fr GPLRank Normal:-44.24 Monster:-124.44

Not necessarily true. Rear wheels slipping more than the front is by definition oversteer, but a heavily understeering car can still be driven at a very high yaw angle if it is allowed sufficient steering lock. Driving sideways does not necessarily mean oversteer, you have to look at the front wheel angle also. "S�bastien Tixier" <[email hidden]> wrote in message news:[email hidden]... > Waow, ;o) > > Well, you have also to take account that when driving with lot of > sideways the rear wheel slip at lot more that the front wheel, so they > loose more lateral grip that the front making the car oversteer.

As far as I know, all road tires have lower coefficients of friction at higher load (tire load sensitivity). If you don't have this effect in a sim, your spring/swaybar adjustments won't have the expected result. (As I discovered from experience! ;) Here's an example from RCVD (Fig 2.30) A P225/70 R15 at 0 camber angle has a coefficient of friction (Fy/Fz) about 1.0 at 200 lbs of load, and 0.9 at 1600 lbs of load. My little "ax + b" function was just a quick empirical fit to that type of data (the graph is pretty close to linear, and anyways I was just trying to establish in which direction the force changes) So a 1.0G tire that didn't exhibit this would have a = 0, b = 1.0 in this case, and the tire I mentioned above would have something like a = 0.1 / (1600 - 200 lbs), b = 1.02 or so. I'm imagining a simple tire model that looks something like Mu = calc_mu(load) = a * load + b tire_force = f(slip_angle) * Mu * load, where f(x) is one of those nondimensional "magic formulae" that peaks at 1.0. (My tire models used the nondimensional tire method as described in Chap 14 of RCVD as their starting point--so my notation may be nonstandard compared to other methods) Since we're talking about max cornering power, I'm assuming that all wheels are at optimum slip angle, thus tire_force = Mu * load. >(1-t)*f(1-t) + (1+t)*f(1+t)This could be written as left_load * calc_mu(left_load) + right_load * calc_mu(right_load) where left_load = (1 - t), right_load = (1 + t), where t is lateral weight transfer fraction (I normalized everything by m*g just to make the math easier) I hope this clarifies things--sorry I'm not very good at using standard notation, plus the fact that I tend to think in pounds and not newtons! ;) -Dave "S�bastien Tixier" <[email hidden]> wrote in message news:[email hidden]... > Waow, ;o) > > Your explanations are really interesting however i can't see form wich > theoreme you get your equation ? > > >Sorry, I wasn't being very clear... by more grip I meant highercoefficient > >of friction, not actual friction force. I guess what it boils down to is > >the behavior along an axle (let's assume a single-axle rally car for > >convenience on a flat road.) > >Let t be the fraction of lateral load transfer, then the normal forces at > >the wheels are > >mg*(1-t) and mg(1+t). For simplicity, let's normalize everything to mg,so > >this becomes > >(1-t) and (1+t) > >Let the coeff of friction be a linear function of load, f(x) = ax + b,which

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> > you mean coefficient of fiction or friction force here ? i would said > friction force. > Fritcion force F = k . Load. > If you mean the coeficient of friction, i didn't know that it was > variable with the load , i thought it was constant at constant > temperatures for a given couple material/surface. > > >will be accurate for these purposes since we're just looking at the

behavior

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> >at a given load +/- a small delta. > >then the max lateral grip should be: > >(1-t)*f(1-t) + (1+t)*f(1+t) = > > hmm, i don't see where did you take this equation from ? > Could you explain me what's is 'a' in your equations. > > >uhmmmm..... (really, I have a point here, bear with me.... ;) > >(1-t)*(a(1-t)+b) + (1+t)*(a(1+t)+b) = > >a-at+b - at + at^2 -bt + a + at + b + at + at^2 + bt = > >2a + 2b + 2at^2 = 2(a + b + at^2) (whew, stuff cancelled out!) > >So this shows that when "a" is negative (which is true of tires on

asphalt, > >or at least of every tire data I've ever seen), the maximum cornerering > >force is at t=0, or no load transfer. This, of course, explains why > >increasing front roll stiffness causes understeer and increasing rearroll > >stiffness causes oversteer: because the total grip at the stiffer end has > >more lateral load transfer, and thus loses more grip. > > Yes this is something all setup's gurus know ;o) > > >So if on dirt, "a" would have to be positive to have the effect oflateral > >load transfer increasing grip, which would also have the effect offlipping > >the understeer/oversteer of a car, which is the behavior I'm havingtrouble > >with imagining or matching up to what I've seen. I think I can imaginethis > >being true for really wet tarmac, as you say, because increasing the load > >would decrease hydroplaning. I suppose the same argument could be madefor > >a tire on dirt making a "furrow" in the ground, this could cause a"digging

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> >in" effect, which could result in positive "a"... > > Well, you have also to take account that when driving with lot of > sideways the rear wheel slip at lot more that the front wheel, so they > loose more lateral grip that the front making the car oversteer. > > After thinking about that i think now that the more important effect > of driving sideway on slippy surface is to use the longitudinal forces > to couter act the centrigal forces. > > regards, > Sebastien TIXIER - Game Developer > Dynamics and Car Physics > http://www.eden-studios.fr > GPLRank Normal:-44.24 Monster:-124.44

For the pavement case, see RCVD Chapter 7 -- Pair Analysis, could have saved you some typing<grin>. As far as tire data on dirt, I think that most of the published work has been done for/by the US Army Tank Automotive Command (TACOM) in Detroit, just down the road from the GM Tech Center. For many years they ran tire tests on different kinds of prepared soil "bins" to study "land locomotion". The key guy (now dead) was named Bekker. Their data may not apply all that well to high speeds... Other tire test rigs have been built onto trucks of various sorts, the big problems might be instrumentation and getting repeatable data in some coordinate system (when everything is bouncing around!)--so that the data is usable. -- Doug Milliken http://www.millikenresearch.com/olley.html <-- new book! On Mon, 22 Apr 2002, Dave Pollatsek wrote:

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> Sorry, I wasn't being very clear... by more grip I meant higher coefficient > of friction, not actual friction force. I guess what it boils down to is > the behavior along an axle (let's assume a single-axle rally car for > convenience on a flat road.) > Let t be the fraction of lateral load transfer, then the normal forces at > the wheels are > mg*(1-t) and mg(1+t). For simplicity, let's normalize everything to mg, so > this becomes > (1-t) and (1+t) > Let the coeff of friction be a linear function of load, f(x) = ax + b, which > will be accurate for these purposes since we're just looking at the behavior > at a given load +/- a small delta. > then the max lateral grip should be: > (1-t)*f(1-t) + (1+t)*f(1+t) = > uhmmmm..... (really, I have a point here, bear with me.... ;) > (1-t)*(a(1-t)+b) + (1+t)*(a(1+t)+b) = > a-at+b - at + at^2 -bt + a + at + b + at + at^2 + bt = > 2a + 2b + 2at^2 = 2(a + b + at^2) (whew, stuff cancelled out!) > So this shows that when "a" is negative (which is true of tires on asphalt, > or at least of every tire data I've ever seen), the maximum cornerering > force is at t=0, or no load transfer. This, of course, explains why > increasing front roll stiffness causes understeer and increasing rear roll > stiffness causes oversteer: because the total grip at the stiffer end has > more lateral load transfer, and thus loses more grip. > So if on dirt, "a" would have to be positive to have the effect of lateral > load transfer increasing grip, which would also have the effect of flipping > the understeer/oversteer of a car, which is the behavior I'm having trouble > with imagining or matching up to what I've seen. I think I can imagine this > being true for really wet tarmac, as you say, because increasing the load > would decrease hydroplaning. I suppose the same argument could be made for > a tire on dirt making a "furrow" in the ground, this could cause a "digging > in" effect, which could result in positive "a"... > > What we really need is a tire dyno for dirt. I'm thinking of an old school > bus driving down a dirt road with a tire mounted on an arm sticking off the > side bolted to some bathroom scales... throw in some C-clamps and a lot of > duct tape for good measure. I'm still pretty sure that the peak is at a > wide slip angle, although until one of us actually coughs up some real data, > we're just ... (insert colorful aphorism of your choice here) > > -Dave P. > PS. I'm actually only *mostly* joking about the bus thing. I just may try > it some day... ;) > > "Sbastien Tixier" <[email hidden]> wrote in message > news:[email hidden]... > > On Sat, 20 Apr 2002 17:00:39 -0500, "Dave Pollatsek" > > <[email hidden]> wrote: > > > > >I'm guessing the "more load = more grip" thing, if true, is nonlinear; if > it > > >were strictly true, than this would mean that the load sensitivity is > > >inverse to asphalt, which would imply that a typical RWD sedan (e.g., 55 > % > > >front weight, same tires front and rear, more roll stiffness in front to > > >make it "safely" understeer) would be oversteer on gravel, while an > > >oversteer car would become understeer. > > > > Well, no. In Asphalt you also have more lateral force with more load. > > Just look at the pacejka formula. However that's true that for very > > strong load the lateral force start to decrase. > > > > >In my experience, production cars > > >still understeer in loose surfaces unless the handbrake or some sort of > > >throttle technique (left-foot brake on FWD, or dipping the clutch on RWD) > is > > >used to degrade rear grip. > > > > That's true, the car always loose lateral force on the driven wheels > > because those wheel tends to slip more. > > > > >Also, I wouldn't expect removing ARBs to change steady-state total > lateral > > >load transfer--isn't that merely a function of lateral force, CG height, > and > > >track width? I would, however, expect removing ARBs to reduce wheel-hop > on > > >bumpy surfaces. > > > > Rally driver usually removes the ARB on wet tarmac because it's a > > really slippy surfaces. Nothing to do with bumps. It's, again, to > > increase the lateral load transfert. > > > > >I think the biggest factor is that the friction peak on loose surfaces is > at > > >an extremely wide slip angle, probably due to a quasi-viscous behavior of > > >dirt building up on the outside edge. I think there's also some > threshhold > > >where for very tight turns, it's faster to rotate the car by sliding than > to > > >actually take the turn "properly"--hence autocrossers use the handbrake > even > > >on asphalt. > > >Whatever is true, there's no doubt that there isn't enough stuff written > > >about offroad tire behavior!!! > > > > > > I think the optimum slip angle on slippy surfaces is tighter, because > > the contact patch on loose surfaces has less friction > > so it's deformation is smaller on slippy surface. > > > > IMO, regards, > > > > Sebastien TIXIER - Game Developer > > Dynamics and Car Physics > > http://www.eden-studios.fr > > GPLRank Normal:-44.24 Monster:-124.44 > > > >

"Dave Pollatsek" Dave can I break into this "FACINATING" discussion and ask you to email me privately at [email hidden]? Thankyouverymuch... btw..the only thing I know about tire models is they stand next to tires at trade shows......right? dave henrie <phobNOdyno@yaSPAMhoo

Thanks for the info! I love the internet--two more clicks and a paper is on the way (fifteen bucks lighter, of course) It looks from my quick search that people are using Bekker's model for things like ATVs and humvees, which seems promising. I know you must have done a far more elegant pair analysis than that pile of dung I came up with, but I wanted to see if I could get the correct result off the top of my head... for the record, found a mostly unlabeled set of slides or something on the Humvee. If I'm reading the graphs right, lateral friction peaked at at least 30 degrees slip angle (that's where the graph stopped, but it looked pretty flat there); surprisingly the coefficient was .347 @ 1500 lbs and .358 @ 3000 lbs! However, longititudinal was .313 @ 1500 lbs and .292 at 3000 lbs. I suppose the load sensitivity could be backwards when in the "digging" mode--this disparity also suggests that you might be better off slowing down by driving sideways, as lateral grip seemed higher across the board. That is, if one had a humvee to take to the track. -Dave "Doug Milliken" <[email hidden]> wrote in message news:[email hidden]... For the pavement case, see RCVD Chapter 7 -- Pair Analysis, could have saved you some typing<grin>. As far as tire data on dirt, I think that most of the published work has been done for/by the US Army Tank Automotive Command (TACOM) in Detroit, just down the road from the GM Tech Center. For many years they ran tire tests on different kinds of prepared soil "bins" to study "land locomotion". The key guy (now dead) was named Bekker. Their data may not apply all that well to high speeds... Other tire test rigs have been built onto trucks of various sorts, the big problems might be instrumentation and getting repeatable data in some coordinate system (when everything is bouncing around!)--so that the data is usable. -- Doug Milliken http://www.millikenresearch.com/olley.html <-- new book! On Mon, 22 Apr 2002, Dave Pollatsek wrote: > Sorry, I wasn't being very clear... by more grip I meant highercoefficient > of friction, not actual friction force. I guess what it boils down to is > the behavior along an axle (let's assume a single-axle rally car for > convenience on a flat road.) > Let t be the fraction of lateral load transfer, then the normal forces at > the wheels are > mg*(1-t) and mg(1+t). For simplicity, let's normalize everything to mg,so > this becomes > (1-t) and (1+t) > Let the coeff of friction be a linear function of load, f(x) = ax + b,which > will be accurate for these purposes since we're just looking at thebehavior

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> at a given load +/- a small delta. > then the max lateral grip should be: > (1-t)*f(1-t) + (1+t)*f(1+t) = > uhmmmm..... (really, I have a point here, bear with me.... ;) > (1-t)*(a(1-t)+b) + (1+t)*(a(1+t)+b) = > a-at+b - at + at^2 -bt + a + at + b + at + at^2 + bt = > 2a + 2b + 2at^2 = 2(a + b + at^2) (whew, stuff cancelled out!) > So this shows that when "a" is negative (which is true of tires on

asphalt,

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> or at least of every tire data I've ever seen), the maximum cornerering > force is at t=0, or no load transfer. This, of course, explains why > increasing front roll stiffness causes understeer and increasing rear roll > stiffness causes oversteer: because the total grip at the stiffer end has > more lateral load transfer, and thus loses more grip. > So if on dirt, "a" would have to be positive to have the effect of lateral > load transfer increasing grip, which would also have the effect of

flipping > the understeer/oversteer of a car, which is the behavior I'm havingtrouble > with imagining or matching up to what I've seen. I think I can imaginethis > being true for really wet tarmac, as you say, because increasing the load > would decrease hydroplaning. I suppose the same argument could be madefor > a tire on dirt making a "furrow" in the ground, this could cause a"digging > in" effect, which could result in positive "a"... > > What we really need is a tire dyno for dirt. I'm thinking of an oldschool > bus driving down a dirt road with a tire mounted on an arm sticking offthe > side bolted to some bathroom scales... throw in some C-clamps and a lot of > duct tape for good measure. I'm still pretty sure that the peak is at a > wide slip angle, although until one of us actually coughs up some realdata, > we're just ... (insert colorful aphorism of your choice here) > > -Dave P. > PS. I'm actually only *mostly* joking about the bus thing. I just maytry

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> it some day... ;) > > "S�bastien Tixier" <[email hidden]> wrote in message > news:[email hidden]... > > On Sat, 20 Apr 2002 17:00:39 -0500, "Dave Pollatsek" > > <[email hidden]> wrote: > > > > >I'm guessing the "more load = more grip" thing, if true, is nonlinear;

if > it > > >were strictly true, than this would mean that the load sensitivity is > > >inverse to asphalt, which would imply that a typical RWD sedan (e.g.,55 > % > > >front weight, same tires front and rear, more roll stiffness in frontto

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> > >make it "safely" understeer) would be oversteer on gravel, while an > > >oversteer car would become understeer. > > > > Well, no. In Asphalt you also have more lateral force with more load. > > Just look at the pacejka formula. However that's true that for very > > strong load the lateral force start to decrase. > > > > >In my experience, production cars > > >still understeer in loose surfaces unless the handbrake or some sort of > > >throttle technique (left-foot brake on FWD, or dipping the clutch on

RWD)

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> is > > >used to degrade rear grip. > > > > That's true, the car always loose lateral force on the driven wheels > > because those wheel tends to slip more. > > > > >Also, I wouldn't expect removing ARBs to change steady-state total > lateral > > >load transfer--isn't that merely a function of lateral force, CG

height, > and > > >track width? I would, however, expect removing ARBs to reducewheel-hop

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> on > > >bumpy surfaces. > > > > Rally driver usually removes the ARB on wet tarmac because it's a > > really slippy surfaces. Nothing to do with bumps. It's, again, to > > increase the lateral load transfert. > > > > >I think the biggest factor is that the friction peak on loose surfaces

is > at > > >an extremely wide slip angle, probably due to a quasi-viscous behaviorof > > >dirt building up on the outside edge. I think there's also some > threshhold > > >where for very tight turns, it's faster to rotate the car by slidingthan > to > > >actually take the turn "properly"--hence autocrossers use the handbrake > even > > >on asphalt. > > >Whatever is true, there's no doubt that there isn't enough stuffwritten

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> > >about offroad tire behavior!!! > > > > > > I think the optimum slip angle on slippy surfaces is tighter, because > > the contact patch on loose surfaces has less friction > > so it's deformation is smaller on slippy surface. > > > > IMO, regards, > > > > Sebastien TIXIER - Game Developer > > Dynamics and Car Physics > > http://www.eden-studios.fr > > GPLRank Normal:-44.24 Monster:-124.44 > > > >

Haqsau <[email hidden]> wrote in message news:aa2769$75idi$[email hidden]... > Lateral weight transfer can be increased slightly by removing bars > because the roll angle will increase, and rally cars have a fairly high > CG, therefore the CG and its acceleration vector are going to point Valid point, forgot about that one. > importantly I think is that by decreasing roll resistance the peak > normal force experienced during roll will decrease, thereby decreasing > the minimum effective traction coefficient as the car is rolling. In > other words grip during transient maneuvering will improve. Also bars ... or at least, the load transfer will happen more gradually and is therefore less likely to upset the tyres and trigger a slide. > are just really nasty on uneven surfaces. My Jeeping buddy took his off > because it decreased suspension compliance too much. Of course he has a > they drive on - lots of one-wheel deflections, which are really nasty if > you are using ARB's. Indeed - although recently I've taken to running ARBs both ends together with soft springs, to keep my 1/10th off-roader shiny side up on my (rather bumpy) local track. Could just be the above-mentioned transient behaviour, though, encouraging it to slide when it catches a pothole rather than grip-rolling ;-) Jonny