Tire load sensitivity revisited

Is tire load sensitivity, or the variation in effective friction coefficient with load, generally approximately the same in both the longitudinal and lateral directions? My thinking is that if you look at the strain pattern in the contact patch of a tire in the longitudinal direction, there is a predictable curve from slip ratio, and there is another similar distortion that would exist even with 0 slip ratio that would take away from the lateral force that's available. (Even at 0 slip ratio/angle, the front of the tread is being bent forwards at the road, and rearwards at the rear of the tread, resulting in essentially 0 longitudinal force if rolling resistance was neglected, but would cause a loss of available lateral force dependent mostly on load, I'm thinking.) However, thinking this way and rotating this system 90 degrees, the longitudinal force would not decrease in the same manner. There is a somewhat equivalent loss laterally I think because the center of the tread is being compressed, which would cause a lateral strain pattern resulting in a loss of available longitudinal force (perhaps the left/right edges might tend to slide outwards a bit during free rolling, similar to how the front/mostly rear might work), but my thinking is that this would not be as severe in this direction because the strain here would not be as great....maybe... So.... This would indicate that as load is increased, a tire would tend to lose more lateral force than longitudinal force capacity, causing the friction circle to become not only larger, but more elongated along the longitudinal direction. I haven't read anywhere even a hint that this is indeed what happens though, so I'm questioning my thinking here. Can anybody shed some light on this? Of course, I'm assuming constant temperature, pressure, camber, etc.. Thanks! Todd Wasson --- Performance Simulations Drag Racing and Top Speed Prediction Software http://PerformanceSimulations.Com My little car sim screenshots: http://performancesimulations.com/scnshot4.htm

I've got some pacejka coefficients for a road tyre (you've got them too Todd, I forwarded them to you last month!). At zero load this tyre has an effective friction coefficient of 1.201 (lateral) and 1.571 (longitudinal) but then these coefficients are reduced by 0.038 per kN load (lateral) or 0.080 per kN load (longitudinal). So if your tyre had a 2 kN load you'd have an effective friction coefficient of 1.125 (lat) and 1.411 (long). So the tyres appear to be twice as sensitive to load in the longitudinal direction. "J. Todd Wasson" <[email hidden]> wrote in message news:[email hidden]... > Is tire load sensitivity, or the variation in effective frictioncoefficient > with load, generally approximately the same in both the longitudinal and > lateral directions? My thinking is that if you look at the strain patternin > the contact patch of a tire in the longitudinal direction, there is a > predictable curve from slip ratio, and there is another similar distortionthat > would exist even with 0 slip ratio that would take away from the lateralforce > that's available. (Even at 0 slip ratio/angle, the front of the tread isbeing > bent forwards at the road, and rearwards at the rear of the tread,resulting in > essentially 0 longitudinal force if rolling resistance was neglected, butwould > cause a loss of available lateral force dependent mostly on load, I'm > thinking.) > > However, thinking this way and rotating this system 90 degrees, the > longitudinal force would not decrease in the same manner. There is asomewhat > equivalent loss laterally I think because the center of the tread is being > compressed, which would cause a lateral strain pattern resulting in a lossof > available longitudinal force (perhaps the left/right edges might tend toslide > outwards a bit during free rolling, similar to how the front/mostly rearmight > work), but my thinking is that this would not be as severe in thisdirection > because the strain here would not be as great....maybe... > > So.... This would indicate that as load is increased, a tire would tendto > lose more lateral force than longitudinal force capacity, causing thefriction > circle to become not only larger, but more elongated along thelongitudinal > direction. I haven't read anywhere even a hint that this is indeed what > happens though, so I'm questioning my thinking here. Can anybody shedsome

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> light on this? > > Of course, I'm assuming constant temperature, pressure, camber, etc.. > > Thanks! > > Todd Wasson > --- > Performance Simulations > Drag Racing and Top Speed Prediction > Software > http://PerformanceSimulations.Com > > My little car sim screenshots: > http://performancesimulations.com/scnshot4.htm >

Hi, I'm no physics guru, but am curious if you've ever come across research on modelling of tire dynamics on gravel surfaces. I'm really keen to experience a rally simulator that emulates dirt/gravel surfaces. Whilst LFS is brilliant on tarmac, and good on gravel, the experience of sliding the car on dirt can't be described as realistic. On tarmac surfaces, modelling is much simpler, as the surface is effectively 2D, with only camber angles to worry about (and of course wet surfaces). However, gravel is a 3D surface, with a tire digging in. As such, the physics are quite different. For instance, a car on a dirt surface will stop fastest when the front wheels are completely locked up, as they build a "wall" of gravel up in front of them. Conversely, a car on tarmac will stop fastest when the wheels are still rotating, but at slower speed than which the car is travelling on the road. One of the biggest differences between tire dynamics on tarmac and unsealed surfaces is when the car has attitude (oversteer or 4 wheel drift). Rally cars have the best grip when going sideways, under either acceleration or braking. This allows for the controlled power slides that are the art of rally driving, and which I've never seen realistically moddeled on a Sim. I'd be very interested into any research into this.. Tim "J. Todd Wasson" <[email hidden]> wrote in message news:[email hidden]... > Is tire load sensitivity, or the variation in effective frictioncoefficient > with load, generally approximately the same in both the longitudinal and > lateral directions? My thinking is that if you look at the strain patternin > the contact patch of a tire in the longitudinal direction, there is a > predictable curve from slip ratio, and there is another similar distortionthat > would exist even with 0 slip ratio that would take away from the lateralforce > that's available. (Even at 0 slip ratio/angle, the front of the tread isbeing > bent forwards at the road, and rearwards at the rear of the tread,resulting in > essentially 0 longitudinal force if rolling resistance was neglected, butwould > cause a loss of available lateral force dependent mostly on load, I'm > thinking.) > > However, thinking this way and rotating this system 90 degrees, the > longitudinal force would not decrease in the same manner. There is asomewhat > equivalent loss laterally I think because the center of the tread is being > compressed, which would cause a lateral strain pattern resulting in a lossof > available longitudinal force (perhaps the left/right edges might tend toslide > outwards a bit during free rolling, similar to how the front/mostly rearmight > work), but my thinking is that this would not be as severe in thisdirection > because the strain here would not be as great....maybe... > > So.... This would indicate that as load is increased, a tire would tendto > lose more lateral force than longitudinal force capacity, causing thefriction > circle to become not only larger, but more elongated along thelongitudinal > direction. I haven't read anywhere even a hint that this is indeed what > happens though, so I'm questioning my thinking here. Can anybody shedsome

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> light on this? > > Of course, I'm assuming constant temperature, pressure, camber, etc.. > > Thanks! > > Todd Wasson > --- > Performance Simulations > Drag Racing and Top Speed Prediction > Software > http://PerformanceSimulations.Com > > My little car sim screenshots: > http://performancesimulations.com/scnshot4.htm >

Try searching the internet for Bekker's soil model. People seem to be using this model for things like ATVs and humvees. The last time I searched, I also found some tire data for humvee's on dirt--from the graphs it looked like load sensitivity was "backwards" for lateral grip (e.g., higher coefficient with more load). Which actually seems plausible, because you can imagine the more vertical sides of the tire digging in better than the sloping front face of the tire, which would probably cause the tire to float up. -Dave "Tim Epstein" <[email hidden]> wrote in message news:0VHe9.1099$[email hidden]... > Hi, I'm no physics guru, but am curious if you've ever come acrossresearch > on modelling of tire dynamics on gravel surfaces. I'm really keen to > experience a rally simulator that emulates dirt/gravel surfaces. WhilstLFS > is brilliant on tarmac, and good on gravel, the experience of sliding the > car on dirt can't be described as realistic. > > On tarmac surfaces, modelling is much simpler, as the surface iseffectively > 2D, with only camber angles to worry about (and of course wet surfaces). > However, gravel is a 3D surface, with a tire digging in. As such, the > physics are quite different. For instance, a car on a dirt surface willstop > fastest when the front wheels are completely locked up, as they build a > "wall" of gravel up in front of them. Conversely, a car on tarmac willstop > fastest when the wheels are still rotating, but at slower speed than which > the car is travelling on the road. > > One of the biggest differences between tire dynamics on tarmac andunsealed

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> surfaces is when the car has attitude (oversteer or 4 wheel drift). Rally > cars have the best grip when going sideways, under either acceleration or > braking. This allows for the controlled power slides that are the art of > rally driving, and which I've never seen realistically moddeled on a Sim. > > I'd be very interested into any research into this.. > > Tim > > > > "J. Todd Wasson" <[email hidden]> wrote in message > news:[email hidden]... > > Is tire load sensitivity, or the variation in effective friction > coefficient > > with load, generally approximately the same in both the longitudinal and > > lateral directions? My thinking is that if you look at the strain

pattern > in > > the contact patch of a tire in the longitudinal direction, there is a > > predictable curve from slip ratio, and there is another similardistortion > that > > would exist even with 0 slip ratio that would take away from the lateral > force > > that's available. (Even at 0 slip ratio/angle, the front of the treadis > being > > bent forwards at the road, and rearwards at the rear of the tread, > resulting in > > essentially 0 longitudinal force if rolling resistance was neglected,but

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> would > > cause a loss of available lateral force dependent mostly on load, I'm > > thinking.) > > > > However, thinking this way and rotating this system 90 degrees, the > > longitudinal force would not decrease in the same manner. There is a > somewhat > > equivalent loss laterally I think because the center of the tread is

being > > compressed, which would cause a lateral strain pattern resulting in aloss

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> of > > available longitudinal force (perhaps the left/right edges might tend to > slide > > outwards a bit during free rolling, similar to how the front/mostly rear > might > > work), but my thinking is that this would not be as severe in this > direction > > because the strain here would not be as great....maybe... > > > > So.... This would indicate that as load is increased, a tire would tend > to > > lose more lateral force than longitudinal force capacity, causing the > friction > > circle to become not only larger, but more elongated along the > longitudinal > > direction. I haven't read anywhere even a hint that this is indeed what > > happens though, so I'm questioning my thinking here. Can anybody shed > some > > light on this? > > > > Of course, I'm assuming constant temperature, pressure, camber, etc.. > > > > Thanks! > > > > Todd Wasson > > --- > > Performance Simulations > > Drag Racing and Top Speed Prediction > > Software > > http://PerformanceSimulations.Com > > > > My little car sim screenshots: > > http://performancesimulations.com/scnshot4.htm > > > >

Thanks very much for the lead. I just found this: http://arc.engin.umich.edu/arc/research/T2RealTimeTire.htm Its only abstract, but means that this area is being seriously looked at. Cheers, Tim "Dave Pollatsek" <[email hidden]> wrote in message news:3d7b7739$0$36736$[email hidden]... > Try searching the internet for Bekker's soil model. People seem to beusing > this model for things like ATVs and humvees. The last time I searched, I > also found some tire data for humvee's on dirt--from the graphs it looked > like load sensitivity was "backwards" for lateral grip (e.g., higher > coefficient with more load). Which actually seems plausible, because you > can imagine the more vertical sides of the tire digging in better than the > sloping front face of the tire, which would probably cause the tire tofloat

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> up. > -Dave > > "Tim Epstein" <[email hidden]> wrote in message > news:0VHe9.1099$[email hidden]... > > Hi, I'm no physics guru, but am curious if you've ever come across > research > > on modelling of tire dynamics on gravel surfaces. I'm really keen to > > experience a rally simulator that emulates dirt/gravel surfaces. Whilst > LFS > > is brilliant on tarmac, and good on gravel, the experience of sliding

the

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> > car on dirt can't be described as realistic. > > > > On tarmac surfaces, modelling is much simpler, as the surface is > effectively > > 2D, with only camber angles to worry about (and of course wet surfaces). > > However, gravel is a 3D surface, with a tire digging in. As such, the > > physics are quite different. For instance, a car on a dirt surface will > stop > > fastest when the front wheels are completely locked up, as they build a > > "wall" of gravel up in front of them. Conversely, a car on tarmac will > stop > > fastest when the wheels are still rotating, but at slower speed than

which > > the car is travelling on the road. > > > > One of the biggest differences between tire dynamics on tarmac and > unsealed > > surfaces is when the car has attitude (oversteer or 4 wheel drift).Rally > > cars have the best grip when going sideways, under either accelerationor > > braking. This allows for the controlled power slides that are the art of > > rally driving, and which I've never seen realistically moddeled on aSim.

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> > > > I'd be very interested into any research into this.. > > > > Tim > > > > > > > > "J. Todd Wasson" <[email hidden]> wrote in message > > news:[email hidden]... > > > Is tire load sensitivity, or the variation in effective friction > > coefficient > > > with load, generally approximately the same in both the longitudinal

and

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> > > lateral directions? My thinking is that if you look at the strain > pattern > > in > > > the contact patch of a tire in the longitudinal direction, there is a > > > predictable curve from slip ratio, and there is another similar > distortion > > that > > > would exist even with 0 slip ratio that would take away from the

lateral

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> > force > > > that's available. (Even at 0 slip ratio/angle, the front of the tread > is > > being > > > bent forwards at the road, and rearwards at the rear of the tread, > > resulting in > > > essentially 0 longitudinal force if rolling resistance was neglected, > but > > would > > > cause a loss of available lateral force dependent mostly on load, I'm > > > thinking.) > > > > > > However, thinking this way and rotating this system 90 degrees, the > > > longitudinal force would not decrease in the same manner. There is a > > somewhat > > > equivalent loss laterally I think because the center of the tread is > being > > > compressed, which would cause a lateral strain pattern resulting in a > loss > > of > > > available longitudinal force (perhaps the left/right edges might tend

to > > slide > > > outwards a bit during free rolling, similar to how the front/mostlyrear > > might > > > work), but my thinking is that this would not be as severe in this > > direction > > > because the strain here would not be as great....maybe... > > > > > > So.... This would indicate that as load is increased, a tire wouldtend > > to > > > lose more lateral force than longitudinal force capacity, causing the > > friction > > > circle to become not only larger, but more elongated along the > > longitudinal > > > direction. I haven't read anywhere even a hint that this is indeedwhat

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> > > happens though, so I'm questioning my thinking here. Can anybody shed > > some > > > light on this? > > > > > > Of course, I'm assuming constant temperature, pressure, camber, etc.. > > > > > > Thanks! > > > > > > Todd Wasson > > > --- > > > Performance Simulations > > > Drag Racing and Top Speed Prediction > > > Software > > > http://PerformanceSimulations.Com > > > > > > My little car sim screenshots: > > > http://performancesimulations.com/scnshot4.htm > > > > > > > > >

Bullseye! http://www.cybernet.co.jp/dads/seminar/userconf1999/Tire-Soil_Interaction.pd f This includes all the equations, including lateral force slip angle, based on how much the surface compresses. Only problem is the CPU performance hit trying to model this in real time - imagine with 8 x AI cars as well! "Dave Pollatsek" <[email hidden]> wrote in message news:3d7b7739$0$36736$[email hidden]... > Try searching the internet for Bekker's soil model. People seem to beusing > this model for things like ATVs and humvees. The last time I searched, I > also found some tire data for humvee's on dirt--from the graphs it looked > like load sensitivity was "backwards" for lateral grip (e.g., higher > coefficient with more load). Which actually seems plausible, because you > can imagine the more vertical sides of the tire digging in better than the > sloping front face of the tire, which would probably cause the tire tofloat

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> up. > -Dave > > "Tim Epstein" <[email hidden]> wrote in message > news:0VHe9.1099$[email hidden]... > > Hi, I'm no physics guru, but am curious if you've ever come across > research > > on modelling of tire dynamics on gravel surfaces. I'm really keen to > > experience a rally simulator that emulates dirt/gravel surfaces. Whilst > LFS > > is brilliant on tarmac, and good on gravel, the experience of sliding

the

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> > car on dirt can't be described as realistic. > > > > On tarmac surfaces, modelling is much simpler, as the surface is > effectively > > 2D, with only camber angles to worry about (and of course wet surfaces). > > However, gravel is a 3D surface, with a tire digging in. As such, the > > physics are quite different. For instance, a car on a dirt surface will > stop > > fastest when the front wheels are completely locked up, as they build a > > "wall" of gravel up in front of them. Conversely, a car on tarmac will > stop > > fastest when the wheels are still rotating, but at slower speed than

which > > the car is travelling on the road. > > > > One of the biggest differences between tire dynamics on tarmac and > unsealed > > surfaces is when the car has attitude (oversteer or 4 wheel drift).Rally > > cars have the best grip when going sideways, under either accelerationor > > braking. This allows for the controlled power slides that are the art of > > rally driving, and which I've never seen realistically moddeled on aSim.

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> > > > I'd be very interested into any research into this.. > > > > Tim > > > > > > > > "J. Todd Wasson" <[email hidden]> wrote in message > > news:[email hidden]... > > > Is tire load sensitivity, or the variation in effective friction > > coefficient > > > with load, generally approximately the same in both the longitudinal

and

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> > > lateral directions? My thinking is that if you look at the strain > pattern > > in > > > the contact patch of a tire in the longitudinal direction, there is a > > > predictable curve from slip ratio, and there is another similar > distortion > > that > > > would exist even with 0 slip ratio that would take away from the

lateral

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> > force > > > that's available. (Even at 0 slip ratio/angle, the front of the tread > is > > being > > > bent forwards at the road, and rearwards at the rear of the tread, > > resulting in > > > essentially 0 longitudinal force if rolling resistance was neglected, > but > > would > > > cause a loss of available lateral force dependent mostly on load, I'm > > > thinking.) > > > > > > However, thinking this way and rotating this system 90 degrees, the > > > longitudinal force would not decrease in the same manner. There is a > > somewhat > > > equivalent loss laterally I think because the center of the tread is > being > > > compressed, which would cause a lateral strain pattern resulting in a > loss > > of > > > available longitudinal force (perhaps the left/right edges might tend

to > > slide > > > outwards a bit during free rolling, similar to how the front/mostlyrear > > might > > > work), but my thinking is that this would not be as severe in this > > direction > > > because the strain here would not be as great....maybe... > > > > > > So.... This would indicate that as load is increased, a tire wouldtend > > to > > > lose more lateral force than longitudinal force capacity, causing the > > friction > > > circle to become not only larger, but more elongated along the > > longitudinal > > > direction. I haven't read anywhere even a hint that this is indeedwhat

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> > > happens though, so I'm questioning my thinking here. Can anybody shed > > some > > > light on this? > > > > > > Of course, I'm assuming constant temperature, pressure, camber, etc.. > > > > > > Thanks! > > > > > > Todd Wasson > > > --- > > > Performance Simulations > > > Drag Racing and Top Speed Prediction > > > Software > > > http://PerformanceSimulations.Com > > > > > > My little car sim screenshots: > > > http://performancesimulations.com/scnshot4.htm > > > > > > > > >

Hey great find Tim! Ruud, are you reading this? :0) "Tim Epstein" <[email hidden]> wrote in message news:7eNe9.1521$[email hidden]... > Bullseye! >http://www.cybernet.co.jp/dads/seminar/userconf1999/Tire-Soil_Interaction.pd > f > > This includes all the equations, including lateral force slip angle, based > on how much the surface compresses. > Only problem is the CPU performance hit trying to model this in realtime - > imagine with 8 x AI cars as well!

Hey great find Tim! Ruud, are you reading this? :0) http://www.***net.co.jp/dads/seminar/userconf1999/Tire-Soil_Interac...

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>I've got some pacejka coefficients for a road tyre (you've got them too >Todd, I forwarded them to you last month!). At zero load this tyre has an >effective friction coefficient of 1.201 (lateral) and 1.571 (longitudinal) >but then these coefficients are reduced by 0.038 per kN load (lateral) or >0.080 per kN load (longitudinal). So if your tyre had a 2 kN load you'd have >an effective friction coefficient of 1.125 (lat) and 1.411 (long). > >So the tyres appear to be twice as sensitive to load in the longitudinal >direction. >

Thanks, Jim :-) Once the 2kN load is applied, the longitudinal capacity is 89.9% of what it was at 0 load, while the lateral is 93.7% of the 0 load value, so is the longitudinal really twice as sensitive? But then again, if you went to 4 kN load, you'd have 1.049--87% lateral and 1.091--77% longitudinal. Will have to look into this a bit further, but it looks like I need to go back to the old drawing board on a thing or two :-) Thanks again Todd Wasson --- Performance Simulations Drag Racing and Top Speed Prediction Software http://PerformanceSimulations.Com My little car sim screenshots: http://performancesimulations.com/scnshot4.htm

>Hi, I'm no physics guru, but am curious if you've ever come across research >on modelling of tire dynamics on gravel surfaces. I'm really keen to >experience a rally simulator that emulates dirt/gravel surfaces. Whilst LFS >is brilliant on tarmac, and good on gravel, the experience of sliding the >car on dirt can't be described as realistic. > I haven't really come across anything that gets into the areas you're looking for (this is the first time I've heard of Bekker's model, thanks Dave). I've seen some limited stuff in SAE papers that looks at pure slip ratio on snow and gravel. Basically, the curves looked the same but occured at a much lower level, and the force didn't drop off with increasing slip ratio (the curves stayed flat after the peak). The main paper that comes to mind didn't get in to the slip angle aspect, though. I just downloaded the Bekker model, but must be using an old Acrobat Reader because I can't view the thing! :-) >On tarmac surfaces, modelling is much simpler, as the surface is effectively >2D, with only camber angles to worry about (and of course wet surfaces). >However, gravel is a 3D surface, with a tire digging in. As such, the >physics are quite different. I think with the type of model I'm developing this would be fairly straightforward to approximate, as I'm already calculating this "digging in" area. It should be pretty simple to calculate a theoretical mass of dirt/whatever moved by this area and take the slip angle into account to get the necessary forces. I haven't done it yet though, and things don't work the way I think they will quite often, so I won't say I've "solved" this yet in my own work :-) Someone (maybe you?) once said that with LFS's knobbly tires the off road behavior was much better. What do you think? Todd Wasson --- Performance Simulations Drag Racing and Top Speed Prediction Software http://PerformanceSimulations.Com My little car sim screenshots: http://performancesimulations.com/scnshot4.htm