Tire physics experiments

[JeffR]

Some experiments I ran with LFS S2 alpha. I chose the LX6 as it has a good power to weight ratio and doesn't have downforce.

Drove in circles until cornering forces approached about 1.15g's, then continued to steer inwards until I reached the steering lock point. As I continued to steer inwards, the cornering forces dropped down to .8g's this is more than 30% loss in grip. Ran another test, initiating a spin, but then centering the steering wheel, grip remained about 1.1g's even with the car going sideways. Last test, slamed on the brakes, locked up the tires, slight loss in grip until the tires over heat. While drifting sideways or with all 4 tires locked up, there is very little loss in grip, but steering inputs can cause an extreme loss of grip. This doesn't make sense to me.

Induced understeer doesn't seem to work as well as it should, especially on the LX6 with front swaybar maxed out, and rear sway bar set to 0. With this setup, it would seem that the much stiffer front end would have significantly less lateral grip than the rear end. If the car starts spinning, pegging the fronts inwards should cause the front end to wash out, because they have a much higher slip angle, and because the suspension is set so much stiffer up front, yet induced understeer doesn't work well or not at all.

My conclusion from these experiments is that something strange is going on when one end of a car loses grip, as opposed to both ends losing grip at about the same time. When one end loses grip, the result is much less grip than when both ends lose grip.

[skiingman]

As I continued to steer inwards, the cornering forces dropped down to .8g's this is more than 30% loss in grip.

Why is this unexpected? The rough spec is that a sliding tyre has 30% less grip than a rolling tyre, but that doesn't directly translate into how much cornering force will be lost at the front wheels when you slam them into the steering lock. They may be producing 30% less grip, but instead of the force vector pointing toward the center of the turn it is pointing to a point well behind the center of the turn. The vector component perpendicular to the car's travel will therefore be (much) more than 30% smaller.

Ran another test, initiating a spin, but then centering the steering wheel, grip remained about 1.1g's even with the car going sideways.

How are you measuring grip in this instance? Via the counter at the bottom of the screen? That figure is AFAIK perpendicular to the vehicle, not the direction of travel. I would therefore assume once again that the component perpendicular to travel would be less than 1.1g.

Last test, slamed on the brakes, locked up the tires, slight loss in grip until the tires over heat. While drifting sideways or with all 4 tires locked up, there is very little loss in grip, but steering inputs can cause an extreme loss of grip. This doesn't make sense to me.

I don't think the locked wheels are dropping in grip as much as they should in straight lines. I think your perception that steering inputs cause huge loss in grip may be related to a less than thorough understanding of what happens when you turn the wheel. Open up the suspension dialog and see how crazy high the camber numbers get at full steering lock. Full lock plus forward travel will equal almost no grip at the front wheels.

[JeffR]

steer inwards ... 30% loss in gripWhy is this unexpected? The rough spec is that a sliding tyre has 30% less grip than a rolling tyreIf so, then why doesn't this show up when all 4 wheels are locked up or the car is sliding sideways with the steering centered?

I read from an actual go-kart racer that a driver can run an entire lap, doing a 4 wheel slide in every turn, and that lap times will hardly differ from a lap without any sliding. He mentioned that you can do this for several laps before the tires get overheated. Apparently, with go-kart slicks, the loss in grip when sliding is minimal. This is also true with the bias ply slicks used on most lighter non-downforce race cars (I hear this from people who race Caterhams and Formula Fords).

sliding the car with steering centeredHow are you measuring grip in this instance? Via the counter at the bottom of the screen? That figure is AFAIK perpendicular to the vehicle, not the direction of travel. I would therefore assume once again that the component perpendicular to travel would be less than 1.1g.It shows 1.1gs even as the car slides sideways (90 degrees). The vector sum seems to remain at 1.1g's as the car rotates with all 4 tires sliding. Why is the grip so much less when only 2 of the tires are sliding?

all 4 tires locked up while braking in straight lineI don't think the locked wheels are dropping in grip as much as they should in straight lines.Well something is off, as I mentioned. Either the all 4 tires locked up grip is too high, or the steering grip is too low.

I edited my original post to include the rear end sliding without the front, a spin. I think that just like the steering loss, rear end loss from oversteer is reacting in a similar matter, making the spins more severe.

[ColeusRattus]

Apparently, with go-kart slicks, the loss in grip when sliding is minimal.

I do not know much about physics in general and tyre and grip behaviour specifically, but that statement sounds fundamentally wrong. If the slicks would grip, they wouldn't slide anyway.
So it is like the loss of grip is responsible for sliding, but not vice versa.

And I don't think that the grip is measurable with the dials at the f9 screen. IIRC they just show the accelerating forces effecting the car. I don't think they are enough information to calculate grip values out of it...

[JeffR]

I do not know much about physics in general and tyre and grip behaviour specifically, but that statement sounds fundamentally wrong. If the slicks would grip, they wouldn't slide anyway. So it is like the loss of grip is responsible for sliding, but not vice versa.A tire produces lateral force based on it's slip angle. The slip angle is the difference between the direction the tire is pointed, and the actual direction the tire is moving. There's always some slippage at the edges of the contact patch, and as slip angle increases, the amount of slippage increases, as well as the amount of lateral force, until you reach a peak amount of lateral force. What happens if the slip angle goes beyond this optimal amount of slip angle depends on the type of tire. In the case of go-kart and bias-ply racing slicks, there's very little loss of grip, the curve for lateral force versus slip angle is almost horizontal from the optimal angle and beyond. Street radials, on the otherhand, have a signifcant downwards slope in the curve beyond the peak force, with a resultant loss in lateral force, and negative stabilty at the rear end of a car (once a spin starts, slip angle increases, lateral force decreases, which increase slip angle even more, which reduces lateral force even more, ... ).

The go-kart racer was forcing the go-kart to go into an oversteer situation with throttle inputs, increasing the slip angle beyond optimal. A good driver can do 4 wheel drifts, which is a lot of fun, but wears out the tires. However, because of the nature of the tires, the cornering force stays about the same even with larger than optimal slip angles.

In terms of recovery, if oversteer occurs under braking, a drivers only hope is to induce understeer by turning the front wheels inwards and maybe stabbing the brakes to cause the front end to break loose. Trying to counter-steer while braking just makes a spin worse. Lifting off the brakes probably means you end up off track. If you use left foot braking (go-karts and game players), and have braking balance set very rearward, adding throttle will reduced the braking on the rear tires and stop the oversteer if caught soon enough, but this type of setup is not that common.

If oversteer occurs under acceleration, then easing off the throttle and countersteering is normally used (induced understeer can be used also, but this slows down a car at a time when the goal is to accelerate).

In the case of the go-kart, a driver can really hang out the rear end without spinning the kart, and run about the same lap times.

And I don't think that the grip is measurable with the dials at the f9 screen. IIRC they just show the accelerating forces effecting the car. I don't think they are enough information to calculate grip values out of it...But what I'm interested in is the acceleration forces on the car, I don't really care about the grip, except for the fact that the grip is what determines the amount of force acting on a car.

The net result from my experiments is that understeering or oversteering grip is much less than the grip from a 4 wheel slide or than with all 4 wheels locked up. Something is wrong here.

[Bob Smith]

Karts do act very strangely to normal cars so I don't think it's fair to do direct comparisons.

One thing I've never sussed out with tyres is knowing you've "got grip" and when you've "lost grip" in terms of physics. From what we've learnt about slip angles there is in fact no loss of grip, and in fact there could even be a slight increase with large slip angles. What is it that makes the car feel like it's cornering normally, and then feel like you're sliding? Is it something to do with the shape of the slip angle grip curve, or some other properties of the tyre?

[Hyperactive]

I was just yesterday experimenting with lx6 as well and noticed some strange behaviour, to me at least... If you set to the forces view by pressing "f" you can see that the force affecting to the inner rear tyre gets red (meaning it's slipping?) very very easily. Also noticed that when the car seems to snap oversteer all the forces suddenly go red? Not that first inner rear, then outer rear and then front ones as I might think...

[Honey]

Some experiments I ran with LFS S2 alpha. I chose the LX6 as it has a good power to weight ratio and doesn't have downforce.

Drove in circles until cornering forces approached about 1.15g's, then continued to steer inwards until I reached the steering lock point. As I continued to steer inwards, the cornering forces dropped down to .8g's this is more than 30% loss in grip. Ran another test, initiating a spin, but then centering the steering wheel, grip remained about 1.1g's even with the car going sideways. Last test, slamed on the brakes, locked up the tires, slight loss in grip until the tires over heat. While drifting sideways or with all 4 tires locked up, there is very little loss in grip, but steering inputs can cause an extreme loss of grip. This doesn't make sense to me.

Induced understeer doesn't seem to work as well as it should, especially on the LX6 with front swaybar maxed out, and rear sway bar set to 0. With this setup, it would seem that the much stiffer front end would have significantly less lateral grip than the rear end. If the car starts spinning, pegging the fronts inwards should cause the front end to wash out, because they have a much higher slip angle, and because the suspension is set so much stiffer up front, yet induced understeer doesn't work well or not at all.

My conclusion from these experiments is that something strange is going on when one end of a car loses grip, as opposed to both ends losing grip at about the same time. When one end loses grip, the result is much less grip than when both ends lose grip.

i didn't read all replies, but your conclusion seems pretty realistic to me:
if u lose grip on both front and rear, at each wheel aren't applied any forces (phisically speaking) and the sliding is only consuming the energy that car have because of velocity, when only one edge is sliding then there is also the centripetal force to compensate, so the sliding wheels (the rear) must dissipate the cinetic energy and also conteract the centripetal force.

it's hard to explain more easily in english for me, i hope you understood anyway :)

[JeffR]

Not sure if this is related, but if the LX6 is stuck in a sand trap, there are times it can not move forwards, but it can move backwards (in reverse).

[xaotik]

Not sure if this is related, but if the LX6 is stuck in a sand trap, there are times it can not move forwards, but it can move backwards (in reverse).

I've found that to be the case with all cars with a rear-biased weight distribution in LFS, but it also might have to do with the ride height of these cars as well as all of them (formulas and LXs) are pretty low riding.

[JeffR]

I've found that to be the case with all cars with a rear-biased weight distribution in LFS, but it also might have to do with the ride height of these cars as well as all of them (formulas and LXs) are pretty low riding.Ok, that would make sense, maybe the front end of the car is getting stuck in the sand.

[jtw62074]

Some experiments I ran with LFS S2 alpha. I chose the LX6 as it has a good power to weight ratio and doesn't have downforce.

Drove in circles until cornering forces approached about 1.15g's, then continued to steer inwards until I reached the steering lock point. As I continued to steer inwards, the cornering forces dropped down to .8g's this is more than 30% loss in grip. Ran another test, initiating a spin, but then centering the steering wheel, grip remained about 1.1g's even with the car going sideways. Last test, slamed on the brakes, locked up the tires, slight loss in grip until the tires over heat. While drifting sideways or with all 4 tires locked up, there is very little loss in grip, but steering inputs can cause an extreme loss of grip. This doesn't make sense to me.

Induced understeer doesn't seem to work as well as it should, especially on the LX6 with front swaybar maxed out, and rear sway bar set to 0. With this setup, it would seem that the much stiffer front end would have significantly less lateral grip than the rear end. If the car starts spinning, pegging the fronts inwards should cause the front end to wash out, because they have a much higher slip angle, and because the suspension is set so much stiffer up front, yet induced understeer doesn't work well or not at all.

My conclusion from these experiments is that something strange is going on when one end of a car loses grip, as opposed to both ends losing grip at about the same time. When one end loses grip, the result is much less grip than when both ends lose grip.

I don't see anything wrong with that at all. As Skiingman said, the lateral force vector is rotated further and further rearward as steering angle is increased. If you steered the front wheels to 90 degrees you aren't going to corner at all, but you haven't "lost grip" by any sense. So this "two wheel grip loss" at the front is not necessarily tire grip loss at all, you might be losing 30% lateral acceleration because you've changed the direction of the force vector so severely.

In addition, LFS likes 0 dynamic camber at all times. Any deviation from this at all reduces grip. So as someone else pointed out, the front "wash out" will be compounded by the severe camber induced by steering the front wheels (caster/kingpin inclination causes this to occur). However, the pushing is mostly due to that lateral force vector pointing rearwards. Remember that the lateral acceleration shown with F9 appears to be in the car plane, so the only time you're actually seeing the real grip level is when all four wheels are pointed straight ahead and you're sliding along sideways somewhat. Not suprisingly, you'll see the highest lateral acceleration then when the car has been pitched into the turn and the front wheels are more or less straight ahead.

This can be confirmed with "mass moment method" (MMM) measurements/calcuations done on the Chaparal racing car in Milliken's "Race Car Vehicle Dynamics." Peak lateral acceleration is not obtained in a trimmed attitude. If it were, the car would have no stability at all in a steady state corner at max lateral acceleration (by definition).

The only issue I'd take with Skiingman's post is the statement that a sliding tire produces 30% less grip than a non sliding one. Of course, this really means that somewhere past the peak force slip angle the force drops 30%. This simply is not true. Street tires on dry pavement peak and are flat, flat, flat, even the radials JeffR mentioned :) Ok, maybe as you get out to 60 degrees at high speed you may see some drop, but it's not anything approaching 30%..

But anyway, Jeff's test that showed the grip dropping from 1.15 to 1.10 when pitched sideways with the wheels straight ahead indicates that the curves in LFS with those tires peak at 1.15 and then drop very slightly to 1.10. Quite realistic for some speeds and tires :)

[jtw62074]

Karts do act very strangely to normal cars so I don't think it's fair to do direct comparisons.

One thing I've never sussed out with tyres is knowing you've "got grip" and when you've "lost grip" in terms of physics. From what we've learnt about slip angles there is in fact no loss of grip, and in fact there could even be a slight increase with large slip angles. What is it that makes the car feel like it's cornering normally, and then feel like you're sliding? Is it something to do with the shape of the slip angle grip curve, or some other properties of the tyre?

As Doug Milliken explained it to me, the feeling that the car has "let go" is not because the force curves fall off after the peak (if they indeed do), but rather that the force merely stops rising. When you hit the peak you can get the sensation that the car has broken away. The shape of the curve leading up to this point will be what tells your butt whether the tire breaks away suddenly or not.

There are really two things there:

1. Where the peak occurs (i.e., at what slip angle does the force stop rising). The greater this slip angle is, the more forgiving the tires will feel. Street tires peak at very high slip angles. I've seen data showing peaks at 20 degrees. One set showed it still rising at 28 degrees!

2. The shape of the curve leading up to the peak. If the force rises very linearily and then suddenly flattens out, the tire will feel like it breaks away very suddenly. If instead it rises normally and quickly starts rolling off towards the peak (the slope decreases to 0 gently) it will feel more forgiving. Tire designers can influence this shape substantially through their choices in cord patterns/angles and so forth. Radials typically rise more quickly than bias ply tires and roll off a bit more suddenly (into a flat peak!) This makes them feel like they break away more suddenly then bias tires do and leads people to believe (falsely) that radial tires are losing a bunch of grip after the peak. T'aint so! :)

This is similar to people's rear ends telling them that cars actually speed up once they hit the grass sideways, which is of course nonsense. The acceleration became lower, that's all.

[skiingman]

The only issue I'd take with Skiingman's post is the statement that a sliding tire produces 30% less grip than a non sliding one. Of course, this really means that somewhere past the peak force slip angle the force drops 30%. This simply is not true. Street tires on dry pavement peak and are flat, flat, flat, even the radials JeffR mentioned :) Ok, maybe as you get out to 60 degrees at high speed you may see some drop, but it's not anything approaching 30%..



I only used the 30% figure as that is what was used by the LFS programmers in the text for the training sessions, regarding straight line deceleration.

[J.B.]

Todd, when are you going to write a book? Nice explanations, as ever. :up:

[JeffR]

leads people to believe (falsely) that radial tires are losing a bunch of grip after the peak. T'aint so! When I do web searches for

radial bias ply racing tire slip angle

most sites reports that there grip remains about the same for bias ply slicks, but does fall off for street radial tires. Racing radials have some fall off but not as much.

Top fuel drag racing tires lose a lot of grip if they spin, but wrinkle wall, high grip tires are in a class by themselves.

The stickiest rubber compounds are found on table tennis rackets, with coefficients of friction 7 or higher.

[JeffR]

Does anyone, other than the developers, know what the slip angle versus grip curve for the tires in LFS looks like?

Lateral grip must fall off somewhat due to slip angle, otherwise a driver couldn't induce understeer by turning the front tires inwards. The main thing I noticed is that in real life (I've tested this with several cars going in circles at a big parking lot), the induced understeer effect is much less than it is with LFS.

[jtw62074]

When I do web searches for

radial bias ply racing tire slip angle

most sites reports that there grip remains about the same for bias ply slicks, but does fall off for street radial tires. Racing radials have some fall off but not as much.

Top fuel drag racing tires lose a lot of grip if they spin, but wrinkle wall, high grip tires are in a class by themselves.

The stickiest rubber compounds are found on table tennis rackets, with coefficients of friction 7 or higher.

Well, this is what Doug Milliken, author of "Race Car Vehicle Dynamics" told me, and he very frequently conducts these tests himself for various clients. He said outright that most of the books that show these swooping graphs of the tire force dropping way off after the peak are flat out wrong about this. As such, it's not surprising that most web sites are too. He'll be here next week for a meeting so I'll be sure to ask him about it again. The only tire data I've ever seen that shows a force drop off after the peak on street tires (bias or radial) is in the wet. Saw some Nascar tire data once that indeed did show a drop off after the peak, but not very much. Nothing approaching 30%. Also, during braking tests when a wheel locks there is drop off as well, but a lot of this is due to the fact that you're frying the part of the tire that's stuck in the contact patch. Overheating it will change things..

For what it's worth, I've seen coefficient of friction in a polymer of 42 in one case. Don't remember what that was though. Most rubber on glass can hit 8+ (much higher in some cases) at extremely light loads. Trivia really :)

Here's a crude drawing that shows how the lateral force swings sideways towards the rear of the car, causing the F9 display to show less lateral acceleration. Indeed the lateral acceleration of the car itself will begin reducing once a critical point has been passed provided the CG is not so high that the extra forward weight transfer doesn't offset it. LFS is absolutely correct in this regard. :)

http://www.performancesimulations.com/files/steering.jpg

[jtw62074]

Todd, when are you going to write a book? Nice explanations, as ever. :up:

Thanks. Maybe some day :p

[jtw62074]

Does anyone, other than the developers, know what the slip angle versus grip curve for the tires in LFS looks like?


Well, your four wheel drift test shows that it hits a peak at some point at about 1.15 and a bit later drops to 1.10 at whatever slip angles you were driving at.



Lateral grip must fall off somewhat due to slip angle, otherwise a driver couldn't induce understeer by turning the front tires inwards.


Sure you can. Turn the front wheels to 89 degrees slip angle. You won't be turning anywhere, regardless of whether the curves drop off or not. Instead, you'll get about the same effect as if you locked the front wheels. The optimum cornering steering angle is going to be somewhere between 0 and 90 then. Any less or more than that optimum and you'll be turning less hard than you otherwise would. I.e., you're probably in an understeer condition.


The main thing I noticed is that in real life (I've tested this with several cars going in circles at a big parking lot), the induced understeer effect is much less than it is with LFS.

This backs up my argument that real force curves don't drop off on dry pavement, and if they do it's usually very minor. Take a look at my diagram. If that red line gets shorter as you dial in more steering/slip angle, the green line will get shorter too. That could mean more understeer than you might get in reality.

Also, the center of gravity height influences this too. With a high CG you'll have more forward weight transfer, which makes the red line longer. Of course the green line would then get longer too. If the CG/wheelbase ratio was great enough you could even turn that understeer into oversteer at extreme steering angles even if the front tire curves dropped off after the peak.

[jtw62074]

Here's another similar diagram that maybe will explain what I mean:

http://www.performancesimulations.com/files/steering3.jpg

The black boxes represent tires in a top down view. The upwards direction is the direction of movement. The difference between the angle that the tire is facing and the direction it's moving (the vertical) is the slip angle, as we all know.

From top to bottom:

#1 - The tire is below the peak (it might be at 5 degree slip angle or something). The red line is the "tire lateral force," which is always measured in the tire's plane. I.e., it's always sticking out the left/right of the tire perpendicular to the direction the tire is facing. When you see a Pacejka graph or a chart of lateral force versus slip angle, you're looking at how the length of this line changes with slip angle. (The larger the force, the longer the line). The green line is the sideways component *in the car's coordinate system.* I.e., when you hit the F9 key in LFS you're looking at the lateral acceleration of the car, not the "tire lateral force." The blue line shows "induced drag." This is what causes the car to slow down more and more as you add steering. The F9 key shows lateral/longitudinal acceleration; the green and blue lines.

#2 - The tire has more slip angle and is producing more lateral force (the red line is longer.) Also, the green line in this case is longer too, so the lateral acceleration is higher than it was in #1.

#3 - The "tire lateral force" has reached its peak. I.e., we're at a slip angle where the red line is as long as it's going to get. If we assume the "tire lateral force" does not drop off at all as we increase slip angle beyond this, all we're doing is taking that same length red line and swinging it further and further towards the rear of the car as we increase slip angle further.

#4 - Here the tire is still producing the same "tire lateral force." However, notice that the lateral component in the vehicle's coordinate frame (the length of the green line) is getting shorter now. At the same time, the blue line (induced drag) is getting much larger now. As this blue line gets longer and longer we are getting more forward weight transfer. This will actually make the red line longer, but it may not increase it so much that the green line increases beyond where it was in #3. If it doesn't, then we are getting more and more understeer as we increase the front slip angle further. If it does, we'll continue to turn in harder. Whether or not this happens depends on the CG height to wheelbase ratio. The higher the CG is in relation to the wheelbase, the more front weight transfer we'll get (which increases the length of all the lines).

#5 - Here's an extreme slip angle. Notice that we've got hardly any lateral force *in the car's coordinate system*. The tire force is still the same and in fact, due to the forward weight transfer, will continue to increase. However, the green line is still very short so we have very little lateral acceleration now. The car is understeering like crazy even though the front tires have not lost any grip at all. It's just that the direction is all pointed rearwards instead of to the left of the car. I.e., the F9 readout will show a very small lateral acceleration. At some angle I'd expect it to be 30% of whatever the peak was :D

Now, here's the kicker. This happens regardless of whether or not the "tire lateral force" curve drops off or not. I.e., if that red line hits a maximum length and stays there as we swing it out towards 90 degrees slip angle, we'll still get understeer at some point (the green line will tend toward 0 length as 90 degrees is approached). In fact, even if the tire lateral force never peaked at all but continued to grow all the way out to 90 degrees, at some point the car will still understeer. I.e., the green line will eventually drop below it's peak.

The F9 display shows the length of the green and blue lines, not the red one.

[JeffR]

OK, so my first experiment was too extreme, however, it only takes a few degrees beyond optimal to drop from 1.15g's to 1.05g's, but this is small enough that you couldn't feel it in real life either.

What I don't understand is that you can create a lot of understeer with higher steering lock settings, but if the rear end starts to slide first, pegging the fronts inwards doesn't seem to cause this same understeer reaction. Maybe an issue with differential settings?

[JeffR]

Inspite of all my experiements, I can make the LX6 stable, but the setup paramters are a bit extreme, starting with race 1 setup. Lower height to .060, increase springs to 40 rear, 35 front. I max out front sway bar and set rear sway bar to 0. I set the camber +1.5 on the fronts, +1.0 on the rears. Which results in about -.2 on the left, -.1 on the right, as viewed from the suspension diagram with driver and 15% fuel. Fronts get hot on the outer 2/3rds, rears are about even. I set the camber a bit too positive on the front to reduce oversteer. I reduced tire pressure to 11 psi. I set differential to 40/40. I'm not that great a driver with LFS, but was able to do 1:24.5 laps at Blackwood with this setup, anything under 1:25 is a good lap time for me.

[Bob Smith]

Jeff, 25/50 on the diff should help, and keep spring rates a bit more equal, you do need plenty of understeer on the springs, perhaps you won't need mad ARB settings then. :)

[J.B.]

What I don't understand is that you can create a lot of understeer with higher steering lock settings, but if the rear end starts to slide first, pegging the fronts inwards doesn't seem to cause this same understeer reaction.

I'll give it a try:
saying the rear end is sliding is the same as saying the rear tyres are running at a high slip angle compared to the fronts and that the force/slip angle curve is past the point of linear increase (which gives you that "oops, it let go feeling").

Understeer is the opposite which means that you have to get the front slip angles higher than the rear slip angles. So just increase the lock until this is the case. You will have to go further to get understeer than you would if the rear end was not already sliding. After all you are trying to "beat" the rear slip angles with the fronts.

This brings us to the GPL-old question of wether inducing understeer is a valid driving technique to combat oversteer. I think this technique works to some degree in every sim out there. My view has always been that it would only be possible if you turn the wheel really quick, that it should not be as easy as in GPL, that counter-steering is a much more sensible reaction and that no one uses this technique in real life (at least in road racing, not sure about others).

So how does this technique work? Let's start with an oversteering car in a left coorner where the front slip angle is still on the linear part of the graph. What happens now when we slowly add some steering lock? If we pretend that things happen one after the other (they do in a digital world) then the direction of motion of the car stays unchanged at first while the steering lock increases. This means the front SA increases which increases the lateral force (red arrow) generated by the front tyres. These effects then determine the new direction of motion of the car.

There are two possibilities. Either the new front end force (green arrow) is larger than before because of the increased SA on the force graph or it is lower because of the force vector diagram in Todd's above posts. This would probably depend mainly on how steep the linear increase of the graph is (I think this is called cornering stiffness). I'll take a guess that high performance tyres have a higher cornering stiffness than lower performance tyres.

Incase the new (green) force is less than before, the car will have less cornering force overall and less anti-clockwise rotational accelleration caused by these forces. This will effectively cause the car to travel on a larger radius curve than before while the SA at the front has increased compared to the rear: we have less oversteer.

If the (green) force is higher the overall cornering force is higher and the car will go round a tighter curve and increase its anti-clockwise rotational accelleration. This would mean that the SA at the rear would actually increase compared to the front as the rear wheels are now actually pointing more to the inside while they are moving more to the outside: we have more oversteer.

So I think to make the induced understeer technique work you would need tyres with a very low cornering stiffness or you would have to turn the wheel so fast that you get state 4) or 5) of Todd's diagram almost instantly so that the understeer kicks in before the oversteer has increased beyond the point of no control.

Right, now I need someone to tell me if this is all a bunch of jibberish or if I have actually learned something reading all those physics threads. :smileypul I'm unsure especially of the second to last paragraph (two up from here). Is it true that an increased green force actually changes the motion of the car such that the rear SA increases by definition? This would seem to make sense to me as it would explain how to get a car to corner in the first place.

[JeffR]

If the rear tires are at an excessive slip angle, then turning the fronts inwards means that they have even more excessive slip angle.

With LFS S2, at least in the case of the LX6, if the rears aren't sliding, then inducing understeer is extremely easy, but once the rears start to go, it's seems to be very difficult. This is what I don't understand. With LFS S1's all or nothing grip, inducing understeer was very easy, even more so than GPL, as shown in this video:

http://jeffareid.net/cgi-bin/lx6.wmv

I've tried messing with setups in attempt to be able to get the front end to wash out after losing the rear end (if I center the steering or steer inwards), and I find I have to max out front sway bar, zero out rear sway bar, and mess with camber settings to just get the car to slide sideways evenly. I'm running minimum pressures in the tires of the LX6 and yet the middles are still the hottest points. I could try increasing front tire pressure while leaving the rears at 11 psi.

[JeffR]

Induced understeer to recover from oversteer works in real life for most cars. If you have access to a large empty parking lot, you can try this for yourself.

[Shotglass]

One thing I've never sussed out with tyres is knowing you've "got grip" and when you've "lost grip" in terms of physics. From what we've learnt about slip angles there is in fact no loss of grip, and in fact there could even be a slight increase with large slip angles. What is it that makes the car feel like it's cornering normally, and then feel like you're sliding? Is it something to do with the shape of the slip angle grip curve, or some other properties of the tyre?

doenst a slipping tyre have a lower slip angle than one that is at its grip maximum ?

[JeffR]

OK, got the induced understeer to work, but it took increasing the steering lock to 30+ degrees (I maxed it out to 36 degrees), I can transition from oversteer to understeer with a lot of inwards steering input. However, I don't need anywhere near this much steering lock to induce understeer if the rears aren't sliding. Still puzzling me why the rears sliding affects what happens at the front end so much, especially when the car is oversteering, the front tires slip angle is already increased.

[JeffR]

So how does this technique work? Let's start with an oversteering car in a left corner where the front slip angle is still on the linear part of the graph.How can this be? Front slip angle is greater than rear slip angle unless you counter steer. The assumption in this case is the fronts are still turned inwards. If the rear tires are spinning because of excessive throttle, there's a small window of time before the slip angle at the rear goes beyond optimal, and you can just lift a bit and/or counter steer.

Generally induced understeer is used in two cases. When a car is slowing, normally you don't get understeer unless the driver downshifts too soon and breaks the rears loose, or the car setup is just too oversteery under lift thottle. In this case induced understeer works well because you can use it while braking. The other case for using induced understeer is in a very high speed turn, where the throttle is floored just to keep the car going against the aerodynamic drag from the high speed. Under this circumstance, the rear tires are driving the car forwards as well as trying to turn the car. Keeping the fronts turned inwards just a bit past optimum can stabilize a car, if the rear end steps out, the increase in slip angle at the front results in the car just drifting instead of oversteering. It's just a bit past optimum, because any more than that and you create too much drag and heat on the tires. It's more of a preventitive method than a recovery method. Again, it's not going to help if the car isn't properly setup for high speed turns.

Countersteering is normally used during acceleration, similar to drifting, or rally style driving, or dirt tracks. You modulate the throttle and steering inputs to control the car. Countersteering and braking don't mix, the reduced slip angle at the fronts allows them to have more available braking grip than the already sliding rear tires and just makes the spin worse.

[J.B.]

How can this be? Front slip angle is greater than rear slip angle unless you counter steer. The assumption in this case is the fronts are still turned inwards. If the rear tires are spinning because of excessive throttle, there's a small window of time before the slip angle at the rear goes beyond optimal, and you can just lift a bit and/or counter steer.



I think front SA > rear SA is the definition of understeer.

EDIT: Wikipedia (http://en.wikipedia.org/wiki/Understeer) seems to agree.

I agree that induced understeer can be used as a preventative measure to stop the rear from stepping out. I saw F1 drivers doing this at Suzuka in the wet two weeks ago. I haven't got any footage though, as it was in FP.

[JeffR]

I think front SA > rear SA is the definition of understeer.
Assuming that the car isn't in the middle of a spin :)

[Bob Smith]

Assuming that the car isn't in the middle of a spin :)
What's that got to do with it? Understeer is how you correct slides. That's why if you drift slightly around a corner at the same speed you will have run wide from your usual line.

And if you're sliding around a lot then it tends to be the middle of the tyre that heats up, I've found (with smooth driving) the LX4/6 to be the easiest car in which to get pretty even temperatures across the tyre.

[jtw62074]

Assuming that the car isn't in the middle of a spin :)

J.B. is right. If the front SA > rear SA then you have understeer by definition. If you're in a spin, the rear SA > front and you're in an oversteer situation. The full SAE definitions are a bit more involved than that, but that's pretty much how it's defined.

[JeffR]

front SA > rear SA is the definition of understeer

Ok, I sit corrected.

Now can someone explain this to the LX6 that so it behaves properly?

[JeffR]

An alternate definition of understeer versus oversteer, at least in terms of defining a car's setup.

You drive a car around in a circle, well below the limits. It takes a certain amount of inwards steering input to maintain the circle. Then, while holding this radius, you speed up the car until it is near the limits. The steering inputs required to maintain the raidus determine if the setup has understeer or oversteer.

Understeer - it takes more inwards steering to maintain the radius.

Neutral - it takes the same amount of inwards steering to maintian the radius.

Oversteer - it takes less inwards steering to maintain the radius.

Critical Oversteer - The steering is centered while maintaining the radius.

Critical oversteer isn't a limit, because rally cars and dirt track racing go beyond this.

[jtw62074]

An alternate definition of understeer versus oversteer, at least in terms of defining a car's setup.

You drive a car around in a circle, well below the limits. It takes a certain amount of inwards steering input to maintain the circle. Then, while holding this radius, you speed up the car until it is near the limits. The steering inputs required to maintain the raidus determine if the setup has understeer or oversteer.

Understeer - it takes more inwards steering to maintain the radius.

Neutral - it takes the same amount of inwards steering to maintian the radius.

Oversteer - it takes less inwards steering to maintain the radius.

Critical Oversteer - The steering is centered while maintaining the radius.

Critical oversteer isn't a limit, because rally cars and dirt track racing go beyond this.

That's close enough for government work and probably good enough for general discussion :)

If we want to be picky though this isn't really right. All that matters is what the slip angles are at the end of the day. Basically, if you're running around the circle and you're not needing to countersteer to keep the car from spinning, then the car is either in an understeer or neutral state. Whether or not you had to increase steering lock into the turn to maintain the circle doesn't really matter. You could have been running 5 degrees front slip angle and 3 rear at low speed, then kept the steering right where it was while speeding up. If you wind up at 10 front and 8 rear slip angles, the car is still understeering even though you haven't steered into the turn at all to maintain the circle. In fact, if you countersteered 1 degree to get 9 front and 8 rear, you're still in an understeer condition.

Neutral technically means the front/rear slip angles are the same.

For all practical purposes, if the car doesn't spin out it's understeering or neutral, so your description is probably close enough :)

As for the LX6 stuff, I'll need to reread some of the later posts to comment.

[JeffR]

Regarding the slip angle versus lateral force (grip), here's a link that describes the differences between radials and bias ply racing tires, along with a graph. I think the radial curve dropts off steeper than it does in real life for racinc radials.

tire_info.htm (http://www.sascosports.com/products/tire_info.htm)

If LFS doesn't allow slicks for all cars, then it should at least allow DOT racing tires, which are typically bias ply, and provide grip aroung 1.2 to 1.3g's, right between the road supers and slicks of LFS, but with the stability of the slicks, because they are usually bias bly. In real life, most racing classes permit the use of the equivalent of a DOT racing tire. (In the USA, DOT just means that the tire is street legal).

Another link including a graph comparing slip angle versus lateral force versus normal force. Note that at low normal force, the curves lost almost no grip when beyond optimal slip angle. These curves are for racing radials. Bias ply slicks have much less or no loss in grip at fairly high slip angles.

http://www.smithees-racetech.com.au/ackerman.html

[Bob Smith]

This is what I (and many others) would call critical oversteer. You're turning with the steering wheel centered.
If the car begins to oversteer, slip angles must be larger at the rear. Just because you can control the slide with the wheel centered makes no difference. It is still oversteer because the radius of turn of the front wheel (infinite) is larger than the rear wheels (something less than infinite). So by definition it's oversteer. It's in the name!

And what do you think the slip angles are when you're driving in a straight line? I'd say they are equal. :)

[JeffR]

If the car begins to oversteer, slip angles must be larger at the rear.I'm talking about what happens after a car stabilizes into a fixed yaw angle while turning. Think of this as the slip angle for the entire car. The car is pointed inwards relative to the direction it's actually traveling while turning.

My impression was that the only way for all 4 tires to have the same slip angle while turning is for the steering wheel to be centered when turning (and no ackerman effects at the front). In LFS, this can be setup with the LX6 with slicks, via the slick mod, with near equal front and rear sway bar settings (I couldn't get this to work with the road supers as they are too unstable).

However what I left out is the rate or rotation of the car as it turns. The car is yawing in the direction of the turn while it is turning. This means the front end is moving inwards, and the rear end is moving outwards relative to the center of rotation of a car in a turn. However, relative to the path of the car, both the front and rear end of a car are moving inwards.

I've found some web sites with questionable diagrams of slip angles when explaining understeer / oversteer, such at this one:

http://www.autozine.org/technical_school/handling/tech_handling_4.htm

The issue is that the diagram shows the front slip angle as inwards versus straight ahead, and rear slip angle as straight ahead versus outwards, for the neutral and oversteer cases. Based on these diagrams, the implied path of the front end of the car is the same as the direction that the car is pointed in, for both the neutral and the oversteer case, while at the same time, the implied path at the rear end of the car is outwards.

Obviously, there is no slip angle at the rear end of a car unless the rear end of a car is yawed inwards relative to it's path. Assuming the car isn't hinged in the middle the entire car is yawed inwards at the same angle. This is pretty obvious in the case of tires with large slip angles, similated in games like GPL or GTL.

[JeffR]

I thought of a good example regarding slip angles.

Take the case of a car moving forwards on a steeply banked straight. The actual path direction vectors for both the front and rear of a car are parallel: their directions are identical front and rear. Yet the car will have an overall yaw angle, pointed slightly up slope on the banked section of track.

In order for slip angles to be the same front and rear, the steering has to be centered.

I prefer a steering input based definition for understeer versus oversteer. Maybe new terms are needed.

Steering understeer - as the amount of bank of the straight is increased, it takes more steering input to keep the car going straight.

Neutral steering - as the amount of bank of the straight is increased, it takes the same steering input to keep the car going straight.

Steering oversteer - as the amount of bank of the straight is increased, it takes less steering input to keep the car going straight.

Steering critical oversteer - as the amount of bank of the straight is increased, the steering wheel approaches being centered.

[Bob Smith]

In order for slip angles to be the same front and rear, the steering has to be centered.
OK, so take this example:
Your rear brakes loose in the GTT for example, and you have to pile on loads of opposite lock to stop the car from spinning. You put on just enough luck to stop the car from spinning, but not enough to reel the rear back in. So you're now pulling a huge powerslide (some call it drifting, I'm sure there's some subtle difference in definition, but anyway). But you're not going to spin. And the rear isn't coming back into line. So slip angles must be equal and you've still got lots of lock on.

[JeffR]

OK, so take this example:
Your rear brakes loose in the GTT for example, and you have to pile on loads of opposite lock to stop the car from spinning. You put on just enough lock to stop the car from spinning, but not enough to reel the rear back in. So you're now pulling a huge powerslide (some call it drifting, I'm sure there's some subtle difference in definition, but anyway). But you're not going to spin. And the rear isn't coming back into line. So slip angles must be equal and you've still got lots of lock on.In this case, the rear slip angles are much higher than front slip angles. You see this all the time at drift events, rally events, and dirt track events. There's no rule that states slip angles have to be the same to prevent a car from rotating (yaw rate) faster or slower than it needs to in order to take a turn.

In the case of donuts, you can have the inside front barely moving at all, virtually no slip angle, while the rear tires have a huge slip angle, as in the second half of this video:

http://home.att.net/~b.a.porter/burnout.wmv

[Bob Smith]

In the case of donuts, you can have the inside front barely moving at all, virtually no slip angle, while the rear tires have a huge slip angle, as in the second half of this video:
Well, that makes sense, I've no argument with that.

[JeffR]

Anyway, all of this is getting a bit off topic. I'm trying to understand why some LFS cars like the LX6 reacts so severely when the rear end gets a bit loose. I tried using the setup parameters so if the LX6 is sliding sideways, the front end has less grip than the rear. In addition to sway bars maxed at front and zero at rear, I had to also mess with the camber settings to get the result I wanted. The net result is a LX6 that is a lot easier to drive, but I don't understand why it takes such an extreme setup to get it to behave itself.

If you want to try it out, here's the test setup I created for Blackwood:

http://jeffareid.net/lfss2/LX6_bl1_jeffr.set

[Bob Smith]

Well, I tried your set, and I wouldn't really call it easy to drive, it's just pig understeer at every corner. OK so it doesn't spin so easily, but you couldn't actually use a set like that for anything other than "this set doesn't spin much".

I fiddled with it a bit to try and make it somewhat sweeter to drive, ended up having to change a lot. I could keep it on the track even with my demented pedals. See what you think - it's not that bad a handler surely?

[Slartibartfast]

As Doug Milliken explained it to me, the feeling that the car has "let go" is not because the force curves fall off after the peak (if they indeed do), but rather that the force merely stops rising. When you hit the peak you can get the sensation that the car has broken away. The shape of the curve leading up to this point will be what tells your butt whether the tire breaks away suddenly or not.

There are really two things there:

1. Where the peak occurs (i.e., at what slip angle does the force stop rising). The greater this slip angle is, the more forgiving the tires will feel. Street tires peak at very high slip angles. I've seen data showing peaks at 20 degrees. One set showed it still rising at 28 degrees!

2. The shape of the curve leading up to the peak. If the force rises very linearily and then suddenly flattens out, the tire will feel like it breaks away very suddenly. If instead it rises normally and quickly starts rolling off towards the peak (the slope decreases to 0 gently) it will feel more forgiving. Tire designers can influence this shape substantially through their choices in cord patterns/angles and so forth. Radials typically rise more quickly than bias ply tires and roll off a bit more suddenly (into a flat peak!) This makes them feel like they break away more suddenly then bias tires do and leads people to believe (falsely) that radial tires are losing a bunch of grip after the peak. T'aint so! :)

This is similar to people's rear ends telling them that cars actually speed up once they hit the grass sideways, which is of course nonsense. The acceleration became lower, that's all.

Wow! Your (and Milliken's) contributions to this thread are making my head spin. Great stuff. So... I've always felt that the car or kart or bike or skateboard is really at it's peak when it starts to get "squirmy". That's been my interpretation. My buddies and I used to trade cars in highschool and we'd talk about where the car "floats". I'd say, "In my car you want it to float around the shifter." Dave would say, "To drive my car right it needs to float around where the backs of the front seats meet the frame." We we're talking about where you put the sense of rotation in a particular chassis to drive it neutral. But to get to float, we were always refering to the tires. Float only happened once the tires were at the limit. If that's making any sense.

So, LX6. LFS. I have no idea where I'm going with this. I've always felt that the fronts over commit, perceptually or actually, in certain cars more than others, in LFS. Like, they get more grip, more quickly, and with less feedback than the rears. I have been lowering and lowering cambers to make the cars "float" more. I find that if the cambers are too high, the car (or end of car) commits at a level that I can't percieve. The imbalance in handling then becomes a result of me providing steering or throttle input that I am not aware of until it's far too late. (Commited.) I was just driving a FOX around that I think is quite nuetral, front cambers are at 1.40, rears at 1.90. I wanted it to oversteer a little more, changed rear camber to 1.83, and got the desired result. I am more and more, tuning the "feel" of the tires with camber.

In the LX6 front camber is key to the radical mid and late corner oversteer for me. I have noticed that a lot of people set up the LX6 to understeer like cazy with very stiff front ends. All fine and dandy under the limit, making the car not spin crazy, but also making a car that doesn't turn in well and becomes a bear by the time it does get pitched. In the LX6 my cambers are well below 1 degree in front. Bars are at a minimum, and spings medium with less sping in front. The car seems to rotate well, and is easy to read front and rear without that over commitment in the front end that I think I'm not feeling.

So what does any of this have to do with any of that? Heck if I know. I don't understand most of this thread. Some of the steering angle setup stuff seems really important, especially in the LX6. I understand what caster is geometrically at the front axle, but am having a hard time picturing how it affects the rest of the car through cornering phases. Add to that, suspension geometry, rates, and slip angle correlations and I'm completely lost.

In the end, I think that a lot of the problem is perceptual. You can set the car up to act properly, with acceptable grip and balance through the cornering phases. Reading what it is doing is entirely another task.

[JeffR]

Well, I tried your set, and I wouldn't really call it easy to drive, it's just pig understeer at every corner. OK so it doesn't spin so easily, but you couldn't actually use a set like that for anything other than "this set doesn't spin much".As someone else posted, reducing front camber (which I did) seems to help. Regarding my understeery setup, on the skid pad the sustained g-forces while cornering remain about the same.

Did you see more than a 1 second per lap difference between the two setups? My issue is I'm just not that fast with LFS, my Blackwood times are barely under 1:25. You're probably a lot faster.

Regarding setups, you can try edgars wr setup.

LX6_bl1_Edgar748.set (http://jeffareid.net/lfss2/LX6_bl1_Edgar748.set)

[Woz]

Wow! Your (and Milliken's) contributions to this thread are making my head spin. Great stuff. So... I've always felt that the car or kart or bike or skateboard is really at it's peak when it starts to get "squirmy". That's been my interpretation. My buddies and I used to trade cars in highschool and we'd talk about where the car "floats". I'd say, "In my car you want it to float around the shifter." Dave would say, "To drive my car right it needs to float around where the backs of the front seats meet the frame." We we're talking about where you put the sense of rotation in a particular chassis to drive it neutral. But to get to float, we were always refering to the tires. Float only happened once the tires were at the limit. If that's making any sense.

So, LX6. LFS. I have no idea where I'm going with this. I've always felt that the fronts over commit, perceptually or actually, in certain cars more than others, in LFS. Like, they get more grip, more quickly, and with less feedback than the rears. I have been lowering and lowering cambers to make the cars "float" more. I find that if the cambers are too high, the car (or end of car) commits at a level that I can't percieve. The imbalance in handling then becomes a result of me providing steering or throttle input that I am not aware of until it's far too late. (Commited.) I was just driving a FOX around that I think is quite nuetral, front cambers are at 1.40, rears at 1.90. I wanted it to oversteer a little more, changed rear camber to 1.83, and got the desired result. I am more and more, tuning the "feel" of the tires with camber.

In the LX6 front camber is key to the radical mid and late corner oversteer for me. I have noticed that a lot of people set up the LX6 to understeer like cazy with very stiff front ends. All fine and dandy under the limit, making the car not spin crazy, but also making a car that doesn't turn in well and becomes a bear by the time it does get pitched. In the LX6 my cambers are well below 1 degree in front. Bars are at a minimum, and spings medium with less sping in front. The car seems to rotate well, and is easy to read front and rear without that over commitment in the front end that I think I'm not feeling.

So what does any of this have to do with any of that? Heck if I know. I don't understand most of this thread. Some of the steering angle setup stuff seems really important, especially in the LX6. I understand what caster is geometrically at the front axle, but am having a hard time picturing how it affects the rest of the car through cornering phases. Add to that, suspension geometry, rates, and slip angle correlations and I'm completely lost.

In the end, I think that a lot of the problem is perceptual. You can set the car up to act properly, with acceptable grip and balance through the cornering phases. Reading what it is doing is entirely another task.

Perception has a massive impact on the LX cars. Cars like the LX series are strange as you have a light car with a massive power to weight ratio. You also sit right over the rear wheels so you feel everything that is going on the instant that it does happen. Everyone I know that owns a real car like this say it tells you exactly what is going on so its very hard to spin out, or to be more exact you correct it as soon as you detect the problem. (the fear of death/injury normally makes you do this :))

All this means that IRL you become far more subtle with your inputs than you would in LFS. That is not to say there are issue with the tire model but the LX really suffer from over driving more than other cars in LFS.

I understand exactly what you mean by the squirmy/float feel cars get on the edge. All of a sudden you get "that" feeling start to build and you know if you push much harder you are in the problem zone. But due to the feedback of what is going on IRL it becomes easy to hold it on that edge in a well setup car. My BWM Mini Cooper was classic in this as it was stiff enough to left you power through roundabouts well into slip angle and move between power understeer and liftoff oversteer to get you round the corner with the most power it was possible to put through the front wheels at any given instant. Trouble is that this feeling and level of feedback is not possible to give through a FF wheel as most of the feeling comes through your body and ears (ears being the key to your feeling of G and balance)

Is there a solution to this? probably not without getting a motion platform. It might be possible to give some clues with fake FF effects but personally I am against this route as it would cloud the great feel the FF currently has.

[JeffR]

I own a Caterham SV with a 2.3 liter Ford Duratech with about 250hp. This is the first car I've ever experienced lift throttle oversteer with (however the car recovered very quickly, much quicker than my heartrate did). Recovery seems much easier than it does in LFS, but as posted, it's because you can feel the rear end getting loose much sooner than you can with LFS. One thing that would help would be for tire scrub sounds to occur further below the limits than they do now with LFS. In a real car, you can hear scrub sounds well below the limits of traction (at slower speeds where wind noise doesn't block out the sounds).

[Woz]

I own a Caterham SV with a 2.3 liter Ford Duratech with about 250hp. This is the first car I've ever experienced lift throttle oversteer with (however the car recovered very quickly, much quicker than my heartrate did). Recovery seems much easier than it does in LFS, but as posted, it's because you can feel the rear end getting loose much sooner than you can with LFS. One thing that would help would be for tire scrub sounds to occur further below the limits than they do now with LFS. In a real car, you can hear scrub sounds well below the limits of traction (at slower speeds where wind noise doesn't block out the sounds).

Yep, tire scrub sounds earlier would help a bucket load as you would get that early warning but not sure how simple that would be as it depends on the tire model and how it is coded.

[JeffR]

Yep, tire scrub sounds earlier would help a bucket load as you would get that early warning but not sure how simple that would be as it depends on the tire model and how it is coded.Base the sounds on slip angle, and for throttle / brake usage, base it on slip ratio. The slip ratio sound may be a bit unrealistic if it kicks in well below the limits, but there isn't any other reasonable means of feedback.

[Slartibartfast]

I'm getting really good feedback off the tire sounds. In the FWDs I'm able to drive as WOZ explained he does in his Mini. I'm not very good with the FWDs yet, my right foot is a major impedement as 27 years of instincts are fighting change. But I'm "feeling" everything.

My tire sounds are all the way up, engine sounds almost all the way down. That may have something to do with it. And I don't use FF. I think I "feel" much, much more without it.

In the RWDs the rear tires seem to be a little more critical than I percieve them to be IRL. I think the fronts are masking to some extent. I'm sure the tire physics need some tweaking, and will get it.