Crazy oversteer / wheelspin with Michelin Pilot Super Sport

[wparsons]

Originally Posted by ZDan

I got into this when this question:
why do heavy cars use larger tires but roughly the same tire pressure?
was incorrectly answered thusly:
Because they have tires with significantly larger contact patches.
(I now see that it wasn't even you who gave that answer)
Anyway, that's incorrect. If larger contact patches increased load carrying capacity, you would lower pressures for higher loads. But you INCREASE pressure for higher loads, which reduces contact patch area.

There's a difference between what pressure is required to hold up a given weight for a given tire size, and what pressure is required when increasing weight without changing tire size.

You increase pressure with heavier loads on the same tire because less pressure results in too much sidewall deformation and excess tire heat.

That doesn't change the relationship between weight, pressure and contact patch area in the slightest though.

If a vehicle weighs X lbs and has Y sq/in of desired contact patch area, then it will require Z psi in the tires. Z is going to be roughly X/Y.

If you increase weight to double, then you'll need roughly double the contact patch area at the same pressure to support the vehicle. If you want the same contact patch size, then you'll need roughly double the pressure.

If you increase the weight and don't touch anything the contact patch area will be larger. Now if the tires aren't happy with that much sidewall deformation, you either need to change tires to wider ones so you get more contact patch area with less sidewall deformation or you need to increase the pressure. Contact patch width is basically fixed for a given tire, regardless of inflation pressure and load. The length of it will change greatly with inflation pressure and load. The only two ways to increase area are more width or more length. More length means more sidewall deformation and more heat.

Again, this has nothing to do with maximum load for a tire, just the relationship between weight, desired contact patch area and pressure.

Bigger contact patches will support more weight at a given pressure, that's simple physics and can't be disputed. The other factor is if a tire is happy running at the desired contact patch size or not.

[Ferrari]

Originally Posted by tYtEn86

Hey OP did u get your answer?

Yes. I lowered the rear tire pressures to 34psi and the back feels more planted. The rear tires also make a bigger contact patch with the road, as I can see more wear on the outer edges (since the rear has negative camber, about 2cm on the outer edge was off the ground under normal driving when the pressures were 37psi).

[Captain Snooze]

Originally Posted by Ferrari

The car is manual and not modified.

Originally Posted by Ferrari

Although, I also changed the rear diff fluid from Motul Gear 300LS to Motul Gear 300. It's a less viscous fluid with no friction modifiers. The diff lockup seems to be less aggressive, so that also helped with the oversteering tendency.

I don't understand why changing the diff oil would make a difference given it's a Torsen type diff, no friction plates are involved.

[wparsons]

Plus, more aggressive lockup would actually make it understeer more, not oversteer.

[Ubersuber]

Originally Posted by stonenewt

Okay one last go at this.

The force triangle -

   F
———————
 P | A

F = Tyre load
P = Tyre pressure
A = Contact patch

If you know two of them you can calculate the other.

If I have
P as the maximum tyre pressure
A as the maximum contact patch
F becomes apparent through simple multiplication.

If I know
F as the tyre load
P as the tyre pressure
A is the division of F by P

When I know
the tyre load (F)
contact patch (A)
I can then calculate the tyre pressure (P)


With the help of a data book I can find enough information to calculate A, at maximum tyre deformation, with bit of simple geometry gives. From the same data book I can find out P which is maximum pressure of a tyre. That gives me the two bits of information I need to calculate F, the load needed on the tyre to cause A & P to be those values.

If you don't get that you're either trolling or simply not that bright. I'm going to give you the doubt and assume the former.

Bye!

Assuming the tyre is a balloon, which it isn't. So, you can't calculate contact patch area using this method.

You can develop a curve of contact patch area using this method for a single tire and you can get a useful approximation of contact patch size variation using this method.

Trouble is, contact patch area isn't a very useful piece of information.

Two tyres with the same contact patch area on the same vehicle (all other things being the same that is) can have radically different grip. Changing the contact patch area on a given tire will not change the grip in the direction you might expect.

Increasing contact patch area by reducing tire pressures will reduce grip if the contact patch was the correct designed size to begin with.

Decreasing contact patch area by increasing tire pressure may increase grip if the resulting slip angles for a given side thrust are reduced as is often the case although much less so now with low profile tires than when tires were nearly as tall as they were wide.

Contact patch shape is rarely elliptical on low profile tires, even statically, and the shapes are getting pretty complicated dynamically. Contact patch shape and area are not as related as you might think.

[Ubersuber]

Originally Posted by wparsons

Plus, more aggressive lockup would actually make it understeer more, not oversteer.

Depends where in the corner you are. Corner entry yes, corner exit the opposite is true. Usually.

[Ubersuber]

Originally Posted by Captain Snooze

I don't understand why changing the diff oil would make a difference given it's a Torsen type diff, no friction plates are involved.

Not quite true. The variability of torque bias depends both on gear angles (primarily) and end thrust plate materials. Some manufacturers will sell you different thrust "washers" to change the bias ratio.

[Ubersuber]

Originally Posted by Ferrari

Yes. I lowered the rear tire pressures to 34psi and the back feels more planted. The rear tires also make a bigger contact patch with the road, as I can see more wear on the outer edges (since the rear has negative camber, about 2cm on the outer edge was off the ground under normal driving when the pressures were 37psi).

And what front pressures did you try?

I would guess that 34 psi would be Michelin's recommended cold pressure if they had one. Close to it anyway.

[Ubersuber]

Originally Posted by wparsons

Bigger contact patches will support more weight at a given pressure, that's simple physics and can't be disputed. The other factor is if a tire is happy running at the desired contact patch size or not.

You were doing quite well until you included this howler.

Contact patch size results from higher load, it in no sense supports anything.

Higher tire pressures support higher loads basically until the tire structure fails.

Now that's physics.

[Ubersuber]

Originally Posted by wparsons

Slip angle is the difference between the direction the tread is pointing and the trajectory of the wheel. It can be from tread deformation or from the tire sliding slightly (or most likely, a bit of both).

That's why running too much or too little pressure at one end of the car can both reduce grip.

None of this makes any sense except you get the technical description of slip angle right.

However, tire engineers only consider slip angles for non sliding tires as the term has no useful meaning once the tire begins to slide.

The useful aspect of "slip angle" compares the vertical centerline of the wheel to the wheel trajectory up to around 10-15% contact patch "slip". Once the contact patch begins to slip by more than this slip angle is irrelevant as the tire is sliding and peak grip has long since disappeared.

Combine the front and rear slip angles and assume no steering angle and you get the yaw angle, which is the single most important factor in vehicle cornering.

This yaw angle issue is what entertains me when "performance" drivers yammer on about turn in, rotation and corner exit. Yup, those yaw angles are important.

Understeer has nothing to do with the way the wheels are oriented relative to the body (and neither does oversteer but that is intuitively simple to understand).

The reason fast cars on modern low profile tires appear to "understeer' is that a small amount of positive lock (apparent understeer but probably not truly understeering) is generally required to allow maximum utilization of rear tire grip.

These BRZ/FRS do not develop maximum cornering power on stock tires because they cannot put all of their power down without oversteering, particularly in tighter corners under hard acceleration.

Slip angle accounts for only part of the slip experienced by the drive wheels. The more power you can put onto the road the higher the slip of the tires. This slip reaches a maximum which can only be utilized by allocating the total slip (which equals grip BTW) among the load directions the driver is asking of the tires. AWD road cars pretty much always understeer all of the time as do FWD cars. It takes some pretty crazy engineering to change these inherent characteristics. There is no doubt that FWD is best for street driving as long as you are not trying to put more than around 250 bhp down, although higher bhp fwd cars have been marketed they are unsatisfying to drive because the expertise to exploit them is rare and if you have that expertise rwd is more enjoyable.

The more power you put down the less grip is available to corner and vice versa. Braking is divided in the same way.

For a given tire there is an ideal pressure range for a given range of tire temperatures.

For a street tire driven normally this is the manufacturer's recommended tire pressure. You will not get more grip by setting these pressures lower than recommended. Under some conditions you may get a bit more grip using higher pressures but unlike the old 78 series days this is very limited for a 45 series tire. 2 psi high is the most I've tried to use unless I was trying to change the handling bias away from understeer. In the old days raising tire pressures did not change the contact patch shape very much, just made it smaller. The resulting stiffer sidewalls gave better grip even though the contact patch was smaller. Modern 50 or shorter sidewall tires are much more sensitive to tire pressure in the sense that too much can distort the contact patch from design size and shape. Lower than recommended tire pressure always distorts the contact patch undesirably and softens the sidewall, both bad for grip. To deflect the irrelevant drivel that seems inevitable in this topic I state the obvious which is the manufacturer's recommended cold tire pressure assumes a "usual temperature range". Boyle's Law applies so if you are going to get your street tires really hot then you need to allow for that when setting pressures. Cold pressures will be set lower in order that hot pressures do not get too high. Duh. Note that the car will handle very poorly then until the heat is obtained. To do this on the street would be highly irresponsible and generally dangerous.

Even the Primacy tire is unlikely to fade away from heat when driven on a street unless the driver is about to be arrested ....

Track conditions are not street conditions.

[OmarGC]

[Ubersuber]

Originally Posted by D_Thissen

So, yesterday it was 5C and tomorrow is supposed to be near 18C, I should adjust my cold tires by 2.5psi ..ish?

Yes.

Ideally, tires would have variable pressure while driving. This is impractical but Porsche did so for its 959.

TPMS could be set up to show real time tire pressures and you would then note the normal running pressure is 2-4 psi above the cold setting, but only after a fairly lengthy drive. Porsche (and others) have this nifty dashboard display but most do not bother to display this TPMS information most such systems actually generate.

Generally, decide what the most likely ambient cold temperature you are going to be driving in as compared to the temperature at which you are checking the tire pressures.

Garage at plus 10C and outside it is minus 30C then you need about 7 psi "too much" inflation pressure as measured in your garage to be safe.

If you do not do this you may notice the TPMS light will go on after a lengthy period parked in very cold conditions (if you drive a car so equipped) because they tend to trigger the fault at around 20% low pressure.

You must not exceed the maximum inflation pressure on the sidewall of the tire.

Also, temperature rise in the tires is less in very cold weather so the effect on tire shape is not as linear as theoretical application of Boyle's Law would indicate. Boyle's Law still applies but the running character of the tires is different in very cold weather, as it is in very hot weather. I run winter pressures about 4-5 psi higher as measured at plus 10C than I do in "summer". Out here "summer" ambient can get lower than 10C even on a day that may reach plus 30 C which makes estimating the true "cold " tire pressure even more difficult.

Generally I have found setting cold tire pressures about 2 psi high in "summer" and 4 psi high in winter works best for my locale assuming I use the garage temperature as the "ambient" cold temperature.

In your location you may find a different estimation will work better. The objective is to maintain safe minimum pressure while keeping contact pactch shape close to the ideal the tire manufacturer has calculated results from its recommended pressure, while actually driving. Cold tire pressure is only relevant as an estimate or prediction of how much higher the actual running pressure will be as temperatures rise in the tire.

Yes, adjust your tire pressures frequently to aim for this ideal.

[stonenewt]

Originally Posted by Ubersuber

Assuming the tyre is a balloon, which it isn't. So, you can't calculate contact patch area using this method.

You can develop a curve of contact patch area using this method for a single tire and you can get a useful approximation of contact patch size variation using this method.

It's good enough for my purposes because effectively void space is effectively road contact for load bearing. I've been told this by two different people who develop special condition tyres for a living.

Trouble is, contact patch area isn't a very useful piece of information.

It's exactly the information I need for what I was wanting, comparative loads between tyres.

Two tyres with the same contact patch area on the same vehicle (all other things being the same that is) can have radically different grip. Changing the contact patch area on a given tire will not change the grip in the direction you might expect.

Increasing contact patch area by reducing tire pressures will reduce grip if the contact patch was the correct designed size to begin with.

Decreasing contact patch area by increasing tire pressure may increase grip if the resulting slip angles for a given side thrust are reduced as is often the case although much less so now with low profile tires than when tires were nearly as tall as they were wide.

The thing is I wasn't using any of this to calculate grip levels.

Contact patch shape is rarely elliptical on low profile tires, even statically, and the shapes are getting pretty complicated dynamically. Contact patch shape and area are not as related as you might think.

It's close enough, for what I'm needing.

[wparsons]

Originally Posted by Ubersuber

And what front pressures did you try?

I would guess that 34 psi would be Michelin's recommended cold pressure if they had one. Close to it anyway.

But earlier in this thread you said running less than 35 psi was dangerous and shouldn't be done under any situation?

Originally Posted by Ubersuber

You were doing quite well until you included this howler.

Contact patch size results from higher load, it in no sense supports anything.

Higher tire pressures support higher loads basically until the tire structure fails.

Now that's physics.

You really don't understand simple concepts, do you?

PSI is pounds per square inch. That means for every square inch of footprint (which on a tire is, drumroll please, the contact patch) it can support whatever the pressure is in weight.

You can look at the flip side of this and look at what the pressure exerted on the ground by the tires is. Shockingly, it will be the vehicle weight divided by the contact patch area.

Want a real world example? What exerts more psi of force on your foot, a woman stepping on your foot with the ball of her foot in a high heel shoe, or a woman stepping on your foot with the heel of a high heel shoe? The weight of the woman hasn't change, just the contact patch of what is applying the pressure. Want to see this in action, ask a woman in heels to stand on soft dirt. There will be more weight on the ball of her foot, but because it is supported by a much larger area the hell will sink first.

Want another real world example? Look at snow shoes, they work by distributing weight over a larger area. The weight of the person wearing them hasn't changed, just the contact patch area has. The difference with snow shoes is that the ground is the deformable variable, instead of the tire. Why the groomers use for ski hills have very wide tracks instead of narrow wheels? Same reason.

You're not wrong that increased weight, without touching pressure or changing anything else, will increase the contact patch area. But if you actually read what I have said multiple times, I've been saying that all along.

tire PSI required ~= vehicle weight / desired contact patch area. It's not 100% exact because the sidewalls do provide some support, but it's pretty negligible when looking at it with the weight of a car on the empty tires.

The whole point of this stemmed from you suggesting that a larger internal volume would require less pressure, when that's actually not the case. Wider tires will support more weight for the same pressure because they have more contact patch area before the tire gets unhappy with the amount of deformation.

The only reason that higher pressures are needed for heavier loads is because the tires aren't happy with the amount of deformation happening under those loads.

Think of it this way... You have a 100mm wide tire and a 200mm wide tire, both supporting 500lbs. Both are at 30psi. Which tire has a bigger contact patch? Answer, neither. Now, is the 100mm wide tire going to be deformed too much and overheat very easily, probably. Doesn't change that for a given weight, pressure is changed to get a desired contact patch size (and sidewall deformation).

Originally Posted by Ubersuber

However, tire engineers only consider slip angles for non sliding tires as the term has no useful meaning once the tire begins to slide.

Modern tires generally generate max grip with a bit of slip angle, and a bit of sliding. Not a lot, and likely too little for most people to feel, but they'll make more grip with a bit of sliding.

Originally Posted by Ubersuber

The useful aspect of "slip angle" compares the vertical centerline of the wheel to the wheel trajectory up to around 10-15% contact patch "slip". Once the contact patch begins to slip by more than this slip angle is irrelevant as the tire is sliding and peak grip has long since disappeared.

15% is a HUGE slip angle, most tires are much happier in the 6-8% range. You also have to factor in acceleration or braking forces, adding either reduces the maximum slip angle a tire is going to stay gripping with.

Originally Posted by Ubersuber

Combine the front and rear slip angles and assume no steering angle and you get the yaw angle, which is the single most important factor in vehicle cornering.

Slip angles by definition ignore the steering angle, so why bring it up? You can't figure out yaw without looking at slip angles and steering angle, because if I'm turned into a corner at low speeds with no slip angles there is still most definitely a yaw angle.

Originally Posted by Ubersuber

This yaw angle issue is what entertains me when "performance" drivers yammer on about turn in, rotation and corner exit. Yup, those yaw angles are important.

You do realize that rotation is just the car yawing into a corner, right? Just because you don't follow the terminology doesn't mean they're wrong.

Turn in is also important, because it is the cars willingness to start yawing into a corner. Turn in and rotation are very closely related.

Originally Posted by Ubersuber

Understeer has nothing to do with the way the wheels are oriented relative to the body (and neither does oversteer but that is intuitively simple to understand).

What is happening with a car that has a yaw angle of 45* but a trajectory of only 30* with all 4 tires pointed straight ahead? Is it understeering or oversteering? What are the slip angles?

A better way of looking at it is to compare the path the vehicle should be following for a given steering input compared to the yaw angle.