Tire pressure and oversteer

[landho]

Can someone explain to me the impact of air pressure on reducing or increasing oversteer.

Why does adding air to the rear tire reduce oversteer? And what increments should I add air? by 1/2 or 1 lb increments.

And when would I add/reduce air to the front tires?

Thanks.

[kmagvette]

I'll take a stab at it. If I screw it up, at least it will get the discussion going.

The Chief Instructor at my DE's always tells the 911 guys to have 8 lb more in the rear than in the front to get more grip to the rear and induce understeer - Watkins Glen is notorious for eating 911s.

I believe this increases the grip in the rear by preventing the sidewalls from rolling over as easily.

Conversely, reduced air pressure allows (in a right turn) the inside of the left tire tread to lose contact with the pavement in hard cornering as the sidewall rolls over, thereby reducing the size of the contact patch and inducing oversteer.

When I went to slicks on the Z06, the general consensus was to get hot pressures of 40 rear and 38 front which roughly correspond to 32,30 cold. At these pressures my tires wear evenly across the tread with the exception of the outside shoulder on the fronts; this probably means I need more negative camber up front.

Hope that helps. I'm sure any errors in my explanation will be corrected by the Track Gods.

[weirdpanda]

Chalk the tires to find out how they react under cornering. Use that info to tell you what your tires are doing and how much pressure to run. In autox, cold, on the stock goodyears i found 35/32-33 was usually adequete and generated a good feel and balance.

[No Doubt]

Decreasing tire pressure will cause the tire sidewall to roll more, not less.

MANY FACTORS can and do contribute to both under and oversteer. Tire pressure is only one of them.

You can check for correct tire pressure by marking the sidewalls with chalk. If the chalk rubs off the tire sidewall, the tire is rolling over and you need to increase pressure. Another method is to use a tire Pyrometer and check tire pressure across the tread in three places. That is a topic for another post which has been discussed here before.

The goal is to maintain as much tire contact patch with the surface as possible. Whatever tire pressures achieve this are nominal.

If you are understeering, chances are the front tire pressure is either too high or too low and the contact patch is reduced. The same applies to the rear tires.

Again, there are many factors that can contribute to either condition and they need to be evaluated.

[landho]

Originally Posted by No Doubt

...If you are understeering...front tire pressure is either too high or too low

How can it be both (too high or too low)?

So this oversteer/understeer thing is related to decreasing/increasing sidewall flex? I know other issues can cause over/under steer...but I am interested in understanding the effect of tire pressure only...

Thanks.

[No Doubt]

Yes. The cause is a lack of tire contact patch and/or tire contact patch difference between the front and rear tires. That assumes the tires are the ONLY cause and that all other possibilities have been evaluated and corrected.

Lack of rear tire grip = oversteer ("loose")
Lack of front tire grip = understeer ("push" or "tight")
Excessive front grip compared to the rear = oversteer
Excessive rear grip compared to the front = understeer

Tire pressures can therefore affect under/oversteer if they are incorrect and no other cause is found. Tire pressures are a fine tuning measure, as are shock settings for example.

If the tire pressure is too great the tires contact patch is reduced and the center of the tread will be too hot when compared to the shoulders.

If the pressure is too low the shoulders will get too hot when compared to the center due to excessive sidewall flex.

Improper alignment can cause the same issues.

[99cobra]

Originally Posted by landho

How can it be both (too high or too low)?

So this oversteer/understeer thing is related to decreasing/increasing sidewall flex? I know other issues can cause over/under steer...but I am interested in understanding the effect of tire pressure only...

Thanks.

It can be both because it there is a "threshhold" correct pressure.

Say your pressure is very low on the front tires. All else equal this causes understeer due to a high slip angle (fancy way of saying tires are rolling over/towards sidewall). Adding pressure will help alleviate this understeer. Now if you add too much, tire gets high in center, reducing contact patch, again causing understeer.

I start with about 32 front 31 rear with Toyos on road courses. When I run my Hoosiers I start at 31/30.

Right now my Z, with has good negative camber alignment, T1sway bars, and warm Hoosiers turns in like a bitch, it's just awesome. Mild oversteer at limit, controllable with throttle say, in a carousel.