Ubermensch

Does the relative ratio of the tire widths (front to rear) have an impact on the understeer/oversteer characteristics of the car? For example, if I'm running 8" fronts and 10" rears and increase the rears to 11" will the car understeer more, less, or the same? Intuitively I would would think that the additional width in the rear would give you more oversteer.

Thanks,

S.

gota911

If you change only the rear wheels and go from 10" to 11" wheel width, it wil INCREASE the understeer.

rountreed

I thought tire width had more to do with creating under/ over steer. Ie not going from 10 to 11 inch rear wheels but going from 265 to 295 tires. I know I now have 19 inch with 235 front and 275 rear on my car and can tell you it looks great and handles ok on the Autobahn but if I go to the track I am putting my 17 inch back on, it was not till I did this cosmetic modification that I truly came to realize that unsprung weight is the worst weight from acceleration to braking. So I guess I am saying that for daily driving use what you want but when it comes time for true performance at the 90% plus use the proper widths, tires and lightest rims possible.

BruceP

gota911 is right. Increasing the width of your rear tires in relation to the front ones will increase the car's tendency to understeer.

Ubermensch

Thanks Bruce and Gota. Can you offer up a brief explanation?

S.

1999Porsche911

Well, obviously, everything else remaining the same (weight and road conditions) a wider tire will grip better. If you increase the grip of the back tires and not the front, you have changed the balance of the car. You will be able to push the car harder in corners and the backend will stay stable, but the frontend will give way....defined as understeer.

To maintain current traction balance, you will need to increase the grip of the front tires through wider tires, air pressure, etc,

BruceP

To grossly oversimplify and make a long story short...

Which end of the car is likely to break traction first under the centrifugal force of turning determines whether it will tend to understeer or oversteer. All kinds of things effect this in the moment, from the energy state of the car to weight distribution to the braking effect of engine on the drive wheels, and on and on. But one of the factors that is designed into this particular car which affects this is the differential in front vs. rear tire widths. Porsche puts big fat tires on the back to control trailing throttle oversteer, but the brilliance of the engineering actually lies in the suspension which is able to keep all that rubber on the ground under even extreme cornering forces.

Understeer is a safety thing. It has no real performance benefit. It just keeps you out of trouble and gives you warning that the car is nearing its limits. And more is not better. You want to be able to manipulate the car to a neutral state or even oversteer by using the throttle (or at least the guys who are good at it can and do). If you over-tire the back and not the front, the fronts will break traction way too long before the backs will and, in theory, the car will just plow and be very reluctant to snap back into line or nicely kick the tail out around a corner.

As a matter of interest, proportionally over-tiring the car at both ends solves the understeer problem, in theory, but may raise the car's overall limit of adhesion to where the throttle can't be used to any effect in real-world cornering. That's why on 'normal' 911s, you don't see cars at the track with vastly larger rubber than what they were designed for. More, yes, but not super giant stylin' more.

gota911

You are correct. I assumed that he would know that an 11" wheel will require a wider tire than his 10" wheel, so I didn't specifically state that fact, but probably should have done so.

Irksome

I went from 295 to 265 in the rear, and went from being unable to induce oversteer/pitch, to being able to occasionally induce understeer/pitch. I'm using a standard street alignment, so there is a lot of understeer dialed in, and I definitely noticed a difference dropping 30 mm.

Mind you, I have never induced oversteer in normal street conditions. I can only pitch the car during autocross, and even then it is very difficult with my current alignment.

Ubermensch

Thanks for the clarification.

S.

Dr_KarlB

Joel from PCA had a story about the guy at AX who would switch the fronts and rears on his old 911 so he would have no understeer and plenty of throttle oversteer to get around sharp low speed corners... this would be dangerous on the road though :-)
As others have said its just balancing grip between the front and back