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View Full Version : Suspension tuning topic discussion: Slip Angle



mitchntx
04-12-2007, 06:50 AM
I always thought "slip angle" referred to the attitude the car took as it negotiated a turn. I was always amazed following Varner through turns at just how far he was able to hang out the rear end, achieving a wide slip angle.

But in reading "How to make your car handle" by Fred Puhn a little more closely, it kind of takes on a new meaning.

Slip angle refers to the angle difference between the direction of actual travel of the sprung weight of the car vs the direction of travle of unsprung weight.

If you have your front wheels turned full lock (unsprung weight) and the car is still traveling straight (sprung weight), the slip angle of the tires is HUGE.

Slip angle is a "condition" and not necessarily a trait. A tire has an optimum slip angle based upon several factors ... inflation, temperature, vertical force ...

And because of this condition, a tire has not yet lost it's grip while in it's "slip angle". But that doesn't mean that there is an immediate change in grip once the tire goes beyond it's slip angle. It's kind of a bell curve where the tire's grip increases to a point and then tapers off. The curve is more pointed in some tires, but still a curve none-the-less

A car with 50/50 weight bias will have front and rear tire slip angles very close to each other, given even temps and pressures.

A car that is nose heavy, will have the front tires with a narrower slip angle while the rear tires have a wider slip angle. The reason Eric can hang the rear end out so far and still be fast.

The more weight up front, the bigger the delta in slip angles.

That might also explain why a nose heavy car, like we drive, has good, crisp "turn in" and will pick up a push on corner exit. The transition from brakes to power changes the vertical forces applied to the tires and the slip angles change. So the slip angles are dynamic.

A tire losing grip at certain points is probably nothing new. But understanding the dynamic and cause will help tune the chassis (or driver) in order to work the dynamic in your favor instead of fighting it.