to curtis73:

if you're talking about this chart:

http://code.eng.buffalo.edu/dat/sites/tire/img55.gif

then I'm going to have to say, "not exactly."

also to curtis73: the following definitions are not intended specifically for you, rather they are for the entire reading audience, who may not have the same background as you and might not understand all the terms used.

this chart is showing normalized longitudinal force on the Y axis, versus longitudinal slip on the X axis, for a variety of slip angles. (edit: i originally said "steer" angles)

Normalized means divided by the vertical force on the tire as tested, which is why it says (Fx/Fz) on the Y axis title.

Longitudinal means fore-aft, ie acceleration or braking. Conventional definition is "translation speed minus rotation speed, divided by translation speed", such that +1.0 on the X axis corresponds to zero rotation speed. in other places you'll see this defined as "100 percent slip". So, to the right of the Y axis is braking, and to the left of the Y axis is acceleration.

the data were taken at specific slip angles and each set of data is graphed, which is why there are curves noted "alpha =" 4.0, 8.0, 12.0, 16.0, and 20.0. there is no calculation of slip angle here.

slip angle is the difference between the direction the tire is pointing and the direction that it is actually traveling. in this case they're measuring it in degrees.

the point of this chart is to show that longitudinal force capability decreases as slip angle increases.

the purpose of normalization is to allow both axes to have the same scaling, from -1 to +1.

from the magnitude of the peaks, we can be fairly sure that the data for this graph were generated by testing on dry high-coefficient surface (highest peak is about 0.87, which occurs at the lowest steer angle tested, alpha = 4 degrees). and it does indeed have something to do with the coefficient of friction between the tire and the road, because on ice the peak normalized longitudinal force would be around 0.1, rather than 0.87.

If you're familiar with the friction circle concept, this graph just shows that concept in another way, ie as steer angle increases, maximum longitudinal force capability decreases.

EDIT: I love this stuff. We cover some of this in the Society of Automotive Engineers' excellent seminar "Applied Vehicle Dynamics." You should all take this three-day class, held each May and November at the BMW Performance Center just outside Greenville SC. Learn more about this class here

did i mention that i'm a co-instructor?