We all know those big showy sideways drifts are not the quick way around a corner. At that point, the tires are no longer gripping and are sliding, just like locking the tires when braking without ABS. But, I think a lot of people outside of motor sports don’t realize that maximum traction does happen when there is a little bit of ‘Slip Angle’; or the angle between where the wheel is actually pointed and the direction of travel.
If you were to plot this out, it would look something like this:
You can see in this graph that the optimum amout of traction (force) happens off center. These curves change with load, so that’s why there is multiple lines; each one representing a different load.
The biggest factor that contributes to this angle is the deformation of the tires carcass (aka: side wall), but other factors include the tire tread squirming, the compound of the tire and the actual internal construction of the tire (bias ply or radial).
This video on YouTube showing a tire dynamometer clearly shows the deformation which contributes to the slip angle.
This phenomenon of the tread flexing and moving the contact patch is referred to as “pneumatic trail”.
Connecting this with my previous posting on reverse ackerman steering: you can see how reverse ackerman is required to optimize a tires traction due to the load difference between the inside and outside tires affecting the slip angle. Thus, the less loaded inside tire needs less steering angle than the highly loaded outside tire.
Leave a Reply