I read something earlier on about tyre slip angles, which really confused the hell out of me as I know contend with two different definitions. Perhaps someone here can enlighten me more?
Here is what I read:
The tyre slip angle results from the tyre being elastic. The centripetal acceleration of the car distorts the tyre around the contact patch. The contact patch is pointing at different angle to the wheel. If a tyre is sliding then this angle loses relevance. The tyre isn’t always sliding. The tread surface is able to work in a regime of static friction. An example of this is driving a car on sand. At low speeds the tyres will leave their tread pattern behind. With sliding the tyres will pump much more sand out and you won’t get a clear pattern.
Under braking a similar thing happens. Optimum braking performance on dry roads occurs when the tyres are rotating at a bit less than road speed, but it is not necessary for braking. The tyres will distort under any braking load, so there will be a brief lag between the braking force being translated to the tyre surface, but the tyre contact patch will not slip until the braking force exceeds the static friction coefficient multiplied by the dynamic weight on the tyre.
The tyre’s ability to key itself into the surface means that it is quite able to exceed a coefficient of friction of 1. Greater forces can be transmitted through sliding, but with additional wasted energy.
For a given tyre greater downforce will allow a greater slip angle before the tyre starts sliding. There isn’t a binary transition from not sliding to sliding, but pretty much by definition the maximum slip angle is the point just before the tyre starts sliding.