Round Black Donuts, or in Other Words, Tires Part 2 Bob White ...

B 245/575 – 15 slick (12 K x 15)
C 265/575 – 15 slick (12 K x 15)
D 215/60 VR XWX (7 J x 15)
E 245/575 – 15 intermediate (12 K x 15)
F 340/575 – 15 slick (15 K x 15)
Notice in comparing the friction values in Figures 3 and 4 that it is important to use the
same load values since the weight scales are different. For the worst race type tires the
friction values are 20% or so higher than the best street values and the best race tire
values are 60% or more better than the street values! For the same radius of a turn the
speed increases with the square root of the mu value which means sqrt(1.6) = 1.26 or a
26% increase in speed, i.e. a 50 mph maximum turn speed becomes 60 mph with race
tires.
For straight line acceleration or braking the change is directly proportional to the friction
value assuming the engine has sufficient torque (and correct gearing) to keep the tire at
its maximum friction value. This leads to another characteristic of tires, there must be
some slip between the tire and the road to produce any braking, acceleration, or turning
force. For braking and acceleration the slip is ratio of the actual rotation speed of the tire
and the ideal rotational speed being the rotation speed if you let the tire roll down a hill
by itself. We will return to what slip is for a cornering tire a bit later. Thus, a free rolling
tire has a slip of zero and a locked up tire has 100% slip. Figure no. 5 below shows how
the friction varies with slip.