Physics Lesson – Friction
By Khiem Dinh
Khiem Dinh is an engineer for Honeywell Turbo Technologies at the time of this writing. All statements and opinions expressed by Khiem Dinh are solely those of Khiem Dinh and not reflective of Honeywell Turbo Technologies.
Fric-tion [frik-shuhn] frik-shuh n
|When a car is hard on the brakes, there's the possibility of locking up the tires which changes the friction from static to kinetic. A car with the tires locked up (kinetic friction) will slide to a much longer distance than a car that is able to prevent lockup (static friction). Maintaining static friction is the raison d'etre for ABS systems.
|On the powered tire, traction control maintains the balance between static and kinetic friction. Here, Casey Stoner powerslides his old Ducati (he now rides for Honda) through a corner with the wheelspin laying down a strip of black rubber. MotoGP and World Superbikes have had traction control for years and now the technology has trickled down to the sportbikes that you can buy today; the traction control can be set to allow varying levels of wheelspin.
|For a quick example, let's look at the above situation where we are looking at one corner of a car with a mass of 250 kg. As with all basic physics problems, we'll ignore stuff like wind drag and weight transfer. I looked up common values for static and kinetic coefficients of friction (mu) for tires on concrete and they are 1.0 and 0.75 respectively. The force normal acting on the tire and road is the mass of 250 kg times the acceleration due to gravity, 9.8 meters per second squared. Knowing the force normal and coefficients of friction, braking forces can be calculated for both cases. Using a velocity of 35 m/s (just under 80 mph), basic equations of motion can be used to calculate braking distances; the stopping distance for static friction is 62.5 m and kinetic friction resulted in a distance of 83.3 m. Those distances convert to 205 ft. and 273.4 ft. After a quick search, a C6 Z06 stops from 80 mph in 199 ft., so it looks like the approximate value of static mu is pretty good. That 70 ft. difference in braking distance between threshold braking and locking up the tires is the reason for ABS!