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

Noun
1. Surface resistance to relative motion, as of a body sliding or rolling.

We like friction between tires and whatever surface they are riding on.  However, there are two types of friction: static and kinetic.  Static friction occurs when there is no relative movement between two surfaces.  Kinetic friction occurs when one surface of an object slides across another.  Using tires and a road surface as our example, and making it overly simplified, static friction occurs when the tire does not slip on the road and kinetic friction is when it does slip relative to the road surface.  In general, static friction has a higher coefficient of friction (resistance to relative motion) than kinetic friction meaning that static friction will provide more traction.  What does this mean to us automotive types?  A tire that is not spinning relative to the road surface (static friction) has more grip. Conversely, the occurrence of wheel spin (kinetic friction) results in a reduction in the coefficient of friction and therefore traction.  Again, this is the super simple explanation.

 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.

Drag racing best shows the two extremes of friction.  The massive burnout uses kinetic friction to heat up the tires.  The coefficient of friction is so great from the hot and gooey tires that they are able to maintain static friction on the launch aided by the wrinkling sidewalls of the tires.  A whole lot of 'stick' is required to maintain traction with some 8000-10000 horsepower!
 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!