View Full Version : Friction: addendum

Jeff Ives
12-04-1994, 10:18 PM

>From Jeff Ives:
Straight from the expert (Jim Sprague) comes this addition to the
tire/friction question posted and responded to earlier:

1. Lateral force generated by a race tire IS proportional to contact area
(among other things). When race tires get up to operating temperature
(200-300 F), the soft compounds become sticky and actually adhere to the
track surface, as evidenced by the gravel which sticks to the tires when
they go off course, or cross non-asphalt areas when being pushed back to
the garage.

2. Increasing tire width allows tires to be operated at a lower
inflation pressure for a given load. This is desirable (to a point) in
race tires as lower pressure allows the footprint to better adapt to the
irregular terrain, and gets the tire up to operating temperature sooner.
The lousy rolling resistance is seldom a problem for high powered cars on
road courses. Inflation pressures range from 8 psi for drag tires to 55
psi for heavy stock cars on oval courses with most road course type
for heavy stock cars on oval courses with most road course type
racing using about 18 to 25 psi.

3. Increasing tire width increases contact patch size which reduces
temperature and unit pressure and therefore slows tire wear... a
desireable trait in a sport where tires frequently have a life span of 45
minutes, or in the extreme case of top-fuel drag cars, 30 seconds.

4. Your argument of "base of support, stability and C. of M. "
unfortunately will need to be retracted as it doesn't really hold water.
Most racing classes have either a track width or a tire width restriction
or both, therefore if the only desired goal of the tire were to increase
effective track width, it could be achieved by using bicycle tires
cantilevered out to the most outward allowable position. Clearly this is
not the case except in some arcane racing classes like formula V (which
are relatively slow and underpowered cars anyhow).

Summary: Automobile tires in general and Racing tires in particular are
exceedingly poor examples of "classical" friction. Friction coefficients
for race tires range from approximately 0.9 to 2.2 and are dependent on a
large number of parameters. First order effects on mu (friction
coefficient) are typically

Slip Angle
Slip Ratio (longitudinal counterpart to slip angle)
Camber Angle
Road Condition (wet, icy, dirty, fresh, rubbered in...)
Road Surface Type
Operating Temperature
Inflation Pressure
Normal Load (mu decreases at high loads)
Tire Condition (wear, number of heat cycles...)

Additionally, lateral force is dependent on longitudinal force, this is
often referred to the traction ellipse concept.

These are mostly practical observations obtained from tire mechanics
research and coursework, and from working in the industry. A bona-fide
tribologist could give you a better explanation of the actual
micro-structure friction mechanisms.

Happy cornering!

................Jim Sprague
Biomechanic and former Race Tire Engineer
U of Michigan biomechanics laboratory
sprague@engin.umich.edu or 71601.2752@compuserve.com