Physics: Principles and Problems

Chapter 5: Forces in Two Dimensions

In the News

NASCAR and your car: tires and friction.

November 2004

I'm rubber, you're glue: the physics of friction:

Solid objects retain their shape because their molecules exert great attractive force on one another. Since the molecules on the surface of a solid are not fully surrounded by others, they tend to attract any other molecules available. This is why most liquids form strong bonds with the surface of most solids: we say that the liquid 'wets' the surface of the solid. The attraction is quite strong, which is why raindrops cling easily to a windowpane. If the liquid then solidifies, the strong bonds will remain: thus does ice stick to a sidewalk, paint to a wall, and glue to two pieces of wood.

Why your All Stars grip the smooth gym floor:

Soft rubber is a very flexible solid. It thus exhibits great friction when in contact with smooth, dry; rigid surfaces because it conforms to the solid surface and yet is very tough. We could say that rubber 'wets' the surface of most solids. The inter-molecular attraction between the solid's surface molecules and the rubber molecules tend to prevent the surfaces from sliding over each other. Other substances do the same thing—putty, for example—but these do not have the strength of rubber and will tear apart.

They don't use Hush Puppies at NASCAR:

The construction of a racing tire depends on its duties. A drag or track racing tire has a smooth tread and a large diameter. This allows the most rubber to contact the track. Though we have seen that force of friction is independent of the surface area of the objects in contact, a large area of contact helps spread the heat energy generated by the friction of the slipping surfaces. In a racing tire, a large area of contact will help reduce the peak temperature on the tire tread and keep the rubber from melting or burning. Even so, racing tires frequently become hot enough to ignite, so they are filled with compressed nitrogen instead of the compressed air we use in our passenger car tires.

Those wiggly grooves:

A tire used in long-distance road racing may encounter rain and therefore must have grooves in its tread. These grooves raise the pressure of the tread on the road and so encourage water to squeeze out from between the road surface and the tire. If these grooves are not present, the pressure of the trapped water will force the tire off the road, much as a speedboat lifts high out of a lake. The coefficient of friction between the tire and trapped water is nearly zero, so the least disturbance will cause the vehicle to go out of control.

And Jeff Gordon would probably melt:

Even with a good tread design the grip of tires is vastly reduced on a wet road. This is because the rubber cannot make intimate contact—in other words, cannot 'wet'—the surface of the concrete road. Thus the number and magnitude of the intermolecular forces are reduced and the tire may skid. We could say that water acts as a lubricant between the tire and the road. This is why track races are typically cancelled in the event of rain.

Observe recommended inflation pressure, or else:

The inflation pressure of racing tires is a serious matter. A tire that is inflated to a high pressure has low rolling friction because it is hard. It will also run cooler than a softer tire because the tire does not flex much as the wheel rolls. A harder, cooler tire has reduced wear and improved fuel economy. These are very important in racing because they determine the number of pit stops a driver must take to replace tires and refuel.

However, a hard tire may tend to hop and lose contact with the road on bumpy turns. A professional race driver can deal with this, but the average driver cannot. While a soft tire will run hot and tend to squirm in tight maneuvers, it is generally more predictable in its behavior than a harder tire. That is why tires on passenger cars are meant to run with low inflation pressures.

Activity:

Look up the various tire designs used for NASCAR, Indy, and road-racing cars.

You've learned that the force of friction is independent of the surface areas in contact. So why would a tire with an aggressive tread grab the road better than a smooth tire in the rain?

Resources:

http://boson.physics.sc.edu/~rjones/phys101/tirefriction.html

http://www.racegoodyear.com/media/race/072204.html

http://www.hoosiertire.com/rrtire.htm

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