Re: Rolling, Static, and Kinetic Friction

[David Dockstader <DRDOCK00@UKCC.UKY.EDU>]

On Fri, 12 Jul 1996 14:53:17 -0700 (PDT) Lowell Herr said:
>                 ...a toy Tonka car was allowed to move down an
> incline, first with the rear wheels taped so they would not move.
> Then the front wheels were taped and the rear wheels were allowed to
> rotate.  When the rear wheels are taped, the car slides out of control
> and falls off the incline but when the front wheels are taped to the
> car, the toy car will ALWAYS slide straight down the incline.
> Your ideas are welcome.
Sketch your car slightly crooked on an incline, with a force acting on the
center of mass and directed  directly down the incline.  If none of the wheels
can turn, but all of them slide they will slide in the direction of the force.
However, wheels that roll, but do not slide, will go in the direction that the
car is pointed.  If the front wheels slide and the back wheels roll the front
slides in a direction which will point the car straighter down the incline.
If the front rolls and the back slides, the back slides in a direction that
makes the car more crooked on the incline.

Front slip, back roll is in general much more stable than front roll back slip.
In driving lingo this is over steer (front roll) and under steer (front slip).
Cornering is very similar to going down a hill and so most cars are set up to
produce some degree of under steer.  It is very difficult to control a car
which oversteers in a hard corner.  In real cars the understeer condition is
produced by adjusting caster, camber, and aligment of the various wheels.
Race cars are usually so "misaligned" that they are hard to push, which is one
reason why tires don't last very long.