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Re: Rolling, Static, and Kinetic Friction



On Sun, 14 Jul 96 12:26:58 EDT Emilio O. Roxin said:
I like to add something about "rolling friction", as for example
when you roll a wheel (without the car, just the wheel with tire).
You may draw an exaggerated picture of an underinflated tire making
contact with the floor over a significant extent of its outside.
The picture might suggest that in order to start the rolling motion,
you must push the wheel so that the sum of push + weight falls
outside the surface of support (= contact tire/floor). THIS IS WRONG.
On this surface of contact, the tire is flat and just transmits the
internal air pressure, which is NORMAL to the floor and cannot
generate any friction force! This is the reason why an underinflated
baloon will roll down an inclined plane even for a slight angle.

Tires have great shear strength and when they roll large shear stresses are
exerted as well as normal stresses. Have you ever seen high speed pictures
of an automobile tire rolling at a high speed (say 100 MPH or more)? It's
pretty scary. The flat spot on a round tire is sort of like the "mythical
tidal buldges" The flat spot propagates around the tire creating a highly
distorted mess that makes you wonder how it rolls at all! I don't know how
important resonances are since the distortions are highly damped. I presume
tire designers vary carefully design tires to avoid resonances. Has anyone
ever seen a discussion of this in print. I have the impression that
the competitive nature of the tire business keeps most of this info in house.