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Re: [Phys-l] Another tire question



To organize some thoughts which have been alluded to here in various
ways:

1) I think there is a consensus that the crucial upward force is the
upward pull of the upper sidewall on the bead.
2) A remaining question seems to be: "where is the glue that bonds the
bead to the upper rim, so that this upward pull is communicated to the
rim?"
3) IMHO this "glue" is provided by the (very large) tesion in the
stretched bead. Even though the sidewall tension tends to pull the
bead away from the rim, this tension would forbid any such separation
by developing a large enough inward radial force on the rim. Instead
of separating from the bead, the rim follows the upward motion of the
bead. This radial component of the bead tension would increase
drastically if any increasing curvature should occur because of the
pull of the sidewall on the beam. (Just like the effect of a
transverse force on a stretched rope.) At the rim top this bead
tension is directed downward on the rim - it serves as a glue force,
it is not the lifting force.
4) In sum, the enormous bead tension "glues" the rim to the bead,
reacting to (and forbidding) any attempt at separation - just like
glue would). This allows the upward tension in the upper sidewal to
pull up on the bead-rim composite, as if on a single object.

Bob Sciamanda
Physics, Edinboro Univ of PA (Emeritus)
www.winbeam.com/~trebor
trebor@winbeam.com