It remains clear to me that the sidewall is always in radial tension; that
is, always pulling radially outward. The lower sidewall is not exerting
any upward force. It is pulling downward. The support of the wheel is
coming from the sidewall tension in the upper half becoming more, and the
sidewall tension in the lower half becoming less, when the car's weight is
added to the wheel.
The thing I still struggle with is the bead. It is not clear to me whether
we should consider the bead/rim as a rigid unit, or if there is some
movement of the bead with respect to the rim. I have watched tire people
remove tires from wheels. It is not easy to break the bead from the rim.
Remember that the air pressure is pushing axially outward on the sidewall
and bead, pushing the bead into the rim sideways. When they mount the tire
they spread gluey goop on this joint. The bead generally becomes stuck to
the rim so hard that when you want to remove the tire you have to work to
break the bead/rim seal. They have motorized equipment to stretch the bead
off the rim once the seal is broken. They also have hydraulic equipment to
push on the bead to get it separated from the rim.
I once observed a person removing one of my tires from the rim. The air
pressure was all released, the tire machine was pressing down on the
sidewall trying to break the bead/rim seal, and not succeeding. While the
tire machine was still trying to break the bead seal, the serviceman grabbed
a very large rubber mallet and starting hitting the sidewall near the rim
with what looked like all his might. It took several hammer blows before
the bead separated from the rim.
In that case, the bead was stuck so hard, without air pressure, that I
wondered if he was going to free my wheel from the tire. When the tire is
pressurized, the bead is pushed sideways into the rim and that would make it
even more difficult to break the bead/rim seal.
Based on that experience I can certainly imagine that the rim can "hang"
from the upper bead. By I can also imagine that if the rubber/metal
interface on the top stretches some, then that upward pull from the sidewall
will "propagate" some distance circumferentially along the bead and down
around the wheel. A few inches? All the way to the bottom of the wheel? I
don't know.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
1 University Drive
Bluffton, OH 45817
419.358.3270
edmiston@bluffton.edu