I think I disagree with John that the sidewalls pushing up from below or
pulling up from above amount to the same thing. I have a drawing and
some explanation posted at the reference below. Please check this out
and see if it makes sense.
www.bluffton.edu/~edmistonm/Car.Tires.pdf
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu