I am not familiar with sew-up tires so I don't know many details.
However, this has reminded me of an earlier picture that John Denker
posted in which he showed a bulged tire on a rim. This is at
In this drawing he shows the bottom cross-section as somewhat of a
horizontal ellipse whereas the top cross-section is somewhat of a
vertical ellipse. In this case the air pressure (not the sidewall
tension) in the lower tire does contribute some upward force that is not
countered by as much downward air pressure in the top cross-section.
I do not think this is the correct profile for an automobile tire
because when I look at mounted auto tired there is practically no
difference in the shape of the sidewall as comes away from the rim.
That is, the bulge is much lower (near the bottom tread). So I think
the effect John is describing in this drawing is a very minor
contributor to auto tires.
It might make more contribution with the low profile tires becoming more
popular, and with bicycle tires in which the sidewall is not very long.
But when I look at the clincher tires on my bicycle, I still see the
bulge pretty low, and the sidewall profile near the rim is almost the
same all around the rim. Perhaps for sew-up tires this is not the case.
Maybe John's drawing is very much appropriate for sew-up tires.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu