Luns Tee said:Quoted message said:The light bulb just lit up. Blow-off is a condition caused by
lateral casing pull from the tire bead, a force that is not
affected by constriction. Therefore, a narrow tire no wider than
the rim, lets say close to 20mm or so, could not be blown off no
matter how high its inflation, also because the casing would burst
before than 300psi.
Quoted message said:It's a lateral pull, but I think it's the pull of the casing at the
edge of the rim's hook, rather than the external pull of the tire
bulging wider than the rim.
Isn't that exactly what I said? I'm hearing echos!
Quoted message said:The external pull is supported by the rim, and is conveyed to the
bead like a rope wrapped around a pole. If we assume the friction
between the casing and rim has failed (no friction), then the
tension of this pull is just the tension of the casing, which for
the unloaded part of the tire is the tire width times air pressure
and some factor involving sqrt(2).
Don't make it so cumbersome. Its the horizontal component of casing
tension that does it and this is always present, explaining why I have
had blow offs with less than 100psi after long rim heating. I don't
know what you mean by "external pull", the casing can only pull in one
direction and at its rim contact. That means it is pulling the
softened tire bead around a corner, something its shape otherwise
resists.
Quoted message said:This is independent of rim width - a 10mm rim, if you could use such
a thing - with that 20mm tire would be no more susceptible to blow
offs, and in fact, with more tire wrapping around the rim to provide
more friction, I would expect it to be less susceptible to blow offs
than a wider rim.
It is not! If the pull were radial it would not lift the tire from
the rim because constriction forces hold that load against the rim.
If you doubt it, do the calculation or try a tubular tire when
inflated. Constriction increases with inflation pressure. I take it
you have not had a tire blow off.
Quoted message said:Also, I believe your comment about blow offs relative to bursting is
off. What causes a burst is the casing tension exceeding the
casing's tensile strength. This is the exact same tension that
pulls on the tire bead. Changing tire width, and adjusting pressure
with it, ends up having no net effect on either blow offs or
bursting as long as the casing tension stays the same.
I am talking about the casing filing in tension, not tire blow off.
Casing stress increases with pressure and cross section. For a small
tire, casings can rupture with sufficient pressure. I have
experienced such explosions and they are not pleasant. The tire
casing rips open over a larger length so it is a sudden explosive
release of air.
Jobst Brandt