Hi Antti,

Don't forget that the force exerted by the tire on the rim scales with
the radius of the tire section (that is, with the surface area of the
casing). A wide tire at lowish pressure can exert more force at the
bead/rim interface than a 700x18 tire at 150 psi.


Hoop stress in a cylindrical pressure vessel is PR/T where P is
pressure, R is the cylinder radius and T is the thickness of the
material. Since we're concerned about force and not stress (which is
force/area), we can dispense with the denominator. This leaves us with
PR.


For an 18mm (0.009 m radius) tire pumped to 150 psi (1.03 megaPascals)
we get a force of 9270N per unit length*,


For a 37 mm (0.0185 m radius) tire pumped to 85 psi (0.59 MPa) we get a
force of 10,915N per unit length--obviously, a larger force.


When one considers a 23mm tire pumped to 115 psi (perhaps the most
common numbers), we get a result of 9118N per unit length.


This calculation incorporates a number of assumptions. I've assumed
that nominal tire width is actual width and that toroidal pressure
vessels (i.e., tires) operate exactly like cylindrical pressure
vessels. However, the forces I've listed should be pretty accurate on a
relative scale.


The point is that low-pressure fat tires can and do exert higher forces
on rims at the bead interface than do high-pressure skinny tires.


This is a very strange failure to me. I'll be interested to hear what
the cause was.


Cheers,


Jason


* N.B.: A Pascal is a unit of pressure, specifically, a force of one
Newton per square meter. Since P*R is (N/m^2)*m, the unit of radius
cancels with one of the units of area, yielding force (Newtons) per
meter.