View Single Post
Old 04-08-2008, 04:35 PM   #44
Tryin Hard
Joined: Apr 2006
Location: sydney
Oddometer: 2,997

look at a basic tyre - lets assume the rear of a GS - 150x80-17.

Total diameter on the inside (ie ignoring the thickness of the tread wall, because the internal pressure doesn't act on it), you have about a 17" + 2*4") diameter - ie 25". Since 150mm is about 6", lets just use that.

Looking at half the tyre, you have a total load produced of pressure (P) * projected area. ie 30psi * 25"*6" = 4500lbs.

Ignoring the shear loads through the tyre sidewall (giving rise to reactionary loads R3 and R4), lets just say that all the load is taken by the tyre carcass that supports the tread (R1 and R2). You now have a 2250lb load in the carcass (equal top and bottom as R1=R2).

Take a look at the construction of a tyre (eg Michelin say thier tyres have between 2 and 4 plies of radial kevlar and another 2-4 of cross nylon or similar.

Since the plies in the circumferential direction are handling this load, we only analyse them. Lets say there are 100 cords across the ply. A puncture takes out 2 cords per ply.

Thus we have 2ply*(100-2cords per ply) = 196cords to support 2250 lbs. (divide by the cosine of the ply offset if not zero degree belted).

You now have 2250/196 = 11.5lbs per cord. A tiny fraction more than the tyre without damage. Compare this to the breaking strain of nylon fishing line, and it is a pitiful load. Kevlar and steel have much greater tensile stress capability to boot.

Where is this bomb you are talking about? That will only be in compressed gas and elastic strain energy - both of which are pretty negligible at 30psi anyway, and the advantage of a tubeless tyre is that it tends to seal around a puncture object, preventing explosive release of the contents - why the flamboyance of 12,000pounds of pressure? There is only 30 pounds of pressure per square inch of applied area, or 2250 lbs of force in the whole tread wall, but it is distributed across the area of the plies, and so the working stress in the tyre is actually much lower than in the bridge you are driving over, or even the engine mounts and floor beams of the car or bike you are driving.
He immediately paid me the one of the highest compliments an Advrider can receive: "You're nuts!" (Alcan Rider)
bonox is offline   Reply With Quote