200 lbs psi

topflite51 wrote:
A good friend claims that he can inflate 16" truck tires to 200 psi without any damage to rim or tire. This is what gets discussed when there is nothing else to do. Any resident genius's care to comment? Tire and rim are both rated for 80 PSI.

OK, here's the scoop.

In engineering circles, there is a thing called fatigue - that is, when a structure has a cycling load (and a rolling tire is cycling the load in and out of the contact patch), the amount of stress before failure is much much lower than the stress needed to cause a failure for a single load application.

The classic way to look at this is to take a bolt and put it in a load machine and pull it until it breaks, then record the load when it breaks.

If that same bolt were to be loaded to some fraction of that breaking load (say 50%), and then a cyclic load were to be applied (say +/- 10%), the bolt wouldn't break right away, but would eventually break after a number of cycles (and the number of cycles would be about 10,000 cycles)

To describe this phenomenon, engineers use what is called an "SN curve" - Stress vs Number of cycles. The SN curve is different for each material, but the curves have very similar shapes.

The same thing applies to tire bursting strength. Tires are by their very nature subjected to a cyclic load. While a single application of pressure will cause the tire to burst at a very high pressure, it will burst at a much lower pressure after many miles. To give you and idea, 50,000 miles is about 6 million cycles. This puts the difference between the usage pressure and the bursting strength for a new tire at about a factor of 4 or 5.

That's why there are maximum usage pressures written on the sidewall of tires. Those pressures are fatigue pressures. Too many people think that because a new tire bursts at a very high pressure, you are safe using a slightly lower pressure - and nothing could be further from the truth.