Quote:
Originally Posted by RobertSmalls
That would be a good first approximation if the pressure throughout the tire were uniform. However, it takes time for the pressure spike from the curb/pothole impact to reach the far side of the tire. Locally, the pressure could be quite high. Suppose I could tell you it's >200 psi for 0.05s. Is that enough to burst a tire or unseat the tire from the rim? Dunno, but as an engineer, I would recommend you close your eyes and ignore the pressure vs. time vs. position question, and pay attention only to the outcome. i.e. instead ask "To what cold PSI can I inflate a tire and still hit a pothole at 75mph without tire failure?"
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That seems partially right - and partially wrong.
Pressure waves travel at the speed of sound - at standard conditions for air over 600 mph - at higher pressures the speed to be even higher. I don't think an object impacting a tire is going to go fast enough to generate more than a couple of 0.1's psi in the form of a wave.
Plus the biggest problem with tires impacting objects is the structural bending. You want enough inflation pressure to prevent the tire's sidewalls from bottoming out, which will cause a localized bending of the sidewall, thus breaking the cords. This is commonly called a "Rim Pinch" - the rounded curb scenario.
On the other extreme, if the object being impacted is pointed - like the edge of a pot hole might be, then the localized bending adds stress to the cords, particularly the inside ply cords. If the cords are already highly stressed due to high inflation pressures (the cords are pre-tensioned by the pressure), the cords could break. Needless to say, the more pointed the impacting object, the more likely the cords will break.
So you want enough pressure to resist bottoming out, but not so much to over-tension the ply cords.
And that, of course, doesn't take into account any other inflation pressure issues, such as fatigue and heat generation.
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