If I have a tire that I inflate to 40 psi, then mount it on a car and take the same measurement, is it still 40 psi? It would seem that the added weight would increase the air pressure.
You would think so, I agree, but in practice it will read the same. Perhaps with a precise high-resolution gauge that could read in the tenths or hundredths of a pound one could see a difference, but with normal tire pressure gauges it’s undetectable.
I have seen tires actually pop from vehicles being grossly overloaded. Looking back it would have been cool to have taken a pressure measurement before the failure occured. I have never detected a difference in heavily loaded trucks as opposed to empty trucks so the difference must be small.
I bet the instances you’ve seen weren’t static overloads. i.e. the empty vehicle was loaded more and more until a tire failed under the weight.
I’m betting you saw a vehicle driven in that overloaded condition and eventually a tire failed. In that case the excess flex on each rotation lead to excess heating. And once the tire rubber got to a few hundred degrees, some combination of increased air pressure due to temperature vs. some reduction in tire strength due to temperature crossed over the threshold & the tire burst.
Pressure is proportional to absolute temperature. So a doubling of absolute temperature will double the pressure. For real round numbers a tire heated to 500F would have doubled the internal pressure. IANA tire engineer, so I don’t know what that much temp would do to tire strength. It might already be melting and catching fire by that point, or it might just be getting close.
I’d sooner bet that extended overloaded operations at lower temps but with massive sidewall & tread flex would lead to sidewall separation & a blowout long before we had a simple overpressure failure.
PSI is a factor of the amount of air in the tire and the volume the tire occupies. The amount of air isn’t going to change due to load so the remaining factor is the volume the tire occupies.
Look at a loaded and unloaded tire - the shape changes slightly but the volume is essentially the same. The only way to change the pressure is if you compress the tire enough to change the volume displaced, and the tire may well fail before that happens.
From heat, as you allude to. Tires can take considerable overpressure for a long time, but when overloading puts extraordinary flex on the sidewalls, the resultant heat rapidly degrades the carcass and structure and - blammo.
Sort of a WTC1 and 2 on your rims. It’s the heat that does the final damage.