The title for this post is essentially the exact question I was asked.
"Tireman9, can you explain why, when the tire pressure drops below a rating
for a tire, that the sidewall of the tire blows out with great force?
What pressure drop will cause this type of damage? When the tire blew it
had enough force to damage the motorhome with the rapid escape of air
that made a large bang when it blew."
OK, let's see if i can cover the details of why and how a tire sidewall fails due to being run without proper inflation.
The mechanics are essentially the same, be it a textile (usually
Polyester) tire as are most P, LT, and ST type tires, or for tires with
steel body cord as most commercial grade LT tires and "TBR" Truck-Bus
Radial" tires. These cords are referred to as the "body ply".
I think we all realize that tire sidewalls bend when loaded. This can be
observed by simply looking at the bottom (near the road) vs. the rest of
the tire sidewall. The amount of bending is essentially just a function
of tire size, load and inflation. This bending includes some stretching
of the outer surface of the tire and of the rubber surrounding the body
ply. This stretching results in some heat being generated. You can
test/experience this heat generation yourself with a simple test of
holding a rubber band against your lip and stretching and releasing the
rubber band rapidly. Your lip is sensitive enough to feel the temperature rise
of the rubber band.
Now the rubber used in tire construction can tolerate some temperature
rise. The heat generated can transfer to outside air at about the same
rate it is being generated. This is what happens for hopefully tens of
thousands of miles and hundreds of thousands of revolutions, i.e.,
So what happens if there is a leak of inflation air? Or if the tire was
not properly inflated in the first place? With lower air pressure the
amount of bending increases and with an increase in bending we see more
heat being generated. Increased heat generation means increased
temperature of the rubber internal to the tire structure. Since rubber
is a good insulator, heat transfer can be slower than heat dissipation
to the outside air so the temperature can continue to rise ever faster.
The strength of the rubber decreases with an increase in temperature
which allows more bending. With slower heat transfer from the internal
structure to the outer surface and increased heat generation as more air
leaks out, I think you can see how it is possible to get to a point
where there is something like a chain reaction or "runaway" temperature
The above heat generation can also result in the polyester experiencing a
rise in temperature with the associated loss of strength. You have seen
the effect of high heat by holding a match near the end of a piece of
Nylon or Polyester rope and see the textile melting. In the steel body ply
tire the increased bending can result in a fatigue failure of a steel
cord. You can test the fatigue with a steel paperclip. Simply bend the
paper-clip a few times and it will break. In the case of a tire the
number of bends to failure can easily be in the thousands.
So what you are seeing when the sidewall fails is the result of dozens, hundreds or thousands of miles of excess rapid flexing of sidewall rubber and sidewall cords. If the bending is great enough the materials that are being bent simply fail. An explosive loss of air can occur if/when the force of the remaining inflation exceeds the strength of the body cord. Since there is an almost infinite combination of load, inflation, bending and speed involved, it is impossible to give an exact answer as to when the sidewall will finally fail.
If we have a large tire we can have significant force involved even at relatively low pressure. 20 or 30 psi can easily generate over 30,000 pounds of force.