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How much does tire sidewall stiffness contribute to the force holding the car up? I know that can vary a lot. Some cars have run-flats that won't sag when they lose pressure (or won't sag as much, anyway). Low profile tires probably transfer more of the load through the sidewall than doughnut tires.

And of course the contribution from the sidewalls (as a percent) will change as tire pressure increases.






Oops that's a great point. I completely forgot about sidewall stiffness. My messing around with treads was probably just me adjusting things to make myself look good then!

Don't worry. Fiddling with your one free parameter to make your model look good is as quintessential to statistics as Student's t-distribution.

It's useful to consider the extremes. If your car was somehow supported only by party balloons on the rims, then their stiffness would contribute nothing and the calculation would be spot on.

On the other hand, if you had hollow steel "tires", the stiffness would be everything and the contact patch would change only imperceptibly with a change in tire pressure.

Car tires are obviously somewhere in this range.


I imagine you could just inflate the tires to the point of bulging at the treads to reduce the sidewall effects...

>How much does tire sidewall stiffness contribute to the force holding the car up?

A car? Not much. At any reasonable driving air pressure the air is doing most of the work.

A pickup/van with E or better rated tires? Eh, some but not much in proportion to the weight said tire is expected to carry, especially if the tires are anywhere near the top of their pressure range.

A skid steer or some other off highway machine with "bajillion ply" tires expressly designed to be resilient to sidewall punctures that runs low pressure because all of the suspension is in the tire? Yeah, sidewall stiffness accounts for a lot in that case.


I'm sure the sidewall of a quality passenger car tire could hold 50 lbs just fine. A decent quality passenger car tire will hold my body weight if I sit on one tilted upright. It could be a significant percentage.

When the tire is inflated, the sidewalls (and the tread area) are under tension, right? (They would just fold up under any appreciable compression.) If so, does that mean there's no downward force from the sidewalls? Perhaps we should be asking if there needs to be extra pressure in the tire to keep the sidewalls under tension?

The pressure of the tires pushes horizontally outward on the sidewalls and helps them hold their shape, so they can absolutely be in compression and not fold up.

I am not convinced by this. It would seem that, if it works the way you say, a bubble's surface could be under compression.

Consider a section of tire wall, with the same pressure on either side. It is curved outwards, and if you put compression on it, it will buckle outwards. If there is higher pressure on the inside of the curve, that would help with the buckling, not prevent it.

In an inflated tire, as in a bubble, it is the tension along the curved surface that resists the internal pressure. What keeps the sidewall of an inflated tire from buckling is that the buckling would decrease the radius of the curve locally, increasing the net inwards force of the tension over the area that is buckling.


Depends a lot on the car and tyre configuration. In F1 cars the suspension is so stiff that the tyre stiffness plays a large role in VD.

I have some data on this for a few different tyres but I don't think I can legally quote it here :(


I don't follow F1 very closely these days, but I think I've read that they recently changed, are will change in the near future, to lower profile tires. Is that right? And that the teams have made, or will made, the suspension softer because low-profile tires contribute less?

Didn't somebody test a couple years ago with a set of wheels/tires that more closely resembled passenger car sidewall heights? You know, like 80-100mm sidewall heights instead of whatever they run currently? It seems nothing came of that; can you shed any light?



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