Question for ya.

So, when its extremely cold out the air pressure in your tires will be affected: cold air takes less volume. But when you drive, doesn't the air in your tire heat up due to friction? And if so, then does that mean if you fill your cold tires up with more air, that this will cause over-inflated tires?

My tires are fine, but it's just something I'm curious about.

From a purely Te approach:

Tire manufacturers ought to have tested their tires in different weather conditions. Anyone who is concerned about over-inflating their tires should strictly follow the manufacturer's recommendations, and not go over the maximum recommend pressure. Also, car owners ought to have a reliable tire gauge in hand.

From a Ti-Se approach:

Naturally, it is possible to over-inflate your tires with cold air and have them burst once too much friction heats up all that air in your tires. But having too much cold air in your tires is not the only practical consideration - since you threw friction into the equation...

Tire pressure affects the shape of your tire, and therefore, its efficiency and the amount of friction generated. Under-inflating your tires, theoretically, increases the tire's footprint/contact with the road, thus increasing friction and cause cause your tire to overheat and burst. (I've seen this happen too often on my car racing video game when my tire pressure is low. ) Well-inflated tires should reduce friction because a rounder-shape actually reduces the tire's footprint. Even slightly over-inflated tires (like in car-racing) should create a rounder shape and thus reduce friction.

So who can say if your tires are over-inflated with cold air? Trust the manufacturer's recommendations and a reliable tire gauge. And leave over-inflating to the experts. But if you want more theoretical possibilities, ask Dr. Feynman.

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I'm not really qualified to be a physics geek, but I have typing fingers and by gosh I'm gonna use 'em! *rolls up sleeves*

My theory is that car tires, being made of rubber, will have some stretch that will accommodate the increase in air pressure. *nods sagely, just in thyme... I'm so oregano!++*

++ I just helped move a helluva lot of furniture out of my place in 37 degree heat today.. and yes the control on my inner pun machine appears to have switched on accidentally.

Hi! I'll put my physics hat on

Everything you said is absolutely correct (well, there's the friction thing). What's needed is to know how temperature affects pressure, fortunately we have the ideal gas law.

If we assume the volume of the tyre is constant (this is a worst case scenario, all tyres expand) the relationship between volume and temperature is:

p=cT

Where p is the pressure, c is a constant, and T is an absolute measure of temperature.
The absolute part is important! The Kelvin scale is the standard measure of absolute temperature so we can use that.

Now we can get to work on the problem!!

0'C = 32'F = 273'K
30'C = 86'F = 303'K

So a change in temperature from 0 to 30'c is an 11% increase in absolute temperature which is an 11% increase in pressure - in a bad theoretical tyre that doesn't expand. In reality the tyre will expand so the pressure increase will not be as big. Also a change of 30'C is rather drastic, the tyre is well ventilated, has lots of air rushing past it and is always contacting new & cold road so temperature increases should be small.

The manufacturer's standard ruling on tyre pressures is they are done before the journey, the tyre is then designed to cope with any temperature changes during the journey (provided you're not driving from Alaska to Mexico or something drastic ).


Tyres & friction. (Edit, sorry almost missed this part of your question.)
Tyres don't heat up because of friction with the road (not unless the tyre is spinning against the surface because it doesn't have proper grip on the road, which is a very different process). What heats them up is the deformation of the material. Like if you get a rubber band and stretch & relax it quickly it will get warm. It's one of the characteristics of rubbers, it exists in other materials but to a much lesser degree. (If you want to look it up it's called elastic hysteresis). If you want to see it with your own eyes attach a weight to the bottom of a rubber band and let it bob up and down and see how quickly the bobbing stops as all the energy gets converted to heat (If the same experiment is done with a steel spring, or a similar experiment done with a pendulum it takes far, far longer for the energy to all go to heat).

Hope I didn't write too much :S

Ahhh. This can happen when driving over icy roads. In winter, some drivers think it's better to under-inflate their tires because it creates more contact with the road - thinking it will give them better grip. But simple increase in contact does not amount to better grip. And well-inflated tires, on the contrary, actually give better maneuverability. So what happens is that the tire spins against the slippery surface thus causing friction and tires that wear out more quickly. In other words, don't under-inflate your tires in winter, and use winter tires for better grip.



So inflating tires well causes them to be stiff enough so that the rubber is less deformed on the road, whereas under-inflating tires (flat tires) allows the rubber to expand and contract more, thus generating more heat, possibly resulting in burst tires.

So all that theory that these auto mechanics give us about tire footprint and friction is bull?

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Exactly right, more heat generation in flat tyres.

The deformation process can also cause fatigue in the tyre, like what happens if you bend a spoon too often and too far, it gets brittle and snaps. This is a property mostly associated with metals though tyres do contain a steel belt under the tread.

Another contributing problem with under inflation is it can allow the wall of the tyre to contact the ground and get worn.



What theory? In what way is the friction acting?
There is friction as a mechanism for heat generation (dynamic friction) (which involves the tyre rubbing against the road, such as occasionally spinning on gravel or on icy roads, or slipping when cornering way too fast, which doesn't happen often enough to generate any appreciable heat).
Then there's friction related to getting an actual solid grip on the road (static friction) which is related to the tyre's footprint.

Maybe it's something to do with those two? Like if you have a stepladder against the wall, it's friction that keeps it in place; but unless the ladder starts to slip and move there's no heat generation.

Thanks that was all quite clear.



So the other explanation I received and shared here attributed too much tire damage on the heat generated by dynamic and static friction. I stand corrected.

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Hey, thanks for the responses all. Sciski, you're punny.

Tragedy, your answer wasn't too long. I found it very interesting and learned something! All in all a good read.