My car (like most nowadays, I suspect) can show me on the dashboard, the current air pressure in each of the four tires. I never bring up that screen except when I actually see that a tire seems to be low on air, and then again after I fill it up. The sensors seem to be fairly accurate, because the numbers are fairly constant, at about 32-33 psi. And when the tire is low or refilled, the car’s numbers are close to the numbers on the air pump. On a cold winter morning they might go down to 30 or so, but then go back to regular after I’ve driven a while.
This afternoon, for no particular reason, I took a look, and the numbers were 38, 38, 40 and 41 psi, instead of the normal 33. I never saw it so high before, and I started to worry that when I start driving they’ll go even higher, so I ought to let some air out to prevent an explosion. Then I thought that it is merely temporary, from the hot pavement (it’s above 90F here today) and that the pressure will go down when the wind from driving cools them off. But even with several miles of driving, the numbers stayed exactly where they were.
SO: Anyone else have similar experiences? Is this a normal summer thing for the tire pressure to go up by 6-8 psi? Or should I have the car looked at? Occam’s Razor would suggest that if all four tires had the same situation, then it is much more likely a normal occurrence than for all four tires (or all four sensors) to malfunction at the same time. But I’d rather hear it from someone who know more about cars than Occam did.
This is completely normal. Driving will warm up tires through friction even when the ambient temperature is cool. Recommended pressures are for cold tires, and tires are designed to be strong enough to withstand the higher pressures that will occur with temperature variation. Underinflation is more dangerous.
I guess I wasn’t clear that the pressure was already high when I first began driving, when the car had been sitting unused - but in the hot sun - for six hours. And yes, friction from driving does raise the pressure, but in my experience only by one or two psi.
Tires are black, so they will quickly get hot if the car is left in the sun. But what I was responding to was this:
The friction from driving will exceed any cooling effect from the passage of air.
It would depend on how you are driving. Slow and steady in a straight line will not warm them up much. Fast cornering on a windy road will warm them a lot more. It can easily be >5 psi.
Watch them trying to get heat into their tires (to get them up to the designed working temperature for grip) in a motor racing warm-up lap. Here’s Kimi Raikkonen not bothering for some reason, while everyone else veers from side to side on the track to create friction:
Concur. Tires will absorb thermal energy from both sunlight and the pavement with the air inside experiencing a corresponding increase in pressure. Tires are designed for this and the burst pressure of the tire is >5X the rated value (for most automotive tires it is 200 to 250 psi) so this increase in pressure will not rupture the tire; the bond at the bead will fail long before that. The only reason for concern is if you have inflated previously tires for low temperature operation and the ‘cold’ pressure of the tire (at 25 °C and not in sunlight) is significantly above the rated temperature.
Underinflation is definitely a problem not only because it impacts handling and potentially makes the tire prone to popping off the bead but also because the sidewalls of the tire require a minimum amount of pressure to have sufficient stiffness to resist lateral loads. The entire weight of the car plus all of the lateral forces are transferred through the sidewalls and if they are too compliant because of underinflation, the material in the sidewalls will begin to weaken and potentially develop interlaminar shear, potentially causing an unexpected sidewall blowout which is the most dangerous form of tire failure because of how sudden it is and usually occurring at high speed or while cornering. You should regularly inspect the sidewalls of the tires (both outside and inside with a mirror) to assure that there are no bulges or cracks but with modern tires this kind of failure can occur even without any inspectable indications, so if you start experiencing mysterious leaks or pressure drops and replacing the valve stem doesn’t fix it, it is probably time to replace those tires promptly. Automotive tires are an incredibly sophisticated and highly engineered device despite their mundane appearance, and next to brakes they are the most safety critical item on an automobile or motorcycle.
As an aside, driving will cause more heat in underinflated tires than in overinflated ones. Rolling friction is mostly from the rubber continually changing shape, as which part is on the bottom (and hence slightly compressed) constantly changes. Higher pressure means a stiffer tire means less shape-changing.
For as long as I have been driving I have been told not to let air out of the tires in the summer just because the pressure is high. The recommended pressure is for cold temperatures.
Not really. A marginally underinflated tire will cause the engine to have to work harder, and that energy theoretically goes into the tire, but tires are aggressively ‘worked’ just by driving over normal pavement as essentially the first element in the suspension. Since the synthetic ‘rubber’ used in modern automotive tires is a hyperelastic material (does not experience compression) the material is going to experience approximately the same amount of strain energy for a given load cycle regardless of its initial preload dictated by the inflation state (provided that it isn’t so underinflated that starts warping across itself) the amount of energy available to become thermal energy is essentially the same. What underinflation does do is pushes the material outside of the neo-Hookian stress-strain model and into a regime where the material will start to experience large scale internal shearing that quickly degrades the structure of the sidewall.
The recommended inflation pressure is at ‘room temperature’, i.e. 25 ºC. If you are in very cold temperatures (consistently below freezing) then the pressure can drop by up to 20% of the rated pressure, in which case they should be inflated up to the rated load at the ambient temperature.
Back in the days of bias ply tires on cars it used to be recommended to let them run at lower pressure with ambient temperature to increase purchase on what was presumably an icy or snow-covered surface. However, you should not do this on radial tires because of potential damage to the sidewalls.
This. when you read “inflate to XX psi cold”, the “cold” means whatever ambient temp the car will be operating in, with you measuring the tire pressure before driving anywhere and warming up the tires. Tires gain or lose about 1 psi per 10 degrees F temperature change, but the average driver need not fuss if they leave for work in the morning when it’s 60F and come home in the afternoon when it’s 90F. Overinflation is far less problematic than underinflation, so just inflate your tires to the pressure recommended in your driver’s door jamb at the lowest ambient temp you’ll be driving in in the near future, and you’ll be good to go.
To put your mind at ease, check the sidewall of your tires for a max pressure rating. Your car manual (and driver’s door jamb) probably specifies a pressure of 30-35 psi, but the tire likely lists a max pressure of 44-50 psi. And as noted upthread, there’s a huge safety factor built into the tire above and beyond that sidewall rating. Operating at 44 psi when the sidewall says “44 PSI max” does not mean your tires are a hair’s breadth from catastrophic failure.
…I don’t understand. With any level of inflation, it’ll be warping across itself, and it’s the warping that leads to the heating. The more warping, the more heating, and the more inflation, the less warping.
Has this always been true? About 35-40 years ago, I was waiting to use an air hose at a gas station when the guy in front of me exploded his tire. Scared the crap out of me. Big puff of dust and he went rolling backwards a couple of feet. He looked at his tire, got in his car and drove off. The remains of the tire looked like a tank tread going around the rim. I always thought it was just overfilling, but maybe some fault in the tire instead.
You know what really made it memorable? This was in a small Arizona town, not much more than the gas station, a bar and a church. The guy was a 50 year old, 100 pound guy wearing lederhosen. You don’t see that much round them parts.
A healthy tire will have a burst pressure well above the rated numbers. A tire that has a blow-out while driving by definition has a burst pressure that is the same as normal inflation pressure.
IOW, all bets are off if you have a well-worn or defective tire. Inflatable things fail by, generally, failing to contain the pressure they’re meant to.
Many years ago, my father was the head compounder for an international tire manufacturer. All the letters with “suggestions” that the company received went directly to him. In those ancient times, he was provided with a secretary, so he mostly just had the secretary respond with standard “thank you and we’ll consider it” letters. (But every letter got a reply.)
The two most common suggestions?
Make tires smooth and groove the roads instead. Loss of tread depth won’t be an issue any longer.
Install trenches filled with water at gas station pumps to cool the tires when you fill up.
There are places where the pavement is grooved, typically on highways, and AIUI, only on concrete. It helps with water control, reducing the loss of grip associated with wet weather. There are tradeoffs:
Cross-grooved pavement, where the grooves run from left to right, is extremely noisy.
longitudinally grooved pavement, where the grooves run parallel to the direction of travel, can interact with tire tread patterns and cause squirrelly handling.
Yes, but what the people writing in meant was that the pavement should have a grooved pattern that performs like tire tread. This is obviously impossible and impractical.
BTW, there were a couple times that my father had tires made with company logos as treads. These stories were part of his “you won’t believe what we got asked to do” collection.
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