Hot tire pressure

[John105]

I don't pay attention to hot, as I have no control over it.

I don't think I've ever come across the explanation as to why if a mfg says 35 cold, car dealers and rental car agencies inflate to like 47. I have found this behavior at BMW on their loaners, and National for rentals. I usually deflate to a few psi over the recommended cold.

p.s. just googled and this was the first thread I found--his pic is what I was used to seeing every time, when I rented a lot up until the pandemic

 

[John105]

People who understand chemistry, understand the universal gas law. PV NRT. Oxygen, and nitrogen, both follow that.

that and v = ir I've been able to apply throughout my entire life.

 

[CarlB]

I don't pay attention to hot, as I have no control over it.

I don't think I've ever come across the explanation as to why if a mfg says 35 cold, car dealers and rental car agencies inflate to like 47. I have found this behavior at BMW on their loaners, and National for rentals. I usually deflate to a few psi over the recommended cold.

If the goal is to be at a specific PSI, then the hot can inform where to set cold. That's why we take hot readings in addition to cold.

 

[PWMDMD]

"In colder weather the hot pressure doesn’t change as much compared to the cold pressure"

Air is close enough to an ideal gas that we can use PV=nRT or specifically, P = nRT/V to determine the pressure. What this says is if we hold n, R, and V constant, pressure is linearly proportional to temperature. In other words, the difference in pressure between 0F and 30F is the same as 70F and 100F.

 

[meep]

The rule of thumb used to be to target a 3psi differential. The tire needs to be 3 psi hotter when driven…. So if it’s more than 3, add air. If it’s less than 3, reduce air.

Idk if that applies here. Also note, this was pre-tpms. The pressures were taken stopped.

 

[Vercingetorix]

This shouldn’t be that hard. TPMS senors read both pressure and temperature. At that point it’s easy to just say add or remove air, like Porsche does.

 

[Pterostylis]

The rule of thumb used to be to target a 3psi differential. The tire needs to be 3 psi hotter when driven…. So if it’s more than 3, add air. If it’s less than 3, reduce air.

Idk if that applies here. Also note, this was pre-tpms. The pressures were taken stopped.

That is similar to what I was taught in a couple of 4x4 and truck driving courses, except to aim for a 4psi difference.

 

[CapriRacer]

The rule of thumb used to be to target a 3psi differential. The tire needs to be 3 psi hotter when driven…. So if it’s more than 3, add air. If it’s less than 3, reduce air.

Idk if that applies here. Also note, this was pre-tpms. The pressures were taken stopped.

Ah ...... Mmm ........ I don't think this is entirely correct.

I used a version of that several decades ago, and it was a MAXIMUM, not a target. Ya' see, the heat being generated is a function of speed, so you get different answers for 50 mph than for 70 mph.

We used the rule of thumb to determine if we were generating too much heat, and the appropriate response was to reduce the load or increase the inflation pressure. This was for a mining operation, but it was used for over-the-road trucks as well.

 

[barryh]

Rather than a fixed number of psi, I've always used 10% pressure increase from cold to hot but you do have to drive or ride hard for this rule of thumb to be applicable. If the pressure increases by more than 10% then you can be reasonably confident that the cold pressure was too low. If it increases by less the 10% it's harder to infer the cold pressure was too high because you might not have driven or ridden hard enough to get the temperature of the tyre up.

 

[meep]

Ah ...... Mmm ........ I don't think this is entirely correct.

I used a version of that several decades ago, and it was a MAXIMUM, not a target. Ya' see, the heat being generated is a function of speed, so you get different answers for 50 mph than for 70 mph.

We used the rule of thumb to determine if we were generating too much heat, and the appropriate response was to reduce the load or increase the inflation pressure. This was for a mining operation, but it was used for over-the-road trucks as well.

@CapriRacer you are welcome to correct me without being coy, it’s ok. My info is dated for sure.

 

[GSCJR]

Easy to remember, it rhymes. If tires are hot, add four to the door; psi of course.

 

[Kernel Potter]

Rather than a fixed number of psi, I've always used 10% pressure increase from cold to hot but you do have to drive or ride hard for this rule of thumb to be applicable. If the pressure increases by more than 10% then you can be reasonably confident that the cold pressure was too low. If it increases by less the 10% it's harder to infer the cold pressure was too high because you might not have driven or ridden hard enough to get the temperature of the tyre up.

Barry, the info you have is basically what I was thinking. I have also read that about a 10% pressure increase between cold and hot operation is the way to verify that cold tire pressures are initially set where they're supposed to be.

When you have your cold pressures dialed in, you could then use it to work backwards from hot to cold in the event you want to fine tune for extreme ambient fluctuations

 

[ammolab]

The reason why the car manufacturer and the tire manufacturer don't set a hot temp is because conditions vary widely. If you're driving across a desert with the outside temp at 120*F the tire temps are going to be much higher than if were driving through slushy roads with an outside temp of 35*F.

Properly inflated tires don’t get hot enough under high temps or normal highway speeds to fail.

The tires that die on hot highway drives are UNDER INFLATED. The flexing of an under inflated fire creates tremendous heat which destroys the carcass of the tire. 33 psi going to 40psi when hot isn’t a danger

 

[TiGeo]

I only worry on track otherwise set cold.