New Tires Drama

[flar]

Also we can do a thought exercise: placard pressure 45, Set one tire to 41, one at 45, and another at 49. Day high is 40C (104F). After an hour or two of highway speed driving, All three tires will be about 53/54 psi as they reach thermal equilibrium**. However the one that started at 41 will be much hotter than the one at 49.

** Tires typically have an 8% pressure increase when driving at North American highway speeds, but might be as high as 15% if there is a big temperature difference between morning and afternoon temperatures. In other words, they're all going to reach the same pressure eventually, so it's better to start high and avoid the excessive heat buildup which is really bad for tires.

Anecdotally, I've had tires that were 3-4 lbs lower than the others and was too busy to fix it for a couple of days. The lower pressure tires never caught up to the higher pressure tires no matter how long I drove (maybe 40 minutes on a highway commute).

I also don't understand how they can reach a pressure equilibrium given one tire has less air. If the temperature comes from flexing and the flexing comes from pressure and the cooling is the same on the tires (because they are on the same vehicle), then less air will mean less pressure which will mean more flex leading to higher temp. That I can see.

But if it also causes the pressures to eventually equal out then the pressure is the same in both tires which means the flexing is the same, which means the induced temp should be the same, which means the tire with less air in it will have a lower pressure. The two fight against each other. In each tire the temp would reach an equilibrium with the pressure in that tire, but the different tires should not reach the same pressures as a result.

I can see the lower pressure tire running hotter, but I don't see the pressure catching up ever.

 

[jerry33]

I can see the lower pressure tire running hotter, but I don't see the pressure catching up ever.

The hotter tire heats the air to a higher temperature which the air to expand and increases the pressure at a greater rate than the tire that started out higher. Eventually the hotter tire will have increased pressure enough to reach thermal equilibrium. Generally this is close to the pressure of the other tire. It might take a couple of hours to achieve this.

 

[flar]

The hotter tire heats the air to a higher temperature which the air to expand and increases the pressure at a greater rate than the tire that started out higher. Eventually the hotter tire will have increased pressure enough to reach thermal equilibrium. Generally this is close to the pressure of the other tire. It might take a couple of hours to achieve this.

I understood all of that and I understand the ideal gas law.

The point I'm making is where is all of that heat coming from? The source you gave was the increased flex due to the lower pressure. But if the pressures get close, then the flexing should be similar which means that the relative temperatures should also become closer, but that means the pressure will become lower in the other tire, which ... its a cycle that never achieves parity.

Again, I can see how the pressure rise in the lower pressured tire will be greater, but don't mistake that for "and so the pressures will get close". They can't ever get close because that would trigger changes in the system that would reduce the temp and thus the pressure in the tire that was originally lower. They can get "closer" in relative terms, but they will never get "close".

Temperature is related to flex
Flex is related to pressure.

If the pressures become similar, then the flexing would become similar, which means the temperature would become similar, but that contradicts the pressure being similar due to the ideal gas law.

 

[jerry33]

I understood all of that and I understand the ideal gas law.

The point I'm making is where is all of that heat coming from? The source you gave was the increased flex due to the lower pressure. But if the pressures get close, then the flexing should be similar which means that the relative temperatures should also become closer, but that means the pressure will become lower in the other tire, which ... its a cycle that never achieves parity.

Again, I can see how the pressure rise in the lower pressured tire will be greater, but don't mistake that for "and so the pressures will get close". They can't ever get close because that would trigger changes in the system that would reduce the temp and thus the pressure in the tire that was originally lower. They can get "closer" in relative terms, but they will never get "close".

Temperature is related to flex
Flex is related to pressure.

If the pressures become similar, then the flexing would become similar, which means the temperature would become similar, but that contradicts the pressure being similar due to the ideal gas law.

I think what you're missing is that the thermal equilibrium temperature will be different for each tire, also it takes tires a long time to cool down once heated. All I can say here is that in a past life I've been involved with many controlled tests that have had this result.

 

[flar]

I think what you're missing is that the thermal equilibrium temperature will be different for each tire, also it takes tires a long time to cool down once heated. All I can say here is that in a past life I've been involved with many controlled tests that have had this result.

I'm combining it with another post that claimed that the higher temperature came from more flex in the tire with lower pressure. When you combine all of that information it doesn't support the claim. But, I went back and looked at that post again and see that there are also claims related to contact patches and the heat transfer to the mass of the air in the tire. The picture isn't so simple.

Also, I see anecdotal evidence from people talking about tracking race tires, trailers, RVs, etc. all of which put higher demands on tires. On the road doing normal street driving I have not seen the same effect.

So, maybe the effect just requires a more extreme condition than regular street driving.