Optimal Tire PSI When Running

[Bruce_H_]

My ST 215/75R14 Maxxis M8008 tires are set to 50 psi cold in the morning. I know that tire pressure increases roughly 1 psi for every 10* increase in ambient air temp as the day heats up. I also know that tire flex while running generates heat; for example, TireRack ran some tests on car tires in their lab and found that tire flex contributes 4 psi.

Typically, my TST tpms indicates that the tire on the shade side of the trailer will be running at 55-56 psi and the sun-side tire 1-2 psi higher. However, on a recent trip, the tires' cold psi of 50 had been set when the temp was 53*, and as we got into 90*+ ambient temp, the sun-side tire got up to 60 psi. As we get into summer and 100*+ ambient temps, I can see the tires going above 60 psi.

My question: is there an upper psi threshold I need to be concerned about? How much psi is too much when running? Is there an optimal running psi range I should try to stay within?

Looking forward to some knowledgeable resposes.

Bruce

[Bruce_H_]

Thank you, CapriRacer. Very helpful info.

Bruce

[CapriRacer]

Bruce H. wrote:
Thank you, CapriRacer, I was hoping you would respond.....

Quite welcome.

Bruce H. wrote:
...... I had to read your response 5 times before this non-engineer's mind could perhaps comprehend what you were saying. 🙂

Let me see if I got it right. After deducting the increase in psi due to increased ambient air temperature, the remaining psi increase should be within 10% of the starting psi, 15% tops. If over that, it is due to too much tire flex, so add air pressure to reduce tire flex and thereby reduce heat and accompanying psi increase due to tire flex.

Hope you will confirm or straighten me out on my layman's interpretation!

Thanks again,

Bruce

You have it exactly right - with one provision.

You need to be aware of the limitations of the tire. So if you have a tire with a maximum allowable pressure of 65 psi, then you should be wary of exceeding that value. You might be able to exceed the max to get you to a place where you can deal with it (say no more than 100 miles), but this should be a MacGyver kind of thing and not to be relied upon.

[Bruce_H_]

CapriRacer wrote:
As a general rule, you want the pressure buildup to be less than 10% of the starting pressure. If you exceed 15%, you should take immediate action - typically adding pressure. Between 10% and 15% you need to consider what possible future action should be taken, but you don't need to react immediately. (But don't put off the decision for more than a few days of operation.)

So in your example, your 53 psi starting point could go as high as 58 or 59 before your should be concerned. If you add 4 psi because of the difference in ambient, then the pressure could be as high as 63 psi. So it sounds like you are OK.

Thank you, CapriRacer, I was hoping you would respond. I had to read your response 5 times before this non-engineer's mind could perhaps comprehend what you were saying. 🙂

Let me see if I got it right. After deducting the increase in psi due to increased ambient air temperature, the remaining psi increase should be within 10% of the starting psi, 15% tops. If over that, it is due to too much tire flex, so add air pressure to reduce tire flex and thereby reduce heat and accompanying psi increase due to tire flex.

Hope you will confirm or straighten me out on my layman's interpretation!

Thanks again,

Bruce

[CapriRacer]

Bruce H. wrote:
My ST 215/75R14 Maxxis M8008 tires are set to 50 psi cold in the morning. I know that tire pressure increases roughly 1 psi for every 10* increase in ambient air temp as the day heats up. I also know that tire flex while running generates heat; for example, TireRack ran some tests on car tires in their lab and found that tire flex contributes 4 psi.

Typically, my TST tpms indicates that the tire on the shade side of the trailer will be running at 55-56 psi and the sun-side tire 1-2 psi higher. However, on a recent trip, the tires' cold psi of 50 had been set when the temp was 53*, and as we got into 90*+ ambient temp, the sun-side tire got up to 60 psi. As we get into summer and 100*+ ambient temps, I can see the tires going above 60 psi.

My question: is there an upper psi threshold I need to be concerned about? How much psi is too much when running? Is there an optimal running psi range I should try to stay within?

Looking forward to some knowledgeable responses.

Bruce

Bruce,

As a general rule, you want the pressure buildup to be less than 10% of the starting pressure. If you exceed 15%, you should take immediate action - typically adding pressure. Between 10% and 15% you need to consider what possible future action should be taken, but you don't need to react immediately. (But don't put off the decision for more than a few days of operation.)

So in your example, your 53 psi starting point could go as high as 58 or 59 before your should be concerned. If you add 4 psi because of the difference in ambient, then the pressure could be as high as 63 psi. So it sounds like you are OK.

[Dog_Folks]

If you really want to get into the details, there should be an online chart for your brand of tires, that takes in numerous factors, such as temperature, load carried, etc. and then will give you a recommendation.

[Turtle_n_Peeps]

Bruce H. wrote:
If I remember correctly, the TST tpms installation literature recommends setting the high pressure alarm at 20% higher than the cold psi. That equates to 60 psi for my tires (I was expecting it to alarm when the one tire hit 60 psi but it didn't). The obvious implication is that TST considers a psi increase of greater than 20% over cold psi to be a safety issue.

If the consensus is that a higher psi range is acceptable, I will reprogram the TST system to alarm at a higher setting, say 30% over cold psi.

Bruce

As stated, don't worry about the high pressure warning. Just set your alarm for low pressure warning and go on vacation. This from a guy that takes dozens and dozens of tire temp and pressures on the weekend. 🙂

[Turtle_n_Peeps]

Bruce H. wrote:
If I remember correctly, the TST tpms installation literature recommends setting the high pressure alarm at 20% higher than the cold psi. That equates to 60 psi for my tires (I was expecting it to alarm when the one tire hit 60 psi but it didn't). The obvious implication is that TST considers a psi increase of greater than 20% over cold psi to be a safety issue.

If the consensus is that a higher psi range is acceptable, I will reprogram the TST system to alarm at a higher setting, say 30% over cold psi.

Bruce

As stated, don't worry about the high pressure warning. Just set your alarm for low pressure warning and go on vacation. This from a guy that takes dozens and dozens of tire temp and pressures on the weekend. 🙂

[Bruce_H_]

If I remember correctly, the TST tpms installation literature recommends setting the high pressure alarm at 20% higher than the cold psi. That equates to 60 psi for my tires (I was expecting it to alarm when the one tire hit 60 psi but it didn't). The obvious implication is that TST considers a psi increase of greater than 20% over cold psi to be a safety issue.

If the consensus is that a higher psi range is acceptable, I will reprogram the TST system to alarm at a higher setting, say 30% over cold psi.

Bruce

[AirForceAngler]

What pappcam said. If they start getting 90+ psi, then you may have a brake dragging that is creating a lot of heat or some other problem.

[pappcam]

The tires are designed to run at 50psi cold pressure which means they are built to handle changes in pressure from heat from that starting point.

In theory anyway.