Question for LonnieB

[bncinwv]

Lonnie,
I recently had an experience with tire pressure readings and temperature on our 614's and the TST TPMS that I am hoping you can provide some guidance on. I had set the TPMS to alarm at 120 psi and had inflated the tires to 105 psi cold. Upon departing for the rally, the temperatures on the tires started climbing and seemed to stabilize around 10 to 15 degrees above the ambient temperature of around 90. When the tire temperatures climbed, the tire pressures climbed accordingly which seemed odd. The pressures went above 120 which resulted in the alarms going off. I adjusted tire pressures to keep the alarms from going off. My question is what is the allowable pressure climb on a tire?? Is a max 120 okay for a 110 psi tire? Is 130?? I am sure there is some safety factor built in and am wondering if your expertise can us what is the safe upper limit. Thanks in advance for your response.
Bingo

[LonnieB]

Good question Bingo.

As the ambient temperature climbs, air expands. Being confined inside the tire, there is nowhere for the expanding air to go, so the pressure naturally increases. This works in reverse also, as the ambient temperature decreases, the air contracts lowering the pressure in the tire. If you want to conduct a fun little experiment, do this. Get a metal can with a metal screw on cap, remove the cap and place the can on a burner for a few minutes. Once the can is hot, screw the lid on and remove it from the burner. As the air inside the can cools, the can will crumple in on itself.

Tires are designed to withstand the increased pressures, up to a certain percentage. I'm sorry to say, I don't know what that percentage is. On a tire rated for 110 psi, I don't think I would worry about a 10 pound increase to 120 psi. Anything over 120 psi would concern me enough to slow down, or even stop for awhile and let the tires cool. Lowering the psi in the tire, and continuing to travel, can cause the tire to become hotter than it already was. Excessive heat is a tires worst enemy, and will likely lead to catastrophic failure.

[bigred715]

Good question Bingo, I've always wondered myself about the increased tire pressure and temperature. During hot weather I am especially concerned because I have the Goodyear Marathon tires. Constantly wathing the tire monitering system.

[Delaine and Lindy]

Bingo my tires tend to increase about 10% and that also depends on what the temps are. We have been in Colorado, Red River New Mexico and Taos New and the tire pressure does change. And if you have a Sleep number bed be sure to let the pressure down when you go up in elevations, we have been in 10,000 ft plus above sea level. Happy Trails...

[HamRad]

Bingo,
Lonnie, of course, has already given you the info you need. The last thing you want to do when a tire starts alarming is to reduce the pressure by letting out air. As Lonnie said this can/will cause the tire to run hotter! Of course as you know temperature and pressure are directly related. More heat equals more pressure.

On a 110psi setting I'd think about 130 to 140 would be about right for alarm trigger. Perhaps someone on here has the same setup will be able to give you their experiences. For low alarm maybe an 90 or 80 setting. My Pressure Pro system has a built in percentage for both low and high alarms. I think we can adjust them if we need/want to do so but so far have had no reason to do so. So since I have NO experience with your system you should take my advice for what you paid for it.

Over on the Escapee's Forum they have had and continue to have great discussions on this very issue. Good reading. Good luck, Dennis.

[Art-n-Marge]

I have always thought of this since I've heard of temperature monitoring on a TPMS. There is information on how to adjust psi when ambient temperatures are high or low (desert like temps and freezing cold environment) but I have not seen a comparable table for what to expect as a tire's temperature rise from friction of a tire being used, versus what it's temperature is expected to be. Then this temperature can also be affected by the current ambients temperatures from the road or ambient air - driving through hot or cold climates.

Like LonnieB alludes, what we don't know is that the tire engineers have designed in a starting point for PSI and how to adjust for temperature as an adjustment. This, by no means, suggests that the pressure and temperature will remain so from driving and actual numbers might complicate things, a lot. There's plenty of knowledge indicating changes occur and that maintaining the PSI keeps it within the "desired range". A tire company might want to keep these close to the vest for competitive reasons. I wish more could be published how temperatures affect PSI in real numbers but then you'll have to know these for each tire, too. I suspect that a tire maintained for 32psi will go through a range of psi/temp changes, and a tire at 110psi will probably go through psi/temps much comparitively different. The exact numbers is what we're missing for temperature ranges compared to driving temps, and ambient temps and I fear this would be way too expensive to produce and make practical. So we must try and use what works for us.

I guess what I should be doing is buy a infrared device and monitor my tire temps when driving around. Then when I get my TPMS, that does temperature monitoring, I'll have something to go by. In the meantime, I'll have to depend on my religiousness about maintaining tire pressure at 80psi exclusively and look for tire wear. Unfortunately, my trailer mostly sits (no outings planned) and this is probably worse for my tires anyway (flatspotting and aging). Awareness and continued education is probably the best medicine, but still no guarantees. At least, I think my abuse, neglect and ignorance days are behind me, regarding tires, anyway.

Better yet, the advantage for MOCers, is posting what each person uses is a start and I thank all who have posted their numbers so far!

Great topic! It keeps the thinking juices flowing on how to stay ahead of problems. That can't be a bad thing.

[sreigle]

Does increase in elevation increase or decrease tire pressure?

[Red939]

At higher altitudes, the same amount of air in a tire filled at sea level will have more pressure because the outside force is reduced due to the lower air pressure at higher altitudes. Bottom line, increased altitude increases tire pressure.

[ole dude]

The big boys down at the pool room tell me that a tire at 193 degrees will be O K, a tire at 194 degrees will pop. On motorcycle tires a 4lb increase in pressure is acceptable, as I understand. Don't know about larger tires, what is O K. I do notice mine will gain 8 to 10 PSI from when I take off in the A M at say 74 degree ambient, to when the ambient reaches 100 degrees.

[Waynem]

Steve,
Altitude and temperature do affect tire pressure. For every 10 degree change in temperature you will experience approximately a 1 psi change in pressure.

Now, consider that the ambient temperature when you fill up your tires is 14.7 psi, and the molecules going into the tire are 14.7 psi. That is the atmospheric pressure at sea level. Now consider traveling 10,000 feet up. At that altitude the atmospheric pressure is about 10.1 or so. However, the molecules inside the tire will still be in a contained space, about 14.7 psi, so as altitude increase, ambient atmospheric pressure decreases, but the molecule structure in the sealed space will be stil around 14.7. psi and cause an increase of pressure when using an air gauge which works off of the ambient temperature. Confused yet?

Now couple that fact that as temperature decreases, most likely due to the higher altitude in this case, for each 10 degrees of temperature decrease there will be about a 1 PSI decrease in temperature.

So in many instances the offset of altitude change will be compensated by the temperature change.

I go by the Michelin load chart tables since I have Michelin tires. Goodyear also has a load chart table. I set my tires to accept the minimum inflation as prescribed by the load chart, and then I increase it one load chart factor. So, if my tire calls for 95 psi, let's say for a load of 4000 pounds per tire on the front axle, and the load factor of 100 psi is good for 4500 pounds, and 110 pounds is good for 5000 pounds, since 110 is my maximum pressure for my tire, and 90 psi is my minimum, I'm happy to set it between 95 to 100 psi as a fudge factor. I can loose 5 psi at 95 psi and still be within load inflation ranges, or I can gain 5 psi and still be with in load range. I don't have to worry about being below minimum or above maximum.

Anyhow, that's my story, and I'm going to stick to it until a better version comes along.

Happy trails (and HELP Lonnie B - especially if I'm all wet)

Edited: All of my inflation are based on first thing in the morning temperatures with a cold tire.

[sreigle]

Thanks to all.