Tires getting hot

[bharrisv65]

I searched but didn't have anything come up regarding this so please forgive me if it's been covered before.

I bought the RVI brake system and Tire Patrol tpms system and so far it all seems to work great. The tire pressures seem to be about 2 PSI different than the gauge I use and I used two different gauges to ensure which is accurate. All reading are so close I don't think it's a big issue. The Tire Patrol has a high and low PSI limit setting that will cause an alarm if either setting exceeds the threshold you set. I initially set the upper PSI at 110 and low at 95 PSI. First trip out, once the tires heated up from driving, the PSI's all increased to 119-120 PSI which, of course caused the alarm to chime. No big deal but I just wanted to check and ask if this is pretty normal for tires with cold inflation of 100 PSI rising to 120 PSI when hot? Tire temps never seemed to go above 90 degrees, ambient temp was in the upper 70's. Thanks for any input.

[brucev]

I don't pay as much attention to actual pressure, rather it is the difference from tire to tire. I find the inside tires run a few PSI higher as do tires on the sun side. I have an IR temp gauge and check the tires and hubs when I stop. My TPMS is usually within a few PSI of the gauge.

[Oldchief]

We have an EEZ tpms on our Axis. Like others, above, I've noticed that tire pressures vary slightly from the manual tire gauge I use when I measure them. As previously, stated, no big deal. I have noticed, however, that our inside right rear tire gets and stays around 10-12 degrees hotter than the other tires when we're traveling. So far, it hasn't gotten hot enough to set off the monitor alarm but I have no idea why that one tire is running hotter.

[16ACE27]

Your alarm was set too low. For my coach which I run at 82 PSI cold I set the high alarm at 100 which usually never gets exceeded. This winter I topped off the tires for a trip when the ambient temperature was in the 30s. Worked fine for that trip. Next trip the ambient temperature was in the 70s -80s and the cold pressure was in the upper 80s PSI. I was too lazy to drop it before we left; after about an hour on the road I got my first high pressure alarm. 30 minutes later another one. The pressures never went over about 103 PSI but if I had dropped my cold pressures to where I normally have them there would not have been any alarms.

Another tip for those who live in the south: I was out in my yard in the morning a couple of weeks ago near where I park the RV and heard the TPMS alarm going off. I had forgot to turn it off but the reason it was alarming is the temperature of the east facing outer rear wheel was indicating over 150 degrees even though ambient temperature was in the 60s. The early morning sun shining at the wheel simulator was like a dish with the sensor close enough to the focal point to really heat up. This can't be good for it so now an appropriately sized piece of plywood blocks that wheel from the morning sun.

[bharrisv65]

Originally Posted by Tfryman

Your alarm was set too low. For my coach which I run at 82 PSI cold I set the high alarm at 100 which usually never gets exceeded.

Thanks for sharing that input. Based off simple math, your tires go up less than 18 PSI, mine go up 20+ PSI. I originally set the high limits at an arbitrary +10 PSI as a starting point. I increased that to +15 PSI as they heated up while driving but I became a little concerned when the PSI's all went above 120 PSI, highest was about 121 PSI. I checked all tires before leaving - with a gauge which read 100 PSI and the Tire Patrol reading 102 PSI, so based off that, the tires went up over or around 20 :SI. This may be okay and really, I'm not too concerned about it because I was driving normal on good roads and tires with only 7,000 miles on them. I just wanted to ask if anyone else has experienced this just as a little additional comfort that it's normal.

Regarding the sun shining on the wheels, I bought tire covers which should be good to keep that "magnifying glass effect" from happening.

Thanks for replying Tfry, safe travels!

[Chance]

Originally Posted by bharrisv65

Thanks for sharing that input. Based off simple math, your tires go up less than 18 PSI, mine go up 20+ PSI. ....cut....

Not sure what you mean by simple math, but I think a temperature rise from 70 to 90 F should not make the pressure go up more than a few PSI (much less than your 20 PSI difference).

I’m not familiar with your sensors, but would imagine that reading the correct pressure is easier than reading the actual tire temperature. In any case, I would expect a loaded-down truck/RV tire to warm up much more than 20 F above ambient.

[JamieGeek]

Originally Posted by Chance

Not sure what you mean by simple math, but I think a temperature rise from 70 to 90 F should not make the pressure go up more than a few PSI (much less than your 20 PSI difference).

I’m not familiar with your sensors, but would imagine that reading the correct pressure is easier than reading the actual tire temperature. In any case, I would expect a loaded-down truck/RV tire to warm up much more than 20 F above ambient.

So far with my fun with TPMS project (see here: http://www.thorforums.com/forums/f27...red-11423.html ) I've measured a PSI rise of about 10 on my first 90 minute drive in the RV while recording data (ambient temps at the time were around 60 degrees, my sensors are at the end of the valve stems so I wouldn't expect them to be able to measure the tire temp--seems more like they pick up the sun than tire temp).

[Chance]

A sensor at end of valve isn’t going to read internal tire temperature very accurately, if at all. If he’s actually seeing +/- 20 PSI rise, I’d start worrying about actual tire temperatures getting too hot.

I just estimated temp/pressure rise based on perfect gas and constant volume. In reality tire volume goes up as pressure climbs, so temperature would have to go even higher to get calculated pressure rise.

[petefoss]

Originally Posted by Chance

A sensor at end of valve isn’t going to read internal tire temperature very accurately, if at all. If he’s actually seeing +/- 20 PSI rise, I’d start worrying about actual tire temperatures getting too hot.

I just estimated temp/pressure rise based on perfect gas and constant volume. In reality tire volume goes up as pressure climbs, so temperature would have to go even higher to get calculated pressure rise.

PV=nRT!

[Chance]

Exactly. To see pressure go up about 20%, the internal temperature of the tire would have to go up around 100 degrees F (more or less). Certainly a lot more than 20 degrees F.


P.S. — That assumes perfect gas, which excludes tire being partially filled with water.

[JamieGeek]

Originally Posted by petefoss

PV=nRT!

Side note, a conversation from my high school chemistry class:

Student: Does R change much over time, or does it stay the same?

Teacher: No, that is why we call it a constant.

[petefoss]

FYI to the non geeks out there, Temperature in this case is Kevin NOT Centegrade or Farenheit!

[Metalgator]

I also have the RVI system and usually keep my tires around 95psi cold. I. have seen the psi rise to around 110-112 after a few hours of driving here in Florida during the summer.

[Chance]

It’s also interesting to think about the fact that the rolling resistance of tires uses a lot of engine energy (hence why it lowers fuel economy), and that much of that energy is converted to heat within the tire itself as it flexes with each revolution.

Anyway, at steady state (after a few hours or less of driving when tire temperatures level off) heat generated within tire must travel to outer surface of tire so it can transfer to the surrounding air, road, etc. I would expect some heat is transferred to air inside tire, which can then transfer to rim (assuming rim isn’t already warmer due to heat from brakes). In any case I would expect that this transfer of heat to inside of tire is minimal, and that that would make the inside surface of the tire, and also the air inside, to be as warm as any part of the tire.

My point in this long post is to say that while the outside temperature of the tire can be felt by touching or measured with instruments, it doesn’t necessarily tell us just how hot the inside of the tire may be. It’s very likely that it’s a lot warmer.

We’d also have to guess that thicker heavier-duty truck and RV tires could have a much greater temperature difference between inside and outer surfaces once steady state is reached.

Also, while on subject of dumb and time-wasting things to think about, it’s also interesting to contemplate that large RV tires have less surface area for cooling relative to the weight they carry compared to car tires. This along with thicker tire that should have greater resistance to heat transfer may explain why RV tire pressures and temperatures increase much higher than in typical auto applications.

For me all this makes a great case for smaller and lighter RVs with smaller tires that run with less pressure.

[Chance]

Originally Posted by petefoss

FYI to the non geeks out there, Temperature in this case is Kevin NOT Centegrade or Farenheit!

Yes, and pressure must be on absolute scale also (PSIA will do). For comparison temperature can also be in degrees Rankine (for us older guys that relate to F scale better).

[Druff]

Originally Posted by Oldchief

We have an EEZ tpms on our Axis. Like others, above, I've noticed that tire pressures vary slightly from the manual tire gauge I use when I measure them. As previously, stated, no big deal. I have noticed, however, that our inside right rear tire gets and stays around 10-12 degrees hotter than the other tires when we're traveling. So far, it hasn't gotten hot enough to set off the monitor alarm but I have no idea why that one tire is running hotter.

On our class c it was because the exhaust runs right behind that tire. Keep an eye on that tire it is also the first to go bad.

[captmetal]

to all the experts on tire pressure, what was forgotten was heat input from brake use, the brakes create heat when resisting rotational force, a lot of this heat is transferred from the disc brake rotor to the aluminum wheel that is mounted to it, so what do think will happen to the tire mounted to the hot wheel??

[Chance]

Originally Posted by captmetal

to all the experts on tire pressure, what was forgotten was heat input from brake use, the brakes create heat when resisting rotational force, a lot of this heat is transferred from the disc brake rotor to the aluminum wheel that is mounted to it, so what do think will happen to the tire mounted to the hot wheel??

Wasn’t really forgotten, as mentioned a couple of posts above, because it’s common knowledge that brakes can affect temperatures. However, I think that motorhomes don’t use brakes like race cars for long periods of time, except when descending mountains. Hopefully skilled motorhome drivers won’t ride brakes for long periods, because they would fade. Even when brakes get hotter than usual due to descends, the RV is likely traveling at much lower speeds, which means heat from rolling resistance of tires would be much lower. Basically there should be an offsetting effect on most descends (more braking but less average speed). I think it’s highly unlikely that a driver would ride the brakes and be traveling at high speeds at same time (like 70 MPH or so). If that were to occur, the driver would have much bigger problems than reducing tire life.

Originally Posted by Chance

.....cut.... which can then transfer to rim (assuming rim isn’t already warmer due to heat from brakes). ....cut....

[lwmcguire]

You guys that are getting the huge temperature rise and pressure increase better weigh your rigs. Sounds to me like your tires are overloaded or are very near to being.

I have had hot tires before but only when overloaded or underinflated.

[Fulltimer50]

A change in altitude will also change pressure readings.