Tire pressure (temperature) increase while driving

[WoodIsGood]

What is considered an excessive temperature for tires? If I set my tire pressure to 5psi higher than the manufacturer's recommendation, when driving 65mph on a hot day (100+*) the pressure of the rear tires will increase by as much as 30psi (go from 85 to 115psi). At 10 degrees per 2psi increase that's a 150 degree increase in internal tire temperature. Starting at 75* the tires will reach 225* internally! This is true with both the original Goodyear G670RV and new Michelin XRV tires in size 245/70R19.5-F. This seems excessive to me, so I end up running the tires at the max recommended pressure (95psi, or 15psi higher than the chart recommends). This cuts the pressure increase while driving in half, but results in a harsher ride.

I've weighed by coach several times over the years on various truck scales (rear axle always less than 13,000 lbs. with less than 200 lbs. difference side-to-side). I've checked my pressure gauge against 5 other gauges and all were within 1psi of each other. I use a TST TPMS system (interestingly all 10 of the sensors read 3psi lower than all of my gauges at all pressures from 35psi to 115psi.).

Is a 30psi (and corresponding 150*) increase normal and I'm worrying about nothing? The front tires never increase more than 15psi.

[CapriRacer]

seekingsunnyskies wrote:
Sorry for the delay. We recommend 10% and 20% above. By going 25% above we will not void a warranty its up to you. Typically you should see 15 to 20% higher psi when tires are hot, and 14 to 22% above ambient on temp.

Feel free to contact me with any questions at 210-420-0132

Thanks,
Mike Benson
TST TPMS
210-420-0132

Mike,

I'm sorry to disagree with you, but both Roger (Tireman9) and I think 10% is a maximum, and anything above 15% is a call for immediate action.

Perhaps your experience is why there are so many problems with ST tires - they are just running too hot because the tires don't have enough load carrying capacity.

And 25% will not void the warranty? May the TPMS warranty will not be voided, but this level will void the tire warranty - and, Yes!, it can be detected when the failed tire is examined.

[TDInewguy]

You may have a dragging brake or axle bearings that are overheating.

Check the lube in your rear axle, and check your brakes.

Rear axle bearings can fail and they can get hot enough to start things on fire...

[CapriRacer]

Blaster Man wrote:
You are getting lots of information, much of it incorrect. There is a tire expert on the forum, maybe he'll come on and give some good information...he was a tire dealer for 40 years and gives seminars at many rallies. His advice is to check the weight on the chart, and if using individual wheel weight, add 10% or 10 lbs to the pressure noted in the manual for the heavy side, and use that pressure for all tires on that axle. If not using individual wheel weight, divide the total weight on the axle by two, and add 10% or 10 lbs PSI to the pressure noted in the manual to both sides...same pressure for all tires on that axle. In either case do all these reading prior to driving, either the day prior or morning before starting.

I think you are referring to Tireman9. He was a tire engineer for a major tire manufacturer - as was I. We've compared notes many times, and I consider him quite knowledgeable and practical.

The problem is that the moderators on this site (and others) object to the fact that his website has advertising on it, so he has been banned several times. This is in spite of the fact that he himself doesn't sell or promote anything. He is just trying to reduce the expenses of running a website.

His website is: RV Tire Safety

And what you've described is the way to properly size a tire for any vehicle. I may not agree with the exact numbers, but they are in the ballpark.

Note for those following this thread: Just taking the value from the tire load charts is a guarantee you are going to have overloaded tires - and overloaded tires fail!! - sometimes catastrophically. The weights need to be adjusted for side to side and front to rear load variation - plus, every tire should be operated with a *reserve (unused) load capacity*. I recommend 15%, but that's just me. Also note that there were (are?) a few trailer manufacturers who produced trailers with - ah, let's just say less than that.

[Blaster_Man]

You are getting lots of information, much of it incorrect. There is a tire expert on the forum, maybe he'll come on and give some good information...he was a tire dealer for 40 years and gives seminars at many rallies. His advice is to check the weight on the chart, and if using individual wheel weight, add 10% or 10 lbs to the pressure noted in the manual for the heavy side, and use that pressure for all tires on that axle. If not using individual wheel weight, divide the total weight on the axle by two, and add 10% or 10 lbs PSI to the pressure noted in the manual to both sides...same pressure for all tires on that axle. In either case do all these reading prior to driving, either the day prior or morning before starting.

[rgatijnet1]

Tire heat is generated not only by outside air temperature, road surface temperature, but also by friction of the tire against the road surface. Overinflating the tire will mean less road surface contact so it makes sense that there will be less temperature rise.
On the other side, a tire that is overinflated will ride rougher, provide less braking resistance and can be dangerous to the handling of the coach, as well as wearing out quicker in the center of the tread.
Follow the manufacturers chart for setting your tire pressure with the vehicle filled with FULL fuel, as well as loaded the way that you travel with all occupants and supplies. If you travel with water and waste liquid, make sure that is in place when you have the vehicle weighed.

[WoodIsGood]

All I could afford wrote:
Please double check your weights, weight distribution and pressures... Something just doesn't seem right there.

I checked axle weights on a certified CAT truck scale that day and the rear axle was 12,820. Later in the trip I encountered an unmanned weigh station scale in Idaho. I drove across 3 times (stopping for each weight to register properly) and checked right, left, and axle; rear axle weight was exactly the same at 12,820 and read 150lbs. different right to left.

In case anyone is wondering, the rear tires wear fine. There is no cupping, rivering, odd wear patterns, or premature wear, so it isn't an alignment problem (and yes, the alignment was checked at a heavy truck shop a while back).

Front tires I have no problem with; just the rears that heat up and increase dramatically in pressure.

[BigSkyBob]

STBRetired wrote:
I run Michelin XRVs on my MH. Fronts at 90, rears at 94 when cold, which is 3 PSI above what the weight charts say. After driving for several hours at expressway speeds, fronts are 104, rears are 108. So a 14 PSI gain. 30 seems excessive.

This has been my experience as well. About a 14 PSI increase, a bit less on the shaded side on the rig.

[Bikeboy57]

As a Chemical Engineer, I thought at first glance that your calcs must be off. So I dusted off the ideal gas law equations and ran them for myself. I confirmed your temperatures for a given pressure rise.

I see pressure rises on my TPMS but not quite that extreme. On the other hand I haven't driven across the desert.

Thinking about this some more, and knowing I never see tire temperatures over about 140 using an IR gun, it hit me that one thing we haven't accounted for is the ambient air rushing over the tire and cooling it as we run down the road.

Conclusions, one the air temperature may indeed be that high, but the rubber is cooled by the ambient air. Two, You did the right thing by adding some air to prevent a problem on the road.

[All_I_could_aff]

Just for comparison Example of pressure increase on the road... But keep in mind this was my single axle hybrid trailer the first season after I bought it used...
with factory spec 175/80/13 Load C rated and running at 50psi, temperatures after a two hour highway run never exceeding 65 mph... Pressure reached 62-64 psi when I checked immediately after stopping.

That's about 25% increase, which seemed high to me, and lead me to stop at a scale on our way home. It turns out each tire was approximately 15% over loaded.
Please double check your weights, weight distribution and pressures... Something just doesn't seem right there.

[JaxDad]

WoodIsGood wrote:
I added air as I believed (and still believe) that they were heating up excessively and didn't want to overheat and damage $2K worth of brand new tires.

Michelin's chart lists the minimum pressure for a given carry capacity. Adding 10 psi just made the volume of air in the tires roughly the same as if I'd started the trip at 95 psi (the sidewall listed max cold pressure). By the time I was rolling down the highway again the pressures were all at or below what they were when I stopped (because the tires had cooled down some). And they continued to drop slightly until settling in at about 108-110psi. In the morning they were all within 2psi of 95psi (verifying my above claim about air volume). I equalized pressures to 95 before resuming my trip.

Thermodynamics is a pretty solid science. At 80-95psi a 10* increase in temperature results in a roughly 2psi increase in tire pressure (at 30-35 psi starting pressure the same 10* increase in temperature results in a roughly 1psi increase in tire pressure). Since the hubs and brakes weren't heating the wheels (yes, I checked them for excess heat) the source of the heat for the air inside the tires had to be from the rubber. And the rubber had to be at least as hot as, if not hotter than, the air inside the tires. And based on the applicable thermodynamic equations, the air was around 250*. That just sounds too darned hot to me.

Be careful making statements like that, the 'tire pressure posse' will show up! LOL.

It's rather surprising the numbers of people who take those pressure charts as gospel and will go so far as describing 'miraculous' improvements in ride and handling by doing so.

The other issue is that as you mentioned tire heat is very much related to pressure. That heat is also very related to tire failure. If you develop a slow leak while rolling down the road, the lower the starting point, the sooner the tire will reach a critically overloaded and overheated point and fail.

[CapriRacer]

Just an old rule of thumb in the Tire Industry:

You want no more than 10% increase in pressure between *cold* and operating pressure. This does not include any pressure increase due to ambient temperature increases, nor those from elevation changes.

If you experience more than that you need to make some changes. Temporarily, slow down. That lowers the rate of heat generation. For the long term, a tire with a larger load carrying capacity - meaning more pressure and/or a size increase.

If you experince more than a 15% increase, you need to do something immediately.

[WoodIsGood]

Tyler0215 wrote:
Why did you add more air to the tires?
If the Michelin chart says 85 psi for the weight you are carrying, thats the pressure you should run. Don't be adjusting the pressure when the tires are hot.

I added air as I believed (and still believe) that they were heating up excessively and didn't want to overheat and damage $2K worth of brand new tires. Michelin's chart lists the minimum pressure for a given carry capacity. Adding 10 psi just made the volume of air in the tires roughly the same as if I'd started the trip at 95 psi (the sidewall listed max cold pressure). By the time I was rolling down the highway again the pressures were all at or below what they were when I stopped (because the tires had cooled down some). And they continued to drop slightly until settling in at about 108-110psi. In the morning they were all within 2psi of 95psi (verifying my above claim about air volume). I equalized pressures to 95 before resuming my trip.

Thermodynamics is a pretty solid science. At 80-95psi a 10* increase in temperature results in a roughly 2psi increase in tire pressure (at 30-35 psi starting pressure the same 10* increase in temperature results in a roughly 1psi increase in tire pressure). Since the hubs and brakes weren't heating the wheels (yes, I checked them for excess heat) the source of the heat for the air inside the tires had to be from the rubber. And the rubber had to be at least as hot as, if not hotter than, the air inside the tires. And based on the applicable thermodynamic equations, the air was around 250*. That just sounds too darned hot to me.

[wolfe10]

Ivylog wrote:
It's normal to have a 10-15% increase in pressure on a hot day BUT you are getting a 30+ % increase. I would try +10 psi.

I agree.

Also, check and make sure the brakes are not dragging, creating heat. Not rocket science-- just put your hand on the center of the hub after driving. Should be warm, but certainly not hot enough to burn your hand unless you just completed a panic stop.

[Ivylog]

It's normal to have a 10-15% increase in pressure on a hot day BUT you are getting a 30+ % increase. I would try +10 psi.

[jadatis]

What counts is the temperature of the rubber that is not allowed to go over a sertain temperature.
The tire is warmed up bij the deflection and flexing back of every segment of it, about 10 times a second at 65m/h. The higher the speed the more heatproduction a nd the more deflection also more heatproduction a cycle.
Cooling down is dependent on temperature differences between rubber and in- and outside- air . so inside air temperature so is in a way related to the temp of rubber.

But your pressure risisng could be explained that high because it was measured riding 65m/h at an ambiënt temp of 100degrF+, while they where filled at 85psi at an abiënt temp of 65 degrF.
Will give a list I made , where the advice pressure fat printed at 65degrF.
Then you can see what 105 psi will give for inside tire temperature.
Estimate that if tire inside gets about 115 degr if ambiënt temp is 65 degr F, when driving about 55m/h.

Then I read in my tabel that 85 psi filled at 65 degr F, gets 50 degrm hotter, then when ambiënt temp is 100 degr F , tire inside will sertainly not get hotter then 145 degr F . 140 is hottest in my tabel.
Then it reads about 99 psi, so 105 for 65m/h can be OK.


End conclusion from me , for what its worth, you most likely have the right pressure for your weight and speed.