Tire pressure

I agree with capri-racer that , because it even needs higher pressure then 90 psi to get a low enaug temperature-rising, this can only be done by ST tires with higher maxload-pressure then 80 psi in same sises, or LT tires with a same or bit higher maximum load then the ST tires, but then need even more pressure needed for the maximum load.
Can be that you have to go to larger tires.
And a higher maxloadpressure means a stiffer tire, so needs more pressure even for the same load .
Could be ending then in pressures of over 100 psi .

Also another problem can rise.
Your TMPS might go only to 100psi . If then it goes over it, it will give for instance at 101 psi a pressure of 1 psi and then the alarm for to low pressure goes off. Then you yourself know its OK and you mayby yust shut down the low alarm and monitor the pressures and yudge yourselfes if they are to low.

I asume you have metal valves wich can stand a pressure of over 145 psi so those wont give problems.

Dave H M wrote:
Jadatis, you are really in there when it comes to the scientific end.

So with all that being said, are you indicating that I am toying around with the so called "critical mass" here.

I was planning to run those tires up to the 5 year mark and then go the LT replacement route.

Depends if you had tire-failures whit this 5th wheeler in the past.
Id you have it long time, with no tire-problems, these temperatures are probably allowed and it is because of the more deflection of a ST tire.
But I you had problems I would try the 90 psi.
And if your tires are pretty new with low milage on them , nothing stands in the way of putting 90 psi in, so its a free try.
But for this I need tire maximum load and pressure given for that on sidewall. From 5th wheeler the GAWR's and configuration( number of axles and tires) . If you have ever weighed , the axle or beter seperate wheel(pair) weights are also handy .
I suspect the 92 psi tire warm has the highest load on it.

Dave H M wrote:
tires are ST235/80/R16

That 92 was fairly constant but just one one tire - occasionally would drop to 91 for a while. Rest of the tires ran 89-90. No braking, running at speed on the road. I tow at 65.

I did not get that scientific on the cold pressure. That one 92 degree tire could have been 1-2 lbs higher when cold. I have not been using a digital pressure gauge for cold airing up.

OK, so it does sound like the tires need to be adjusted in some way.

As a start, I'd recommend you get scientific - and I's also recommend using 90 psi and see how much build up you get.

But I think the real answer is using a larger tire size.

Jadatis, you are really in there when it comes to the scientific end.

So with all that being said, are you indicating that I am toying around with the so called "critical mass" here.

I was planning to run those tires up to the 5 year mark and then go the LT replacement route.

OK then the pressure rising by temperature rising of your 5th wheeler tires is from 50degr/80psi to 109degr/91psi and 115degr/92psi. This all determined with my spreadsheet as tool.
this only courced by the about 10 to 15 times a second deflecting and flexing back of everey segment of the tire-sidewall and a bit heatening the treath by the resistence treath/road.So no extra heating by braking.
this is a heatening up of between 59 and 65 degr , while it should be only 47 degrees, as far as I concluded.

Now I state that the temperature rising goes linear with the deflection of the tire, and I found a relation between the deflection and the surface on the ground, if surface gets 2times as small the deflection gets 2*2 times so 4 times as small so quadratical.

These ST tires have more loadcapacity mainly because for lower speed of 65m/h there is more deflection allowed, wich will give more heat-production, if you drive the same speed. So I state 2 times as much deflection gives 2 times as much degrees temperature rising. All verry discussable I know.

Now in other topics I already found proove that for ST tires also 10 psi extra above the pressure written on sidewall is allowed by TRA.
It is adviced for higher speed then 65m/h, but now we use it for lower temperature.
So I calculated what would happen when you would put your tires at a cold pressure of 90 psi instead of 80 psi wich I asume is written on your tires so they are E load/10PR tires .
You must be carefull with that , because if your tires are already damaged by to much deflection , wich I expect because of the temperature rising, they will still blow-up soon and you will blame it to the high pressure. But if you only have then for a 1000 miles under the 5th wheeler you can do it to my opinion. When the tires are new and so not damaged, the 90 psi is sertainly ok.

When cold pressure 90 psi its 90/80 times the old pressure, and so the surface on the ground is 80/90 times the old surface.
square(80/90)=0,79 x deflection old so temperature chanche old
So 59 degrees gets 46,61 degrees
And 65degrees old gets 51.35 degrees higher temperature warm then cold what the temperatures will get in the new situation.
This is still somewhat high but about that 47 degrees I determined to be ok.
Then I filled this in in my spreadsheet and made a picture of it. fooled the programm a little to get all the temperatures in one picture, so swiched warm and cold .
Then the 90 psi cold filled will rise to between 99.6 and 100.5 psi when driving. To my opinion then we give the tires a deflection a LT tire would have gotten and so saver for the tires.


If you do this filling your tires to 90 psi,you would measure about these warm pressures and temperatures, and if so it prooves that the temperature rising is linear to the deflection and the deflection is quadratical to the surface on the ground.

But do you also have TPMS on the tow-vehicle you tow the 5th wheeler with. Because there the pressure are determined for LT tires the temperature rising and so pressure -rising should be lower.
Would be a good comparison.

tires are ST235/80/R16

That 92 was fairly constant but just one one tire - occasionally would drop to 91 for a while. Rest of the tires ran 89-90. No braking, running at speed on the road. I tow at 65.

I did not get that scientific on the cold pressure. That one 92 degree tire could have been 1-2 lbs higher when cold. I have not been using a digital pressure gauge for cold airing up.

CapriRacer wrote:

Dave H M wrote:
Just got my TST system cranked up and also just came off the road from a 920 mile run. Have 16's on the fifth wheel so I always start at 80 psi cold. It was generally around 56 degrees or cooler outside. One morning it was 40 outside and I noticed the tires generally had dropped from 80 to 78. The highest pressure indicated was 92 psi. Temp ran about 12 degrees higher than ambient.

I been wondering on this subject also so this was a good read.

I did not time it but sure was surprised how the tire temp and pressure quickly climbed close to the final running numbers when initially starting out.

The rule of thumb is that you'd like no more than a 10% pressure build up - and if you get 15% or more, you need to take action.

You are AT 15%, so you need to take some action to improve the load carrying capacity of the tires - which means a larger size or more inflation pressure.

You said you were using 16" tires at 80 psi, but you didn't say what size. If they are ST235/80R16 Load Range E's, I'd recommend you move up to the larger LT235/85R16's Load Range E's.

Could be that the 92 psi read by Dave H was after some severe braking.
The normal warm pressure could be in the 80 somewhere .
Then the pressure rising is pretty normal.
So to determine if the cold pressure needs to be higher or tires with higher maximum load should be used, he should yust do 4 point weighing and calculate the pressure with a comfortable reserve.
So Dave, was this 92 incidental or constant when driving?
You also write that temps ran about 12 degrees higher then ambiënt.
So this is the inside tire temperature read from your TPMS?
Then this is way lower then the about 30 degrees I assumed for RV tires of these sises and so not that bad.
If you would have read the temperature when the pressure was 92 psi , it would prove to be higher. Filled 50degrF cold and 80psi cold in in my spreadsheet I mentioned , and at 92 psi it gave an inside tire temperature of 115 degrees, wich is 65 degrees hotter so not even that bad. Still think that 92 was exeptional and short time.

Played a little further with my spreadsheet and and when temperature goes from 56 to 40 degr , pressure lowers from 80 to 77.1psi, yust about what was measured, becouse the 78 can be a inacuracy of the device or rounding to 1 degree the TPMS mayby does.

Dave H M wrote:
Just got my TST system cranked up and also just came off the road from a 920 mile run. Have 16's on the fifth wheel so I always start at 80 psi cold. It was generally around 56 degrees or cooler outside. One morning it was 40 outside and I noticed the tires generally had dropped from 80 to 78. The highest pressure indicated was 92 psi. Temp ran about 12 degrees higher than ambient.

I been wondering on this subject also so this was a good read.

I did not time it but sure was surprised how the tire temp and pressure quickly climbed close to the final running numbers when initially starting out.

The rule of thumb is that you'd like no more than a 10% pressure build up - and if you get 15% or more, you need to take action.

You are AT 15%, so you need to take some action to improve the load carrying capacity of the tires - which means a larger size or more inflation pressure.

You said you were using 16" tires at 80 psi, but you didn't say what size. If they are ST235/80R16 Load Range E's, I'd recommend you move up to the larger LT235/85R16's Load Range E's.

stickdog wrote:

Popsie wrote:
Note that if you run your tires over inflated based on the load they carry, you will tend to wear out the center of the tread early, opposite for under inflated.

Unless you travel continually odds are your tires will need replacing due to age before they wear out.

For some reason, the RV community seems almost paranoid on the age of tires. I have a good friend who operates and rents equipment that includes dump trucks and other heavy vehicles. They replace truck tires when they can't be retreaded any more. He has twenty year old tires on some of his trucks. He says a good inspection will tell you when to replace a tire. The recap guy X-rays the tires to determine if the carcass is sound before retreading.

Just got my TST system cranked up and also just came off the road from a 920 mile run. Have 16's on the fifth wheel so I always start at 80 psi cold. It was generally around 56 degrees or cooler outside. One morning it was 40 outside and I noticed the tires generally had dropped from 80 to 78. The highest pressure indicated was 92 psi. Temp ran about 12 degrees higher than ambient.

I been wondering on this subject also so this was a good read.

I did not time it but sure was surprised how the tire temp and pressure quickly climbed close to the final running numbers when initially starting out.