tire pressure and extreme heat

Tires should be cold inflated to the tire's max. sidewall pressure in the morning before heading off or after at least 3 hours of sitting in full shade. Don't go by the placard figure. The tires may have been upgraded by an owner or the factory. We have factory optional LRD tires that should be run at 65 psi however the factory used their standard placard for LRC tires (50 psi). Using the placard psi in our case would result in serious damage and a blowout(s).

The term "maximum" used on the sidewall tires creates confusion. It is actually the temp. that the tire should be cold inflated to in order to carry the load rating indicated on the sidewall. Lowering the pressure can result in overloaded tires (not good).

Cold inflation pressure is the inflation pressure taken with tires at the prevailing atmospheric temperatures. Tire pressure only changes about 2% for every 10F change in temp. Running your tires low will damage them from overheating and is a good invitation for an eventual blowout. You should never bleed them down when they are hot. Tires are designed and tested to be able to withstand up to around 3 times the sidewall pressure so no worries about exploding tires from running in high ambient temps.

For some tires (only a couple I think) such as GY Marathons, you can raise the psi by 10 psi and be able to run at 75 mph max.

Go and google "RVtiresafety" and you will find your answer there. Lots of reading on anything you ever wanted to know about RV tires. Remember, ST trailer tires are more "delicate" in comparison to other tire types and you want to take all the recommended measures to take care of them to minimize premature wear and reduce the chances of a blowout. Heat is the biggest enemy of ST tires by under-inflating and/or running above 65 mph.

Did some calculating with use of my made spreadsheet.
Asuming inside tire is totally free of water ( not gas not liquid).

Asuming your AT-pressure to be 80 psi.

Then filled at 65 degr F at 80 psi will become 81.8 psi at 75 degr F and 86.3 at 100 degr F.

The other way around:
Filled at 100 degr F 80 psi.
Becomes at 75 degr F 75.7 psi
and at 65 degr F 74 psi.

So yudge for your selfes if that is worth it.

It would mean that if 80 psi is calculated to be the right presssure for your combination and situation and is meanth for that 65 psi, you need to fill it cold when ambiënt temp is 75 psi at 81.8 say 82 psi and at 100 degr Ambiënt temp at 86.3 psi.
This to get the same savety for the tires.

A tire whith some water in it fluctuates more in pressure .
Have list of partial pressure for 100% humidity at temperature and this shows that the extra rising from 65 to 100 degr F is about 0.67 psi. You then have to add this to the pressure difference for dry air. So the 80 psi at 65 degrF will get 87psi at 100 degr F.


But if you give your AT- pressure , I will do the calculation for that

If higher ambiënt temp the inside tire will get also higher when warm by the heatproduction of bending rubber.
Then the rubber also gets hotter , so gets closer to the temperature where it hardens and so get damaged by next bendings.

The lesser deflection the lesser heat is produced by the tire.
Lesser deflection is what you get when inside tire temp gets higher. So also when the tire inside temp goes up when driving at speed.
So to some extend and mayby totally, the heatening up takes care that the tire produces less heat, so critical spots of tire still wont get to hot that damage begins.

Even though that seems like a wild swing, you're only looking at a pressure change somewhere around 2 PSI. At 60 PSI you'd be about 3% over the Max Cold PSI. The Max Cold PSI spec accounts for these variations.

AS mentioned don't over think this. While cold may be relative to ambient conditions, It is still recommended to fill tires to pressure while cold. The ratings allow for expansion/higher pressures due to traveling in hot weather.

In any case you dont have to be afraid you will distroy your tires by the higher pressure,... but no you then give lower pressure for the temp then when colder ambiënt temp filled at max pressure.
Your "what you think"maximum pressure of tire is in fact the pressure needed for the maximum load up to maximum speed of tire (or if lower 99m/h).

for your trailer tires mayby even higher pressure is needed then that max, and is allowed by most tire-makers.

For Towingcar you should calculate the needed pressure with help of my spreadsheet, and this advice is officially for 18 degr C/65 degr F.
But still the difference is small so you can give same pressure when hot ambiënt temp filled. Little difference is compensated by taking enaug reserve but not to much things go bumping.

End advice is to be carefull with the TT tires , hot ambiënt temp filled at AT-pressure could mean that at 65 degrees its to low if you would calculate it.

If you give more details of TT and car I can give you an advice with enaug reserve for also this temperature fluctuation.

kaydeejay wrote:
If they are OEM size tires, inflate to the cold pressure shown on your tire placard on the trailer.
This may or may not be the same as the max pressure on the tire sidewall.
After an overnight cool-off in Florida, check again and adjust to the same pressure as before. You may need to let some air OUT if the new ambient temp is higher than when you left home.

x2

That's what I did recently.

If they are OEM size tires, inflate to the cold pressure shown on your tire placard on the trailer.
This may or may not be the same as the max pressure on the tire sidewall.
After an overnight cool-off in Florida, check again and adjust to the same pressure as before. You may need to let some air OUT if the new ambient temp is higher than when you left home.

Check and fill the tires when cold (cold is a relative term depending on where you are). The tires will be fine if you follow manufacturer's recommendations.
You could weigh your trailer to find the recommended tire pressure too.

downtheroad wrote:
Don't make this more complicated than it is....(Rv'er tend to make things complicated.)

Early morning - check and fill your tires to the max as recommended on the side wall and the placard on your tow vehicle...

I'm pretty much in agreement with this but not everyone fills tires to the maximum pressure listed on the sidewall and there may be reasons for that.

I think some folks get nervous when they measure pressures after a run in hot weather. The pressures will be above the sidewall recommendations. There's no reason for alarm as the mfg has built the tire and calculated the tire pressures to endure that rise. There is far more stress on the tire from under inflation than being a few pounds over.