Driving on "Hot Roads"

[tireman9]

I got this question and thought that others might be wondering about the same topic.

Jim K asked
Subject: tire pressure
Message: I will be traveling in the desert for the first time and I am
wondering if I should reduce the tire pressure before I go. The hot road will
increase the pressure and I am afraid of damaging my tires.
++++++++++++++++++++++

Hi Jim,

No you don't have to worry about hot roads.
IF you run the correct cold pressure

Also you should NOT reduce the cold pressure to "compensate" for the hot roads.

Now you didn't say if you have a standard RV trailer or a Motorhome so I will give you the short answer for each application.

(FMCA members can share this part with friends with towables)

Trailers: You should set the Cold Inflation to the pressure on the tire sidewall. If you look at the sticker on the side of your trailer you should find the tire size, type, Load Range and pressure recommendation from the manufacturer. In almost all cases the recommended inflation is the inflation on the sidewall of the tires.
Have you confirmed you are not overloading any of your tires? Simply guessing or looking at the tires is not good enough you need to get the trailer on a scale and at a minimum get the total load on the tires. Now you can't assume the load is equally distributed side to side or axle to axle Measurements of thousands of trailers suggests you need to assume at least 53/47 to 55/45 split axle to axle and split side to side so you need to calculate the heaviest load based on an estimate of 27% to 30% of the total being on one of the 4 tires. A better method is to get individual tire loading. You can learn more HERE.

Motorhomes are a bit different than towables. Here you need to get the "corner" loading as the side to side difference is affected by the placement of things like generator, water tanks, refrigerators etc. The Front/ Rear loading is obviously different and for most motorhomes the number of tires on each axle is also different. You can use the information on your placard but a better method is to get the actual tire loading and then using Load/Inflation charts establish the MINIMUM cold inflation then add 10% to get your Cold Set inflation. THIS post has some info and a link in it.

Bottom Line

When tires are designed, we tire engineers know that some vehicles will be driven on hot roads.

Tires will normally run +20°F to +50° above ambient.

You should run a TPMS to get warning of air leak due to puncture.

If you are driving in the USA you should have no problems.
If you are traveling to Saudi Arabia, the Sahara or Australian outback then we need to take some additional steps and precautions.

I think that if you look at some of the dozens of posts on Load, Inflation and Temperature on my blog you will find more details and expamded answers to your questions.

[jlandon13399]

Great info Tireman. Thanks.

[desertdeals69]

Hey tireman, I meant to ask you this question when I saw you in Redmond. If I fill my tires first thing in the morning to the correct pressure, what happens when I get to colder climate. We sometimes wake up to 95 plus degrees and if I set my pressures and then go to the Oregon coast where its 38 degrees I am off somewhat.

Thanks,

Bruce Chong

[tireman9]

Hey tireman, I meant to ask you this question when I saw you in Redmond. If I fill my tires first thing in the morning to the correct pressure, what happens when I get to colder climate. We sometimes wake up to 95 plus degrees and if I set my pressures and then go to the Oregon coast where its 38 degrees I am off somewhat.

Thanks,

Bruce Chong

Glad you were able to make it to Redmond.

Good question with relatively simple naswer. First let me relate my experience.

After leaving Redmond I spent a number of weeks in OR, WA, BC AB, Glacier Nat Park & Yellowstone and had a number of days when it was warm followed by snow or frost the next AM. As I mentioned in my Seminar I have two TPM systems on my coach (internal and external) so I can compare the readings for both pressure and temperature.

Yes temperature will affect the tire pressure. I have a few posts on Temperature and pressure, including one with all the technical formulas for those so inclined.

The rule of thumb is about 2 percent change for every 10°F change. An example would be if you set your cold pressure to 80 psi in Oklahoma City in November and it's 45°F you may see your cold pressure has increased to about 86 psi in Phoenix the next morning when it is 85°F.

BUT

Operating temperature is really a function of the heat generated by the tire flexing. The upper limit of tire temperature when properly inflated and loaded, is reached when the heat (BTU) flowing out of a tire is equal to the heat being generated by driving. The flow of heat out is faster when the outside air is colder then when it is hotter so there are some complex interactions. I'm not going to put everyone to sleep with the details on heat flow.

BOTTOM LINE: When you set the pressure in the morning the tire will partially adjust for the change in outside temperature as outlined in your example so you should not make change when the tire is hot or until the tire has had at least a couple hours to reach ambient temperature. I know this based on my direct observations made during my travels in Aug & Sept.

You should still be withing +/- a couple psi of your goal "cold" pressure each morning of a travel day. Remember you establish your goal pressure bu knowing your actual tire loads (corner loads), using your tire mfg Load/Infl tables to learn the MINIMUM cold pressure and then add 10% to the Min. Inflation to get your goal pressure.

[desertdeals69]

Thanks for the explaination. When I got home 2 weeks ago and drove my 2 cars out of the garage both factory tire indicators came on. When I checked all 8 tires they were all exactly 6 lbs low. When I had filled them in April with close to 100 degrees they were at 36 lbs. I found it odd that they all were still equal. The cars are 07 and 08 Prius with the same size tires.

[tireman9]

Thanks for the explaination. When I got home 2 weeks ago and drove my 2 cars out of the garage both factory tire indicators came on. When I checked all 8 tires they were all exactly 6 lbs low. When I had filled them in April with close to 100 degrees they were at 36 lbs. I found it odd that they all were still equal. The cars are 07 and 08 Prius with the same size tires.

If they were all 36 psi @ 100F I am not surprised that 4 months later at cooler? temp they measures 30.

In general tires loose 1% to 3% air each month when temperature compensated in a stationary test in a lab. What was the temp when you measured Oct 1?

[desertdeals69]

Roughly 40 degrees cooler.

[tireman9]

Roughly 40 degrees cooler.

Here is a post with the math. Simple answer is 2% pressure for each 10F