I received this question from an RVtravel.com reader regarding tire cold inflation pressure:
I have read a lot of your posts and that makes me think my manufacturer has my Cold Inflation Pressure (CIP) wrong or used too much tire on my unit (maybe not a bad thing).
5th wheel, GVWR (gross vehicle weight rating) 12,025 lbs.
Tires—Goodyear Endurance ST235/80R/16
CIP—80 psi per label and max CIP on tire
Per the GY load/inflation chart, 80 psi will support 3,420 lbs. per tire.
Fully loaded before a trip on a CAT scale, trailer axles weighed 4,060 front and 4,080 rear, tongue weight 1,800 lbs.
This is what I come up with. Being conservative, I used 2,270 from the chart at 40 psi for my operating pressure. Using your guidelines I used 50 psi as 25% reserve.
I think 40-50 psi is too little pressure. I think I should run 65 psi as CIP rather than the 80 psi on the label. I’m running 80 psi CIP and I had pressure go to 100 psi and temp as high as 130 degrees on TPMS, which both seemed high to me.
Is 65 a good CIP with MOP at 50 based on the info I provided? Does a high (80 psi) CIP cause tires to run hot with high pressures?
I hope I didn’t muddy the waters with my numbers, but I’m finding it difficult to ask the right questions. I would greatly appreciate it if you could help me clear this up. Thank you.
My reply
Thanks for the question. Yes, understanding the Certification Label psi versus information in the tables can be confusing. We need to remember that these different inflations are based on different requirements.
You didn’t provide the Max load that is stated on your Certification label. I am guessing it says something like GAWR 6,850# for each axle. The inflation stated on the label, by law, must be sufficient to support the stated GAWR (gross axle weight rating). ST235/80R16 LR-E tires are rated for 3,420# each at 80 psi.
If you do a search on “MAX inflation” on the RVtravel.com Maintenance posts you will get a number of posts explaining the meaning of “MAX” and why you may be misunderstanding what it means.
I agree that 40 psi is way too low. I would consider 65 to 70 psi CIP more appropriate, assuming that no tire is supporting more than 3,000#. BUT if you are seeing a psi increase from 80 to 100, that might indicate you need CIP of 80. Have you checked your digital pressure gauge for accuracy?
I do not think your RV company put “too much tire” on your RV. If anything, I would be recommending Load Range F LT-type tires for heavy 5th wheel trailers.
No, running “a high psi of 80” will not result in a tire running hot. Excess psi comes from low inflation and excess speed. I consider 65 mph a MAX operation speed for ST-type tires. I also consider that running any CIP below the Certification Label CIP as “consuming” tire life due to interply shear.
You can learn more about tires in RV application on my Blog www.RVTireSafety.net and by reviewing the posts regarding RV Tire Safety on RVtravel.com.
Roger Marble
MORE POSTS FROM ROGER ON TIRE PRESSURE
- How to set tire pressure when there are wide swings in temperature
- Learn the basics of setting tire pressure and testing TPMS
- My RV tire pressure is showing a significant increase. Is this OK?
- Is the tire pressure too high?
- Should you adjust tire pressure when temps get colder?
- Do not adjust your tire pressure for ambient temperature
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If you have tire questions, check out Roger’s posts on RVtravel.com. There are hundreds of posts covering everything to do with tires.
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RVT1254
I always am a little uncomfortable when we see the very large 5th wheels fly past us at speeds in excess of 70 mph. Last trip, saw a 5th on side of road with 2 tires blown out on the passenger side of the 5th wheel. Luckily, it was not on it’s side. Ruined Spring Break.
Agree.
A couple years ago we were traveling through Wyoming on a 100°F+ afternoon. I had slowed my motorhome to 60 mph to help keep tire temps down. A very large toy hauler 5ver passed us at what I would estimate as over 80 mph. About 30 miles down the road he was pulled over looking at his two blown rear axle tires. With those tires gone, the rear end of the trailer was almost on the ground. I’m sure the rest of his afternoon didn’t go well. He probably ruined the two front axle tires as well.
Roger, SOME manufacturers are providing more tire than absolutely necessary. For example, we were just looking at an Airstream with 3800-pound axles and ST225/75R15 E tires, maximum load 2830 pounds at 80 psi. That’s 5660 pounds of tire capacity per axle, and the heaviest load on any tire was only 1600 pounds.
I think you will discover that not every size tire comes in LR-C & LR-D & LR-E. So sometimes the tire is selected based on size and you end up with more Load Capacity then the minimum necessary.