Thor Forums

[Beau388]

Originally Posted by Chateau_Nomad

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What is NOT clear to me... why the MAX COLD 80 PSI on the sidewall? Possibly because this tire might be used in a different application?? The (possibly flawed) math I'm doing shows all tires on the unloaded coach within 300# of the specified MAX load rating. Is there a situation where I would want to use UP TO 80# per tire instead of the 75#/65#?

Finally an OPINION:

To me, tire inflation is about safety and using the tires as a load bearing medium as they are intended. Adjusting tire inflation to achieve aesthetics like a softer ride is dangerous - IF those inflations are outside of those specified by the tire OR chassis manufacturer.

Tire manufacturers seldom make a single tire for a single application. An exception like Corvette summer tires. I use Sumitomo tires (ST719 SE) made for a cross-country box truck. The side wall shows 110 lbs max cold pressure. Thor door sticker for Goodyear RV tires of the same size shows 82 lbs. A little arithmetic shows that the recommended 82 psi cold comes from the rear axle being loaded to exactly.12,000 lbs. The tire chart shows if the front axle is loaded to 6,000 lbs (max allowable weight for the coach is 18,000 lbs) the recommended tire pressure is 74 psi for the front. Goodyear recommends 80 psi as the minimum for the tire rim combination.
Starting out with the pressure set on 72 degree morning, the fronts will gain 12 psi when the black pavement hits 120 degrees. (Yes, I have a infrared heat gun), the inside rears 22 degrees and the outside rears 18 degrees. I ended putting the rears to 88 psi which gives a tire pressure gain of 18 psi on the inside and 16 on the outside.
It is the flexing of the tire that generates the heat. The less the pressure the more heat the tire generates and the more fuel the coach uses.

If you worry about tire wear you an chalk the surface of the tire (not the full tire but a good size patch across the tire, then drive about 20 revolutions in a straight line. Where the tire is scrubbing will wear the chalk off.
For you info, Ford in 2015 put the same rim and tire on the 27 ft 16,000 lb stripped F-53 chassis; 29 /31 ft 18,000 lbs chassis and the 34/35 foot 20,500lb chassis.