Tire pressure in trailers....
 

Is the reading on the sidewall (80 psi max) a cold temperature rating or a maximum the tire can withstand when it is hot? I have always wondered about this.

If I had to guess it would be a cold (rested) reading and the tire should be designed to withstand pressure increase due to temperature increase caused by on the road use. But this is my guess. Does anyone really know?

I have always run 65 PSI on D rated tires.

The rule of thumb is for every 10° Fahrenheit change in air temperature, your tire's inflation pressure will change by about 1 psi (up with higher temperatures and down with lower).

Most tires have a raitng in () parentheses....most state (MAX LOAD COLD XXpsi)

The number is not standard to the size....We had Firestone D load that needed 30 pounds more than our current Michelin.

Its the Maximum period !!!! NEVER go over the MAX rating written on the tire. So it should set under that pressure if filled cold. Anticipating it will go up when running. To be sure some trailer people check them after running for a while.

BUT....If the tire gets hot while running (like the link FF put up) the tire is likely underinflated so you add some. But not beyond the MAX written on the tire.

This is why on cars you go by the sticker insdie the door, not whats on the tire.

WAIT.........I talked to an RV guy here he suggested the Goodyear RV tire site has lots of info.

From Michelin
For RV use only, Michelin displays tire loads per axle end in the load and inflation tables, as we recommend weighing each axle end separately and using the heaviest end weight to determine the axle's cold inflation tire pressure. For control of your RV, it is critical the tire pressures be the same across an axle, while NEVER exceeding the maximum air pressure limit stamped on the wheels.



The Inflation Loading
Proper Tire InflationCorrect tire inflation is a key component in tire care. The recommended maximum inflation pressures for your tires are indicated on the certification label or in your owner's manual. Since RVs can be loaded with many different configurations, the load on each tire will vary. For this reason, actual air pressure required should be determined based on the load on each individual tire. Inflation pressure should be adjusted to handle the tire carrying the heaviest load, and all tires on the axle should be adjusted to this standard.

Each manufacturer provides load and inflation tables specific to their products to help you determine the correct tire inflation pressure for your vehicle's loading.

Underinflation brings a higher risk of susceptibility to damage due to road hazards, reduces casing durability, and causes a loss in fuel economy, plus uneven or irregular tire wear. Severe or prolonged underinflation brings about an increased risk of tread separation.

BUT see how they skirt the issue of MAX pressure rating on the tire. (Not mentioned).......

Now I am really confused.

I think the only sensible reading to put on the sidewall is the cold rating . No one is going to weigh each axle and adjust tire pressure as a function of temp and weight, that is absurd.
One has to assume there is a significant safety factor on over-pressure on this reading and that the over pressure that is created by heat will be within that margin.

I have an uncle that invented the run-flat tire (sold the patent to a giant tire manfuc) The testing for this tire was like 12 years. (Believe it or not it stemmed from research used to develop the wheels on the lunar rover that drove on the moon) We use to joke that he was "re-inventing the wheel" (an engineering joke) I will ask him, maybe he knows what the safety factors are, I would bet they are at least 2X the sidewall rating.

This is from my Atlantic Trailer Manual.

Maximum Inflation Pressure

A tire's maximum inflation pressure is the highest "cold" inflation pressure that the tire is designed to contain. However the tire's maximum inflation pressure should only be used when called for on the vehicle's tire placard or in the vehicle's owners manual. It is also important to remember that the vehicle's recommended tire inflation pressure is always to be measured and set when the tire is "cold." Cold conditions are defined as early in the morning before the day's ambient temperature, sun's radiant heat or the heat generated while driving have caused the tire pressure to temporarily increase.

For the reasons indicated above, It is also normal to experience "hot" tire pressures that are up to 5 to 6 psi above the tire's recommended "cold" pressure during the day if the vehicle is parked in the sun or has been extensively driven. Therefore, if the vehicle's recommended "cold" inflation pressures correspond with the tire's maximum inflation pressure, it will often appear that too much tire pressure is present. However, this extra "hot" tire pressure is temporary and should NOT be bled off to return the tire pressure to within the maximum inflation pressure value branded on the tire. If the "cold" tire pressure was correctly set initially, the temporary "hot" tire pressure will have returned to the tire's maximum inflation pressure when next measured in "cold" conditions.