Tire Pressure - but wait... a different question!

Airbags are not the same as a tire. The entire airbag is compressed from weight above while a tire is only compressed on the lower portion of the tire. That's why a tire only goes flat on the bottom, lol.

No, it doesn't matter. The volume in the tire is virually the same, therefore the pressure does not change. There is a minor increase / decrease, but nothing that you would notice. You will have a greater increase in pressure from building up heat in the tire, that is why it states a "cold" psi rating.

I autocross occassionaly and we will build 2-4 lbs of pressure in a 60 second run in tires just from the heat buildup. So the miniscule amount of pressure difference loaded vs. unloaded won't impact the overall performance of the tire.

I'm assuming empty but a co-worker says the pressure in the tires will be higher after it is loaded (if you pump tires to max psi, they will be over max psi and then when they heat up they will be way over). He is probably right but is it a negligible difference, anyone know?

Your co-workers owners manual for his/her truck provides inflation values for the load carried. The lighter the load the less air inflation value, the more the load the higher the inflation value. I would think that if your co-worker was correct the lower values unloaded would increase naturally with the load to the higher values, but it does not. Adding more air for more load is counter to his/her reasoning.

When I carried a 4000 pound cabover camper on my truck I ran 80 psi cold in the tires. Lightly loaded with no camper or loaded with the camper 80 psi was 80 psi -- the value was the same.

The cold air pressure check is the tire manufacturer's recommendation and that value takes into consideration that heat will increase the pressure a little once on the road. Never release pressure to compensate for heat or the tire will be essentially operating underinflated.

Altitude slightly changes tire pressures but it is insignificant for most RVers making it is a non issue.

If you want to run nitrogen in tires there are some benefits -- less water content than air, less heat increase re pressure and altitude makes a lesser difference.

iawoody2 wrote:
65lbs is 65lbs, no different weight or no weight. Adding weight cannot add more air.

This is not always true. My airbags with an empty truck normally have 5-7psi of pressure. Just by lowering the camper onto the truck, pressure jumps to 17-20psi. It's because of the small volumn of air. If it affects tires, I believe it would be very minimal.

Mark

I've actually wondered the same thing. I even try to air up when not hitched up but won't stress if I do it while I am hitched.

Air pressure is measured by the amount of compacted (density) air inside a vessel (tire) compared to the air outside the vessel. Anything that reduces the volume of the vessel (tire) would therefore increase the pressure.

Therefore, the answer to the question boils down to, "Does the inside of the tire (its volume) become smaller when it is carrying a load?" If yes, then your co-worker is right. I actually think the tire retains the same volume but in a different shape.

65lbs is 65lbs, no different weight or no weight. Adding weight cannot add more air.

Next time measure the pressure before and after loading. Then you will know. I have doubts your coworker has actually done this.

It is normal for RUNNING tires to have a pressure above max COLD inflation.

Too complicated for my simple mind. Just like the question about air pressure and altitude.

I set them/check them cold in the morning and that's the end of it.