Well I'm an Engineer...Google up "ideal gas law" and you can make yourself believe that the pressure doesn't change. However, it's not that simple since there's outside factors in it all.
Theoretically, if temp doesn't change P1V1/T1=P2V2/T2 turns into P1V1=P2V2.
You could say as long as the tire deforms (and they all change shape), the overall volume doesn't change, and the pressure after load does not change.
However, by ideal gas laws, if you applied a load and it deformed the tire such that it didn't increasing the volume (read: flat spotted, but didn't expand in diameter or width any)- then the volume would have changed, and the internal pressure would have to change as well. That doesn't happen though. The tire changes shape for sure.
Reality is you'll find it's not a linear effect. At some point the tire resists the volume change and then the pressure changes. More importantly though, the tire deforms such that the shape induces more drag and scrubbing. More scrubbing (friction) leads to increase in internal temp. As the temp goes up, so does the pressure and/or the volume of the tire. This is what leads to a tires ultimate failure / seperation / blah blah.
Bottom line: tire manufactures recommend applying the load and then monitoring the pressure. It says so on the tire - "MAX PSI at load".
GPSHEMI
06' 3500 Dodge Ram Megacab 4x4 w/5.9L Bully Dog Triple Dog
16K Pullrite SuperGlide w/super rails
05' Laredo 29RL (32'10")
The bottom line is, in all practicality there is an insignificant change in tire pressure for our use, either way is OK. The important thing to remember is: ALL the load/inflation charts say in fine print somewhere that the chart reflects the minimum safe operating air pressure for the corresponding load, not the optimum pressure.
Ray,IN wrote: The bottom line is, in all practicality there is an insignificant change in tire pressure for our use, either way is OK. The important thing to remember is: ALL the load/inflation charts say in fine print somewhere that the chart reflects the minimum safe operating air pressure for the corresponding load, not the optimum pressure.
Thanks Ray;
I been thinking that those load/inflation charts were just giving the bear minimum pressure for a given load.
Ray,IN wrote: The bottom line is, in all practicality there is an insignificant change in tire pressure for our use, either way is OK. The important thing to remember is: ALL the load/inflation charts say in fine print somewhere that the chart reflects the minimum safe operating air pressure for the corresponding load, not the optimum pressure.
Thanks Ray;
I been thinking that those load/inflation charts were just giving the bear minimum pressure for a given load.
If one had an airbag with no load on it set at a given pressure, a 50% compression of that bag should yield a 100% increase in pressure, because the bag itself doesn't stretch, like a balloon would. The rubber is just a sealant to contain the air within a corded structure. 20 psi, for instance, would go to 40 psi. With a properly inflated tire, the dimensional change (if any) is so small when compared to overall volume that no material change in pressure should occur. It's not noticeable from a guage on my truck, at least.
As an aside, that's what makes an airspring superior in ride quality to a metal spring. It's infinitely variable. When the vehicle hits a bump, load on the air spring increases, compression occurs and resistance (pressure) rises. On the rebound, as the volume of the airspring increases the pressure drops, continually lessening the force as the load passes its statically loaded point and once again "settles" on the airspring. That's not what happens with a metal spring as they are commonly constructed. They deflect a given amount for a given application of force, say one inch for 250 lbs., another inch for the next 250 lbs. and so forth. When they rebound, the unloading occurs much more violently, as there is no lessening of force until the point of no deflection from an unloaded state is reached, whereupon the pressure is actually reversed (if allowed to go that far) and the spring actually begins to pull the load back to equilibrium.
I just got finished hitching up my SunnyBrook and made the following observations.
I checked and adjusted the air pressure in my rear duals to 65 psig before hitching up the fiver. After hitching up the fiver, which added about 2600 lbs to the rear axle, I checked the air pressure again in each dual, and guess what? They were still at 65 lbs.
Did the shape of the tire change? Yea, maybe a little bit. Did the volume in the tires change. Probably a small amount. But obviously not enough to make the pressure change.
The pressure gauge that I used was the kind with the round dial face with needle. The number graduations are in 2 lb increments. Could I have detected less that a one pound increase in pressure. No way, not with this type of gauge accuracy.
So from this experiment I have concluded that the tire's volume did not change significantly enough to cause a change in pressure. And my instrument for measuring the pressure was not accurate enough to detect that change, if any.
The OPs question was about proper truck tire pressure but someone brought up aircraft tires which confuses the subject. Due to the difference in construction, a tire under the load of an aircraft has 4 percent higher pressure than it does when unloaded. Therefore, if unloaded pressure has been specified in the manual but the tire is being checked on aircraft, that number should be increased by 4 percent to obtain the equivalent loaded inflation pressure.
That's why I questioned LIJohnnyH figures he gave for aircraft tire inflation pressure. They look to me to be load inflations pressures and not the 4% normally gained by putting a load on them.
ryoung
2004 Dodge 3500 Dually HO 6 sp 4.10, Jordan Controller
2005 SunnyBrook 31BWKS
2004 Lance Lite 915
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