Tire psi planted versus lifted

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Originally Posted By: mjoekingz28
Just wondering does having weight on the tire affect the pressure reading?

It doesn't.
 
Yes but you can't measure it. The footprint smooshes in less than 1% of the air space, and some of that balloons outwards elsewhere, like the sidewalls. So, negligible.
 
Yep, you'd think it would be more given the weight of a vehicle, but because of the shape and design of the tire, it's a very small difference.
 
I was curious about this as well, the inner tire volume is NOT a constant closed system, and I could not measure the change on a jacked FWD car with 60/40 weight distribution on a front wheel assy after I lowered the car and re-checked the pressure - though I was using a low resolution pencil gauge. This might be a fun experiment here - especially on a 3 ton diesel truck front wheel - maybe HERE it would be seen given the more volume restrictive and concomitant loss of elasticity on a LT tire.

Sorry I feel been using the concomitant word too much
smile.gif

Off to the Thesaurus!
 
I will challenge anyone who doesn't believe the internal tire volume doesn't change (and therefore the pressure) loaded vs unloaded, by doing what I did - measure the pressure both ways. You'll need a pressure gauge that reads in 0.1 psi increments in order to see the difference. It's real and you can demonstrate it for yourself.

BTW, the engineering principle is that things try to get to their lowest energy state - and an inflated object (like a tire) tries get to a sphere - the most efficient shape - but is prevented by the tension of the tire construction. When loaded, the shape changes for the worse, and the tires returns to its original shape when unloaded BECAUSE the loaded shape is smaller in internal volume which causes additional internal pressure. If that additional pressure wasn't there, the tire would not return to it's original shape.
 
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It is real, as you indicate, but it's not terribly significant, I would suggest. The drop in cold pressure from ambient day to night temperatures can be more. When I say it's not significant, I mean that when I'd be mounting a tire onto the rim for the cabs years back, when I got the cold pressure where I wanted it on the mounting machine, I left it as such.
 
Originally Posted By: Pontual
Sure, but to know this "small difference" you need to use the Clapeyron equation:

P.V=n.r.t


Isn't Volume staying the same though? Just because it is smooshed doesn't mean there is less volume. The tire buldges to compensate for that.
 
Originally Posted By: ccap41
Originally Posted By: Pontual
Sure, but to know this "small difference" you need to use the Clapeyron equation:

P.V=n.r.t


Isn't Volume staying the same though? Just because it is smooshed doesn't mean there is less volume. The tire bulges to compensate for that.


Nope - and the fact that the pressure increases proves that the volume is decreasing.
 
Originally Posted By: ccap41
Originally Posted By: Pontual
Sure, but to know this "small difference" you need to use the Clapeyron equation:

P.V=n.r.t


Isn't Volume staying the same though? Just because it is smooshed doesn't mean there is less volume. The tire buldges to compensate for that.


The tire would rather not bulge, though. Put a balloon on a table. Put a book on it. Put another on. Balloon keeps changing shape but also fighting back more and more. If the sidewall was soft enough it would bulge empty, it would not support the vehicle.
 
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