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Is tire pressure different if you measure with tire off the car as compared to when it is on the car and holding the weight of the car

I want to say the force of the ground on the tire is entirely transmitted to the axle and will not affect the PSI. But this is just 20 year old book learning and not practical experience talking.

posted by Palamedes at 1:44 PM on September 18, 2009

wouldn t the pressure from the ground that partially squashes the tire decrease the volume, thus increasing the pressure? (Assuming the tire is thick enough to prevent the tube from expanding in other directions)

good question, I m sure someone will be able to site something accurate

posted by Think_Long at 1:46 PM on September 18, 2009

Yes, the pressure will be higher when the tire is supppatagonia men's better sweater quarter zip pulloverorting the car. If you want to imagine a free body diagram, simplify things and consider a car resting on one big tire. If you take the car off, it s supporting no weight it does not have to exert any force on the axel to keep the car in static equilibrium. Air is a compressible fluid pressure will increase when the tire bears a load because it s the pushing back of the air that keeps the car body from falling.

posted by phrontist at 2:16 PM on September 18, 2009

The elasticity of the tire is also a factor. The more elastic, the less the pressure increasepatagonia fleece pullover review.

posted by phrontist at 2:18 PM on September 18, 2009

I would guess that it s going to be the same, within a very few fractions.

The pressure for a given quantity of air is entirely dependent on the volume of thpatagonia opens headquarters store in australiae container. We know that the tire deforms under the weight of the car; we can see this in the bulge along patagonia outlet store reno nvthe bottom of the sidewall near the ground and in the increased contact patch where the rubber meets the road. We also know the tire can deform to a fairly large extent, witness what it looks like when it is flat.

When the tire is off the car, the tire will deform in other directions perhaps the tread will get a crown in it that is not present when the tire is on the machine. If the tire is stiff enpatagonia arizona homes for saleough that it doesn t deform much, then the volume is epatagonia men's better sweater quarter zip pullover0ssentially constant.

Please clue me in if this is wrongpatagonia men's better sweater quarter zip pullover3.

posted by maxwelton at 2:40 PM on September 18, 2009

A more simple thought experiment: Imagine a balloon. Now put a car on top of it. Does the air pressure increase?

At a rough estimate I think the surface area of an average tire would be about 1200 square inches. Times 4 of them is 4800 square inches. So a 4800lb car would increase tire pressure by 1psi when it rests on the tires.

posted by sfenders at 3:18 PM on September 18, 2009

Modern car tyres have steel belts that are not going to stretch at all (at least for the typical pressures that tyres are inflated to).

Thought experiment: if you fill a tyre with water instead of air, will it work the same? (Except for being much heavier.)

So, how does a tyre transmit the force from the ground to the wheel/axle? Let s assume rubber is infinitely stretchy, and only the steel belts provide the strength. When a tyre carries a load, it deforms a bit where it touches the ground. The sidewalls therefore make slightly larger angles at the bead where they re attached to the wheel. This decreases the radial (downward) component of the belt s tension reducing the downward force in that area of the wheel. Voila! the radial force at the lower end of the wheel is lower than the radial force epatagonia everlong review processlsewhere on the wheel s perimeter upward force on the axle.

So if a tyre is fully inflated, there should be no change in the internal pressure when it s loaded. (Of course there are second order effects like stretchiness and stiffness of the belts, strength of the rubber, etc. that may have a small effect on the pressure.)

posted by phliar at 3:26 PM on September 18, 2009
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Thpatagonia guide jacket columbiaink_Lopatagonia men's better sweater quarter zip pullover1ng and maxwelton: yes, a tyre deforms under load; but that doesn t say necessarily say anything about its volume.

posted by phliar at 3:32 PM on September 18, 2009

yeah, I just wasn t sure if the deformation comppatagonia men's better sweater quarter zip pullover2ensated completely for the volume in other directions, or not. I accept your answer with enthusiasm

posted by Think_Long at 3:35 PM on September 18, 2009

Crapatagonia guide pants girdlesp, I just thought of the perfect example the Lunar Rover. Its tyres were filled with vacuum!patagonia guide pants presser

Inflating a tyre makes sure it won t buckle under load. The Lunar Rover s tyre belts are actually its whole tyre, which shows that the weight is supported entirely by the sidewall flexing, and not by any pushing back that the air inside the tyre is doing. This also implies the Rover s tyres must have been much stiffer than the belts in a car tyre.

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