Tire Pressure: Hoist vs. On the Ground

I know temperature affects tire pressure (about 1psi for a 10 degree change) being a former frequent bracket racer.

But....can the tire pressure vary between no weight vs. weight? Such as, a car on a hoist vs. the car being on the ground with the cars weight applied

Someone I work with asked me this question.

I think the tire pressure is the same, as long as the tire is properly sealed and mounted.

Give me your 2 cents (or more)

It shouldn't affect it. The tire changes shape when you add weight to it, but it doesn't change size. Same volume, same mass, therefore same pressure.

I"m not 100% convinced the pressure would not change from loaded to unloaded.

Why is there an impression that the tire does not change size when you load it up?

I'd like to see one of us check it out to be sure. I'll check it out next time I put a tire in the air, but that could be a while.

Personally, I think the pressure will change, but not significantly, i.e. only a few psi.

Jack

curtis73 wrote: It shouldn't affect it. The tire changes shape when you add weight to it, but it doesn't change size. Same volume, same mass, therefore same pressure.

Agreed. I don't think it will change at all.

Its easy enough to test...

check the pressure, jack up the car, and check again.

I bet the difference will be so small it will be hard to measure.

A few years ago, had a conversation on this subject referencing cargo aircraft tires. Does the tire pressure change empty aircraft vs loaded aircraft? Answer then was no, same volume. And we double checked it with tire gauges once and found it true, tire pressure won't change because of wieght. Fairly recently when we had access to engineers asked them the same question, the answer again was the same, no, same volume. Can't increase pressure when the volume remains the same.

Tire shape will change loaded versus unloaded. The flat contact spot and the sidewall bulge is a change in shape when loaded.

Load will not increase tire pressure, Heat, or lack of it is the only outside factor that can change the pressure of a sealed wheel and tire assembly.

PV=nRT

You are pervin on an RT? You kids and your internet speaking!

normal load no change if you over load it it will go up as tires only streach so far.

The air chamber most definately changes volume with a load. You can observe this directly by placing the tire unpressurized on the ground with the car on it. The bottom gets quite squished, reducing the total volume by the amount squished.

So from that quick little study can we conclude that tire pressure changes substantially when we lower a car from the rack? In a word, no.

The change in volume for a fully inflated tire is minute, and effectively undetectable with a standard hand held tire pressure gauge.

foxtrapper wrote: The air chamber most definately changes volume with a load. You can observe this directly by placing the tire unpressurized on the ground with the car on it. The bottom gets quite squished, reducing the total volume by the amount squished.

I disagree... the tire's contact patch elongates and the sidewall bulges. Sure, they squish, but that doesn't mean that the volume decreases, it just means that the tire has changed shape. The shape changes, but the volume doesn't... at least not enough to show a measurable change in PSI.

The carcass of the tire is remarkably rigid. This isn't like squeezing a balloon. Tires are supported by the pressure inside, not "inflated" like is commonly thought.

curtis73 wrote: I disagree... the tire's contact patch elongates and the sidewall bulges. Sure, they squish, but that doesn't mean that the volume decreases, it just means that the tire has changed shape. The shape changes, but the volume doesn't... at least not enough to show a measurable change in PSI.

If this was strictly true, you'd never blow a tire by overloading it.

However, I will agree that in practice (and at normal load ranges) your analysis is correct.

I don't think overloaded tires blow due to 'higher pressure' so much as excessive heat created by the deformation of tire.

If this was strictly true, you'd never blow a tire by overloading it. However, I will agree that in practice (and at normal load ranges) your analysis is correct.

That's true... its not absolute, but for practical purposes its safe to say that additional weight won't increase pressure. But, your demonstration isn't entirely accurate.

The reason tires blow from too much weight is not because you have overloaded them and increased the pressure, its because you have added more weight than the tire can support with its rated pressure. The tire then deforms and the heat buildup from friction in the sidewall causes the tire to fail. Even if you add crazy weight without the car moving, if the tire fails its because you have deformed the tire beyond its reasonable limits.

Think about E-range truck tires. The carcass of the tire itself doesn't hold any more weight than a passenger tire, but the design of the tire is made to hold more pressure. That means the tire can use more pressure to hold its SHAPE under heavier loads. The air isn't suspending the weight as much as it is properly allowing the tire to do its job. That is why proper tire pressure is more a function of vehicle weight than the actual tire on the car. If I put the same 215/65-14 tire on my E30 and my 66 Pontiac, the proper pressure will be very different. That is why manufacturers publish tire pressures on the CAR, not the tire. Tires have a max pressure and a max weight, but its the weight of the CAR that determines proper tire pressure. Again, we technical GRM kinda people know that its not absolute, but the pressure is only there to hold the tire where it should be.

Now picture a balloon. If you inflate it to 1 psi and squeeze it until it bursts, its not that you've increased the pressure inside the balloon, its because you've deformed the latex to the point where it fails. If you put a pressure gauge on a balloon you would see that the pressure changes very little when you squeeze it.

But, like you say, its not absolute. If you add a couple tons of weight to your tires, the overall volume of the tire may decrease slightly, but for practical purposes you won't be able to tell a difference with a pressure gauge.

People think that tire pressure is what supports the weight of the vehicle. It doesn't... it simply supports the tire so that it can operate properly.

curtis73 wrote: I disagree... the tire's contact patch elongates and the sidewall bulges. Sure, they squish, but that doesn't mean that the volume decreases

Actually, it does. Do your geometry math on the shapes. Volume changes with the shape change.

A 1L object remains a 1L object no matter what shape you make it unless you cause it to leak.

Not correct.

foxtrapper wrote:

curtis73 wrote: I disagree... the tire's contact patch elongates and the sidewall bulges. Sure, they squish, but that doesn't mean that the volume decreases

Actually, it does. Do your geometry math on the shapes. Volume changes with the shape change.

That's correct.

Picture a container the shape of a hip flask (some of you may relate). Fill it with water. Now grab the two sides and stretch it out of shape so it has basically a round opening instead of a flat oblong opening.

It will now hold more water. The exterior skin has not changed in area at all. The volume has increased with the different shape.

The pressure difference in a tire loaded/not loaded may be small but it's there.

So we can agree that the pressure can vary, but at the tire sizes & car weights involved with normal use it is minimal?

Let me go over this one more time. We built up an aircraft tire and inflated it to 88psi (standard pressure for the application). Installed it on a 25,000lb aircraft, rechecked pressure - it was still 88psi. Then loaded 25,000lb of cargo on the aircraft, naturally the tire compressed but the pressure gage still read 88psi when checked. That is a variation of 0 to 50,000 lbs and no tire pressure deviation. A car is nowhere near that weight so no pressure deviation would occur there either, and yes I know that cars don't run anywhere near that tire pressure. Nothing would change at 32psi either. The only thing that could affect tire pressure would be atmoshperic pressure change but again that is so small that the change would be in the .00X of a pound difference and not measurable.

So what sort of plane has a mono-gear?

Just messin' with ya...

...of course you will want to divide that weight by the number of tires (approximately). The point is still the same though.

So you step on a balloon. What makes it pop?

;-)

Bill

When you step on a balloon you do not change the internal dimension until one of inner walls of the chamber makes contact with another inner wall. Once your footprint causes a large segment of the balloon to contact itself then the reduced chamber capacity causes the air to stretch against the chamber and eventually tear the chamber.

The problem with the balloon analogy is that a balloon has a lot more give than a tire.

A balloon generally pops because the surface rubber stretches too much to hold itself together.

And if you squish from two opposite directions, it will stretch.