# Tire Pressure: On the lift=on the ground?

#1

I get a new tire. They test the pressure while it’s on the lift. They say it’s perfect. Then they put my 3000 pound car down from the lift onto the tires and say see ya’ later.

I say, “Doesn’t the pressure in the tire go up when you put the weight of the car down on it? Shouldn’t you recheck it?”

They look at me like I am an idiot. Who’s right?

#2

The pressure does not change when you set the car down. Tire pressure is PSI, pounds per square inch. When you set the car down, the amount of rubber in the tire (square inches) does not change, and the rim does not change. And the amount of air in the tire does not change. So the pressure is the same. The shape of the tire is a bit different though, there is a flat spot where it sits on the ground, and what gets displaced from the flat spot gets pushed out to the sides.

#3

They look at me like I am an idiot. Who’s right?

They were right on all counts. BTW- at what pressure do you maintain your spare?

#4

If that is true, then take the argument to an absurd but real test level.

Say I have a single tire at 32 pounds pressure with a real time pressure monitor on it.It is upright on the ground.

I then put a 20,000 pound stack of lead weights on top of the tire. You are saying the pressure gauge won’t go up?

What if I double the pressure on top of the tire to 40,000 pounds. No change in the tire pressure?

#5

No difference in pressure on the lift or on the ground.

Tire pressure DOES change with temperature, however, and the tires should always be checked when cold.

As the winter approaches and the ambient temperature drops, the tire pressure will also drop, so get in the habit of checking the tire pressures once a month. Proper inflation saves fuel.

#6

If the entire outside of the tire were compressed rather than just the contact patch, like if you put the inflated tire in a 500 PSI pressure chamber, then
the air inside would be further compressed and what you say would be true.

But that’s not what happens. The tire pressure stays the same. What happens is that the tire distorts until the total amount of force of all the square inches of rubber in contact with the pavement, at the pressure that you filled it to, equals the total weight of the car. The air inside the tire stays at 30 PSI (or whatever you filled it to.

If, for example, you had a 3000 pound car with the tires filled to 30 PSI, the total force of all the square inches against the pavement would be 3000 pounds of force. At 30 Pounds per Square Inch that would be 100 square inches, or 25 square inches per tire. That would be roughly a 5 inch X 5 inch patch of rubber at each wheel.

• mountainbike

#7

You are both right. You are right: for the tire to support weight the air inside has to be compressed, i.e. have work done on it, so the pressure rises slightly and the temperature will rise slightly which will cool to ambient as it sits. The mechanic is right that the pressure rise is so slight that it does not need to be rechecked. The usual recommended pressure on the door sticker is the manufacturer’s recommended lowest pressure for the weight on the tire. The pressure is going to go up as you drive and as the hot road heats the tire. Mild over pressure is more tolerable than under pressure so err on the side of high.

As an experiment, you might try putting the car in the garage; let the tires cool to ambient; jack up the chosen wheel; measure the pressure with an accurate dial or digital pressure gauge; lower that wheel; remeasure the pressure; repeat the above steps (make sure you don’t lose too much air during each measurement) for stastical significance; and post your findings here. You may prove us wrong.

Researcher

#8

I agree, the difference is negligible. In theory, the volume of the tire will decrease slightly when it is loaded resulting in a slightly higher pressure (based on the mass and temperature staying constant). In practice, there are plenty of “real world” effects that will have a more significant impact on the pressure (most notably, the operating temperature). Don’t worry about it.

#9

You are correct, the reason is that the weight distorts the tire and reduces the internal volume, resulting in higher pressure. However, the change due to the weight of the car is insignificant.

#10

So… when you put the weight of the car on the tires, the pressure in the tires does go up, but not significantly…

#11

Well, it has been reported that Mark Twain once said that when it comes to learning, there are two types of people,most read, research, observe and theorize, the rest of us just have to pee on the fence for ourselves.

I had to jack up one of the cars this morning, so just for the heck of it, I checked the pressure with a quality dial gauge, 32psi, on the mark. After jacking it up, 32 psi, on the mark. I can generally discriminate to about a 10th of a psi with this gauge.

#12

Just the other half of the picture.

This subject came up a few years ago on another web site. I happened to be next door to the test garage for a major tire manufacturer (my employer!). There was a Buick LeSabre on the lift.

I borrowed one of the gauges that reads to the nearest 0.1 psi (didn’t need to “eyeball” it - it was digital and this one was certified - traceble back to the National Bureau of Standards). The fronts (the more highly loaded) increased 0.5 psi. The rears hardly changed (0.1 and 0.0)

#13

I remember once at Home Depot they put a pallet of pavers in the back of an old Toyota pickup and one of the tires went flat. Wouldn’t that indicate some sort of pressure increase?

#14

No, it probably meant that the tire did not start out with enough air pressure
but it was high enough to allow the tire to appear round when the bed
was empty. The added weight changed its shape enough to warrant concern.

#15

I think the pressure goes up, but not significantly. If you calculate the surface area of the 4 tires and multiply that by 30 or so pounds per square inch, you get a pretty big number compared to the weight of the car. My back-of-the-napkin shows a car weight on the order of 2 or 3% of the total pressure in the tires. Since just driving a couple of miles raises the pressure a pound or two, the weight of the car is probably not significant.

Another way to think of it is to assume a car tire has a maximum volume (it’s not stretchy like a balloon). All the weight of the car can do is make a flat spot on the bottom of the tire compared to when the car is on the lift. That flat spot reduces the volume inside the tire, thus raises the pressure. But how much of the volume do you think the flat spot takes? Again, I’d say not more that a percent or two.

A question for those of you who think the weight of the car is irrelevant: Why do heavier vehicles need more pressure in their tires?

#16

I think those tire guys were the idiots for not considering the practical and theoretical applicability of your question! One wouldn’t want to suffer a tire blowout, would one? And, after all, physics is what makes the world go 'round, and mathematics is the “grease”. It shouldn’t be THAT much of a problem for them to calculate the surface area of a torus (tire/rim) of 20 inches mean diameter by 4 inches cross-section, multiplied by the tire air pressure (say, 32 psi), and arrive at the total air pressure loading on the inside of the tire/rim of about 25,000 pounds. THAT should hold up your car. Boy! Some people just don’t think! (Watch for prominent cheek bulge from tongue insertion).

#17

Another way to think of it is to assume a car tire has a maximum volume (it’s not stretchy like a balloon). All the weight of the car can do is make a flat spot on the bottom of the tire compared to when the car is on the lift. That flat spot reduces the volume inside the tire, thus raises the pressure. But how much of the volume do you think the flat spot takes? Again, I’d say not more that a percent or two.

The flat spot does not reduce the volume inside the tire, at least with the car sitting on the ground and not some ridiculous situation. What gets displaced from the flat spot gets pushed out to the sides at the flat spot.

A question for those of you who think the weight of the car is irrelevant: Why do heavier vehicles need more pressure in their tires?

That depends on the width and number of tires, and the weight of the vehicle, and how much deformation is allowable for the application.

My mountain bike tires take 35 psi. So do the tires on my 4000 lb van.

By increasing the pressure, you reduce the number of square inches in contact with the road, and you also reduce tire deformation. This leads to reduced rolling resistance and better gas mileage. It also makes for a harder ride. And it the tire is not made for the pressure, it can cause tire failure.

#18

The flat spot does not reduce the volume inside the tire, at least with the car sitting on the ground and not some ridiculous situation. What gets displaced from the flat spot gets pushed out to the sides at the flat spot.

Actually, a flat spot does reduce the volume, just not by very much. To make this simple, think of a two dimensional cross-section of the tire (a circle if the tire is unloaded, a circle with a flat bottom if the tire is loaded). The circumference of the circle is assumed to be constant (not exactly true, but close enough).

OK, what two dimensional shape has the maximum area for a given circumference? Answer: a circle. Any distortion of the circle (including a flat spot) will reduce the area. If you extrapolate to three dimensions, any flat spot on the tire will reduce the volume compared to a perfectly round tire of the same circumference. However, the difference will be very small.