The volume doesn’t change.
Hey Sniper - The tires on your SUBARU are overinflated.
Rodd,
I would be very careful slinging arrows. Some of them are barbed and tend to come back to hunt the ones that launched them.
I submit that the air does nothing except to stiffen the tire - and I can prove it. Plus in Septemeber there was a whole room of tire engineers who agreed with me. (probably a couple hundred.)
So to try to prove my point - if the air is 100% holding up the vehicle:
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What is it pushing against?
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What is the pressure it is pushing against?
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What is the area of the object and what object is it that it is pushing against?
This last question is a reality check, because the the object that the pressure ought to be pushing upwards is the rim and the that force ought to be area times the pressure. For example: a 15 X 7" rim is 105 square inches - and an inflation pressure of 35 psi ought to yield. 3675 pounds - and I can assure you my van doesn’t weigh 14,700 pounds
If a vehicle has a weight of 2400 lbs and
assuming the weight distribution is equal to
all 4 tires then each tire is holding up 600
lbs. If each tire has 40 psi of pressure
then it in turn must have a contact patch of
150 square inches. 2400lbs divided by 4
divided by 40 psi. If you lift the car off
the ground you decrease the load on the tires
to 0 and create a contact patch of 0 square
inches leaving the pressure in the tire the
same. The contact patch will increase or
decrease with the load to support the load
depending on the tire pressure.
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The air pressure is pressing against the earth on the bottom. Air pressure is also pushing against the load on the top which would be the car.
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" "
3)The area doesn’t matter. Air is a gas. It pushes out equally in all directions. Don’t let all these numbers confuse you. We are dealing with relativity here. It is all relative to atmospheric pressure.
I think what you are leading up to by comparing areas is a misconception. If the fluid were a liquid such as non-compressible hydraulic fluid, then areas matter. In that case you wouls be setting up a master cylinder-slave cylinder situation.
Sorry, dog, but any fluid works - air, nitrogen, water, oil.
Any fluid under pressure exerts a force equal to the pressure times the area it is acting on.
your half there. any distortion in the tire will alter volume. when you alter volume you alter pressure so you not entirely correct. as was stated earlier the tire changes pressure when loaded but by only .02-.05 psi. i roughly calculated tire distortion required to produce the proper contact patch to support the weight of the car. my math shows under .200" of compression with some liberal #'s. and then figured the difference in volume again using a worst case scenario as it would be difficult to account for sidewall flex. then i converted psi to atmospheric bars multiplied my “compression ratio” and back to psi. it accounted for a .06 psi change in pressure. i think that coincides with what is shown in practice with a very sensitive gage.
I think you mean : pressure divided by the area that it is acting on.
True, the pressure is distributed throughout. But, the similarities end there. Air and nitrogen are both a gas at room temperature. Water and oil are both liquid at room temperature. The physical properties of a gas are different than a liquid. For a gas to do work, it must be compressed (relative to atmospheric pressure). For a liquid to do work, something must be displaced.
With a gas twice the amount of air = twice the amount of pressure, all other things being equal. If 10psi can lift a 10lb bowling ball 10ft, then 20psi can lift a 10lb bowling ball 20ft.
With a liquid there is a multiplicity effect. If you push a 10" cylinder with a force of 10lbs., and on the other end there is a 1" cylinder, 100lbs. of force will be generated.
So, let’s go back to the shop. I am sure you know what a hydraulic floor jack is. As long as the scissors mechanism can take the load, it is possible to generate enough force to lift the car with only a small amount of liquid. Less common, but you have probably seen one, is a pneumatic bumper jack. If the tires are at 32psi and you put more than 32psi into the jack (It’s all relative), then the tires come off the ground. After that, twice the pressure = twice the height.
The pneumatic bumper jack operates the same as a tire.
Sometimes it is not very useful to describe gas pressures in terms of Pounds Per Square Inch. It can even be confusing. It’s just a point of reference. Something for us to visualize. The square inch in your tire is the same as a square inch on the earth. It can sometimes be used for comparison. But we are talking about the same tire on the same car. The area inside your tire doesn’t change. It is a constant.
A more useful term is Barre. 1 Barre = 1 atmosphere. It’s kind of a nebulous term really. Itis used to describe the amount of energy present in a given volume of air. It relates this energy to atmospheric pressure. A gas will only expand as far as atmospheric pressure will allow.
I have to guess that you are interested in going fast. Get a good Physics book and check it out. If you can understand how a tire works, then maybe you can understand how a car engine works. To get the most out of your ride you have to understand how the whole vehicle works, including the tires.
Dog,
First, I am a degreed engineer with 35 years experience in the tire industry. I analyze tires for a living. I know how they work. My challenge in this forum is to present discussion points that are simple enough and clear enough so that everyone who is interested can understand the concepts being presented.
Not to put to fine a point on this, but there are some items in your posting that need to be clarified:
“…I think you mean : pressure divided by the area that it is acting on…”
Nope, pounds per square inch times square inches equals pounds.
“…but you have probably seen one, is a pneumatic bumper jack. If the tires are at 32psi and you put more than 32psi into the jack (It’s all relative), then the tires come off the ground…”
Uh…the pressure in the jack that is needed to lift the tire off the ground is going to be dependent on the cross section area of the pressurized cylinder. My bottle jack works on that principle and it’s about 1 1/2" in diameter - certainly a lot smaller that the footprint of a tire.
“…The pneumatic bumper jack operates the same as a tire…”
Sorry, but no, the jack is in between the object being lifted and the ground. A tire completely surrounds the object being lifted, so the pressure is pushing inwards on the rim 360 degrees. Net effect = no net vertical force, no net horizontal force, no net force at all.
“…A more useful term is Barre…”
BTW it is spelled “bar” and it is defined very precisely: 1 bar = 100kPa (kiloPascals) = 14.5033… pounds per square inch - or at least that is how the European Tire and Rim Technical Organization defines it.
“…I have to guess that you are interested in going fast…”
Was the race car in the garage a hint?
“…Get a good Physics book and check it out…”
Got one - Halliday and Resnik. It’s about 2 1/2" thick, and it’s a bit old, but the laws of physics don’t really change over time - may be our understanding might, but at college entry level, the stuff presented in fairly stable.
So please bear with me while I try to walk everyone through the mechanics of tires. There are some important concepts and one of them is they don’t behave like hydrualic jacks.
Correction : With a hydraulic jack, push on the smaller cylinder and you get a much larger pressure in the larger cylinder.
Correction to the correction : The pressure is the samme in both cylinders. The total force acting on the larger piston is bigger.
Wow! This is the most spectacular display of “male answer syndrome” I’ve ever witnessed. What a collection of wonderful, well reasoned, scientifically sound, WRONG answers. Everybody seems to have whiffed on this one. Here’s the real answer. The air in the tire does not really change in volume so the pressure doesn’t change. The bulge in the sidewall almost entirely compensates for the flat spot on the bottom. Anyway, any minute pressure change is totally beside the point; you can find and explain the pressure change but it a red herring. Before you start arguing with that, try this. The air in the tire does not support the car AND the car does not rest on the tire! I’ll explain.
We have all been culturally trained to think in medieval terms about wheels and tires. For thousands of years vehicles have rested on axles, which have rested in wheels, which have rested on the ground. The weight of the vehicle pushed straight down on the axle, which pushed straight through the wheel to the ground. Soon wooden spoked wheels had tires. At first they were thin bands of metal that helped the wheels stay together, wear more slowly, and have less rolling resistance. Later these tires were fitted with a solid rubber surface to absorb some of the carriage wrecking shock of cobblestone pavement. But when pneumatic tires came along, everything changed? except for human perception.
To be completely fair, wire spoked wheels were the real revolution (pun intended) and they are the best way to explain the situation. Vehicles don?t rest on their tires, they HANG from them. You don?t think a Ferrari Testa Rosa is supported on top of the dozen or so little wire pokes at the bottom of those gorgeous Borani wire wheels?! No, the bottom wires carry almost none of the load and would collapse instantly without the rest of the wheel. The spokes act in tension. The tensile strength of the spokes under the upper arch of the rim is more than enough to carry the car. The same principle applies to your Uniroyal radials. The thousands of fibers in the tire carcass act exactly like the spokes of a wire wheel with the big exception that they curve sideways between the solid wheel rim and the tread band of the tire. The tread band of the tire, usually reinforced with steel or nylon belts, acts as an arch from which all those fibers encased in the rubber side-wall are suspended like shrouds from a balloon. The 30 PSI air does only one thing. It keeps the tread band arch up and gives the side-walls enough tension to keep their shape.
All this time we thought our cars sat on a cushion of air wrapped in rubber. The shocking truth is that all that mobile metal is suspended by a handful of thread!
Dear Oh My Aching Head,
I sympathize. Please see comment 51, take 2 asprin, and get some rest.
Magneto, Welcome to the club.
Please read over the above posts again.
Please explain why I have been able to consistently measure a pressure change?
And before you post back, please conduct the measurements yourself to verify what you say is true.
BTW, here’s a similar thread:
I don?t wish to be indelicate. This is a bit like stepping into a conversation at a party when the other guests are verging on testiness. I have no doubt that you are able to consistently measure pressure change because there is a pressure change. I am equally sure that, as a tire analyst, you appreciate tires as fairly complex engineered structures and, upon careful consideration, you will admit that the pressure change is incidental and misleading. Misleading, that is, in terms of answering the original question, which is obviously predicated on the mistaken assumption that the air in the tire supports the weight of the car. The air in the tire supports the structure of the tire. When that structure is stressed under the weight of the vehicle, yes, it distorts (per design) and the elastic nature of the air seal, the rubber, allows for the decrease in internal volume that accounts for the pressure change. All this is great fodder for conversation but it misses the point, which, to me, would be addressing the initial misconception. Oh and why would I want to ??conduct the measurements (myself) to verify what (I) say is true?? You are ??a degreed engineer with 35 years experience in the tire industry??
You know what I?m saying is true.
Well CapriRacer, I hate to beat a dead horse, but here goes. I bought an Accutire gauge with .1 psi increments and I’m very disappointed with it. Its not the same model as the one at tire rack. I found it to be very difficult to get a reading because the nozzle does not seat well against the valve stem. It has almost no lead in. Good gauges have at least a 1/4 lead in, that is the indentation that just slip fits over the threads of the valve stem to insure the mating surfaces will be parallel.
Every time I used the gauge, I lost at least .2 psi, and that was when I got lucky and got a good mate the first time. I suggest that you try your experiment again in the opposite direction. Raise the vehicle, measure the tire pressure, then set it down and measure again. I’m not going to try this test with the gauge I bought because it is just too unreliable. The gauge was a waste of money.
Been there. Done that!
I’m going to beat this dead horse one more time, this time I did get results closer to yours. Instead of measuring the air pressure on the ground and raising the vehicle, I pressurized the tires in the air and lowered the vehicle to the ground. The vehicle is a 97 Nissan Pick Up, regular cab. Started with 36psi on the front and 31.3 on the rear. When I set the vehicle down, I got 36.2 on the front and 31.5 on the rear. Still don’t like that Accutire gauge though, hard to get it to seat right.
I don’t know if the type of tire makes any difference. The truck has Michelin tires which are true radials. The Saturn has Bridgestone Turanza’s, the Tercel has Dunlop. Neither of those are true radials in that they have two ply sidewalls. Two ply sidewalls have the plies at a slight angle, up to 15?, that make the sidewalls stiffer. Interestingly, Michelin is going to this design on some of its newest tires.
Keith,
I’m glad you were able to duplicate the results.
I’m sorry, but I have to correct you on one point:
“…Two ply sidewalls have the plies at a slight angle, up to 15?, that make the sidewalls stiffer…”
Having 2 plies doesn’t mean necessarily that the plies are at a bias. I’d go so far as to say that it would be fairly rare.
"Having 2 plies doesn't mean necessarily that the plies are at a bias. I'd go so far as to say that it would be fairly rare."
They don’t have to be on a slight bias, they just are. Thats the Pirelli patent. The Michelin patent is a single ply at exactly 90?