How is a car with tubeless tires supported?

Maintenance/Repairs
41

Mountainbike,

With all due respect:

The air bag analogy doesn’t work here. With the air bag, the thing being pushed up is on top of the bag. With a tire, the thing being pushed up is in the middle completely surrounded by pressurized air. No matter how you slice it, this pressurized air is acting in all directions and is completely taken out of the equation.

I suspect that many folks think the air pressure is acting on the rim - just like the bottle jack acts on the cylinder. So let’s do a reality check:

My van using 15 X 7" rims and the tires are inflated to 35 psi. So if the air pressure is acting on the rim, it is pushing against a 15" by 7" platform at 35 pounds per square inch - or with a force of 3,675 pounds - and that would be 7,350 pounds per axle. Opps, way too much!!.

Don’t forget that the pressure is there whether the tire is loaded or unloaded.

Also, try a couple of thought experiments:

  1. What would happen if I use a larger diameter rim at the same pressure? How about a smaller diameter rim? What about a rim so small in diameter you can almost take it out of the equation - like one inch.

  2. How do bicycle rims work? If you take off the tire and only have the rim and the spokes - clearly the spokes on the bottom would buckle under the weight if the weight wasn’t suspended from the top of the rim. So in theory, the only spoke that matters is the top most spoke and the axle is suspended from it.

But seriously, in the big scheme of things, the actual working mechanism here is only of value to tire engineers and rim engineers. I’m sure they have figured it out.

For everyone else, it is just an academic game.

42

You call my thought experiment a “false analogy,” but I’m pretty sure it isn’t. The axle puts downward pressure on the tire or tube. It makes no difference. The air pressure equalizes throughout the container. If it’s perfectly flexible the area of the contact patch on the ground would equal the weight divided by the PSI. Now, real tires are not perfectly flexible so there is some support provided by the structure of the tire, but in the main the air pressure is holding the axle up.

Again, if you used just a plain inner tube around the axle, how is it different if the top inner edge is touching the axle or not? It isn’t. The part that seems to be confusing you is that with a tubeless tire part of the “tube” is the non-flexible wheel. Yes, the air pressure on the inner surface of the wheel is equal all the way around, but that is just the same as the air pressure in a flexible inner tube.

Let’s try one more thought experiment. Suppose you have a nice tubeless tire mounted on a wheel. Now, take the center part out of the wheel leaving just the rim such that is still holds air. (some truck wheels are like this) If you put the axle into this wheel it would rest on the inside of the rim at the bottom. Do you still think the wheel is hanging from the top of the tire in this case? If not, then how is it different when the axle is suspended in the center?

43

You actually started an interesting thread. The key point I think everyone seems to be missing is that air is matter. It has mass. It has weight. It’s comprised of atoms and molecules. It occupies space. It resists conpression. It is no less matter than hydraulic fluid. The tire controls the shape of the space that this fluid matter occupies.

Just like the resistance of hydraulic fluid to compression allows it to do work and lift the bucket of an excavator when the shape of its containment is controlled, the resistance of the air to compression allows it to do work and keep the axle suspended when the shape of its containment is controlled.

The place I differ in the example of the basket ball is that IMHO the ball isn’t lifting the axle…the air is. The ball is simply the container that allows us to use the air to lift the axle.

Think of it from a design standpoint. You want to lift 700 pounds, yet still retain in the lifting system the ability to absorb impact. It needs to rotate. It needs to be able to be angled relative to the main chassis to accomodate turning. What do you use: hydraulics? Pneumatics? A mechanical system such as springs? You settle on pneumatics because of its simplicity and abilities. What’s the best system to use? Ultimately, that will evolve to the tire. The tire and wheel assembled is a simple pneumatic system with the ability to rotate, be articulated relative to the chassis, and absorb impact.

People are overthinking the problem.

44

C=Racer, the air pressure theory works thusly; the reason the wheel stays up is that the air inside the tire has no place to be pushed to to get out of the way of the weight of the loaded wheel. If the tire were made of the same rubber as a balloon the rubber would expand on the sides to provide a space for the air to be pushed into and the wheel would fall. But in the case of the tire the air in the entire envelope, the entire tire, is resisting being compressed and is holding the wheel up by not allowing the air below the wheel to move up. The air under the wheel is trying to move up the tire but it cannot.

The bicycle rim analogy falls because that’s a comparison to the rim only.

It’s just an academic mind game, but I’ve found it to be one of the most thought provolking and funnest (funnest?) we’ve had for a while. I’m enjoying this one. And, having come to know you as a technically astute individual that’s not afraid of a challenge, I’m betting you are too!

45

First, I think that you guys are all looking at this in the same, wrong, static way. You have to consider the dynamic situation for a robust explanation of the mechanism. There is far more force within the tire (compressed air) than is needed to support ? the weight of the car as Capri calculated. Only a portion of that force is required to support the axle (wheel). What happens as the tire and air are compressed by the downward force of the weight on the axle (wheel)? The tire on both the top AND bottom of the wheel are distorted by gravity and the mass of the vehicle. This is also true in other directions when the tire ?hits bumps?.

What is the function of the air pressure? It certainly does not support the mass of the car in any way. It has to be analogous to the preload tension within a spoke wheel which is much higher than the actual load on the wheel. No preload = collapsed spoke wheel = collapsed pneumatic tire. The function of tires is more analogous to spoke wheel than a solid wheel. Why does the tire get flat on the bottom, because it is supporting weight? NO, because it can not support weight. It is collapsing to the point that it must, due to the stretch in the casing above the wheel until the weight is supported. There is no way flexible rubber-like material or fabric or steel wires can support weight. It is no good in compression. The casing above the axle is taking the force in tension. Robertlipp is correct, of course.

I have three questions:

Where the heck are Robert Thomson and John Dunlop when you need them?

If compressed air can not support weight, how do airfoil boats work. How about the analogous support of a similar body over land?

Have I exposed myself to too much crack today?

46

axles have nothing to do with tubeless tires…it is the airpressure that seams the tire to the rim

47

Beadsandbeads and mountainbike have made some interesting points. Ultimately of course it is the ground that supports the vehicle, with the air and tire merely links in the chain of things supporting the car. But very interesting and important links! Mtbikes comments the other day about bike spokes being a suspension system have been rattling around in my head all day. I logged on to comment, and I found B&B practically took the words from my mouth, plus adding a few other good thoughts. Bike spokes are always in tension. When supporting weight, the tension on the top ones is increased while the tension on the ones on the bottom is decreased, BUT they all remain under tension! When a tire is inflated off the car, the sidewalls are under tension equally just like spokes on a wheel. Under load, the tension on the upper half increases while the tension on the lower half decreases, But the sidewalls remain under tension everywhere.

BTW - airfoil boats are just big leaky balloons - tubeless of course :slight_smile:

48

Interesting thoughts about the sidewall tension. Perhaps our assumption that only one mechanism is at work to hold up the car is shortsighted. Perhaps it’s both the properties of the fluid within the tire and its dynamics as well as the tensile strength of the sidewalls, both acting together as a system.

A seperate thread about the airfoil boats might also be interesting. It isn’t the leak that holds the boat up, it’s the pressurized air still under the skirt. Escaping air, as in a leaky baloon, can support weight via thrust, but the support mechanism at work in the air boat is entirely different, more like an intact baloon supporting a mass…more like those emergency lift bags I “linked” to. More like the air bags in an air suspension system.

49

Geez, I had three paragraphs when I posted this. What happened?

50

My car weights 4,000 pounds. Each tire supports 1,000 pounds. The air pressure in the tire, is 32 pounds per square inch (psi) which is pressing against the 1,500 square inches of the inner sidewall surfaces, for a total force of 48,000 pounds against those sidewalls. The wheel carries that 1,000 pounds to the sidewalls. That force (gravity) is carried in compression by the lower half of the sidewalls, and by tension by the upper half of the sidewalls. Deformation of the sidewalls, by the 1,000 pound, is resisted by the 48,000 pounds of force acting against the sidewalls. The 1,000 pounds of force adds 2/3 (two thirds) of one pound per square inch (psi) of force to the 32 pounds per square inch (psi) of force already acting against the sidewalls. The sidewalls are buttressed by the 48,000 pounds of force exerted by the 32 pounds per square inch (psi). So buttressed, the sidewalls are, easily, able to support the 1,000 pounds. The force is transmitted to the ground by the sidewalls (and tread).

51

Interesting thought. Two comments. There is another thread floating around where someone asked if air pressure that was measured with the wheel off the car needed checking after installation. Someone went out and checked and did not find a measurable difference. (There probably is, but it is very small, much less than the 2/3 pound you mentioned which is very measureable). Second comment. Adding more weight makes the bottom of the tire bow out more. It seems buttressing would need to be applied to the outside of the tire to make your theory work as the deformation actually occurs in the same direction buttressing force is applied.

52

Yup. Things are almost always more complicated than they seem. There are always second and third order effects that occasionally can be very important. For now I’m happy with the tension idea as the primary force on a normally operated car tire.

I agree about the airfoil boats. Poor attempt at humor on my part.

53

The “buttressing” is the air pressure (32 psi x 1500 sq.in. inner sidewalls and inner thread surface) for a total of 48,000 pounds force. The force acts against both sidewalls, and tread area, simultaneously. They are all connected as a unit.

54

It’s obvious there are 2 camps on this issue - and I don’t think one side is going to convince the other without a breakthrough in the explanations.

Here’s one from the “Not the air, It’s the tire” camp:

For the tire to bulge at the bottom when loaded, the rim has to be offcenter relative to the hoop formed by the tread. This also means that at the top, the sidewalls are sucked in a little bit.

55

Air pressure stiffens the tire and the bottom half of the stiffened tire supports the wheel/axle.
Cut the tire in half horizontally, cap the ends and fill the bottom half with air, the car is supported. But, cap the ends and fill the top haf with air and the axle hits the ground.

You are correct when you say the air pressure does not directly support the tire.
You are incorrect when you say the axle/wheel hangs on the tire (but, BTW, that IS how bicycle wheels work…). There is no mechanical connection between the tire and the wheel.

56

I disagree with your methods. Folks are trying to simplify this to a basic statics problem, which it is not; its a more complicated mechanics problem. If you “cut” members in a mechanics problem, you have to replace any internal forces and moments. Cut off the bottom half of a bike wheel and it falls to the ground too, yet you do agree that the bike axle is hanging from the spokes. First, do your mechanics cutting correctly on the bike: if you cut the bottom half below the axle, there is the large downward force of the bike’s weight and the downward force of the spokes you’ve cut. For the sum of forces to be zero, you have to put an upward force somewhere–on the rim. Now, it won’t fall to the ground. Then ask what supplies the upward force: We know there’s an upward force of the air pressure (any time you cut a pressure vessle in half you add the vector of the pressure force you removed). Then there are internal forces in the tire and rim. The rim could feasibly be in compression, and a tension force in the tire could counteract the pressure force of the air. It’s statically indeterminate to untie those three forces in the tire and rim, which means you have to understand the mechanical properties of materials, not just the force diagram. Same thing with the car tire. And there IS a mechanical connection between the tire and the wheel in a car. That’s the bead, and it allows the tire to be in tension without popping off the rim. At least you have to agree that the tire is completely in tension on an unloaded car tire, pulling radially away from the rim to counteract the pressure. Now you can start thinking about all those internal tension and pressure forces on a loaded tire and really confuse yourself.

57

Easy hypotheses to test. Let the air out of your tire and see if the sidewalls alone hold the car up! Let us know your findings!

58

The force is transmitted to the axle/wheel, then, to the sidewalls. The sidewalls transmit the force (press against) the air volume. The air volume presses back and presses against the tread. The tread presses against the ground.

59

Regarding the listener who wondered why tire pressure did not change when it was lifted onto a jack, I think that there is a better explanation than that given by your resident expert, Wolfgang.

Using trigonometry, and assuming that my tires are shaped like a torus, the surface area of each tire
A = 4(pi)(pi)Rr, where R = the radius to the center of the torus, and r = the radius of the tube
R = 10 inches, r = 3 inches, A = 300 square inches.
The area of four tires then is 1200 square inches.

If the car is filled to 30 lbs/square inch while on a jack, the total air pressure on tires and wheels will be 36000 lbs. It then follows that the pressure exerted by the tires and wheels will be the same.

If the car, which in my case, weighs about 3000 lbs, is lowered to the ground, the tire will be flattened to equal the weight of the car. In this example, the area flattened will equal 3000/30 or 100 square inches. The volume of the tire will be reduced to account for this flattening, and the pressure will increase according to Boyle’s Law.

Unfortunately, I don’t know how to calculate the loss in volume, and went about it in a different manner. Assume, in the extreme case, that the weight of the car is added to the total pressure of the tires plus wheels on the gas entrapped. The total pressure then becomes 36000 + 300 = 39000 lbs. If we then divide the total pressure by the area of the tires, we then get 39000/1200 = 32.5 lbs/square inch.

In reality, the total pressure will not increase because the pressure exerted by the car will offset the pressure that would have been exerted by the part of the tire which is flattened.

If Wolfgang, can calculate the volume shrinkage caused by lowering the car to the ground, I suspect that he will probable come up with a number closer to 30.5 lbs/square inch.

Hope this helps.
Stan Storfer

60

I always enjoy reading these type of threads. To your point, this is akin to asking the question; what moves the car, the wheels or the engine? One does no good without the other. It is a SYSTEM. Components of a system interact to produce a result.