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 Posted by: martin_uk ® 03/01/2005, 19:45:16 Author Profile Mail author Edit I am hoping someone can answer a technical question, to resolve a long standing argument regarding car tyres.The question is - what supports the weight of a car, the sidewall of the tyre or the air.My understanding is that the air is there only to maintain the shape of the tyre, and allow the sidewall to do it's job, holding the weight.Thinking about it, with a vehicle sitting on the ground, the force due to the weight of the car acts downwards and is equalled by the force of the groud on the tyre. This is acting at the point of contact (ie. the bottom of the tyre) - if the air was supporting the weight of the car then would not the pressure (of the air) be greater at the bottom of the tyre than the top (?) which of course it isn't as pressure in a gas is constant. Further, if say an average tyre pressure is 30psi and the car is jacked up, the pressure remains the same (30psi). Again, if the air was supporting the weight of the car, would this pressure not be expected to increase when the car is on the ground. Effectivly the weight of the car would cause the air to be compressed causing a higher pressure - which is not the case.If I am wrong, how is it that the pressure stays constant, with or without the weight of the car.I would be most grateful if you could resolve this for me.Kindest regards,Martin Holt

 Replies to this message

 Posted by: deepalcatrazz ® 09/08/2005, 14:15:03 Author Profile Mail author Edit Hi MartinThe question is misleading. The weight of the car can be held by tube tyres i.e tyres with no air in them as well. Think of the tyre assembly as a single elastic body with a certain prescribed stiffness. Then, isolate this body as a free body in space. Once you have done that, You will understand that the rubber of the tyre is in a constant state of prestress. Thus, there is an interplay between the pressure exerted by the air and the strain energy stored in the tyre/tube. This pressure does not change..i.e when the car is jacked up, the tyre material restates itself and consequently expands when in contact with the ground. However, coming back to the question about which one takes the load..its both of them acting as a system!! Hope this helpsCheers deepak

 Posted by: Cragyon ® 03/02/2005, 09:23:50 Author Profile Mail author Edit My understanding is that the air is supporting the weight of the vehicle. Obviously, a tire without internal air pressure would not support the weight of the car. I do know that the design and manufacture of the tire is what maintains the proper shape of the tire under correct internal pressure and when loads are applied during manuvering. In fact, if you have a structural failure in the side wall of the tire, every time you make a turn into the failed tire, the tire will partially lay or bend over due to the side force applied on the tire. I beleive the pressure in a tire is approximately constant. This is due to the changing of shape of the tire under loading. Now, as the air within the tire heats up, you will see an increase in pressure due to the expansion of the the air gas.

 Posted by: martin_uk ® 03/02/2005, 14:33:57 Author Profile Mail author Edit I see your point - but consider the following,If you lowered the weight of the car onto four (one for each wheel) cylinders with a free moving piston, full of air, the weight would then be supported by the air, and the pressure would indeed rise inside the piston - therefore you would expect this to happen when a car is lowered onto its tyres, but the air pressure does not rise ??? Does this lack of increase in air pressure not show that the air is doing nothing (apart from maintaining the shape).Or yet another way of looking at it, suppose the tyres were hollow, but very very stiff rubber, almost solid. There is a vacuum inside the 'tyre' - clearly the rubber of the tyre supports the car. Now, replace that vacuum with air, nothing changes, still the rubber supports the car. Now make the sidewalls thinner, eventually they will 'flex' but still support the weight of the car - there is no point at which the support suddenly jumps from the sidewall to the air !! What we have 'created' moving from a solid structure to one with thinner walls that flex, is indeed a tyre !!!If the air does support the weight, how do you explain te lack of increase in pressure when the car is on the ground.Indeed a tyre rotating maintains a almost constant volume, yes it changes shape, but volue is near constant. The increse in preesure due to temp. change is valid, but fairly minimal (at least under normal useage.I was originally told of this as a potential trick university / job interview question, by a mechanical engineer who specialised in fluid mechanics so I have very good reason to believe it is true. My engineering days are 'behind me' (I'm in IT) but have a degree in mech eng. I looked at a couple of tyre manufactures websites, and I really believe that one has it wrong as they state that the air supports the weight, but to me, putting across the views above, do I not have a point.Thank you for your interest and reply - I may get the books out and see if I can remember enough to prove by maths.Its a bit like the bike spokes question, the ones 'hanging down' from the top support the weight (well transfer it to the wheel), not the ones 'pointing up' - thats a difficult one to get your head round also.Thanks,Martin

 Posted by: zekeman ® 03/04/2005, 20:36:59 Author Profile Mail author Edit The answer is both air and tire contribute to the forces sustainng the load. Assume that the flat area of contact of tire on ground is A. then a free body equilibrium analysis on the flat material yields W=A*p+s*l where W= reaction force p=air pressure and where A is the measured area of contact and s is the shear force along the periphery and l is perimeter of contact. Since the tire deforms negligibly overall, the volume remains essentially constant under the loads we are dealing with and therefore pressure p remains essentially constant for this discussion.(obviousy under ambient/driving conditions the temperature changes would change the pressure proportionately) The way that the reaction force W is delivered to the rim of the tire is thru the tire wall due to the local shear forces and compressive forces manifested in the distorted tire and are very complex but are entirely solvable by finite element analysis. In the absence of sufficient air pressure the natural forces on the tire at the rim would be compressive and shear which would tend to collapse the tire. Under air pressure the "hoop" stress due to to the air is tensile which reverses the effect of the compressive force , thus holding the tire configuration and preventing collapse.

 Posted by: randykimball ® 03/04/2005, 23:13:48 Author Profile Mail author Edit A simple down to earth answer, if you please....The air in the tire forces the tire to hold shape. The tire can not stretch much, but it can be depressed. The air pressure resists this depressing of the tire. When the forces of mass weight and depressing of the foot print reach equality the tire holds up its share of the car to that height.BOTH hold up the car. The side walls have little effect unless the tire is out or near out of air.-randy- The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them.