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Q: Tyre Pressure again ( Answered ,   1 Comment )
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 Subject: Tyre Pressure again Category: Science > Math Asked by: russell2002-ga List Price: \$12.00 Posted: 20 Jul 2003 10:14 PDT Expires: 19 Aug 2003 10:14 PDT Question ID: 233041
 ```I previously asked - http://answers.google.com/answers/main?cmd=threadview&id=232922 My next related question - If my rear tyres are 275/40/18 and 245/45/18 for the front on my car, What is the total volume inside each tyre, and as the tyre pressure is 35 PSI what is the total volume of pressure inside each tyre, I apreciate you can not answer this 100% as the rim design, sidewall size ect may distort the answer a little. Now, what would the answer be if the tyres were 275/35/18 and 245/40/18, and with that, by how much would the tyre pressure need to be adjusted to maintain the same volume of air in each tyre.``` Request for Question Clarification by smudgy-ga on 20 Jul 2003 20:41 PDT ```Hi, When you ask, "What is the total volume of pressure inside each tyre," what exactly are you looking for? It would be relatively straightforward to calculate, for instance, what the volume of the air in the tyre would be if the air were at normal air pressure. Is this what you are looking for? Or are you looking for something else entirely? Thanks, smudgy.``` Request for Question Clarification by smudgy-ga on 20 Jul 2003 21:55 PDT ```Hi Russell2002, Given that the 275/40/18's are at 35psi, I have calculated the pressure at which the 275/35/18's must be to hold the same volume of air, and likewise for the 245/45/18's and 245/40/18s. In the process I calculated the volumes of all four types of tyre. If this is the information you are seeking, please let me know and I will post it. (If you are seeking other information as in my previous request for clarification, please let me know and I will include it as well.) Thanks, Smudgy.``` Clarification of Question by russell2002-ga on 20 Jul 2003 22:45 PDT ```You are correct, I basically want to know by how much to adjust the PSI in each tyre to maintain the same volume of air.```
 ```Hi Russell2002, I hope you find the following answer meets your needs. If not, please request a clarification and I will address any problems or concerns you have. There is extensive math that I have included for your interest, but if you don't care to see it, the results are summarized as follows. Note that these results involve some simplifications and approximations and are not meant to be exact (details about the nature of the simplifications is at the beginning of the justification section below). 275/40/18 tyres at a pressure of 35psi hold as much air as 275/35/18's at 43.5psi. 245/45/18 tyres at a pressure of 35psi hold as much air as 245/40/18's at 42.2psi. Justification below: We will idealize the situation slightly to make calculations easier. Let's assume that the tyres are perfectly cylindrical and that the sidewalls are perpendicular to the tread, etc. The resulting volumes and pressures will not be off from the actual answers by more than a couple of percent. Also note that the measurements neglect the thickness of the rubber; this causes us to overestimate the volume of the tyre, but our ignoring the bulging at the sidewalls and the volume of the rim causes us to underestimate the volume of the tyre, so the combination of the two factors will not cause our estimate to be -too- far astray. Decoding the tyre sizes, we seek information on the following four sizes: 275/40/18: Tread width 275mm, sidewall height 110mm, inside diameter 18 inches=457.2mm 245/45/18: Tread width 245mm, sidewall height 110.25mm, inside diameter 457.2mm 275/35/18: Tread width 275mm, sidewall height 96.25mm, inside diameter 457.2mm 245/40/18: Tread width 245mm, sidewall height 98mm, inside diameter 457.2mm So imagining the tyres as cylinders with holes in the center, we can get their volume thusly: Volume of whole cylinder - volume of hole = volume of tyre. We will work in meters since this will make the numbers easier to work with: The volume formula for a cylinder is pi*radius squared*height, where height in this case is the tread width, radius is the radius of the tyre, and pi = approximately 3.1415926... Volume of whole cylinders: 275/40/18: pi * .3386^2 * .275 = .0991 cubic meters 245/45/18: pi * .33885^2 * .245 = .0884 cubic meters 275/35/18: pi * .32485^2 * .275 = .0912 cubic meters 245/40/18: pi * .3266^2 * .245 = .0922 cubic meters Now from each of these volumes we must take away the volume of the hole in the middle, which for the tyres of 275mm tread width is .0451 cubic meters and for the tyres of 245mm width is .0402 cubic meters. This gives the volume of the four tyres as 275/40/18: .0539 cubic meters 245/45/18: .0480 cubic meters 275/35/18: .0460 cubic meters 245/40/18: .0419 cubic meters Now, using the Ideal Gas Law, we will be able to determine how much air is in 275/40's and the 245/45's, and then work backwards and find what the resulting pressure will be in the other tyres given the same volume of air. The ideal gas law tells us that if temperature and total amount of air stay the same, there is an inverse relationship between pressure and volume-- that is, the smaller the volume, the higher the pressure. Specifically: pressure * volume = a constant, for a given temperature and amount of air. (I am glossing over some details here.) Using the convenient ideal gas law calculator at http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/idegasc.html#c1 we see that if the pressure reads 35psi at the gauge on the 275/40's, the same volume of air in the 275/35's will produce a pressure reading of 43.5psi on the gauge (assuming an ambient temperature of a balmy 25 degrees Celsius). Likewise, the 245/45's at a pressure reading of 35psi hold the same amount of air as the 245/40's at a pressure reading of 42.2psi. (It is interesting to note here that as far as the gas law is concerned, we need to take into account the nominal 14 psi that the tire and gauge are both under due to the pressure of the atmosphere. The gas law calculator takes this into account if you enter the psi in the entry marked "gauge".) To summarize: 275/40/18 tyres at a pressure of 35psi hold as much air as 275/35/18's at 43.5psi. 245/45/18 tyres at a pressure of 35psi hold as much air as 245/40/18's at 42.2psi. I hope this answers your question satisfactorily. If not, please request a clarification and I will do my best to answer more completely! Good luck, smudgy. Google search terms: Relevant results used for above: http://www.tirerack.com/tires/tiretech/general/size.htm Relevant results: http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/idegasc.html#c1```
 russell2002-ga rated this answer: ```Excellent answer, However, an answer which only opens more questions, its now obvious you cant simply scale up the PSI, the highest I have tried is 41PSI which causes the tyre to start to bulge and cause some handeling problems..... O well.....```
 ```Very impressed by the detailed explanation of the calculations! My point/question is what would one hope to gain by equalizing volume? Keep in mind that your tyres basically have two functions; the first is to serve as the friction interface area (contact patch) while also acting as a (pneumatic spring) component of the suspension. Increasing the pressure in a smaller tyre to equalize volumes will effectively reduce the friction/traction of that axle, while increasing the spring rate, resulting in handling changes as you noted - which I would imagine was more exciting than you'd prefer. Rather than worry about volumes, it would be more valuable to consider tyre temperatures; if the tires on both axles are approximately the same temperature after a vigorous drive, you can be confident that neither axle is being 'overworked'. Just keep in mind the relationship between T, P, and V, and use your air pressure as the variable to achieve an equalized temperature, and ignore the volume issue altogether. Hope that is of some interest in regard to the basis of your original question.```