cdsuggs Report post Posted October 29, 2017 I noticed something on our way back from the South Central Area rally yesterday. I had checked all of the tire pressures before I left, and they were all set to what was intended. A couple of hours into the ride home I noticed that one of the rear tires had actually built pressure to 1 pound above the limit for the tire and the wheel. That got me to thinking, hard as that may be. Everything that I read refers to the COLD pressures. I'm assuming that the limits on the tires and wheels make allowance for the expected pressure increase as the tire warms up to the expected temperature. I realize that this is more of a theoretical question, but does anyone have an opinion on how that is determined, especially since most of us don't use N2 to inflate our tires, don't always operate at sea level, and the difference in the maximum temperature and the cold temperature can differ due to conditions such as weight, relative wind and speed? Share this post Link to post Share on other sites
wolfe10 Report post Posted October 29, 2017 Correct. Both tire and wheel PSI's are for COLD. They are very familiar with the Ideal Gas Law and took that into consideration when developing the specs. Share this post Link to post Share on other sites
bm02tj Report post Posted October 29, 2017 If you did not get your air from a source with an air drier then use your dry air from your motor home or find a shop with an air drier to give you air with no moisture or go Nitrogen the expansion is less for dry air than moist air Share this post Link to post Share on other sites
tireman9 Report post Posted October 29, 2017 You didn't provide the numbers. The % or psi increase can be used as a rough indication of how correct your cold inflation is based on your actual load. It's hard for me to guess as your "1 psi" margin might be the result of a pressure increase of 10 psi or of 20 psi. If you are seeing a pressure increase of 20% you need to have things checked out.add link What were your TPMS Temperature and pressure readings in the AM before you started driving and before the tires were in direct sunlight? What were the readings 2 hours later? It would help me if you could also provide tire size, Load on each end of each axle if you have a tag. Here is a link to posts related to temperature and pressure growth. HERE is a post on Max pressure. Share this post Link to post Share on other sites
cdsuggs Report post Posted October 30, 2017 The two links went a long way in letting me know that others may also have minds full of very interesting thoughts that are of somewhat less use than they are interesting. Some of what I read was information that I knew. My major background is in military strategic/tactical aviation. Large aircraft tires are similar to large truck or RV tires, except that they get worn out much faster and are filled with nitrogen which is normally converted from liquid nitrogen. This makes the pressures much more stable, especially since tire temperatures can vary between as low as -50 degrees centigrade and +200 degrees centigrade. Convert that to Fahrenheit and you find temperatures that you would definitely not wish to place your bare hand on. In both the C-141 and the C-17, the safety factor for tire limiting speed was 150%. I don’t know what the bursting pressure would be, but I suspect it would be rather high. In the case that I was referring to, after weighing the coach, we came up with a minimum pressure on the rear tires of 95 psi. Adding a 10% safety factor gives me a pressure of 105. That is what I had the rear tires set at. The wheel is stamped for a 120-psi temperature in a dual configuration. I believe that it does say cold, but am not at the coach right now to check. The tire pressures were set early Saturday morning. The tires were correct at the time. About 2 hours after leaving Texarkana, the left outside tire was 88 degrees Fahrenheit and the pressure read 121 psi. It just jumped into my mind that “hey, that’s above the pressure that it says on the wheel.” I wasn’t concerned that the wheel would break. I used to do some rather outlandish things to oval track stock car tires go get them around the corner. A broken wheel there at an inopportune time could be sort of interesting too. My question is intended to stimulate interest in something that we should, and sometimes do, take seriously. If it’s worth doing, it’s worth understanding. Thanks for the info. By the way, the tires are Michelin XZE2+ 275/70R22.5's. Share this post Link to post Share on other sites
manholt Report post Posted October 30, 2017 The pressure stamp on tires are for COLD. Since Saturday morning was 33 degrees and never got above 55 here, you was running a cool tire. I suspect you might have put more air in that tire than what your gauge indicated....also, you said nothing about checking any other tires, so we have nothing to compare with! Share this post Link to post Share on other sites
rossboyer Report post Posted October 30, 2017 I checked tire this morning at 33 degrees, 107 psi. After driving for about 4 hours at 56 degrees, all tires 120-122 psi. Went into rain outside temp 48, all tires 116-118 psi. I have never seen above 122 psi in 90,000 miles. The above seems to be normal. Share this post Link to post Share on other sites
cdsuggs Report post Posted October 30, 2017 33 degrees. No wonder my butt was cold sitting on the asphalt. Just to cover the bases that Carl mentioned, both front tires were set at 115 psi. The highest temp I saw on any tire all day was 88 degrees. The rears were set at 105 and the temps were similar. Those pressures were taken using my electronic gauge from Tire Minder. The running pressures were read on my A1A model Tire Minder. Of course, they only guarantee a 3% accuracy. Two readings could be 6 psi different and still be within the system capability. Not trying to cause a ruckus. Just relating a random thought that sprung into my mind. The two articles tireman9 directed me to pretty much spelled out the answers to my queries. However, I never would have found them if I hadn't asked. By the way, the air that I used to adjust the tires before leaving home was taken from the coach air. With about 20% relative humidity, it should have been pretty dry. Can't say much about the air already in the tires though. Last time I went up any about 6 months ago I used my Viair. Share this post Link to post Share on other sites
wildebill308 Report post Posted October 31, 2017 I set my pressure cold and don't care if it warms up. Having the extra 10# will help protect you if the temperature drops. I would be much more worried about running on underinflated tires than ones set properly when cold and go up because of temperature increases. Bill Share this post Link to post Share on other sites
rossboyer Report post Posted October 31, 2017 X-2, Amen. Share this post Link to post Share on other sites
fredcrump Report post Posted November 1, 2017 What is the manufacturers standard cold temperature suggested? If it is only 30 degrees to you inflate higher than if it is 80? Share this post Link to post Share on other sites
wolfe10 Report post Posted November 1, 2017 33 minutes ago, fredcrump said: What is the manufacturers standard cold temperature suggested? If it is only 30 degrees to you inflate higher than if it is 80? fredcrump, Welcome to the FMCA Forum. No such thing. Cold is not a specific temperature. Rather, it is at whatever ambient temperature is before driving, exposing tire to external heat source such as sun. Share this post Link to post Share on other sites
tireman9 Report post Posted November 2, 2017 7 hours ago, wolfe10 said: fredcrump, Welcome to the FMCA Forum. No such thing. Cold is not a specific temperature. Rather, it is at whatever ambient temperature is before driving, exposing tire to external heat source such as sun. x2 to Brett's comment A few more blog posts that may provide answers to some questions Ambient Cold Inflation and part 2 Share this post Link to post Share on other sites
