Well I really stepped in it last week and as Sven pointed out about a minute after I posted that I did not answer the question in the title. That was what is the correct tire pressure? Well, if we hang in a bit I promise that I will answer the question. A hint is that there is no one answer but perhaps we can clear up the formula.
Larry wrote that the “air limit is stamped on the side wall of the tire”. That is basicly true but there is also a weight stamped with the PSI number and usually more to the statement. larry has it half right. The Firestone T559 tires sized 9R22.5 12PR on my old Foretravel says the following: “max load single 4540 LBS at 105 PSI cold,” and then a second sentence, “max load 4200 at 105 PSI cold.” First of all why the difference in the tire pressure between single and dual? According to my sources it is the clearance between the dual wheels to make sure the tire side walls do not touch during deflection.
The statements on the tire side walls says that you can carry 4540 pounds of weight at a specific pressure and the tire will have the correct foot print and side wall deflection for a combination of low heat build up, good ride and handling and good tread wear. The entire statement must be taken in context and does not mean that this is the correct tire pressure under all conditions for this tire.
Jerry wrote, “OK under inflated is not good. So where do I get an accurate tire gauge? Where do I find a air pump that will properly fill my tires?” Well Jerry I use a digital tire gauge that I bought from the Northern Tool catalogue. My compressor is a Campbell Hausfield 110 volt 100 PSI that came off the shelf at Wal-Mart. It is a bit slow but it does do the job.
Roger, a tire designer, wrote, “I am a tire design engineer with 38 years experience. I teach tire failure analysis ( Think CSI for tires) so let me correct a few minor points and answer some of the questions.
The correct inflation is based on the actual load you are running. The inflation information in your owners manual is based on some assumptions on how much weight you are carrying. I have heard of some vehicles being only capable of adding 5oo# over their empty weight before the tire is overloaded so the only way for you to know the real answer is to get your vehicle weighed for each position. Then contact a tire dealer and ask if he has a load inflation chart (or check the manufacturers web site). If he doesn’t find a different tire store. The correct inflation should be something below the max infl on the sidewall of the tire. Only check your inflation when it is cold – not in the sun and not driven on for at least 4 hours.”
Roger says it all. Weight the rig, at least each end and preferably each corner with your travel load, and then compare the weight with the makers tire chart and determine the correct pressure for conditions.
Now stay tuned for next weeks blog when we will hear Gordon say, “I recommend doing a Google search on Nitrogen in tires. The information will make the choice a no brainer”.