By RV Safety & Education Foundation
Setting RV tires to the correct inflation pressure is critical to optimum tire life, tire performance, vehicle handling, and most important SAFETY. Checking tire pressure should be part of the service of every RV. The important question of course: What pressure should be used? Tire pressure should be based on the load being carried by the tire. This puts the tire in the correct shape (profile), ensures that the tire flexes properly, manages heat build-up, and provides adequate carrying capacity (strength) without sacrificing tire patch (contact with the road).
Tire Load Information
If tire load information is available from the unit having been weighed by wheel position, then that data should be used by referring to the tire manufacturers’ load/inflation tables. The basic rule is that all tires on the same axle are set to the same pressure, based on the heaviest side. It is not good practice to weigh an RV by axle positions and assume that the load is equal on both sides (trailers and tag axle motorhomes are not able to separate their axles on axle scales). One other thing to keep in mind is that you need to have all wheels of unit on same plan. There is no calculation you can use to determine your air pressure without individual wheel weights, anything else from an axle weighing is still just a guess. In fact, it is not uncommon for a difference of 1,000 pounds or more to exist between sides on the same axle. (CAT scales DO NOT recommend trying to get off side of scales to achieve individual wheel position weights, see www.cat.com
If tire load information is not available, then the next best resource is the RV dataplate. RV manufacturers are required to post a recommended inflation pressure on the dataplate; however, since they can only assume that the owner will load the unit to maximum weight, they have no choice but to post tire pressures based on maximum weight. The disadvantage of using this source is that the owner may have the unit loaded lightly, which will result in overinflated tires. Over inflation results in a harsh ride, poor tire wear, unstable handling in wind and with passing vehicles, and reduced tire patch which reduces braking effectiveness, especially on wet roads. On the other hand, the owner may have unit loaded beyond Gross Vehicle Weight Rating (GVWR) or an axle rating (GAWR), in which case, using the dataplate pressure will result in under inflation. Having under inflated tires can result in poor tire wear, excessive tire flexing, excessive heat build-up, and eventual tire failure. Finally, using this source does not take into consideration that the RV may be loaded to a significantly unbalanced condition, quite common in our industry.
Tires can be set to the pressure indicated on the sidewall if you lack tire load information and confidence in the dataplate information. This will result in achieving the design load rating of the tire, but ignores all of the factors discussed above, and may not assume optimum tire performance or safety. The pressure indicated on the sidewall is NOT the maximum pressure for the tire, but is the minimum pressure required to achieve the maximum load rating of the tire.
Check Pressure When Tires are Cold
Whatever pressure you choose, it must be set when tires are COLD. Cold means that the tire is the same temperature as the outside air, and has not heated up from traveling. This means that you CANNOT check tire pressures when a unit rolls into the shop right off the road. NEVER reduce tire pressure of a warm tire just because it is higher than the pressure indicated on the sidewall. This is a normal condition. An accurate tire gauge is essential!
The above relates primarily to motorhomes and tow vehicles. RV trailer tires should normally be set to the pressure indicated on the sidewall unless it causes a severe over inflation situation (20psi over recommended inflation pressure to carry the load) often referred to as the “Basketball effect”. If this is the situation allow a 10-15psi safety margin above the minimum required inflation pressure this will help to resolve ride quality problems or its causes. Trailers squirm and weave as they encounter cross winds, passing vehicles, and road variations. All of this motion is arrested by the tires, resulting in substantial lateral forces that create heat and stress in the tire. Having the maximum load capacity will give the tire the best chance to survive this environment.