Revised by Walter Cannon Executive Director, RV Safety and Education Foundation
Sometimes it takes a tragedy to wake us up to something that we should have been concerned about but just never considered a priority. After all, bad things happen only to other people. Right? After weighing more than 40,000 RVs during a twenty-year span, we can tell you with confidence that a significant number of RVers are traveling down the road on overloaded or under-inflated tires that could fail at any time, with potentially catastrophic results.
Finding the ratings
Tires, like most manufactured products, are designed to operate within a specific load; we call this “tire load rating.” On the sidewall of your RV or tow vehicle’s tires you will find a rating for a single application (the front axle, tag axle, or most trailer axles) and a dual tire rating (usually the drive wheels and some trailer axles). Each rating is accompanied by an inflation pressure that, if used, will give you the stated rating. This does not mean that this inflation pressure is correct for your RV. Because RVs can be configured and loaded in various ways, inflation pressure must be set based on the load being carried by the tire, if you wish to achieve optimum life and performance from you tires. Tire manufacturers publish load/inflation tables for their products, available at www.rvsafety.com, which provides us with this critical information. Also, remember that the specified pressures are cold pressures; in other words, pressure in the tires before the tires start rolling down the road and heat becomes involved.
Understanding RV Loads
I suspect that many RVers already understand basic tire rating information. Where I fear the message is being lost is in the manner in which we use the load-inflation tables. We need to recognize that RV’s are not built symmetrically. Of all the RVs we’ve weighed, we’ve never come across one (the first time we weighed it) that was loaded equally on both sides. RV’s are equipped with slide outs, generators, refrigerators, holding tanks, fuel and water tanks, and much more. The only way to properly weigh an RV to ensure that you are within your tire ratings is to weigh it by individual wheel position, (NHTSA has an unpublished study, showing drastic differences from flat scale weights and individual wheel weights). (CAT does not recommend trying to weigh by wheel position on their scales, see www.cat.com) Once you have weighed by individual wheel position, you can refer to the load/inflation table (available at www.rvsafety.com) to determine the correct inflation pressure for your tires. If the load on a tire is greater than the maximum rating shown on the sidewall, you must correct the situation. This can be done by reducing the load on the tire, either by unloading or redistributing the equipment in the unit, or by upgrading to tires with a higher load carrying capacity. Caution: Never exceed rim ratings for maximum load carrying capacity and inflation pressure (psi).
Tires come in many different load ranges, so it may be possible to change to a higher-load-rated tire within the same size. If you do so, be sure not to exceed the load and inflation rating of the wheel. If you change tire sizes, make sure you consult an industry expert to ensure that proper fit and vehicle compatibility are maintained. Keep in mind that installing tires with a higher carrying capacity solves only the problem of tire overload. It does not increase the gross axle weight rating, and does not resolve issues related to the overloading of other components — axles, suspension components, etc.
Reading Inflation Tables
Be certain that you have the correct load/inflation table for the manufacturer of your tire. Different tire manufacturers use different specifications. For example, a Michelin table designed for most sizes larger than 16 inches cannot be applied to other manufacturers’ tires. If the tire size matches and the data on the far right of the table for your load range matches the information on the sidewall of your tire, then you have the correct table. On the table, find the load you measured on the applicable line, dual or single, and move up to the corresponding MINIMUM inflation pressure for that load. Since your RV weight varies considerably with fuel, water, groceries, etc, it is important that you weigh your unit in its heaviest configuration, or how you normally travel. Since inflation tables give us the MINIMUM pressure to carry the load, we suggest that you move one block to the right on the chart, or 5 psi, to give yourself a safety margin. If you find that your load exceeds the chart rating, address the cause — your coach weight. Do not try to compensate by straining the capacity of your tires. Note: Towable – Travel Trailer/ 5th Wheel owners Due to the sever use conditions experienced by tires when axles are very close together – tire industry experts recommend maximum (sidewall) inflation pressure for towable tires unless this causes a sever over-inflation situation (20psi+), often referred to as the ‘basketball effect’. If this is your situation allow a 10 – 15psi safety margin above the minimum required inflation pressure.
Expect to find a different load on each side of the axle. It’s important that all tires on the same axle be inflated to the same pressure, based on the heavier side. If doing so results in an over inflated tire on the light side, correct the situation by balancing your load. An over inflated tire has reduced tire patch, or contact with the road, and may result in unsatisfactory braking, particularly on a wet road. If you’re unable to configure your RV so that it does not exceed a tire rating, contact your RV dealer or manufacturer immediately and ask for assistance. They do not want your safety to be at risk, but keep in mind that your safety ultimately is your responsibility.
Air Suspension Chassis
Our data indicates that 31 percent of all Motorhomes we have weighed exceeded a tire rating without exceeding an axle rating, which means that a tire overload would not be detected on a truck scale when the coach is weighed by axle only. However, almost 50 percent of some motorhome models with air suspension exceeded a tire rating, and it is not uncommon for us to find tires that exceed their rating by more than 1,000 pounds. This will virtually guarantee a tire failure. If you have a motorhome equipped with air suspension and have not weighed it by individual wheel position, I urge you to do so before your next trip to verify your safety. If you have been advised to select your tire pressure based on ride quality, how hot they feel to your touch, how the tread wears — or anything other than measuring the load on your tires to ensure that you are not exceeding a tire limitation — the advice is not in the best interest of your safety. Tire durability is directly related to proper inflation pressure.
A Few Tips:
Know and maintain your proper tire inflation pressure Check inflation pressure often (every driving day) If checking inflation pressure is difficult consider a ‘tire pressure monitoring system’ (TPMS) Use an accurate tire gauge The RV Safety & Education Foundation sponsors RV safety programs and conducts educational seminars. For schedule and additional information please visit www.rvsafety.com.
Recreation Vehicle Safety & Education Foundation
RVSEF is a non-profit safety education foundation, providing safety education materials and services to the RV industry. RVSEF publishes a comprehensive safety education program and an Understanding & Testing Your Motorhome Airbrakes, which can be ordered on line or by calling them at (321) 453-7673.