Motorhome Basics | FMCA
By John Anderson, Executive Director, Recreation Vehicle Safety Education Foundation (RVSEF)
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 10,000 RVs during a seven-year span, I can tell you with confidence that a significant number of motorhomers are traveling down the road on overloaded 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." One the sidewall of your motorhome's tires you will find a rating for a single application (the front axle or tag axle of your motorhome) and a dual tire rating (usually the drive wheels).
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 motorhome. 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.
Every tire manufacturer publishes load/inflation tables for their products, 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 become involved.
Understanding motorhome loads
I suspect that many motorhomers 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 motorhomes are not built symmetrically. Of all the RVs we've weighed, we've never come across one that was loaded equally on both sides.
Motorhomes are equipped with slideouts, generators, refrigerators, holding tanks, fuel and water tanks, and much more. The only way to properly weigh a motorhome to ensure that you are within your tire ratings is to weigh it by individual wheel position. Once you have done this, you can refer to the load/inflation table 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 coach, or by upgrading to tires with a higher load capacity.
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 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 motorhome weight varies considerably with fuel, water, groceries, etc, it is important that you weigh your unit in its heaviest configurations, 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.
A manual published by the Tire & Rim Association indicates that we can inflate our tires 10 psi over the pressure indicated on the sidewall for light truck (LT) tires, and 20 psi for truck and bus tires. However, this does not increase the rating of the tire; it simply helps it to run cooler. 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.
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 overinflated tire on the light side, correct the situation by balancing your coach.
An overinflated 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 coach so that it does not exceed a tire rating, contact your motorhome 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 indicate that 11 percent of all of the 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 so as 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.
It's also important to remember that tires od not last forever. We rarely need to worry about the age of our passenger car tires, because we replace them every few years after the tread has worn out. But in the RV world, it's common to find motorhomes that are perhaps 10 years old with fewer than 20,000 miles on them, so the tire tread looks good. Yet the owner may experience frequent tire failure and, of course, it seems logical to blame the manufacturer for building defective tires.
We can't tell you exactly how long a tire will last, because so many variables affect the aging of the tire casing, but we know from statistics that the average life expectancy of a tire in RV service is five to seven years. In fact, it's important to note that tires age more quickly when not used. Tires are designed to roll, heat up, and release anti-weathering chemicals that help to keep the tire supple and resist aging.
RVers frequently subject tires to the absolute worse-case scenario: We let the RV sit in the sun and ozone for a season, then pull onto the interstate and dash down the highway at speeds in excess of 65 miles per hour. The, we leave our RV parked for a few months, and so on, thus creating an extreme-duty cycle.
Determining a tire's age
Every FMCA member should know the age of the tires on his or her coach and be alert fo signs of aging. The U.S. Department of Transportation (DOT) requires that every tire be marked on the sidewall with its "birthday," or when it was in the mold.
Find a string of numbers and letters that starts with "DOT" the sidewall. The numbers appear on only one side of the tire, so you may have to crawl underneath the coach to see them.
At the end of the string you will find three numbers; the first two indicate the calendar week of the birthday year, starting with "01" for the first week of January. The third number indicates the year, so "089" might signify that the tire was born on the eighth week of 1999.
Unfortunately, not all manufacturers supply the decade, so this tire might be a 1999 or a 1989 model.
Motorhome owners also should have some knowledge of when their tires were installed. Michelin provides a decade indicator; if you see a small arrowhead just after the last digit 9 on the sidewall, it's a 1999 tire.
Fortunately, our government has seen fit to resolve this confusion in the new millennium, so tires being built now have four digits: two for the week and two for the year. Be sure you know the age of your tires, and don't risk your safety for the few dollars (relative to an accident) that a new set of tires will cost.
I urge you to find someplace to weigh your coach properly, and then use the data to ensure your safety.