Under Pressure

Unlike passenger-car tires, where the maximum inflation pressure on the sidewall should never be exceeded, the pressure figure on the sidewall of a light-truck or medium-duty truck tire is the minimum pressure necessary to carry the maximum stated load. In other words, if a tire reads “Max Load single: 3650 lbs at 65 psi cold” then 65 psi is the minimum cold-air inflation pressure necessary to carry the maximum load figure. Some RVers may be inclined to simply inflate such a tire to 65 psi – but unless that tire is carrying its maximum load, all this will result in is a rough ride.

In fact, tire manufacturers like Goodyear and Michelin publish load/inflation tables (available online) that indicate how much weight a tire can carry based on inflation pressure. Therefore, it is recommended that you first weigh your coach loaded and ready for travel (including passenger weight, fuel, freshwater, supplies, etc.) to determine the amount of air necessary to carry your load.

Axle weight and tire pressure

It’s best to use individual wheel scales, giving you weight figures at each corner, but even a typical platform scale – the most common – allows for weighing the front and rear axles separately. While you may find that you need to inflate the front and rear tires to different pressures for their different loads, it is critical that tire pressures remain the same across an axle (even if weight differs from side-to-side) to ensure safe handling.

It’s also vital to continue monitoring tire performance on the road. Tires are permeable; as such, they lose air at varying rates and under different conditions. For trips of a week or longer, they should be checked every few days; for day trips, they should be checked before you leave.

Even stored motorhomes are susceptible to tire air pressure lose. Tires can lose 1 PSI per month, and about 1 PSI for every 10-degree F drop in ambient temperature, which is why tire manufacturers recommend that you check it at least monthly.

Tire pressure should always be checked when the tires are cold – in other words, not driven for more than one mile. If you must check tire pressure when the tires are warm, allow for an increase in air pressure and be sure that tires on the same axle carry the same air pressure. The easiest way to do this is to use a truck tire air-pressure gauge that has an angled, dual head that allows you to check inflation pressure on the inner and outer dual wheels of your coach (if so equipped).

Causes of RV & Truck tire failure

Apart from the fact that correct inflation pressure will yield the best fuel economy, ride, handling and longevity, it will also help prevent damage that can cause sudden tire failure. Under-inflation can cause a variety of problems, including tread separation, fatigue rupture and “dual kissing.”

Tread separation is all too common; separated truck-tire treads can be seen along the sides of all major highways. Fatigue rupture, also known as zipper rip, occurs when the steel casing is fatigued to the point where one or more cords break, subjecting all the other cords in the casing to even more stress. This can create a chain reaction of sorts, whereby the other cords begin to break, creating a zipper-like hole in the sidewall. Dual kissing, as the name would imply, is when the rear dual tires actually make contact with one another. The heat generated by the friction between the two tires severely weakens the casing, causing premature tire failure.

Avoid excessive flex in your RV tires

Moreover, any time an under-inflated tire is run at normal highway speeds, the excessive flex builds heat that can damage the inner liner, casing and outer sidewall of the tire. According to guidelines published by the Rubber Manufacturers Association, any tire that has been run at less than 80 percent of the recommended air pressure for the load it is carrying should be inspected for possible damage. What’s more, when one tire in a dual configuration is underinflated, the other tire can be overloaded, and should be inspected for damage, as well.

Incidentally, the improver use of leveling blocks can cause damage similar to that created by overloading and underinflation. When using leveling blocks with your coach, make sure that they are wider than the tire’s tread, and longer than its footprint. In the case of rear duals, make sure both tires are supported equally.

Numbers Game

Besides load and inflation, another thing RVers must become familiar with are the numbers molded into the tire’s sidewalls. What you need to concern yourself with in particular are size, load ratings, load index/speed symbol, and the DOT number that indicates (among other things) the week and year the tire was built.

Let’s start with size. A typical Class A motorhome tire would be a 275/70R22.5. Here, “275” is the cross section width of the tire, measured at its widest part (not the tread) and expressed in millimeters; “70” is the aspect ratio, also expressed in millimeters. The aspect ratio indicates the height of the sidewall relative to the cross section width; in this instance, the sidewall is 70 percent as tall as the cross section is wide. “R” indicates radial, and 22.5 is the rim diameter (in inches).

As we noted earlier, air pressure and load-carrying capacity go hand-in-hand, so the two figures are displayed together on the sidewall Again, a typical example might read, “Max load single: 3640 lbs at 65 psi cold; Max load dual: 3415 lbs at 65 psi cold.” Note the tire in question is capable of carrying more load as a single-tire application than it is when paired with another tire; there is a logical reason for this, and it has to do with road crown and heat.

Load index

As you know, almost every road is built with a crown (curvature) to promote water runoff. If you can picture a motorhome with dual rare wheels straddling a crowned road, it is easy to see that the inner tires are carrying more load than the outers. Also, the close proximity of dual tires makes them run hotter than single tires. For both reasons, tire manufacturers reduce the load-carrying rating of tires in dual configuration to prevent overloading.

The load index reflects the maximum load ratings of the tire (discussed above), and is expressed in an alphanumeric manner. It may appear something like “143/141L.” In this instance, 143 represents the maximum load for a single tire, 141 for dual tires. The letter is the speed symbol, representing the maximum speed that the tire is rated for. The letter L, for example, is a common speed symbol for a Class A motorhome tire, and indicates a maximum speed of about 75 mph.

If the letter is lower than that (K, for example), the top speed is only 69 mph. If you are driving faster than the maximum speed rating for the tire, you are exceeding that tire’s design limit – and inviting disaster. Keeping your speed within a safe margin in crucial.

Five year life span

Finally, remember that truck/motorhome tires are designed for long life – and you may only put 5,000 miles a year on your coach. As such, it is likely that your tires will “time out” before they wear out.

This is where those last four digits of the DOT number on the sidewall come in. The first two figures indicate the week the tire was made, the second two indicate its year of manufacture. So, if the last four numbers read “5004,” it translates to a tire built in the 50th week of 2004. Tires built prior to 2000 had a more confusing three-digit code –two for the week, and the last for the year (which could have meant the 1990’s ‘80s or ‘70s).

At the end of five years of service, you should have your tires closely inspected, regardless of miles they have traveled. Moreover, seven years is becoming recognized by many tire manufacturers as the age at which a tire should be replaced (including the spare) regardless of apparent condition.

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