Trailer Tire Safety

IMPROPER INFLATION

A tire’s ability to carry weight and its heat–building characteristics are directly related to inflation pressure, and maintaining it is one of the most important safety procedures on any RV owner’s checklist.

The higher the pressure, the more weight the tire can support, up to, but not exceeding, its maximum cold-inflation pressure listed on the tire sidewall; the only exception is light-truck tires, which may be inflated as much as 10 psi over the cold-inflation pressure listed on the sidewall.

Allow inflation pressure to drop, and the tire can become dangerously overloaded, resulting in excessive heat buildup and possibly resulting in a blowout. Even a short period of significant underinflation can cause damage that is not immediately evident but can result in the destruction of the tire somewhere down the road.

Using the correct air pressure for the load also means a cooler-running, longer-lasting trailer tire. Proper inflation assures the best fuel-and-tire mileage, not to mention overall handling. In fact, the Rubber Manufacturers Association says that any tire run at less than 80 percent of the inflation pressure required for a given load should be inspected for damage.

Tires that are not loaded to their maximums do not require maximum air pressure, and load-inflation tables provide the values that can be used to set air pressure for vehicles whose tire loads may be considerably less than maximum — rear tires on lightly loaded pickup trucks, for example. However, trailer weight does not fluctuate significantly with variances in fresh, and waste water and supplies, and trailer owners should always inflate to the maximum pressure listed on the tire sidewall. The inflation figures are for cold tires; if you check pressure after the tire has been driven and thus warmed, you’ll find it higher. This pressure rise is normal and accounted for in the maximum cold– pressure rating.

THINGS TO CONSIDER WHEN CHECKING YOUR TIRES FOR PROPER PRESSURE

Adjusting tire pressure can take a few minutes, but it isn’t difficult. Having a small compressor at home is a help because the work can be done at your leisure when the tires are cold. If you must move the trailer to a source of compressed air, the strictest definition of a cold tire is one that has traveled a mile or less or has cooled for three to four hours. Ambient temperature is also a consideration when setting tire pressures. A 10-degree rise in ambient temperature equates to a 1 psi change in tire pressure. Thus, if you set your tire pressures on a cool 58-degree F spring day, they will magically rise 2 psi the following day should a weather front come through and pop the temperature up to 78 F. Dropping temperature lowers tire pressure the same 1 psi per 10 degrees.

A very real variable is sunlight. Tires in the shade versus those sitting in the sun can have definite pressure differences. Testing by a large tire retailer has shown a 3 psi rise in tires left in the sun all day versus those in the shade. This applies to shade versus sun sides of the trailer, moving the trailer from a cool building into the warm sun, and so on. Clearly, setting pressures in even morning light and temperatures simplifies the job.

An accurate pressure gauge is mandatory. Service station gauges found on the end of air hoses are often inaccurate; the common pencil-type gauge is a better choice. Better yet, it is a round-dial gauge with a short hose length. Coupled with its large, accurate, and easy-to-read dials, it makes measuring pressures much easier. A selection of air chucks may also prove useful, depending on the style of valve stems on your wheels. A visit to a local auto-supply store should net you the necessary air chuck if the common acorn head won’t work.

Because ambient temperature affects tire pressure and tires naturally leak a little air – 1 to 2 psi a month is considered normal – it’s important to check tire pressures once a month. Weekly pressure checks are advisable during trips, along with visual inspection every day.

INCORRECT TIRE SIZE & CHOICE

In theory, tires installed by trailer manufacturers should be capable of carrying at least the maximum load the trailer is rated to carry (its gross vehicle weight rating, or (GVWR). If you suspect that your tires are overloaded – indicated by tire failures or improper wear (which also may be caused by improper suspension alignment or a bent axle) – and have decided to upgrade with higher-rated tires, you’ll need to know how much weight your tires are carrying to select tires with appropriate maximum load capacity, and this requires a trip to a public scale.

Weighing the trailer is required to see if the manufacturer has made the right choice or if your collection of antique books has pushed weight beyond what the manufacturer had intended. If so, the selection of larger tires may seem apparent, but the higher-capacity tires should not lead to overloading of wheels (check ratings on inside of wheel rims) or axles (ratings for which are posted on identification stickers on trailer exteriors, usually toward the front).

If an overload potential exists, you’re better off reducing weight in the trailer to retain the original tire size/load rating. In a tire size change, wheel suitability must be checked, and clearance in wheel housings must be adequate. Hitch adjustment may be necessary if the replacement tire is larger in diameter than the original.

All tires should be the same size. Unmatched tire sizes guarantee uneven tire loading, which may mean at least one overloaded tire. The trailer’s dynamics and stability can be adversely affected by mismatched tires as well. Likewise, mixing bias-ply with radial–ply, for example may lead to handling problems. Of course, there are contingencies where the only spare may be a mismatch. If at all possible, wait for the proper replacement tire to be brought to the trailer. Failing that, use a mismatched spare like the mini-spare in a car. Limit speed to 35 mph or less, and keep the mileage as low as possible.

In most cases, replacement tires can be chosen based on the load rating of the original tire (listed on the sidewall), if it provided good service, and if weight readings indicate that overload is not a factor.

If you suspect that your tires are overloaded — indicated by tire failures or improper wear (which also may be caused by improper suspension alignment or a bent axle) — and have decided to upgrade with higher-rated tires, you’ll need to know how much weight your tires are carrying to select tires with appropriate maximum load capacity, and this requires a trip to a public scale. It will not only reveal any possible overload but also allow weight bias to be measured on one side. Weigh your fully loaded trailer axle-by-axle and, if possible, side-to-side. This is easily done on truck scales by simply rolling onto the platform one axle at a time and then doing a little subtraction from the total weight. Make sure the aprons of the scale are level with the scale. Side-to-side measurements are possible when there is sufficient room on the side of the scale to run one side of the truck and trailer on the scale at a time.

If you’re lucky, you may encounter a segmented platform scale, where the scale is divided to allow axle measurements simultaneously. Going in the other direction, a single-axle scale can only measure one axle at a time, but a little addition can furnish any combination of axle or total weights you’d like to see.

If it turns out that the trailer is overloaded, you can face the music and remove some weight or choose to increase tire capacity while taking care not to overload wheels or axles.

IMPROPER STORAGE

Trailer tires spend most of their life in storage. This is ideally done in a dark, cool garage at maximum inflation pressure. Anything you can do to replicate these conditions is good, such as tire covers or shade from a tree or building. Avoid heat and ultraviolet radiation caused by sunlight or certain types of welding, and keep the tires away from ozone-producing electric motors, generators, and transformers. The same inflation pressure used for travel should be used for storage.

Finally, because some common pavement surfaces are not compatible with rubber in long-term storage, a thin piece of wood under the tires will slow the inevitable tire rot.

While a relatively small number of tire failures are caused by faulty manufacturing, most result from overloading and/or under-inflation. By avoiding those two pitfalls, you’re well on the way to trouble-free travel.

TIRE CONSTRUCTION

Commonly encountered tires are P (passenger car), LT (light truck), and ST (special trailer). While each tire series shares basic construction methods, such as bias or radial belts, a bead, tread plies, and so on, the details vary meaningfully among the three.

Considerations in designing tires used on trailers include carrying heavy loads, the relative lack of cornering loads, and long duty cycles (the tires can be expected to sit for a year and then rotate for days on end during long trips). Furthermore, because the design of most trailer suspensions dates back many decades, ST tires are designed for as soft a ride as possible so they don’t transmit too much shock to the trailer and its contents.

Trailer-tire sidewall stiffness is a compromise between P and LT designs. The desire for stiffer sidewalls is still occasionally cited as the reason for choosing a bias-belted trailer tire. While passenger-car tires are nearly all radials these days, ST tires are still available in bias-belted construction. Radial trailer tires are superior in all respects to bias-belted tires except in sidewall stiffness. Reduced tire heat, lower rolling resistance, and softer ride are among the benefits of radials, not to mention extended wear.

On the road, ST tires share some characteristics of passenger-car tires but are closer to the design of light-truck tires. Trailer tires typically employ heavier steel or polyester cords and somewhat lighter sidewall construction than light-truck tires, and trailer tires typically run lower air pressures than their truck counterparts. This gives ST tires good load-carrying capacity, but with the desired softer ride, ST tires also have the advantage of rubber compounds that are specifically designed to resist deterioration from the elements, including sunlight and ozone, during extended storage.

INSPECTING & MAINTAINING TRAILER TIRES – Helpful Ideas!
Not all tire problems are immediately visible, so it pays to inspect tires periodically. A good time for this is while checking inflation pressure. Run a hand across the tread to check for excessive feathering. That is where each tread block has a distinct raised edge caused by uneven wear. More typical of tires on the tow vehicle, feathering is a sign of an out-of-alignment axle or possibly loose spindle nuts. a

Check for cracks that could mean your tires require replacement. For example, Michelin’s Recreational Vehicle Tire Guide states that most often, the cracks are 360 degrees around the tire, and are acceptable if less than 1/32 inch deep. Between Y32 inch and 2/32 inch, the tire is suspect and should be examined. If the cracks are more than 2/32 inch deep, the tire should be replaced immediately.

Look also for sidewall and tread bulges indicative of carcass (cord) failure, and don’t forget to verify that the valve stem and cap are in good shape. An old, cracked valve stem can break off, leading to a sudden loss of pressure and a real handful of trouble for the driver.

Keep your tires clean by washing them with a soft scrub brush, mild soap, and water. Use caution when selecting tire–care products, and do not use any that contain alcohol or petroleum distillates, which can accelerate the breakdown of the tire compound.

WEAR & TEAR – Other factors to be aware of!
It’s a rare trailer tire that wears out, tread-wise. Lack of cornering loads and low annual mileage mean trailer tires generally wear out first from the effects of sunlight, ozone, and other environmental factors. The tire industry advises five years as a tire’s lifespan due to rubber oxidation. Given low annual mileage, often around 2,000 miles, that means a typical trailer tire accumulates only 10,000 miles in five years when its time is up, even though a considerable amount of tread rubber remains.

Tire companies say time-related damage is difficult, if not impossible, to see. Constant exposure to the pressurized oxygen inside the tire causes rubber breakdown relatively deep in the tire carcass. Outside, ozone accelerates the oxidation process, which is more severe in smoggy urban areas where ozone is prevalent. A stored tire deteriorates faster from interior breakdown than a tire in use because flexing the sidewall tends to release beneficial lubricants from within the rubber. A stationary tire, thus, has a greater tendency to dry on the surface.

We’ve already pointed out low tire pressures as the prime heat source, but environmental heat is also a player. Tires in the egg-frying desert states often die early, especially when combined with long, high-speed tows. Well-cared-for tires in cooler climates will likely exceed the five-year limit, but they are on borrowed time. Remember that impact damage from curbs, potholes, and leveling boards is cumulative, and if you want to avoid trouble, adhering to the time limit is wise. To avoid leveling–block–induced stress, ensure the leveling blocks are wider and longer than the tire contact patch, and use ramped blocks if ascending more than a few inches. This distributes the tire load evenly.
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