Time to Re-Tire?
There’s a lot to know about keeping the rubber on the road
Tires are a vital asset to any motorhome. Considering the high cost of replacement, the more you know about care and maintenance of tires, the better the chances are you’ll realize maximum service life and safe travels.
A flat tire or a sudden blowout can lead to collateral damage that’s expensive and inconvenient to repair (not to mention dangerous). Fortunately, many tire problems are avoidable, with a little preparation and care by you.
Truck tires can be less expensive than equivalent motorhome tires, and it is acceptable to use truck tires on a coach, as long as they are the correct size, type, load rating, etc. However, motorhome-specific tires are engineered to deliver a smoother, quieter ride than regular truck tires and they’re also made to handle the specific needs of RVers, such as extended storage periods. Many motorhome tires also incorporate extra antioxidation additives in their compounds to minimize dry rot and weather cracking. Tires deteriorate faster when exposed to intense sunlight and ozone, which are found in greater concentrations in polluted city air and near large electric motors, welders, etc.
Depending on the type of roads on which you drive and the prevailing weather, you may need tires that have different tread designs on the front and rear. In many cases, a straight-rib highway tire works best on the front, as this type of tread offers the lowest rolling resistance and best wear characteristics. If you only plan to drive in mild weather and on good roads, straight-rib highway tires at the rear will also yield the best mileage and fuel economy. If you intend to drive in muddy conditions and/or winter with some snow, tires with lug or traction patterns should be used on the rear.
In any given size, the higher the load range, the more weight the tire can carry. How much weight depends on the size and construction of the tire, and also on the inflation pressure. In general, the cumulative maximum load capacity of the tires on your coach should meet or exceed the maximum load rating of your RV (total weight of the RV, plus fuel, cargo, passengers, etc.). Wheels must also meet or exceed this rating.
Motorhomes are notorious for having uneven weight distribution due to slideout rooms, tank locations, loading, etc. That’s why it’s best to obtain individual wheel weights on the coach so you can keep the weight as balanced as possible. This will ensure that each tire carries a proportionate (or as close as possible) load and can be inflated accordingly.
Older tire designs used diagonal or bias-ply construction, whereas most modern tires use radial construction. Radial inner belts cross the tire tread perpendicularly, and the shoulders tend to have a rounder shape than bias-type tires. Radials also tend to have lower rolling resistance and last longer than bias-ply designs. Belts may be made of synthetic cords such as polyester, or metal. For motorhome use, look for all-steel construction, which is more durable and resistant to impacts and road hazards.
Radials and bias-ply tires have different handling characteristics and should not be mixed. Goodyear, for example, recommends using the same size and type of tire on the same axle — i.e., all radial-ply or all bias-ply, and all should be the same size, tread pattern and tread depth. If you decide to change to radials, do all positions at once. Always choose tires that have the same (or higher) weight rating than the tires you are replacing.
Manufacturers also vary rubber compounds by mixing various components such as silica, carbon black and other chemicals. This allows tires to be designed for different types of vehicles, driving styles and weather, while maintaining a good balance between wear and traction. Harder compounds wear longer but have less grip, and vice versa.
Motorhome tires come in a variety of sizes to fit the various wheel diameters and widths used by manufacturers over the years.
The metric standard of motorhome tire sizing uses three numbers followed by a slash, then two more numbers, then an “R,” then three more numbers. Using 295/80R22.5 for an example, this tire would be 295 millimeters wide, with an aspect ratio of 80 (80 percent as tall as is it is wide) and the 22.5 is the rim diameter in inches.
The numeric (inch) method is typically listed as two numbers, then a letter and three numbers; for example 11R22.5. This tire is 11 inches wide and has a rim diameter of 22.5 inches, and the “R” indicates the tire is a radial construction. Although not written on the tire, the profile, or aspect ratio in this type is 88. In other words, the sidewall of the tire is 88 percent as tall as the tire is wide.
- Sidewall Codes
There are a number of codes stamped on sidewalls. The ones that are of most interest to motorhome owners are date of manufacture and country of manufacture. The U.S. Department of Transportation (DOT) tire identification number is a combination of up to 12 numbers and letters. The first two are the plant code where the tire was manufactured. (Plant codes are listed online at this website: www.tire
safetygroup.com/tire-dot-plant-codes-sorted-plant-code.) The last four numbers (1999 and earlier codes used three numbers, but these tires should be well out of circulation by now) represent the week and year the tire was built. For example, a tire with a DOT code ending in 2609 was manufactured the 26th week of 2009.
- Weight, Loading and Inflation
Tires are designed to carry a certain amount of weight according to inflation pressure, and in the case of tires designed for either dual- or single-mounting configurations, they have different ratings for each type of service. The weight ratings for single and dual applications are printed on the sidewalls.
Inflation pressures printed on tire sidewalls are not maximum pressures; rather, they are the pressures necessary at maximum-rated loads. Never reduce tire pressure just to get a softer ride, as this may lead to catastrophic tire failure. Always carry a quality tire gauge, and if you have dual rear wheels, make sure the gauge has an offset double head that can reach both outer and inner dual tire valves.
As a tire’s temperature increases, so does the pressure in the tire. Typically, for every 10 degrees Fahrenheit increase, the tire’s internal pressure will increase by 0.7 psi.
Changes in altitude also affect pressure readings. Starting at sea level, pressures subsequently measured at 5,000 feet would indicate about 2-3 psi higher. Conversely, descending from 5,000 feet to sea level would result in an apparent loss of pressure of about 2-3 psi. However, the differences above assume tire pressures are measured at the same ambient temperatures. Since tire pressures change about 1 psi for every 10 degrees Fahrenheit change in ambient temperature, the tire pressure measured at sea level will likely change when exposed to the cooler temperatures associated with higher elevations. This means that in many cases, differences in ambient temperature may come close to offsetting the differences due to the change in altitude. Drivers may want to simply set their cold tire pressures the morning after arriving at their destination, as well as reset them the morning after they return home.
The proper way to determine how much pressure to run in each tire should be determined by weighing the fully loaded coach (with fuel, water, cargo and passengers) on a truck scale. Each manufacturer provides load-inflation tables specific to its products to help you determine the correct tire inflation pressure for your coach’s loading. These may be obtained online at the manufacturer’s websites, and at tire dealers for the brand. Tire inflation placards on the coach, if equipped, only reflect typical recommended pressures and don’t take into account individual loading conditions.
- Tire Wear
Most states have laws regarding minimum allowable tread depth, and those laws should always be followed. While the law may only require the front tires to be replaced at a tread depth of 4/32 inch (on large vehicles such as motorhomes) and the rears at 2/32 inch, wet-weather braking distances and handling are significantly diminished with tires having less than 4/32 inch of remaining tread. Tires should also be replaced if they show any signs of bubbles, separation, significant weathering or other types of damage.
Tire rotation on motorhomes can be expensive, and there is some difference of opinion on when and if it should be done. If the rear tires have special deep-traction lugs and the fronts have a smoother highway tread, rotation is not recommended. If all of the tires are the same type, it can be done. However, steering tires by law must have more tread depth than rear tires. Front tires should also match each other. Rear dual tires should have similar amounts of wear. The rotation patterns also vary between models and manufacturers. Check the owner’s manual for a diagram. If there is a spare tire and it is in good condition and compatible, it should also be rotated.
To optimize tire performance, tire and wheels should be balanced. Out-of-balance wheels put excessive force on suspension and steering components, and tend to cup and wear excessively at the heavy spot. You should have wheel balancing performed when new tires are mounted, after a flat repair and any time a tire is dismounted and remounted.
When tires are loaded and go over bumps and uneven surfaces, they bulge considerably at the sidewalls. To prevent damaging contact and friction, dual wheels must have a certain amount of space between them; this is called dual spacing. When vehicles leave the factory with original-equipment tires fitted, the dual spacing is sufficient. It only becomes a concern when wheels or tires are changed from the original configuration, such as when oversize tires or custom wheels are fitted. Tire manufacturers publish recommended dual-spacing specifications. Check with a reputable tire dealer before changing types or sizes of wheels or tires.
Wheel manufacturers stamp the rims with the maximum inflation pressure and maximum rated weight they are designed to carry. Always use wheels with ratings at least as high as original equipment when replacing them.
Some older motorhomes had 16.5-inch diameter wheels, which have become obsolete. Many owners have switched to newer-style 16-inch wheels. This of course requires a change of tires, so it’s best done when new tires are needed anyway. Always use wheels and tires that have ratings equal or higher than the originals, and make sure they are compatible with the vehicle as far as offset, width and mounting to the hubs.
- Nitrogen Inflation
There has been a lot of talk about nitrogen inflation for tires recently. Green tire valve caps denote the use of nitrogen. Nitrogen has some limited advantages over air, although no manufacturer requires its use. Atmospheric air contains about 79 percent nitrogen, and it’s impractical to inflate tires with 100 percent pure nitrogen (because the tire contains some air even when deflated). The bottom line is, if your tires have nitrogen in them, leave it in. However, you can use air to maintain pressure if needed; never run low pressure due to unavailability of nitrogen.
- Tire-Pressure-Monitoring Systems (TPMS)
Many blowouts occur because of a slow leak that allows tire pressure to drop while the vehicle is in motion, heating the tire to the point of failure. In some cases, drivers are unaware of tire failures on dinghy vehicles and continue to drive, causing the tire to catch fire and in extreme cases, result in loss of vehicle control or even igniting wildfires along the road.
Aftermarket TPMS units are available to monitor both the motorhome and trailer or dinghy vehicle tire pressures. These provide peace of mind and greatly reduce the chance that low tire pressure will result in a blowout. (See our TPMS roundup, “Air Apparent,” beginning on page 52.)
Tires that run high pressures require special tire valves, and valves used with dual tires must be accessible for filling and checking pressure. Special hoses are available to extend the reach of the inner dual tire valves and make it easier to check and adjust pressure. However, these hoses and fittings are subject to leaks, and if a dual loses pressure while driving it may overheat and fail catastrophically. One solution is to use both the hoses for convenience and a TPMS for safety.
Laws prohibit the use of recapped tires on the steering axle. Many commercial trucking fleets prefer recaps for other axles because of their cost savings. However, trucks are typically used daily and rack up many miles in a short time. Motorhomes tend to sit idle for extended periods of time and recaps may already have older sidewalls, so aging is a concern, as is tread peeling. Therefore, retreads are not recommended for motorhomes.
Except for some full-timers who are on the road almost all the time, many motorhomes sit idle for the majority of the year. If possible, keep your motorhome in a cool, dry storage area out of direct sunlight and the sun’s UV rays.
Most modern motorhome tires are comprised of natural and synthetic rubber, and cords made of steel and fabrics such as polyester. These products tend to oxidize over time with exposure to sunlight, rain and other elements. This causes weathering, or dry rot — typically between the tread elements, or on the sidewall of the tire. Dry rot that extends to the cord, fabric or steel of the tire is a severe safety hazard, and the tires must immediately be removed from service.
According to Goodyear, during storage, owners should inflate tires to normal recommended pressure plus 25 percent (to allow for leakage loss), but should not exceed the rim’s inflation capacity. Thoroughly clean tires with soap and water before storage, and move the motorhome at least every three months to help prevent cracking and flat-spotting (but avoid moving it during extremely cold weather). Ensure the storage surface is firm, clean, well-drained and reasonably level. Reset pressures before taking the motorhome out of storage.
- Tire Protection
Tire covers can keep the sun off tires and reduce UV damage. They can be obtained from RV dealers and Camping World stores or online. Use them if you must store the coach outdoors. Most tire manufacturers do not recommend the use of tire dressings because some contain chemicals (petroleum distillates) that may contribute to tire deterioration. However, some products such as 303 Protectant have been noted to provide good protection to tires from sunlight and UV rays.
- Dual Matching
Mismatching dual tires can result in rapid, uneven tread wear and overheating and failure. According to Goodyear, duals should be within the following tolerances for tire circumference (measured around the tire at the tread surface): 8.25R20 and smaller — within 0 to ¾-inch tolerance; 9.00R20 and larger — within 0 to 1½ inches of tolerance.
Before making a final decision to convert to a larger size, contact your local RV tire retailer to verify that the following considerations have been met:
- Check with the vehicle manufacturer to ensure approval of the tire/wheel clearance and fitment.
- Verify that the current rim width is compatible to the new tire size.
- A minimum dual spacing is required and should be confirmed or met.
Due to a tire diameter and static loaded radius, there may be a change in the number of revolutions per mile, which means that the vehicle speedometer and odometer will change. Generally, if the difference in RPM is less than 3 percent, no changes to gearing are required.
- Final Thoughts
Every motorhome owner should carry a portable 12-volt DC air compressor or use the onboard air system (diesel pushers with air brakes/suspensions), which can produce enough pressure to top off tires when needed. It always seems like you can never find a fuel station with a compressor when you need one. Having your own is especially handy, since tires should be inflated when cold, before driving.
For best tire life, check inflation frequently, consider getting a TPMS, be careful to avoid hitting potholes, curbs and other hazards, watch for uneven or unusual tire wear, and store your coach properly, taking the time to protect tires.