Trailer Tire Anatomy

Some people claim that a tire is a tire is a tire: It's round and black and holds air. But that's not really the whole story. Tires are manufactured for different purposes and load-bearing capacities. They come in different sizes and vary in quality, pricing, and materials.

It takes an informed consumer to make good buying decisions, but when it comes to trailer tires, being an informed buyer means more than spending wisely. The safety and, indeed, the lives of the horses on board a moving trailer rest on the integrity of your trailer tires.

Here's how to get up to speed on your next trailer tire purchase.

Size and Weight Matter

One of the most important elements in purchasing trailer tires is buying tires that can support the weight of the loaded trailer and those that are the correct size for the trailer: A tire that is incorrectly sized and/or unable to carry the weight of the trailer could fail, and the consequences of a blown tire range from merely inconvenient to catastrophic.

First, find out the maximum loaded weight your trailer can carry. That information (as well as recommended inflation pressure for the tires) is provided by the trailer manufacturer on a placard or sticker placed on the trailer, and it is sometimes covered in the trailer manual. Look for the "GVWR" (gross vehicle weight rating) or "GAWRs" (gross axle weight ratings).

"The GVWR is the value specified by the manufacturer as the maximum loaded weight that that particular vehicle can carry safely," explains Tom Scheve, owner of EquiSpirit Trailer Company and the author of three books on trailering horses, including The Complete Guide to Buying, Maintaining, and Servicing a Horse Trailer ( "The weight of the trailer plus everything in it--horses, mats, tack, water, spare tire, hay, and any equipment--should never exceed the manufacturer's gross vehicle weight rating."

The GAWR is the maximum weight that each axle can support. "Since most horse trailers have two axles," says Scheve, "the GVWR will most always be the sum total of each individual axle rating. However, horse trailer couplers also have ratings, and if the coupler rating is less, the GVWR will be determined by the trailer's 'weakest link.' Even so, you should match the wheel and tire ratings to the combined total ratings of each axle."

Knowing the GAWRs or GVWR gives you the means for determining the carrying capacity you need for the tires you choose.

"The individual weight rating of each of the tires, when added all together, should be equal to or slightly greater than the gross vehicle weight rating of the trailer," Scheve says. "For example, if a trailer has a GVWR of 7,000 pounds, it should have four tires rated at around 1,800 to 2,000 pounds each. Therefore, all four tires together will carry 7,200 or 8,000 pounds of weight. This is why it is very important to have your trailer as level as possible, so all four tires can carry the total weight equally.

"In addition," Scheve continues, "don't overdo the tire and wheel size, thinking more is better. This will provide a rougher ride for the horses and will raise the height of the trailer, making the ramp and/or step in steeper."

The tire's carrying capacity is printed on the side of the tire (along with the maximum tire pressure). Tire capacity is also available from the tire manufacturer or dealer.

Next (and this is equally important), purchase the right size tire. Explains Todd Hershberger, general marketing manager for light truck tires at The Goodyear Tire & Rubber Company, "Select the proper tire that is the same tire size and rim diameter as the tire you are replacing."

Switching to a tire size different from the original equipment that came on the trailer is not a good idea, states John Soule, North American brand category manager for light truck tires at Michelin North America.

He says, "Changing tire sizes affects the load-carrying capacity. If you go to a smaller tire in particular, you risk that that tire won't be rated to carry the weight of the trailer. It's very, very dangerous to change tire sizes."

Notes Hershberger, "Some people may simply try to put on a tire that they already possess--especially in a farm setting where there may be extra truck, trailer, or implement tires lying around. But just because it fits on the rim doesn't mean it's the right tire for the application."

If you must go with a different size tire, make sure the tire fits in the wheel well and does not contact any part of the vehicle, Hershberger advises.

"The alternate-sized tire should be properly mounted on the rim with a new valve stem and core," he says. "The rim should be rated to handle the tire's rated load and inflation pressure."

For example, don't mount a tire rated at 2,000 pounds on a rim rated at 1,500 pounds.

Types and Features

Having determined the capacity and size of the tires you need, the next step is choosing the type and features of your trailer tires. Considerations include tire type, ply, materials, tread, cost, and brand-name vs. no-name.

Type The three basic tire types are ST (special tires for trailer highway service), P-Metric (tires for passenger vehicles), and LT (light truck) tires.

Explains Hershberger, "Trailer tires are considered a special category in the current standards of the Tire and Rim Association. The major difference is that ST tires are required to carry 10% more load than an equivalent P-metric tire."

Experts agree that P-Metric tires should not be used on trailers, as P-Metrics are designed for lighter loads and purposed to provide a comfortable ride for human passengers.

However, there is some disagreement concerning the suitability for trailer use of LT tires, which are more heavy-duty than P-Metrics, but deliver some of the comfort of P-Metric tires. Says Soule, "Michelin recommends LT tires instead of ST tires--never P tires. LT tires are more readily available in case of emergencies on the road. LT tires are much more robust and able to resist overloading type situations."

In contrast, Hershberger says LTs should not be used interchangeably with STs due to potential differences in load capacity.

"LT type tires may require a significant size or load change to meet the trailer load requirements," Hershberger says. "For example, a 205/75R15 Wrangler HP tire used primarily on pickups is rated for a maximum load of 1,598 pounds at a tire pressure of 35 psi. Contrast that with our Marathon Radial, intended for trailer usage: also in a 205/75R15 size, but as an ST type, this tire is rated at a maximum load of 1,820 pounds at 50 psi."

Radial vs. bias tires Developed by Michelin in 1946, radial ply tires have replaced bias ply tires as the industry standard. If you peeled back the tread of a radial tire, you would see a steel fiber belt covering a weave of polyester or steel cords that run perpendicular from bead (the part of the tire that fits on the wheel's rim) to bead. A bias-ply tire has diagonal nylon plies.

Hershberger says, "Bias tires are still used, but are available only in limited sizes and types. Radial tires are the most commonly used and available in many sizes and types. The advantages of the radial tires are better ride and handling, better tire mileage, more fuel efficiency, improved weathering, and overall, they're more cost-effective."

Fabric vs. steel cords "Fabric/steel radials have two fabric body plies with steel belts, while steel/steel tires have a single steel body ply with steel belts," explains Hershberger. "The fabric/steel tires are lighter in weight, normally lower in price, and more common and available. Some users prefer steel/steel tires due to potential longer wear and increased retreadability."

Tread design "Circumvential grooves--grooves that run all the way around the tire, not across the tire--are more appropriate for a trailer application," Soule states. "The trailer tire doesn't go through a lot of torque, and only a little bit of braking if there are brakes on the trailer. But they don't accelerate; they typically just roll down the road in a straight line. As such they need to have a rib tread design--circumvential grooves."

Avoid the very aggressive, luggy, massive tread blocks often seen on off-road light truck vehicles, Soule suggests. "A trailer tire doesn't need those sorts of features."

Cost "You get what you pay for," says Soule. "There is a huge difference between the economy or low-end tire versus the more premium tire: The quality of the materials, the craftsmanship, and the technology that goes into the manufacturing process all play a contributing role in the longevity of the tire and the ability of the tire to withstand overloading, underinflation, and sidewall cracking."

Name brands Scheve recommends sticking with brand-name tires rather than off-brand ones.

"You can be assured of the quality, and it will be easier to locate tires when you need them," which is especially important if you suddenly have to replace a tire or two. "Some trailer tires are not easy to find even during working hours," Scheve warns.

About That Spare

Your spare tire should match the size and rating of the other tires unless it's an emergency-use mini spare that's intended for short-term use until you can replace or repair your original tire.

Scheve says, "It's not a bad idea to carry two spares, should you have an accident that destroys more than one tire. USRider, which provides national roadside emergency service for drivers with horse trailers, has statistics showing numerous customers having two flat tires."

Scheve also suggests carrying the tools that make emergency tire changing a simpler, safer task: "Always carry a jack; a Trailer-Aid jack is recommended because you just drive the front or back wheel up onto the Trailer-Aid, and the alternate wheel will be off the ground. Also carry a tire gauge to check tire pressure, flares and triangles in case you're stuck on a busy road, extra caps for tire stems (caps keep the dirt from clogging up the spring mechanism), and a tire iron that fits the lug nuts of the trailer."

In the event of a ruined tire that you must replace, replacement of the remaining tires depends on the condition of those tires.

"Each situation is relatively unique," says Soule. "If the other tire on the same axle is relatively new--one or two years old and less than half worn--it's okay to just replace the one damaged tire, matching up the exact size, exact brand, and exact model of tire with the other side. But if the undamaged tire is more than two or three years old, we recommend changing both tires across the axle; it's not worth economizing on a problem that may soon follow in the other tire."

Along the same lines, if the tire on the other axle is still in good condition, there's no need to change out all four.

"But if you have a problem and the tread is getting worn on the remaining three tires, why not replace all three at the same time?" Soule suggests.

Time for a Change

There comes a time when tires should be replaced because of tread wear or tire deterioration.

With trailer tires, the bigger issue tends to be degradation due to sidewall cracking related to inactivity. Most trailers do not incur high mileage, and they're often only used on the weekends or once month, perhaps sitting for half a year at a time.

Explains Soule, "If the tire sits inactive for an extended period of time--six months, for example--some of the natural oil in the rubber seeps out, leading to cracking. The tire cracks and degrades over time. Daily use flexes the sidewall, keeping the rubber molecules moist and fresh and the oils in the tire."

Cracks typically appear in the area where the tire comes in contact with the rim or where the tread meets the shoulder or the sidewall of the tire.

"If you see cracks, have an expert at the tire shop look at the tires and evaluate how deep those cracks are to see if there is any immediate risk," advises Soule. "Small hairline cracks are no problem, but the deeper the crack, the bigger the problem and the potential tire failure of any kind, specifically blow-outs, could be."

Tires that do rack up the miles will eventually incur tread wear. Replace tires when tread is less than two-thirty-seconds of an inch, says Soule.

"One quick test to measure tread wear is to take a penny, turn it upside so Lincoln's head is pointing downward, and insert it in anywhere in the groove of the tire," suggests Soule. "If you can see all of Lincoln's head, that means the tread depth is less than two-thirty-seconds of an inch. Check the tread depth in a few different spots across the tread to ensure there is no one area of the tire that is more worn than the other."

Take-Home Message

You can buy horse trailer tires at most commercial truck tire centers and many general retail tire outlets. Hershberger says, "Ordering from Internet is okay, too."

Just take care that wherever you buy your trailer tires, you double-check your trailer's GVWR or GAWR first and purchase the correct tire type in the right size and carrying capacity. Tires must be of the correct size to support the weight of the trailer and contents (horses, tack, etc.) and be of the correct size. Doing so ensures you'll get maximum longevity from your tires and enhanced safety for you and your horses.


What does that jumble of letters and numbers stamped on the tire or display plaque mean? Todd Hershberger, general marketing manager at The Goodyear Tire & Rubber Company, translates: "For an ST225/75R15 tire, 'ST' stands for Special Tire for Trailers, '225' is the tire's cross section in millimeters, '75' is the aspect ratio (section height divided by section width; a lower number equals a shorter sidewall and wider tread), 'R' represents radial, and '15' tells the rim diameter in inches."--Marcia King


The care and maintenance you provide your trailer tires directly affects how long your tires will last. Here are some tips to get the most life from your tires.

1. Don't overload your trailer beyond its maximum recommended weight. This could lead to overheating of the tires and increased wear, warns Todd Hershberger, general marketing manager for light truck tires at The Goodyear Tire & Rubber Company.

2. Maintain correct air pressure. "An underinflated tire will not be able to support the load and could cause tire failure due to overheating," says John Soule, North American brand category manager for light truck tires at Michelin North America. (The less psi, or pounds per square inch tire pressure, the less carrying capacity of the tire.) Premature or irregular wear could also result.

3. Keep in mind that air temperatures affect tire pressure, reports Tom Scheve, owner of EquiSpirit Trailer Company. "Traveling from colder weather to hotter weather increases the air pressure in the tires," says Scheve.

Every 10°F increase/decrease in air temperature causes a corresponding 2% increase/decrease in psi.

4. Inspect your tires regularly. Suggests Hershberger, "Check tire inflation pressure when tires are cold at least once a month and before extended trips. Look for air loss, unusual sidewall cracking, irregular tread wear patterns, damage, and (reduced) tread depth."

5. Avoid turning a gooseneck trailer so sharply that the wheels quit moving forward and start to drag sideways, cautions Scheve. "This will not only damage the tires, but the wheels as well."

6. Place the trailer up on blocks to take the weight off the tires when storing your trailer, says Soule. The second-best method is to unload the trailer to reduce weight on the tires. Protect the tires from ultraviolet light (which degrades tires) by parking the trailer indoors or on the shady side of a building, or by covering the tires with dark plastic bags or tire covers.

View other tire care tips at --Marcia King  

About the Author

Marcia King

Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.

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