Trailer Tire Frequently Asked Questions

Special Trailer (ST) Tires and Air Pressure

Some tires are specially designed to be used on trailers. These tires include the letters "ST" in the size specification that is listed on the sidewall. The "ST" stands for "special trailer". This kind of tire has a stronger sidewall than an automobile or truck tire so it can handle higher air pressures and higher loads.

Special trailer tires should be inflated to their maximum air pressure. The value for the maximum air pressure should be listed along with the value for the maximum load on the tire's sidewall. These values will usually designate the maximum load at a maximum psi. For example, if the tire is rated at 1,610 lbs maximum at 65 psi, the tire can carry 1,610 lbs of weight if the air pressure is at the maximum of 65 psi. At the maximum air pressure, the tires will perform and wear best, and get the best gas mileage.

If a trailer tire is inflated to a lower air pressure than the maximum, the amount of weight that the tire can carry will be reduced. If a heavier load is put on the trailer tire than what is recommended for that air pressure, the sidewall of the tire could heat up and cause the tire to fail.

You should always inflate your tires when they are cold.

If a tire is over-inflated, it will wear more in the center of the tread, all the way around the tire. If a tire is under-inflated, it will wear on the outside edges of the tread, all the way around the tire.

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Speed Rating of ST Tires

Special trailer tires have a maximum speed rating, just like passenger car and truck tires. Older tires have no special marking to designate a limit. Their maximum speed is 65 mph. Newer tires, those manufactured in 2015 and later, should have one of the following codes on the sidewall to show their rating:

• M - 81 mph
• L - 75 mph
• J - 62 mph

Note: If a trailer tire has any other marking to designate maximum speed rating, follow it.

Matching ST Tires

You should always use tires of the same size, load range, and construction on a trailer. If you use tires that don't match, you could experience problems of overloading and overheating, which can lead to tire failure.

Select Correct Load Range

When you replace trailer tires, you should replace them with tires that have the correct load capacity. In most cases, you should select tires with a load capacity that matches the load capacity of the tires that came with the trailer. If you wish to select tires with a higher load capacity, keep in mind that other components of the trailer also have a weight limit, including the wheels. This means that upgrading to higher capacity tires won't necessarily increase your trailer's load capacity. And higher capacity tires usually cost more.

Radial Trailer Tires vs. Bias Ply Trailer Tires

The main difference between a radial ply and a bias ply tire is how the tire is constructed. The cords inside a bias ply tire run at a 32 degree angle to the direction of travel, and the cords on a radial tire run at a 90 degree angle to the direction of travel, or across the tire from wheel lip to wheel lip.

A radial tire flexes more than a bias tire, giving it better ground contact, traction, stability, and tread wear.

A radial tire will normally run cooler than a bias ply tire, especially when the tire is under a load. A tire that runs cooler will last longer.

A bias ply tire has a stiffer sidewall than a radial, a feature that might be useful for off road applications like farm use.

At one time radials were more expensive than bias ply tires, but now they are closer in price. This makes radials a much better value.

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DOT Tire ID Number

The Department of Transportation (DOT) identification number code is stamped on one side of the tire, near the bead. The code starts with the letters "DOT", then has two numbers or letters to identify where the tire was manufactured. The last four numbers represent the week and year the tire was built. For example, the number 0418 means the fourth week of 2018.

Tire Balancing

At etrailer.com, our tire and wheel combinations are mounted so that the high heavy spot on the tire is aligned with the low light spot on the wheel. This provides adequate balance for trailer tires.

If you buy new tires for a set of rims, you can have them balanced. If you do balance trailer tires, remember that many trailer wheels are centered by the position of the lug bolts (these wheels are lug-centric) and not the center bore of the wheel. To get the best tire balance for lug-centric wheels, you should have them balanced using a pin plate adapter. This mimics the way a lug-centric wheel is mounted to a hub.

Determining Wheel Offset

Wheel offset refers to the location of a wheel's rim in relation to the wheel's center. The rim can be centered on the wheel or it can be offset to either side. The position of the rim determines where the tire lines up - either closer to or farther from the vehicle or trailer on which it is mounted. Trailer wheels usually have zero offset, meaning that the rim of a trailer wheel lines up directly under the center of the wheel. This alignment puts the tire directly under the center of the wheel, giving the wheel and tire combination the greatest ability to handle load.

To determine if your trailer wheels have an offset, remove a wheel from your trailer and measure from the mounting surface of the wheel (where the wheel mounts to the hub) to the inside and outside beads. The difference between these two measurements is the offset.

Because some trailer wheel offsets can be as small as 5 mm, they can be difficult to measure. If you can't measure the wheel, contact your trailer manufacturer with your trailer's serial number.

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