Trailer Tires: Radial vs Bias Construction

Radial and bias construction defined

The U.S. Federal Motor Vehicle Safety Standards define both construction types in terms of how the ply cords are oriented. A bias ply tire has ply cords that extend to the beads at alternate angles substantially less than 90 degrees to the centerline of the tread. A radial ply tire has ply cords that extend to the beads at substantially 90 degrees to the centerline of the tread.

Because the cords cross one another in bias tires, the casing is stiff in all directions. This stiff casing resists flexing in the sidewall and makes the tread area less able to conform to the road surface. Radial tires allow the sidewall to flex independently while the belts under the tread stabilize the footprint, which changes how the tire contacts the road, how heat is generated, and how the tread wears.

For a full breakdown of how construction affects other aspects of the tire, see our tire construction types article and our guide to bias-ply versus radial for off-road use.

Load capacity: same rating, different behavior

Load capacity is the maximum load a tire is rated to carry for a given inflation pressure. Both radial and bias trailer tires carry load ratings, and a tire of either construction with the same size and load range carries the same rated load. The differences show up in how they support that load.

Bias-ply tires carry the load through a stiff, diagonal casing that flexes less under load. This stiffness makes them predictable when heavily loaded and when the trailer sits for long periods. However, the casing works harder at highway speed because the crossing plies rub against one another as the tire flexes, generating internal heat.

Radial tires carry the load through the belt package and the air pressure in the tire. The flexible sidewall allows the tire to bulge more under the same load, which softens the ride and keeps the contact patch flatter. On the road a radial trailer tire can often carry the same rated load at the same pressure as a comparable bias tire, while running cooler at sustained highway speeds.

The standard requires that the sum of the maximum load ratings of tires fitted to an axle be at least the gross axle weight rating of that axle. That rule applies to trailers as well as trucks. Always confirm that the tires you choose have enough combined load rating for the axle's GAWR, regardless of construction. Our load range article explains how to match ratings.

For an overview of trailer-specific tire selection, see our trailer tire guide.

Heat resistance and highway speed

Heat is the number one killer of trailer tires. A trailer tire is usually smaller than a passenger tire for the same load, so it flexes more per revolution, and the flexing produces heat. Bias-ply construction with crisscrossed plies generates more internal friction during that flex than radial construction, which means bias tires run hotter at the same speed and load.

The FMVSS 119 standard requires new tires for trailers, including special-trailer (ST) tires, to pass a high-speed performance test and an endurance test without showing tread, sidewall, ply, cord, innerliner, or bead separation, chunking, broken cords, cracking, or open splices. Both radial and bias ST tires must pass these tests. The tests, however, only establish a minimum. Bias-plies that pass at near their limit will have less margin for speed and load than radials that pass the same test.

For trailers that regularly tow at highway speeds for long stretches, radial construction is the safer choice because it builds less heat. Bias-plies still work well for intermittent or low-speed duty at or below 55 mph. Always respect the tire's speed rating and never exceed the rating for the best heat management.

Tread wear and evenness

Radial trailer tires have a flatter, more uniform contact patch under load because the flexible sidewall allows the tread to stay square to the road. That uniform footprint distributes wear evenly across the tread, so radial trailer tires typically show longer and more even tread life than equivalent bias tires at highway speeds.

Bias-ply tires have a rounded footprint because the diagonal casing resists flattening. The center of the tread carries more load than the shoulders, which causes noticeable center wear on bias tires, especially on trailers that are run at higher speeds or higher inflation pressures. Bias tires also tend to rock slightly from side to side as the stiff sidewall tries to keep a flat footprint, which can scrub the shoulder blocks.

Tread wear on any trailer tire is accelerated by underinflation, overloading, and misalignment. Both constructions require the recommended cold inflation pressure for the load you actually carry. The U.S. Department of Energy notes that properly inflated tires improve fuel economy and under-inflated tires lower it; for trailers, the same principle applies to the tow vehicle, which feels the extra drag.

Regular visual checks for uneven wear are important for trailers since they usually sit for long periods between trips.

ST, LT, and passenger tires on trailers

The regulations separate trailer tires into classes. Under FMVSS 110, trailers may be equipped with ST tires, FI tires (farm implements), or tires with a rim diameter code of 12 or below that meet the requirements of FMVSS 119. This means that even small trailer tires are held to a specialty standard, not the light-vehicle radial standard FMVSS 139.

FMVSS 119 applies to new pneumatic tires for motor vehicles over 10,000 pounds, to light truck tires with a tread depth of 18/32 inch or greater, and to tires for use on special-use trailers (ST, FI, and 8-12 rim or lower diameter code). ST tires are built specifically for trailer service, with stiffer sidewalls and a higher load capacity at a lower speed rating than a comparable passenger tire. See our ST versus LT trailer tires comparison.

LT tires are light truck tires designated by the manufacturer as primarily intended for light trucks or multipurpose passenger vehicles. FMVSS 139 covers radial tires for vehicles up to 10,000 pounds and does not apply to ST tires in highway service. That distinction is important when you are selecting replacements. Check the sidewall marking for ST or LT; the construction type (radial or bias) is separate from the service class.

Which construction should you choose

Pick radial trailer tires when: you tow on highways at 55 mph or more for more than an hour at a stretch, you want even tread wear over many seasons, you care about fuel economy, or your trailer sits on a tandem axle where equal load sharing matters. Radial tires are the standard for newer trailers and recreational vehicles.

Pick bias-ply trailer tires when: you tow at low speed, your trailer is used off-road or on rough farm tracks where a bias tire holds up to curb impact, you have a single-axle or very lightly loaded trailer, or you are on a budget. Bias tires also tolerate a flat spot better if the trailer sits in one place for months.

For heavy trailers, dual axles, or any load over 10,000 pounds combined with regular highway work, radial construction is the safer choice. For small utility trailers, boat trailers under maybe 6,000 pounds, and farm trailers that never see the interstate, bias construction remains perfectly adequate and less costly.

Our best trailer tires roundup covers the top picks in both constructions, and the 16-inch tire guide lists options if you are replacing wheels as well as tires.

What to pick for your use

Questions

Can I mix radial and bias tires on the same trailer axle?

No. Do not mix radial and bias tires on the same axle, and avoid mixing them across axles on a tandem trailer. The different flex characteristics and footprint shapes cause uneven load sharing, irregular wear, and potential handling problems. If you switch, replace all tires at once.

Do radial trailer tires cost more than bias?

Yes, radial trailer tires usually have a higher initial price than comparable bias tires. The higher price is offset by longer, more even tread wear and lower heat buildup at highway speeds, which means they often last longer on road-going trailers. Bias tires are a more economical choice for slow-speed or low-mileage use.

Is a radial tire always the safest choice for towing?

Not necessarily. Radial tires run cooler at highway speeds and are generally the safer choice for sustained fast towing. But bias tires can be perfectly safe at low speed and are often preferred for rough terrain where a radial sidewall is more vulnerable. The key is to match the tire's load range and speed rating to the actual trailer load and towing speed.

Can bias-ply tires handle highway speeds?

Bias-ply tires with an appropriate speed rating can be used at highway speeds, but they generate more heat than radials, which shortens their life and increases the risk of failure. If you routinely tow at 65 mph for hours, a radial is the better choice. Check the speed symbol on the sidewall and do not exceed it.

How do I know if my trailer tire is radial or bias?

Look at the size marking on the sidewall. A radial tire includes the letter R in the size, for example ST225/75R15. A bias tire uses a hyphen or no R, for example 7.00-15 or ST225/75D15. The construction type is also molded into the sidewall. See our tire sidewall markings article for decoding help.