Radial trailer tires — marked with an "R" in the size code, like ST205/75R15 — run cooler and wear more evenly, which makes them the better choice for long-distance highway towing. Bias-ply tires, marked "D" or "B," have stiffer sidewalls that shrug off cuts and scuffing, so they suit farm, construction, and other rough-service trailers. Both are legitimate choices; the deciding factor is how and where you tow. One firm rule: never mix the two types on the same axle.
How Bias and Radial Trailer Tires Are Built
The difference is in how the internal cord plies are arranged. In a bias-ply tire, the plies run diagonally from bead to bead, with each layer crisscrossing the one below it. The sidewall and tread end up working as a single unit, which is why a bias tire feels stiff all over — and why flexing it generates more internal heat.
In a radial tire, the plies run straight across the tire, perpendicular to the direction of travel, with stabilizing belts under the tread. The sidewall and tread can flex independently. That lets the tread stay flatter on the pavement, spreads wear more evenly, and sheds heat better at sustained speed.
Bias vs Radial at a Glance
| Factor | Bias (D / B) | Radial (R) |
|---|---|---|
| Construction | Plies crisscross diagonally bead to bead | Plies run straight across, with belts under the tread |
| Sidewall | Stiff; resists cuts, punctures, and scuffing | More flexible; absorbs bumps and tracks straighter |
| Heat at highway speed | Builds up faster | Dissipates better |
| Tread wear | Faster and less even | Slower and more uniform |
| Towing feel | Firmer, can bounce on rough pavement | Smoother ride behind the tow vehicle |
| Long-term parking | More prone to temporary flat-spotting | Less prone to flat-spotting |
| Typical price | Usually lower | Usually higher |
| Best suited for | Farm, construction, off-road, short local hauls | Highway travel, long trips, frequent towing |
Which Type Fits Your Towing
Choose bias if your trailer works for a living. Equipment trailers, farm wagons, and construction trailers spend their time on gravel, dirt, and jobsites where sidewalls rub against curbs, ruts, and debris. The stiff bias sidewall takes that abuse better, and since these trailers usually run short distances at lower speeds, heat buildup is less of a concern. The lower price also matters when a tire is likely to die from a cut long before it wears out.
Choose radial if your trailer travels. Boat trailers headed to the coast, campers, and car haulers that log real highway miles benefit from the radial's cooler running temperature and even wear. Heat is one of the main things that kills trailer tires at sustained speed, so the construction that manages heat best wins for distance towing.
Whichever type you pick, it still needs to be a trailer-rated tire — see our guide on ST vs LT tires for trailers for why that matters. And remember that trailer tires often age out before they wear out; our guide on when to replace trailer tires covers the warning signs.
How to Tell Which One You Have
Look at the letter in the middle of the size code on the sidewall:
ST205/75R15 — the "R" means radial.
ST205/75D15 — the "D" means diagonal, i.e. bias-ply.
B in the same position means belted bias, an older construction you'll see less often today.
Older numeric sizes that use a dash instead of a letter — such as 4.80-12 — are bias-ply tires. For a full breakdown of every part of the size code, see our guide to trailer tire sizes explained. Note that the construction letter is separate from the load range letter (B, C, D, E) printed elsewhere on the sidewall — that letter describes load capacity, not construction, and is covered in our trailer tire load range guide.
Can You Mix Bias and Radial Tires?
Never on the same axle. The two constructions flex and grip differently, so pairing them side by side makes the trailer handle unpredictably, especially in braking and crosswinds. On a tandem-axle trailer, keep each axle matched at minimum — and the safest practice is to run the same construction on every position, spare included. Our trailer spare tire guide covers how to keep the spare consistent with your road tires. Some states also have their own rules on tire mixing and condition, so check your state's regulations before running a mixed set between axles.
FAQ
Are radial trailer tires worth the extra cost?
For highway towing, usually yes — cooler running and more even wear mean the tire is likelier to deliver its full service life. For a work trailer that crawls around a farm or jobsite, a bias tire often makes more financial sense, since sidewall damage, not tread wear, is the likelier end of its life.
What does the "D" in my trailer tire size mean?
The "D" stands for diagonal — a bias-ply construction where the internal plies crisscross at an angle. An "R" in the same spot means radial, and a "B" means belted bias, which is uncommon on newer tires.
Can I put radials on one axle and bias tires on the other?
It's legal in some places but not recommended: the axles will flex and grip differently, which can affect tracking and sway. Mixing on the same axle should never be done. When in doubt, match all positions and check your state's rules.
Whether your trailer calls for bias toughness or radial road manners, you can browse both constructions in our trailer tires collection — every tire ships free from our Orlando, FL warehouse, and you can call (407) 821-7008 if you'd like help matching a type to your setup.