Tires, Wheels, Bearings & Hubs

ST tires versus passenger tires

Trailers need tires engineered for sustained load and heat, not steering and braking like a car. Special trailer (ST) tires use heavier sidewalls and construction aimed at carrying vertical load over long distances. Passenger (P-metric) or light truck (LT) tires may fit the same rim size but are designed around different duty cycles — steering response, ride comfort, and traction under acceleration and braking.

Using passenger tires on a loaded utility trailer is a common shortcut that works until it does not — usually on a hot highway day when sidewall heat builds faster than the tire was built to shed. ST tires also carry speed ratings that assume trailer use; exceeding those speeds reduces margin even when the tire looks fine at rest.

Radial ST tires dominate modern trailers. Bias-ply ST still appears on some equipment and older rigs. Match what the axle and rim are rated for, not whatever is cheapest at the auto parts store. If you inherit a trailer with mismatched tire types across axles, treat that as a correction item before long trips.

Load range, load index, and inflation

Every tire sidewall lists a load range (often C, D, or E on smaller trailers) or equivalent ply rating. That letter ties to a maximum load at a specific pressure — not at whatever pressure feels firm. Higher load range does not mean you should run lower pressure; it means the tire can carry more weight when inflated correctly.

Load index is a numeric code translating to pounds per tire at rated pressure. Add the per-tire load of everything on that axle — trailer weight plus cargo allocated to that axle — and compare to the tire rating with margin. Running at 100% of rated load with no spare capacity leaves no room for imbalance or a slightly low tire.

Inflation must follow the tire manufacturer’s chart for the load you carry, or the trailer placard if it specifies a minimum. Check pressure when tires are cold — before driving or after sitting several hours. Driving heats air inside the tire and raises pressure readings; adjusting hot tires leads to underinflation when cold.

Carry a quality gauge and check all tires including the spare at the start of trip days. A few PSI low on one corner can cause heat, uneven wear, and blowouts on dual-axle trailers where the low tire carries more than its share.

Speed rating, age, and sidewall condition

ST tires carry speed symbols (often J, K, or L — roughly 62–75 mph depending on tire). Tow slower when loaded heavy, in heat, or on rough roads even if the rating is higher. Trailer stability and brake balance matter more than the tire’s theoretical maximum.

Tire age matters independent of tread depth. Rubber oxidizes. Many trailer tires age out before they wear out because annual mileage is low. Check the four-digit DOT date code on the sidewall — week and year of manufacture. Industry guidance often suggests careful inspection after six years and replacement by ten years regardless of tread, especially on sun-exposed spare tires. A deep-tread spare from 2014 is not a reliable spare.

Inspect for cracking in sidewalls and between tread blocks, bulges, cuts exposing cord, and flat spots from sitting loaded on one surface. Replace tires with structural damage; plug kits are for temporary car emergencies, not a plan for trailer sidewall failures.

Uneven wear and what it signals

Wear patterns point to problems cheaper to fix before the tire is destroyed.

• Center wear: chronic overinflation or light load on high-pressure setting
• Both edges worn: underinflation or overload
• One edge: misalignment, bent axle, or worn suspension components
• Scalloped or cupped tread: balance issues, loose bearings, or worn shocks on torsion axles
• Single tire on dual axle hotter or more worn: low pressure, brake drag, or bearing failure starting

Rotate tires when the manufacturer allows and when wear is even enough to make rotation meaningful. Document wear photos at oil-change intervals so you notice drift over months.

Wheel rating, bolt pattern, and lug torque

Rims have load ratings separate from tires. A tire rated for 2,150 pounds on a corroded rim with unknown origin is a weak link. Use rims matched to axle capacity — stamped steel or rated aluminum designed for trailer hubs, not generic automotive wheels chosen only for appearance.

Bolt pattern (for example 5 on 4.5 inches or 5 on 4.75 inches) must match the hub exactly. One wrong lug hole alignment means the wheel is not centered; lug nuts can loosen and studs can shear. Verify pattern with a template or known-good wheel before buying replacements.

Lug torque is specified in foot-pounds for your stud size and wheel type. Under-torqued nuts loosen with vibration; over-torqued nuts stretch studs. Torque in a star pattern with a calibrated wrench after the first 10–25 miles on a new wheel or after any hub service, then again after a full day of driving. Never use impact guns for final torque without a torque stick and verification — uneven torque is worse than hand tight alone on a short trip to a shop.

Replace studs and nuts that show galling, rust swelling, or stretched threads. Carry a spare lug nut set matched to your studs.

Bearings, grease, seals, and adjustment

Trailer wheel bearings carry radial and thrust load while generating heat from rolling friction. They run inside hubs bathed in grease, sealed by inner and outer seals that keep grease in and water out.

Bearing adjustment on many trailers uses a nut and cotter pin: tighten while rotating the wheel until slight drag, back off to the manufacturer’s spec, then install the pin. Too loose lets the wheel wobble and destroys bearings; too tight generates heat and cooks grease. Oil-bath hubs use a different fill level and sight glass — follow the hub manufacturer’s procedure, not a generic YouTube clip from a different hub type.

Repack bearings on a schedule driven by immersion (boat trailers), mileage, and age. At minimum, inspect grease color and consistency at annual service — milkshake-colored grease often means water intrusion. Replace seals whenever bearings are serviced; seals are cheap compared to hub replacement.

Use grease rated for wheel bearings, not general chassis grease alone unless the product explicitly covers both. Mixing incompatible greases can soften or harden the combined pack.

Hub temperature as a diagnostic

After 10–15 miles at highway speed, hub surfaces should be warm but not too hot to hold briefly. One hub noticeably hotter than its partner on the same axle suggests brake drag, bearing failure, or low tire pressure on that corner. Stop and investigate — do not “see if it cools down” across another hundred miles.

Infrared thermometers make comparison easy at rest stops. Without one, compare by hand carefully — if you cannot keep a flat palm on the hub dome for five seconds, it is hotter than normal. Smell of hot grease or visible grease splatter on the inside of the wheel means seal failure or overheating; service before continuing.

Spare, jack, and field tools

A spare is only useful if it matches bolt pattern, load range, and inflation, and if you can actually mount it. Store the spare out of direct sun when possible; cover it if mounted on the trailer tongue or gate.

Carry a scissor or bottle jack rated for trailer weight, a lug wrench or breaker bar matching your lug size, wheel chocks, and a torque wrench if you will re-torque after roadside changes. A thin board under the jack on soft ground prevents sinking. For multi-axle trailers, know whether you must lift on the frame or axle — lifting wrong bends fenders or axle tubes.

Add bearing grease, a spare cotter pin or cap set, and disposable gloves if you might repack at the roadside. A 12-volt air compressor helps correct pressure after changes or morning temperature swings.

Common mistakes

• Passenger tires because they were on sale. Load and heat duty differ from trailer service.
• Checking pressure after hours on the highway. Hot readings hide underinflation.
• Ignoring spare age. The spare fails at the worst moment because it never rotated into service.
• Impact-wrench lug nuts without re-torque. Uneven clamp load loosens on vibration.
• One hot hub “for later.” Bearing and brake failures escalate quickly.
• Boat-trailer bearings without immersion service. Water in hubs destroys grease within weeks.