Special Trailer (ST)
Special Trailer Service (ST) tires are developed specifically for use on travel trailers, pop-up campers, boat trailers and fifth wheel trailers. Designed to deliver on-highway service for towed trailers, ST-sized tires are not intended to be used on cars or light trucks.
Special Trailer Service (ST) tires typically feature a symmetric tread design with a solid center rib to provide constant rubber-to-road contact to enhance highway stability, traction and wear. Their internal structures most often feature twin steel belts with larger rim diameter sizes often adding nylon cord reinforcement (belt edge strips or full width cap ply) that further stabilizes the tread to increase durability.
Special Trailer Service (ST) tires are not required to be UTQG rated.
Trailer Tire Sizing
Tire sizes installed by trailer manufacturers are selected for their ability to carry the maximum load the trailer is rated to carry (Gross Vehicle Weight Rating - GVWR). Exceeding the GVWR or allowing the load to be unevenly distributed on multi-axle trailers will possibly overload the tires along with the wheels, axles and frame.
If drivers suspect their load nears/exceeds the GVWR, it requires a trip to a public scale to weigh the loaded trailer. If found overloaded, it is necessary to reduce the load in the trailer.
If drivers are considering upgrading their tires to a heavier load range that allows higher inflation pressures or larger-sized tires, wheel suitability must be checked, fender clearances confirmed and frame strength verified. Hitch adjustment may also be necessary if the replacement tire is larger in diameter than the original.
All tires should be the same size. Unmatched tire sizes guarantee uneven tire loading, which may result in at least one overloaded tire.
Special Trailer (ST) Tire Maintenance
Allowing inflation pressure to drop can dangerously overload a trailer tire, resulting in excessive heat buildup and possibly a blowout. And while not always resulting in immediate tire failure, even a short period of operating a significantly under-inflated trailer tire can cause hidden internal structural damage that can result in tire failure. Tire load capacity is reduced while tread/sidewall deflection and heat buildup are increased anytime a tire is operated without enough inflation pressure to carry its load. The chance of failure greatly increases if trailer tires are underinflated or overloaded.
Since all tires lose inflation pressure over time or due to dropping ambient temperatures, it's important to check trailer tire inflation pressures before each trip. It is best to set inflation pressures in the morning before tires are driven on or exposed to direct sunlight.
While checking and setting tire pressure can take a few minutes, it isn't difficult. Having a compressor at home or at the shop allows checking and resetting pressures to be done conveniently when the tires are cold. If the trailer must be driven to a source of compressed air you may need to wait to set pressures. The definition of a cold tire is one that has traveled one mile or less, or has cooled for three to four hours.
Ambient temperatures are a consideration when setting tire pressures. A 10-degree Fahrenheit (F) change in ambient temperature equates to about a 2% change in tire pressure. For example, 50 psi tire pressures set on a 60-degree F morning will naturally rise by about 4 psi in the afternoon when the day's temperatures top out at 80 degrees F. Conversely, tire pressures set in September's heat will fall as ambient temperatures drop in October.
Another very real variable is sunlight. Tires in the shade versus those sitting in the sun have definite pressure differences. Tire Rack's testing has shown a 3 psi additional rise in tires left in the sun versus those in the shade (this applies to shade versus sun sides of the trailer), as well as after moving the trailer from a cool building into the sun's radiant heat.
Setting tire pressures cold in the morning simplifies the job. As a rule of thumb, add four psi to the recommended cold inflation pressure if the tires are already warm/hot to the touch.
Special Trailer (ST) Tire Replacement Following a Failure
All tires on a multi-axle trailer share responsibility for carrying the load. However if one tire is underinflated, punctured or fails, it no longer carries its share of the weight. If a multi-axle trailer is fully loaded, it means the remaining tires on adjacent axles will likely be overloaded.
Unfortunately it is difficult for a driver to detect impending tire failure on a tandem- or triple-axle trailer. Often the first signs of a problem are when the driver sees smoke or tire debris when looking in the rear-view mirror.
Depending on how long it took the tire to fail and how far it was driven after being compromised, it's possible overloading may have caused irreversible internal structural damage to the remaining tires. In this situation, drivers that replace just the one failed tire may experience additional failures due to the unseen damage from temporarily overloading the other tires.
The industry recommendation is if a tire fails on one side of a tandem-axle trailer, the adjacent tire on the same side should also be replaced as that tire likely bore excessive weight when the tire next to it failed.
Unrecognized, multiple failures can be frustrating as drivers feel their tires are failing for no reason. In fact, it was the first failure that lead to the other tires being compromised and eventually failing.
Special Trailer (ST) Tire Speed Ratings
Unless assigned a specific Speed Rating, industry standards dictate tires with the ST designation are speed rated to 65 mph (104 km/h) under normal inflation and load conditions.
However, Goodyear Marathon and Power King Towmax STR tires featuring the ST size designation may be used at speeds between 66 and 75 mph (106 and 121 km/h) by increasing their cold inflation pressure by 10 psi (69 kPa) above the recommended pressure for the rated maximum load.
Do not exceed the wheel's maximum rated pressure. If the maximum pressure for the wheel prohibits the increase of air pressure, then maximum speed must be restricted to 65 mph (104 km/h).
The cold inflation pressure must not exceed 10 psi (69 kPa) beyond the inflation specified for the maximum load of the tire.
Increasing the inflation pressure by 10 psi (69 kPa) does not provide any additional load carrying capacity.