tireman9
Well-known member
Special Trailer (“ST”) Tires
Trailer tires are widely used in a variety of towable trailer applications and are designed and marked as “ST” (Special Trailer) tires.
• Special Trailer tires, with the “ST” size designation, are speed rated at 65 MPH (105 km/h) under normal inflation and load conditions as shown in TRA table ST-M1. Prior to 2016 almost none of these tires had a Speed Symbol letter.
• This speed rating is based on Tire and Rim Association loads and inflation pressures for ST trailer tires without a Service Description for speeds up through 65 mph.
• If tires, with the ST designation and a Speed Symbol are used at speeds between 66 and 75 mph (106 km/h and 120 km/h), TRA directs the cold inflation pressure be increased by 10 psi (70 kPa) above the recommended pressure based on the trailer placard for normal inflation and load conditions.*
• If tires, with the ST designation, are used at speeds between 76 and 87 mph (122 km/h and 140 km/h), it is recommend the cold inflation pressure be increased by 10 psi (70 kPa) above the recommended pressure based on the trailer placard for normal inflation and load conditions and the load is to be decreased by 10%.*
You should also consult your trailer manufacturer’s Owner’s Manual for maximum operating speed recommendations for the trailer.
• No matter the inflation and load adjustments tires should never be operated at a speed higher than what is indicated by the speed symbol.
o Even if Inflation and Load adjustments have been made to a tire with a Speed Symbol, it is never appropriate to use an ST tire at a speed above 87 mph.
o Increasing the inflation pressure by 10 psi (70 kPa) does not provide any additional load carrying capacity.
o Do not exceed the maximum pressure for the wheel or the valve.
o If the maximum pressure for the wheel or valve prohibits the increase of inflation pressure, then the 65 mph rated speed must observed as the maximum speed.
o It may be necessary to dismount the tire from the wheel to determine the maximum rated inflation for the wheel and valve.*
o All the references to inflation here are about the "cold inflation pressure".
*Details about wheel and valve inspection:
a) If the rated (maximum allowable) inflation pressure is not visible on the wheel, consult the wheel or trailer manufacturer. Lacking documentation for a higher inflation the maximum allowable inflation pressure should be considered to be the maximum inflation on the OE tires.
b) Tubeless snap-in type valves, depending on the type valve, are rated for a maximum inflation pressure of 65 (TR413) or 80 (600HP) psi for nominal 0.453” diameter valve holes. If a higher cold inflation pressure is used, a clamp-in valve such as TR416S valve (200 psi maximum inflation) must be installed.
The above is a summary of my understanding and opinion as a tire engineer of steps that need to be taken for operating ST tires. Some will recognize that much of the above were covered in the Goodyear technical bulletin on ST tires
Important new information.
There are now a number of ST tires with a Service Description and various speed symbols above 65. While it is possible to make better quality tires we need to remember that ST tires are already rated for load capacities of 10% to 20% greater than LT tires of identical size but those load ratings were based on the max speed of 65. As an engineer I do not understand the physics used to allow a tire with "ST" on the sidewall the be rated to carry 10 to 20% more load than a tire with "LT" on the sidewall given that LT tires already have some of the best construction features as part of their specification as they have to pass much more stringent regulatory requirements than ST tires do.
Trailer tires are widely used in a variety of towable trailer applications and are designed and marked as “ST” (Special Trailer) tires.
• Special Trailer tires, with the “ST” size designation, are speed rated at 65 MPH (105 km/h) under normal inflation and load conditions as shown in TRA table ST-M1. Prior to 2016 almost none of these tires had a Speed Symbol letter.
• This speed rating is based on Tire and Rim Association loads and inflation pressures for ST trailer tires without a Service Description for speeds up through 65 mph.
• If tires, with the ST designation and a Speed Symbol are used at speeds between 66 and 75 mph (106 km/h and 120 km/h), TRA directs the cold inflation pressure be increased by 10 psi (70 kPa) above the recommended pressure based on the trailer placard for normal inflation and load conditions.*
• If tires, with the ST designation, are used at speeds between 76 and 87 mph (122 km/h and 140 km/h), it is recommend the cold inflation pressure be increased by 10 psi (70 kPa) above the recommended pressure based on the trailer placard for normal inflation and load conditions and the load is to be decreased by 10%.*
You should also consult your trailer manufacturer’s Owner’s Manual for maximum operating speed recommendations for the trailer.
• No matter the inflation and load adjustments tires should never be operated at a speed higher than what is indicated by the speed symbol.
o Even if Inflation and Load adjustments have been made to a tire with a Speed Symbol, it is never appropriate to use an ST tire at a speed above 87 mph.
o Increasing the inflation pressure by 10 psi (70 kPa) does not provide any additional load carrying capacity.
o Do not exceed the maximum pressure for the wheel or the valve.
o If the maximum pressure for the wheel or valve prohibits the increase of inflation pressure, then the 65 mph rated speed must observed as the maximum speed.
o It may be necessary to dismount the tire from the wheel to determine the maximum rated inflation for the wheel and valve.*
o All the references to inflation here are about the "cold inflation pressure".
*Details about wheel and valve inspection:
a) If the rated (maximum allowable) inflation pressure is not visible on the wheel, consult the wheel or trailer manufacturer. Lacking documentation for a higher inflation the maximum allowable inflation pressure should be considered to be the maximum inflation on the OE tires.
b) Tubeless snap-in type valves, depending on the type valve, are rated for a maximum inflation pressure of 65 (TR413) or 80 (600HP) psi for nominal 0.453” diameter valve holes. If a higher cold inflation pressure is used, a clamp-in valve such as TR416S valve (200 psi maximum inflation) must be installed.
The above is a summary of my understanding and opinion as a tire engineer of steps that need to be taken for operating ST tires. Some will recognize that much of the above were covered in the Goodyear technical bulletin on ST tires
Important new information.
There are now a number of ST tires with a Service Description and various speed symbols above 65. While it is possible to make better quality tires we need to remember that ST tires are already rated for load capacities of 10% to 20% greater than LT tires of identical size but those load ratings were based on the max speed of 65. As an engineer I do not understand the physics used to allow a tire with "ST" on the sidewall the be rated to carry 10 to 20% more load than a tire with "LT" on the sidewall given that LT tires already have some of the best construction features as part of their specification as they have to pass much more stringent regulatory requirements than ST tires do.
