How to correctly measure an ST Trailer Tire

Posted by Petra on 11/15/2011 to Trailer Tires FAQ
Towing a trailer is an undertaking that must be taken seriously for safety reasons. The load being towed and the size of the trailer must be able to handle the job. The trailer tires should be sufficient to handle the load being hauled.

Measuring a trailer tire involves sets of numbers that stand for the measurements and specifications of the tire. Understanding these markings will help in deciding what trailer tire is needed.

Begin by laying the trailer tire on its side on the ground. When measured from the ground to the edge of the tire, that equals the section width. In this example we will use a common trailer tire with the markings ST225/75R15. This is a metric value that is found on the sidewall of the tire. This measurement from the ground up should correspond to the first value, in this case 225, in millimeters. To convert this value to numeric, the formula is M x .0394. To convert our example, 225 millimeters x .0394 = 8.865 inches.

Measure aspect ratio from the top of the trailer rim to the top of the tire. In our example, ST225/75R15, this is the second number or 75. This stands for the number, as a percentage, to subtract from the section width, which results in the value of the trailer tire's section height. When using our example, the formula would be .75 x 225=169. Our section height is now 169 millimeters. The conversion formula to inches is again, M x .0394. Take 169 x .0394 = 6.658 inches.

Internal construction is contained in a metric label only. This is identified by one letter, being R or B or D. R is radial construction, so the tire's body plies extend from the center. D is for diagonal or bias ply construction, so the tire's body plies pattern in a crisscross. B is for belted construction, so the tire's body plies pattern in a crisscross and is strengthened with belts. In our example, the R is for radial construction.
Following this letter is a number representing the diameter of the trailer wheel. The third value is the wheel diameter stamped on a trailer tire. In our example, ST225/75R15, this value is 15. Even though this is called a metric measurement value, the last value in these labels are inches. In our example, 15 stands for 15 inches. This value is obtained by measuring the space in the middle of the tire or the trailer rim.

Using ST225/75R15 again as an example, it shows two letters, ST, which signifies the application type. In this example, ST is the value for special trailer. There are other codes that represent other application types. P =passenger vehicle, T= temporary spares, LT=lightweight trucks and C=commercial vans.

In summary, there are three areas to measure to decide the correct trailer tire size required. The width of the trailer tire, the diameter of the rim and the height from the rim top. The letters correspond to the appropriate load use or the materials used in making the tire. The best way to ensure road safety is to have these values as accurate as possible. Use these values to determine that the load being hauled by the trailer have sufficient capabilities; and are appropriate to the load being hauled.