Decoding the Motorcycle Tire Sidewall: Anatomy of a Metric Code
Looking at the sidewall of a modern sport or touring tire, you will find an alphanumeric code such as 120/70ZR17 M/C (58W). Each alphanumeric segment specifies an exact engineering tolerance:
Breakdown of 120/70ZR17 (58W)
- 120: Nominal Section Width in millimeters (120mm at widest point).
- 70: Aspect Ratio (Sidewall height is 70% of the section width, or 84mm).
- Z: High-speed capability indicator (rated for speeds in excess of 149 mph).
- R: Radial carcass construction (cords run 90 degrees across the direction of travel).
- 17: Rim diameter in inches (designed to fit a 17-inch wheel).
- M/C: Motorcycle-specific tire designation (bead profile engineered strictly for motorcycle rims).
- 58: Load Index (Maximum weight rating of 520 lbs / 236 kg).
- (W): Speed Symbol (Parentheses indicate tested speeds exceeding 168 mph / 270 km/h).
Motorcycle Tire Speed Rating Chart
Radial (R) vs. Bias-Ply (-) Construction
Radial Tires: Carcass plies run straight across from bead to bead at a 90-degree angle to the tread centerline. This allows sidewall flexing without transferring heat to the tread crown, keeping the contact patch stable during high-speed cornering and braking.
Bias-Ply Tires: Carcass cords are layered diagonally in crisscrossing ply patterns (typically 30 to 45 degrees). This creates an exceptionally stiff, rugged sidewall capable of supporting heavy loads on large touring cruisers and dual-sport adventure bikes over rocky terrain.
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