Understanding Motorcycle Tire Sizes
Motorcycle tire sizes use a standardized code system indicating width, aspect ratio, construction type, and rim diameter. A common tire size like 120/70R17 breaks down as 120 millimeters wide, 70 percent aspect ratio sidewall height, radial construction, and 17 inch rim diameter. Additional codes indicate load index and speed rating, such as 120/70R17 58W where 58 is the load index for 520 pounds maximum load and W is the speed rating for 168 mph. Understanding these codes ensures you select tires matching your motorcycle specifications and riding style. Incorrect tire sizing affects handling, safety, and speedometer accuracy, making proper size selection critical.
Tire size codes are molded into the tire sidewall and must match manufacturer specifications in your motorcycle owner's manual. The first number is tire width measured in millimeters at the widest point. The second number is aspect ratio, expressing sidewall height as a percentage of width. A 120/70 tire has 120 millimeter width and sidewall height of 84 millimeters, calculated as 120 times 0.70. The letter indicates construction, with R for radial and B for bias ply. The final number is rim diameter in inches, not millimeters, creating a mixed metric and imperial system used worldwide.
Tire Width and Its Impact
Tire width, the first number in the size code, ranges from 80 millimeters for small scooters to 240 millimeters for cruisers and high performance bikes. Width affects contact patch size, grip, and handling characteristics. Wider tires provide larger contact patches, improving traction during acceleration and cornering. However, wider tires also increase rolling resistance, weight, and steering effort. Sport bikes use 120 millimeter front and 180 to 200 millimeter rear tires, balancing agility and grip. Cruisers favor wide rear tires of 180 to 240 millimeters for visual impact and straight line stability.
Changing tire width by more than 10 millimeters from stock can alter handling, clearance, and speedometer readings. A wider tire may rub against swingarms, fenders, or brake components. It also changes the gearing effect, making the speedometer read slower than actual speed. Narrower tires reduce contact patch and grip but improve fuel economy and maneuverability. Always consult your motorcycle manual for acceptable tire size ranges before deviating from stock specifications. Many manufacturers specify minimum and maximum widths to maintain proper handling and clearances.
Aspect Ratio Explained
Aspect ratio, the second number after the slash, indicates sidewall height as a percentage of tire width. A 70 aspect ratio on a 120 millimeter wide tire has sidewalls 84 millimeters tall, calculated as 120 times 0.70. Lower aspect ratios create shorter stiffer sidewalls, improving handling responsiveness and high speed stability. Sport bikes use aspect ratios of 50 to 70 for quick steering and minimal sidewall flex. Higher aspect ratios of 80 to 90 provide taller softer sidewalls, improving ride comfort and absorbing road irregularities on cruisers and touring bikes.
Aspect ratio affects overall tire diameter, which impacts speedometer accuracy and gear ratios. A tire with lower aspect ratio has smaller diameter than one with higher aspect ratio, even if widths match. For example, a 120/70 tire is shorter than a 120/90 tire. This changes the effective gearing, altering acceleration and top speed slightly. Speedometers calibrated for specific tire diameters read incorrectly with different aspect ratios. Most motorcycles allow aspect ratio variation of plus or minus 5 percent without significant impact, but verify compatibility before changing sizes.
Radial vs Bias Ply Construction
The construction letter, R for radial or B for bias ply, indicates how the tire carcass is built. Radial tires have cord plies running perpendicular to the direction of travel, creating flexible sidewalls and stiff tread. This design provides better grip, heat dissipation, and tire life, making radials standard on modern sport and touring bikes. Radials offer superior high speed stability and handling precision, though they cost more than bias ply tires.
Bias ply tires have cord plies running diagonally at angles to each other, creating stiffer sidewalls and more flex in the tread. Bias tires are cheaper, more durable under heavy loads, and better for low speed off road use. Many cruisers, vintage bikes, and dirt bikes use bias ply tires. Some bikes specify bias belted tires, denoted by B or dash symbol like 130/90-16, combining bias construction with belts for improved performance. Never mix radial and bias tires on the same motorcycle, as differing handling characteristics create dangerous instability.
Rim Diameter and Fitment
Rim diameter, the number after the construction letter, indicates wheel size in inches. Common sizes are 16, 17, 18, 19, and 21 inches. Sport bikes predominantly use 17 inch wheels front and rear for quick steering and wide tire availability. Cruisers often use 16 inch wheels, while touring and adventure bikes may have 17 inch rear and 19 inch front wheels for stability and comfort. Dirt bikes use larger diameter wheels, typically 21 inch front and 18 or 19 inch rear, to roll over obstacles and provide ground clearance.
Rim diameter must match exactly between tire and wheel. A 17 inch tire only fits a 17 inch wheel, with no substitution possible. Installing mismatched sizes creates catastrophic failure as the tire bead cannot seat properly. Wheel width also matters, as wider wheels require wider tires. Most tire manufacturers specify acceptable rim width ranges for each tire size, typically varying by half inch increments. Consult fitment charts to ensure your tire width matches your wheel width, preventing bead seating issues and handling problems.
Load Index and Weight Capacity
Load index is a numerical code following the rim diameter, indicating maximum weight capacity. Each index number corresponds to a specific load in kilograms or pounds. For example, load index 58 equals 236 kilograms or 520 pounds. Motorcycles require tires rated for at least the combined weight of bike, rider, passenger, and cargo. Exceeding load ratings causes excessive heat buildup, tire failure, and blowouts.
Most sport bikes use load indexes from 54 to 62, while touring bikes and cruisers require 62 to 75 for heavier bike weights and luggage capacity. Calculate your maximum load by adding bike curb weight, rider and passenger weight, and luggage weight. Select tires with load index exceeding this total. Some manufacturers specify different load indexes for front and rear tires, as rear tires carry more weight. Always match or exceed OEM load ratings when replacing tires, never downgrade to lower capacity.
Speed Ratings and Performance
Speed rating is a letter code indicating maximum sustained speed capability. Ratings range from J for 62 mph to W for 168 mph and beyond. The rating reflects tire construction, materials, and heat resistance at speed. Sport bikes require high speed ratings of V, W, or Z for sustained high speed capability. Cruisers and touring bikes typically use H or V ratings, while scooters may have lower S or T ratings.
Speed ratings account for heat buildup from friction at high speeds. Exceeding speed ratings risks tire failure from overheating, tread separation, or blowouts. The Z rating originally indicated speeds over 149 mph but is now sometimes used with W or Y in parentheses to specify exact capability. For example, ZR rated tires may include W (168 mph) or Y (186 mph) designations. Always match or exceed OEM speed ratings, especially on high performance bikes. Using lower speed rated tires than specified creates serious safety risks.
Alternative Sizing Systems
Some motorcycles, particularly older models and certain cruisers, use alpha numeric sizing like MH90-21 or inch based sizing like 4.00-18. Alpha numeric codes combine tire width letter, aspect ratio, construction, and rim diameter. The letter indicates approximate width in a less precise system than metric codes. Inch based sizing uses tire width and rim diameter in inches, common on vintage bikes and some dirt bikes.
Converting between sizing systems requires charts or calculators, as direct equivalencies don't exist. For example, an MH90-21 roughly equals a 90/90-21 metric tire. Always verify conversions with fitment guides rather than guessing, as incorrect sizes affect safety. Many tire manufacturers provide cross reference charts showing metric equivalents to alpha numeric and inch sizes. When in doubt, consult a motorcycle tire specialist or dealer to ensure proper sizing.
Practical Tips for Tire Selection
Before purchasing tires, locate the size code on your current tires or in your owner's manual. Write down the complete code including width, aspect ratio, construction, rim diameter, load index, and speed rating. Verify that replacement tires match all specifications unless you intentionally change sizes within manufacturer approved ranges. Consider your riding style when selecting tire types, as sport, touring, and cruiser tires differ in tread patterns and rubber compounds beyond size.
Finally, replace tires in pairs when possible, ensuring front and rear tires have similar age and wear patterns for consistent handling. Never mix tire types or constructions. A motorcycle tire size guide with code explanations ensures you select correct tires, maintaining safety, performance, and manufacturer warranty compliance for your motorcycle.