The second number in a motorcycle tire size is a key detail that affects performance, handling, and safety. This article explains what that number represents, how it fits into the overall tire sizing system, and why riders should pay attention to it when selecting tires or replacing worn ones. By understanding these markings, riders can make informed choices that match their riding style and bike specifications.
The standard tire size marking on most motorcycle tires follows a pattern such as 120/70 ZR17. In this example, the numbers convey specific measurements: width, aspect ratio, construction, and wheel diameter. The first number (120) is the tire’s width in millimeters. The second number (70) is the aspect ratio, expressed as a percentage of the tire’s section height relative to its width. The third part (ZR) indicates radial construction and speed rating, and the final number (17) is the wheel diameter in inches. This article concentrates on the second number and how it influences riding dynamics.
What The Second Number Denotes
Aspect Ratio is the second number in most metric tire sizes and represents the sidewall height as a percentage of the tire’s width. For example, in 120/70, the 70 means the sidewall height is 70% of 120 mm. A higher aspect ratio results in a taller sidewall, which generally provides more cushioning and a softer ride but can reduce steering precision and cornering feel. A lower aspect ratio yields a shorter sidewall, improving stability and sharpness of turn-in but potentially increasing ride harshness over rough surfaces.
How The Second Number Affects Handling
Riders should consider the aspect ratio when balancing ride quality with performance. A larger second number (higher aspect ratio) typically offers:
- Better shock absorption over potholes and rough road surfaces
- More flexible sidewalls that can improve grip on uneven terrain
- Less precise steering and slower turn-in due to taller sidewalls
Conversely, a smaller second number (lower aspect ratio) often provides:
- Enhanced cornering stability and precise steering response
- More aggressive profile that can improve road feel
- Potentially stiffer ride on rough pavement due to shorter sidewalls
These effects vary with motorcycle type, weight, and tire brand. Sport bikes and performance riders may favor lower aspect ratios for quicker handling, while touring motorcycles or riders who prioritize comfort may opt for higher aspect ratios.
Choosing The Right Second Number For Your Bike
When selecting tires, the second number should align with the motorcycle’s design specifications and the rider’s preferences. Consider these factors:
- Owner’s manual and tire manufacturer guidelines
- Riding conditions (city commuting, sport riding, long-distance touring)
- Load and axle weight distribution
- Wheel and suspension setup, including front-rear balance
If in doubt, consult a trusted technician or refer to the motorcycle’s recommended tire sizes. Deviating from OEM (original equipment) specifications can alter handling characteristics and, in some cases, affect safety systems.
Related Tire Measurements In The Marking
To fully interpret the second number, it helps to understand other common tire markings:
- Width (first number): The tire’s width in millimeters from sidewall to sidewall. It influences contact patch area and traction.
- Aspect Ratio (second number): The height of the sidewall expressed as a percentage of the width.
- Construction (letter after the ratio): “R” for radial construction is the most common in modern motorcycles; “D” indicates bias-ply in rare cases.
- Rim Diameter (final number): The wheel diameter in inches that the tire is designed to fit.
- Load Index and Speed Rating: Often printed after the rim size, such as 58W, indicating maximum load per tire and maximum sustained speed.
For a marking like 120/70 ZR17 58W, the 58 is the load index and W is the speed rating. These values complement the second number by detailing the tire’s load-carrying capacity and speed capability, which are critical for safe operation at highway speeds and under heavy loads.
Practical Examples
Example 1: 110/80 R19. Here, the second number is 80, meaning the sidewall height is 80% of 110 mm. The taller sidewall provides a comfortable ride with forgiving handling, suitable for adventure or touring motorcycles.
Example 2: 180/55 ZR17. The second number 55 indicates a relatively low sidewall height for a sport or sport-touring tire, prioritizing cornering stability and precise steering at the expense of some ride comfort.
Example 3: 150/60 R17 66V. A 60% sidewall region offers a balance between performance and comfort, while the 66V load/speed rating supports moderate weight and spirited riding at speed.
Maintenance And Safety Considerations
Maintaining appropriate tire pressures is essential, as pressure affects the actual profile and the perceived handling linked to the second number. Regularly check tire pressure per manufacturer recommendations, adjust for load and riding conditions, and inspect tires for wear indicators that signal nearing limits.
Wearing tires with mismatched or incorrect aspect ratios can disrupt ABS calibration, traction control, and overall stability. When replacing tires, pairing front and rear tires with compatible aspect ratios helps maintain balanced handling characteristics and predictable responses during braking and cornering.
Common Misconceptions
Myth: The second number only affects look and aesthetics. Reality: It directly influences ride quality, handling, and performance characteristics. Myth: Any tire size with the same width and wheel diameter will behave the same. Reality: The aspect ratio changes the sidewall height and stiffness, altering feel and contact patch geometry.
Conclusion
The second number in a motorcycle tire size, known as the aspect ratio, represents the sidewall height as a percentage of the tire’s width. This value significantly shapes ride comfort, handling precision, and overall tire performance. When selecting tires, aligning the second number with the motorcycle design and riding goals ensures safer, more predictable behavior across roads and weather conditions. By understanding tire markings beyond the second number, riders can make informed choices that optimize traction, stability, and riding enjoyment.