A motorcycle tire sidewall carries a lot of information in a short string: width, aspect ratio, construction, rim diameter, load index and speed rating. The numbers are not interchangeable. A tire that is 10 mm wider can change steering and may not fit the rim or the mudguard.
What it does
The size string tells you the tire's dimensions. A typical size is 120/70 ZR 17. The first number is the nominal width in millimetres, the second is the aspect ratio as a percentage of the width, and the last is the rim diameter in inches.
The letters and numbers after the size give the construction, load and speed rating. A higher load index means the tire can carry more weight. A higher speed rating means it can sustain higher speed without overheating.
How it is built
The width and aspect ratio together set the tire's height and profile. A lower aspect ratio gives a shorter sidewall and quicker steering, but less comfort. A higher aspect ratio gives a taller sidewall and more cushioning.
The load index and speed rating must meet or exceed the bike's requirements. Fitting a lower-rated tire can be dangerous and may be illegal. The vehicle plate or manual lists the original specification.
Types and markings
- Metric sizes. 120/70-17 style. The common modern format.
- Inch sizes. 3.00-17 style. Common on older and small bikes.
- Alpha sizes. MT 90-16 style. Found on some cruisers and classics.
- Load index. A number that maps to a maximum load. Higher is stronger.
- Speed rating. A letter that maps to a maximum sustained speed. Higher is faster.
Wear and failure signs
- A size that does not match the rim width. The tire will pinch or distort.
- A load rating below the bike's requirement, causing sidewall stress.
- A speed rating below the bike's capability.
- A front and rear mismatch that upsets the balance.
- A tire that rubs the mudguard or swingarm.
- An old date code on a tire sold as new.
Choosing a replacement
- Use the size in the manual or on the tire label, not a guess from a forum.
- Match the load and speed rating to the original or higher.
- Keep the original aspect ratio unless you understand the geometry change.
- Check rim width compatibility. A wide tire on a narrow rim is not an upgrade.
- Confirm front and rear sizes separately. They are often different.
Fit and compatibility
- Check clearance to the mudguard, fork and swingarm with the tire fitted and inflated.
- Make sure the tire does not contact the chain or brake caliper.
- Balance the wheel after fitting.
- Set the pressure to the manual's figure for the load.
- Check the date code and direction arrow before fitting.
Inspection and care
- Record the sizes so replacement is straightforward.
- Check pressure and wear at the same time.
- Do not mix radial and bias-ply tires unless the maker allows it.
- Replace tires in matched pairs where possible.
- Inspect the sidewall for cracking as the tire ages.
Safety and limits
A tire with the wrong load or speed rating can fail under load. Match the specification.
A tire that rubs the frame or fork can fail suddenly. Check clearance.
If you are unsure about a size change, ask a tire specialist.
Questions
What does 120/70 R17 mean?
A nominal width of 120 mm, an aspect ratio of 70 percent, radial construction and a 17 inch rim.
Can I fit a wider tire?
Sometimes, if the rim and clearances allow it. A wider tire can slow steering and change the profile. It is not automatically better.
What is a load index?
A code for the maximum load the tire can carry at its rated pressure. It must meet or exceed the bike's requirement.
Do front and rear tires need to match?
They should be compatible in construction and intended use. Matching a sport front with a touring rear can upset the balance.
