To ensure that a bicycle function correctly, it is important to ensure that the width of the rim and the width of the tire is compatible. If the width of the rim and the width of the tire are not compatible with one another, the bicycle may become unstable when ridden, the bicycle can exhibit poor aerodynamics, or the tire may blow out while in use. The internal width of the rim is the measurement of a narrow channel in the rim.
This is the measurement of the tire that is compatible with that rim. Many peoples, however, use the external width of the rim instead. For example, a rim with an external width of 21mm may have an internal width of 17mm.
How to Match Bicycle Tire and Rim Width
Using a narrow tire with a wide rim cause the bead of the tire to not remain steady. Using a wide tire with a narrow rim causes the sidewall of the tire to flex excessive when descending. The internal width of the rim can impact the shape of the tire.
If the internal width of the rim is narrow at approximately 13mm, the tire will take the shape of a lightbulb. A lightbulb shape tire has a small area of the tire in contact with the ground (contact patch). A small contact patch will cause the tire to roll at a slower rate.
Using an internal rim width of 17mm with a moddern tire will cause the tire to be more rounded. This allows for better airflow over the tire. Using an internal rim width of more than 21mm will cause the tire to have a square-off profile.
A squared-off tire profile will have a broad contact patch with the ground, which will provide better grip around corners, as well as better absorption of vibration from the ground. To ensure proper function of a bicycle, there are compatibility guideline between the rim and tire. These guidelines indicate the minimum, ideal, and maximum tire sizes for specific rim width.
Using a tire that is too narrow for a rim with a wide internal width will result in instability of the bicycle. The type of rim can also impact compatibility between the tire and rim. For example, if the rim has a hook (such as a hooked rim), then the rim can use tube and have higher air pressures on the tires.
If it is a hookless rim, then specific tires is required for compatibility, and the air pressure in the tires will have to be lower then rims with hooks (often under 73 PSI). Air pressure in the tires is another factor that can impact the performance of a bicycle. The wider the rim, the lower the air pressure that should of be use in the tires.
High air pressure will cause the bicycle to bounce when ridden over uneven ground. Low air pressure will increase the rolling resistance of the bicycle. An individual that will ride the bicycle can adjust air pressure based on the type of terrain the bicycle will encounter, as well as the body weight of an individual.
The weight of the rim and the depth of the rim are also factor to consider in the compatibility of the tires and rims. Carbon rims that are narrow in width are often lighter than carbon rims that are wide in width. Because of the less mass of narrow rims, they are often preferred for individuals that intend to climb hill.
Wide carbon rims hold more mass, but offer advantage in aerodynamics. Deep rims will experience more impact from crosswinds than shallow rim. The rule of thumb for tire and rim width is to use a tire whose internal width is 105% of the internal width of the rim.
Another factor to consider is the tension of the spokes in the rim. Wider rims will distribute the mass of the bicycle differently than narrow rim. Finally, another factor to consider is how an individual plan to ride the bicycle.
For instance, if high speed will be encountered, narrow and light rims and tires is preferred. For long ride, wide and compliant tires are better for comfort. If an individual can ensure that the width of the tire and rim are matched appropriately, and that the air pressure is set to the proper level, the bicycle will perform as intended.
