Condensed answer: If a rim has a 25mm internal width, then it’s too wide for a 28mm tire by conventional standards.
If the 25mm measurement refers to the external/total width of the rim, then the combination can operate fine.
The final performance and ease of tire mounting and removal depend on the rim and tire model.
External Rim Width
The external/total rim width represents the perpendicular distance between the outer walls of the rim.
The total rim width depends on the rim thickness and the tire size that the rim is designed for.
Internal Rim Width
The most popular bike tires are known as clinchers and have beads that hook onto parts of the rim called bead hooks.
The perpendicular distance between the bead hooks is the internal rim width. Sometimes people put the letter “i” for internal to indicate the internal rim width (e.g., i28mm).
The internal rim width is often the measurement found in rim description as it has a direct influence on the tire size that a rim can accept. The greater the internal rim width, the wider the tire has to be to preserve its shape and perform as intended.
If a wide tire is combined with a rim that has an extremely narrow inner width, one would face the following problems:
a. The tire will have a hard time remaining attached to the rim due to the narrow space. The most problematic region is near the valve. In extreme cases, the tire may unhook in that region.
When this happens the bike becomes unsafe to ride, and the problem should be taken care of immediately. Otherwise, the tire may come off and cause the wheel to bind. If it’s the front wheel, an accident is close to 100% guaranteed.
b. The tire may roll off to one side when cornering. The lower the air pressure, the greater the likelihood of this outcome.
If a narrow tire is combined with a rim that’s too wide, one may face the following issues:
- Faster tire wear
- Exposed side walls
- Less than ideal cornering
- More stress on the rim
- Wider effective tire width (reduced clearance)
- Difficult installation
Rim and Tire Compatibility Chart (conservative)
The table below shows tire and inner rim width compatibility. The numbers are on the conservative side and some deviations are allowed.
|INNER RIM WIDTH||COMPATIBLE TIRE SIZES (MM)||COMPATIBLE TIRE SIZES (IN)|
The data reveals that a 28mm tire can be combined with a rim that has a 15-20.9mm inner width.
If the rim in question has a 25mm total width, then its inner width is probably 4-6m less. In that case, the rim is within the guidelines presented in the table.
Keep in mind that tire sizes are known to be inaccurate. One 28mm model could be wider or narrower than an equivalent made by another brand. Hence why the numbers above are not set in stone and should serve as a general guideline.
If the internal rim width of the rim is 25mm, then the rim is too wide for a 28mm tire, and the user may face the problems mentioned above.