Condensed Answer: A 26″ fork with long legs may accept a 27.5″ wheel. However, the change may require the user to replace the tire with a slimmer one to reduce the wheel’s circumference.
If the fork uses disc brakes, the brake system will operate as intended as long as the new brake rotor matches the old one in size.
If the fork relies on rim brakes, the brake shoes will sit too low to grab the brake track of the new wheel.
To learn if a specific 27.5″ wheel will fit in a 26″ fork, it’s necessary to acquire the following measurements and compare them:
A. The radius of the 27.5″ wheel with the tire pumped to the needed air pressure.
The radius of the wheel is measured from the center of the axle to the top of the tire. It’s necessary to pump the tire to the required air pressure because higher air pressure equals more volume and thus increases wheel size.
B. The distance between the middle of the fork’s dropouts and the bottom of the crown or arch.
If the fork is rigid, then the user needs to measure the distance between the middle of the fork’s dropouts and the bottom of the fork’s crown. (The crown is the part of the fork connecting the fork’s legs.)
If the fork is suspended, it’s necessary to measure the distance between the middle of dropouts and the bottom of the arch. (The arch sits underneath the crown and connects the lowers (fatter legs) of the fork.)
Note: Forks have a measurement known as Axle-Crown-Length which equals the distance between the middle of the fork’s dropouts and the top of the crown. Thus, you can’t use that measurement to determine clearance accurately.
If measurement B (axle to below the crown or arch) is smaller than A (wheel radius), then the 27.5″ wheel will not fit. Ideally, the B measurement will be at least a few millimeters bigger than A because the wheel needs some clearance for safe operation.
If the clearance is less than 2mm, a small stone may get caught between the wheel and the fork. When that happens, the front wheel could lock and cause the rider to fall over the handlebars.
C. Lateral Clearance
It’s also necessary to measure the width of the tire and compare it to the distance between the fork’s legs near the crown. If the tire is too fat, it could rub against the upper inner sides of the fork’s legs.
If the bike uses rim brakes, the brake shoes will not fully reach the brake track of the new rim as it will sit higher. Consequently, the brakes will be practically unusable as they are.
The solutions to this problem are:
1. Purchase rim brakes that allow you to move the brake shoes higher.
2. Get V-brake risers:
It’s possible to install V-brake risers – special adapters that mount to the existing braze-ons and provide their own elevated brake spots. The downside of this solution is the extra point of failure introduced to the brake system.
3. Get V-brake Clamp Adapters
Another option would be to install V-brake clamp adapters and place them at an appropriate level.
The con of this method is the price. Some clamp adapters are expensive.
If the bike is equipped with disc brakes, no changes are required as long as the new brake rotor is of the right size.
A disc brake operates by grabbing a rotor that’s bolted to the hub. Thus, the rotor is always at the same location (the center of the wheel) regardless of the wheel’s size.
For that reason, people who like to switch wheels frequently get forks and frames with disc brake mounts to facilitate the process.
Changes To The Geometry
By installing a 27.5″ wheel at the front, the user will make the following changes to the bike’s geometry:
- Slacker Head Tube Angle
The head tube angle (HTA) is the angle between the bike’s head tube and the ground. By installing a larger tire at the front, the user will elevate the front-end slightly and slacken the head tube angle.
A slacker head tube angle is considered beneficial for off-road descending and the performance of stunts such as bunny hops which require the user to aggressively lift the front wheel. Hence why modern MTBs come with extremely slack head tube angles.
The downsides are decreased maneuverability at slow speeds and increased drag (the rider’s torso is more upright).
- Higher Bottom Bracket
By elevating the front-end, one will raise the bottom bracket too. A higher bottom bracket increases clearance but hurts the bike’s cornering ability due to the higher center of gravity.
In this case, the change is too small to elicit a drastic performance difference.
- More Weight On the Rear Wheel
The front-end rise positions the rider’s weight closer to the rear wheel. The extra weight on the rear wheel combined with the slack head tube angle make climbing more difficult.
The Advantages of Installing a 27.5″ Wheel On a 26″ Fork
- Greater Roll-over-ability + Cushioning
A larger wheel at the front makes it easier to overcome obstacles.
- Potential Money Savings
If you already have a 27.5″ wheel and need to replace a damaged 26″ unit, you can save money.
The Disadvantages of Installing a 27.5″ Wheel On a 26″ Fork
- Reduced front wheel clearance that could make riding dangerous and prevent the usage of accessories such as full fenders.
- Potential brake incompatibility (only in the case of rim brakes)
- Altered bike geometry
- Extra weight (when all parameters are equal, a larger wheel is a heavier wheel).