Condensed Answer: A 27.5″ fork can be installed on a 26″ frame as long as the head tube and the steerer of the fork are compatible.
The new fork is longer and will therefore raise the front end of the bicycle. As a result, the handling could feel different, although a massive change is not expected.
Fork and Frame Compatibility
First, it’s necessary to determine whether the fork and the frame are compatible.
27.5″ forks are fairly new and come in more “updated” sizes. The two options are 1 1/8″ as well as 1 1/8″ – 1.5″. (The numbers refer to the outer diameter of the fork’s steerer.)
In the first case, the steerer of the fork is 1 1/8″ thick across its entire length. In the second, the fork’s steerer starts at 1 1/8″ but tapers to 1.5″. The second format is newer and meant to add stiffness to the fork which will then result in snappier handling.
If you’re planning on installing a 27.5″ fork on a very old 26″ frame, the headtube of the frame may fail to accept the fork.
Many retro 26″ frames are designed for forks with 1″ streerers. If you have such a frame, you will be unable to install any 27.5″ fork on it.
That said, there are also retro 26″ frames with oversized head tubes which can readily accept a modern 1 1/8″ fork.
If the fork is tapered to 1.5″, then a standard non-tapered head tube will not accept it by default. Ideally, it’s recommended to get a new fork or frame.
That said, it’s possible to install a tapered fork on a non-tapered frame via adapter headset cups.
This approach requires you to get a fork with a longer steerer as the tapered part will technically sit outside of the head tube.
If the frame is tapered and the fork is non-tapered, you can still use the fork, but you will need a headset with a reducer which makes up for the extra space that the fork will otherwise leave.
Threadless or threaded?
27.5″ forks are threadless. Or in other words, they aren’t secured to the frame via the headset. It’s the stem in combination with a star-fangled nut (or a compression plug for carbon forks) that keeps the headset packed and the fork secured.
If the current fork is threaded, and the headtube can accept the new one, you can still make the swap, but you will have to get a non-threaded headset and a new stem.
Changes To The Bike’s Geometry
- Slacker Head Tube Angle
А 27.5″ fork has to accept a larger wheel and will therefore be longer than a 26″ model.
The length of a fork is known as Axle to Crown Length (AXL) and represents the distance between the hub’s axle and the top part of the crown.
The long ACL of a 27.5″ fork will elevate the front end of the bike and will therefore slacken the so-called head tube angle (HTA).
A slacker head tube angle is considered beneficial for overcoming obstacles. Hence why modern MTBs have extra slack HTAs.
However, that comes at the expense of extra drag (the body is more upright) and worse maneuverability at slow speeds.
- Higher Bottom Bracket
The bottom bracket of the bike will get higher too. The extra clearance between the frame and the ground will reduce the chance of hitting an obstacle on the trail, but the higher center of mass will result in instability. (Another bonus is the lower likelihood of hitting the ground with the pedals a.k.a. pedal strike).
FAQ: How can I reduce the changes to the geometry?
Even though the changes aren’t dramatic, some people may still want to minimize them.
There are two options. A – limit the travel of the fork; B – install a narrower tire.
By reducing the travel, you will decrease the ACL and thus keep the HTA steeper. Lowering the travel by about 20mm (e.g., from 120mm to 100mm) will be enough “counterbalance”.
Alternatively, you can switch to a narrow tire which would give a smaller radius to the wheel and thus keep the front end lower.
Note: If you want to limit the change maximally, you can also use the 27.5″ fork in conjunction with a 26″ wheel. If the wheel uses disc brakes, it will be compatible with the fork right away as the brake rotor is mounted to the hub and is always in the center of the wheel.
However, if the fork uses rim brakes (unlikely), you will have to use a 27.5″ wheel because the brake bosses will be too high for a 26″ wheel, and the brake won’t be able to grab the rim.
FAQ: What are the advantages of installing a 27.5″ fork on a 26″ frame?
- Updated fork
The main incentive to perform the described operation is to get access to a newer fork. 26″ wheels are no longer found on modern MTBs. As a result, fork manufacturers have a low incentive to produce newer and better forks for that wheel size. By switching to 27.5″, you will gain access to more forks.