Condensed Answer: In most cases, a 29″ frame can readily accept a 27.5″ fork. However, the new fork will steepen the headtube of the frame and may also be unable to accept a 29″ wheel.
If the fork is combined with a 27.5″ wheel, the head tube angle will get even steeper.
Often, it’s best to stay away from this conversion due to the geometry changes that it comes with.
The Problems With Installing a 27.5″ Fork On a 29″ Frame
1. Lowered Front End
A fork is usually measured from the center of the axle to the top of the crown. That distance is known as Axle to Crown Length (ACL).
Since 27.5″ wheels are smaller, 27.5″ forks have a slightly shorter ACL than 29″ models. (There are some exceptions.)
As a result, the front end of the bike will be lowered and the so-called head tube angle will get steeper.
The head tube angle is the angle formed by the bike’s head tube and the ground. A steeper head tube angle decreases the drag created by the body and makes climbing easier.
However, it also positions more mass on the front wheel and makes it more difficult to overcome road irregularities. For that reason, modern MTBs have much slacker head tube angles than before.
That said, there are ways to minimize the steeping of the HTA when installing a 27.5″ fork on a 29″ frame, namely:
- Install a 27.5″ wheel at the back too. (Riding with a smaller wheel at the front is not recommended for MTBs and full-size commuters.)
- Increase the travel of the fork. (If the fork has adjustable travel, you can increase it to a higher setting. This will increase the ACL of the fork and raise the front end.)
- Install a beefier tire (A wider tire will increase the diameter of the wheel.)
- Lower Bottom Bracket
A shorter fork and smaller wheel(s) will lower the bottom brаcket of the bicycle.
A lower bottom bracket increases stability during turns at the expense of clearance. As a result, it becomes easier for the bike to hit an obstacle. The chances for pedal strikes (the pedals hitting the ground during turns) are also higher.
- Reduced Arch Clearance
A 27.5″ fork will not accept a 29″ wheel with a beefy tire because the arch sits lower.
Head Tube and Fork Compatibility
27.5″ forks are fairly new and come in “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 the fork is non-tapered, the frame will ideally be non-tapered too. However, it’s still possible to install a non-tapered fork on a tapered frame with the help of a headset cup reducer which eats up the free space that the fork’s steerer will otherwise leave.
If the fork is tapered and the frame isn’t, you will need an adapter that sits below/outside of the head tube.
The Ideal Scenario
The ideal scenario for installing a 27.5″ fork on a 29″ bike with minimal negative change is:
1. The frame already has a fairly high bottom bracket. This is necessary to minimize the negative effect of the reduced clearance that occurs with the new fork.
2. A 27.5″ wheel is installed at the front and the back. This method greatly reduces the changes to the HTA.
3. The travel of the new fork matches that of the old.
If the above conditions are met, the changes will be minimal. In fact, the only change will be the slightly lower bottom bracket.
What Are The Advantages Of Installing A 27.5 Fork On a 29″ Frame
Truth be told, the incentive to perform this transition is low to none. First, both 27.5″ and 29″ wheels are the current standard and fairly easy to find. As a result, manufacturers produce plenty of forks for both sizes.
Second, if you simply want to install 27.5″ wheels on a 29″ bike, you can do it without replacing the fork. The vast majority of modern suspension forks use disc brakes which grab a rotor mounted to the hub. This allows you to change the wheels without experiencing “brake incompatibility”.
(If the bike uses rim brakes, however, you cannot swap the wheels right away as the brake bosses sit higher, and the brake shoes won’t be able to grab the rim of the 27.5″ wheel.)
The only logical situations for performing this change are:
- You currently don’t have access to another fork and simply want to ride your bike despite the effects on handling.
- You have a 27.5″ bike with a damaged frame and access to a 29″ frame on which you want to transplant the parts to acquire an operational machine.