Condensed Answer: A 29″ fork can be installed on a 27.5″ bike, but it will have a notable effect on the bike’s geometry. The new fork will slacken the head tube and seat tube angle. Consequently, the handling of the bicycle will change in a manner that the rider may not like.
Switching To a 29-inch Fork
To measure the impact that a 29″ fork will create, one first needs to examine a measurement known as axle-to-crown-length (ACL).
The axle to crown length of a suspension fork is determined by the fork’s travel (how much it moves upon compressing) and the wheels that it’s designed for.
For that reason, the axle to crown length varies between brands and models.
For a more accurate comparison, it’s logical to use models of the same brand designed for 29″ and 27.5″ wheels.
For example, the axle to crown length of FOX 34 27.5″ fork is 532mm whereas that of FOX 34 29″ is 550m. This amounts to an 18mm/1.8cm/0.7in difference.
Thus, if you keep everything else but replace the fork, it will raise the front end of the bicycle by 1.8cm.
The increase will create the following changes to the geometry:
A Slacker Head Tube Angle (HTA)
The head tube angle of a bike is the angle formed by the head tube and the ground.
Modern MTB geometry is focused on slack head tube angles for the following benefits:
A slacker head tube angle puts the wheel further in front of the rider. As a result, the front end has an easier time overcoming obstacles and irregularities. This greatly reduces the chances of going over the handlebars.
- The Front Wheel Becomes Easier to Lift
Another benefit is that it’s easier to lift the front wheel which is beneficial for riders doing massive jumps and bunny hops.
That said, an extra slack head tube angle has downsides too, especially when it’s the result of installing a fork that isn’t technically designed for the frame.
The cons are:
- Extra Stress on the Headtube
- Slower steering
- Harder to maneuver at slow speeds
- Slower climbing
A Slacker Seat tube Angle
The raised front end will result in a slacker seat tube angle (STA) too.
The seat tube angle is the angle formed by the seat tube and a horizontal line passing through the bottom bracket.
A steep seat tube angle brings the seat forwards whereas a slack seat tube angle pushes it back.
Steep seat tube angles are the current MTB trend, for they provide the following benefits:
- Balanced geometry
A steep seat tube angle is needed to compensate for the slack head tube angle and long top tubes common for modern MTBs.
- More efficient pedaling
A steep seat tube angle positions the rider closer to cranks. This makes pedaling easier.
Thus, by putting a 29″ fork on a 27.5″ frame one will be effectively throwing the bike slightly out of balance while making pedaling harder.
Luckily, it’s possible to compensate for the slacker seat tube angle somewhat by pushing the saddle forward (saddles can slide on their rails).
Fork Rake Offset
The term fork rake refers to the hub’s offset from the steering axis (image below)
In most cases, the fork rake (offset) of a 29″ fork is greater than that of a 27.5″ fork.
For example, the fork rake of FOX 34 29″ is 51mm whereas that of FOX 34 27.5″ is 44mm.
The fork rake is important because it greatly influences the trail of the bike.
The term trail refers to the tire patch behind the extended steering axis (image below).
A shorter rake (offset) brings the axle closer to the steering axis and thus increases the bike’s trail.
Conversely, a long rake (offset) pushes the axle further away from the steering axis and decreases the bike’s trail. This is what would happen upon installing a 29″ fork on a 27.5″ bike.
The decreased trail will “sharpen” the bike’s cornering but also make it more unstable when moving in a straight line.
However, the extent to which this will manifest is difficult to predict because the head tube angle will change too because the fork is longer.
The steering axis of the bike will move forward due to the slacker head tube angle.
Thus, the slacker head tube angle will actually increase the trail of the bike and compensate to a degree for the longer rake.
The above may feel a bit complicated. Below are the main points:
- 29″ inch forks tend to have extra offset/rake. (The axle is further away from the steering axis.)
- The greater offset will reduce the trail. (The tire patch behind the steering axis.)
- The longer axle to crown length of the bike will slacken the head tube angle.
- The slacker head tube angle will increase the trail and compensate for the extra fork rake.
Ultimately, it’s hard to predict how much the bike’s handling will change due to the great number of factors involved, namely:
- The original head tube angle
- The offset of the new fork
- The travel of the new fork
- Tire size
A Taller Bottom Bracket
A 29″ fork on a 27.5″ bike will raise the bottom bracket too.
A taller bottom bracket has some benefits (e.g., more frame clearance), but it inhibits sharp cornering and could make getting on the bike difficult if the rider is short.
Switching To a 29-inch Fork and 29″ Wheel
If you change both the fork and wheel size, the changes will be the same but on a greater level because the 29″ wheel will raise the front even more.
A 27″ wheel uses rims with a 23inch/585mm diameter.
A 29″ wheel uses rims with a 24.5inch/622mm diameter.
This amounts to an 18.5mm/0.72in. radius difference.
Thus, if a 27″ and 29″ wheel use the same tire, the 29″ wheel will raise the front end of the bike by 18.5mm. (It’s necessary to use the radius rather than the diameter difference because the wheel contacts only the ground.)
If we continue to use the FOX 34 fork as an example, the total rise of the front end will amount to:
18mm (axle-to-crown-length) + 18.5mm (radius difference) = 36.5mm/3.65cm/1.43in
This is a substantial rise that will greatly affect the handling of the bike.
And if the 29″ wheel uses a larger tire, the increase will be even greater.
To minimize the change, one has to use a 27.5″ wheel.
In order for a 29″ fork to accept 27.5″ wheels, both the wheel and the fork have to operate with disc brakes which is the case for modern MTBs.
If the fork uses rim brakes (a rarity), the shoes of the pads won’t align with the brake track on the wheels because 27.5″ wheels are smaller.
There are other methods to achieving the changes created by a 29″ fork. For example, you could install a fatter tire that will slacken the bike’s front end and the head tube angle.
Another option is to switch or adjust the settings of the existing fork to a longer travel if that’s an option. (Some forks e.g., Manitou Markhor have the option of changing the travel from 80 to 100 or 120mm.)
You could also switch to a different 27.5″ fork with more travel and maybe even combine it with a fatter tire for maximum front-end rising.
Summary: What You Need To Know
- A 29″ fork can be installed on a 27.5″ frame when both use disc brakes.
- A 29″ fork will slacken the head tube angle. The change will make the geometry of the bike more modern, but it will also increase the stress on the head tube and hurt maneuverability at low speeds.
- A 29″ fork will slacken the seat tube angle and thus hurt pedaling efficiency.
- 29″ forks have a greater fork rake (offset) which will reduce the trail of the bike. The slacker head tube angle will counter the change to a certain extent.
- If the rider uses the fork with a 29″ wheel and a massive tire, the changes to the geometry could make the bike unpleasant and even unpredictable to ride.
- In general, there’s little to gain from installing a 29″ fork on a 27.5″ bike. Thus, the change makes sense only when one can obtain the fork at a decent price.
- The changes that a 29″ fork creates can be mimicked to a degree by using a 27.5″ fork with a greater travel and/or installing a larger tire.