Condensed answer: A 26-inch wheel can be easily installed on a 27.5 fork if both are disc brake ready. If the fork uses rim brakes, however, the braze-ons would be a bit higher, and the calipers won’t catch the rim.
Installing a 26-inch wheel on a 27.5 fork will cause undeniable but not critical modifications to the geometry of the bike. To minimize the changes, it’s necessary to install a 26-inch wheel on the back too.
Requirements To Install a 26-Inch Wheel on a 27.5 Fork
27.5 forks offer plenty of clearance for 26-inch wheels.
Having said that, it’s advisable to measure the distance between the fork’s blades near the crown if you’re running a road/urban fork and plan to use larger 26-inch tires.
Conversely, if you have an MTB fork, you should be able to fit even wider tires than before thanks to the smaller rim diameter.
2. Proper Hub Width
The hubs and axles should match the dropouts of the frame and fork. If the wheels are too narrow or wide for the dropouts, you won’t be able to install them securely.
3. Disc Brakes
Disc brakes allow the easiest wheel swapping because the rotors are always at the same place regardless of the wheel size. As a consequence, you don’t need new mounts when installing a smaller or larger wheel than what the fork is designed for.
Conversely, V-brakes, cantilever brakes and Mini V-brakes create problems because the level of the bosses that they mount to is dependent on the wheel’s size.
Smaller wheels require the braze-ons to be lower on the fork in order for the brake shoes to grab the rim.
There are three main ways to lower the bosses, but none of them can be classified as convenient. The options are:
- Cut the braze-ons and re-weld them
Theoretically, one can cut the brake mounts and re-weld them lower on the fork’s legs. However, if the fork is made of aluminum, this procedure requires special equipment and heat treatment. In most cases, the hassle isn’t worth it monetarily and time wise.
If the fork is made of steel, the process is a bit easier, but it’s still necessary to find an experienced frame welder who can get the job done.
- Cut the braze-ons and install V-brake clamp adapters
Another option is to cut the existing braze-ons and install V-brake clamp adapters lower on the fork. The downside of this approach is that V-brake clamp adapters are expensive.
In conclusion, this conversion makes the most sense if you’re operating with disc brakes.
Changes To The Geometry From Installing a 26-inch Wheel on a 27.5-inch Fork
If you put a 26-inch wheel on a 27.5-inch fork, you can expect the following changes to the geometry of the bike:
Lower bottom bracket
26-inch wheels installed on a 27-inch bike will lower the bottom bracket unless they’re equipped with significantly fatter tires.
It’s logical to conclude that the drop of the bottom bracket will equal half of the size difference between the wheels (1.5/2=0.75 inches), but in practice, the numbers don’t work quite like that.
The actual diameters of the rims are:
The difference in rim diameter between 26″ and 27.5″ wheels is closer to 1in/25mm than 1.5in/37.5mm.
This is the number that one has to work with when calculating the lowering of the bottom bracket because it indicates the difference in diameter when both (26″ and 27″) wheels are equipped with the same tires.
Therefore, the bottom bracket is expected to drop by roughly 1in/2=0.5in or 12.7mm.
Obviously, the difference would be even smaller if the 26-inch wheels have fatter tires than the 27-inch models.
Is a 0.5-inch bottom bracket lowering a big deal?
It sounds like little, but it could be noticeable when riding on terrain with lots of obstacles. Hence why it’s recommended to install a bash guard to protect the chainring(s) and the bike.
In some cases, the lower bottom bracket may get the frame and consequently the cranks a bit closer to the ground to the point where the pedals hit the ground when leaning to turn.
The Importance Of Having Matching Wheels
If you have a 27.5-inch bike and install a 26-inch wheel only on the front, the head tube angle of the bike will get steeper, and the rider’s weight will be shifted forward. In the world of modern mountain biking, this geometrical change is undesirable.
To keep the bike’s new geometry as close as possible to the original one, it’s recommended to ride with matching wheels and tires.
The requirements for a 27.5-inch frame to accept a 26-inch wheel are the same as for the fork – if the frame uses disc brakes, you can proceed right away; if the frame relies on rim brakes, it would be wiser to abort the mission since the pads won’t align with the rim.
The Advantages Of Running 26-inch Wheels
26-inch wheels offer the following benefits:
- Greater mud clearance
By installing a 26-inch wheel on a 27.5-inch fork, you would greatly increase the mud clearance of the bike.
- Faster acceleration
Smaller wheels accelerate faster than bigger ones because it takes less torque to get them rolling.
- Nimble handling
The smaller diameter of 26-inch wheels makes it easier to make tight technical turns.
- Fatter Tires
26-inch rims allow you to fit fatter tires than the original 27.5-inch ones.
- Stronger Wheels
All things equal (rim, spoke quality…etc.), a smaller wheel is a stronger wheel thanks to the reduced diameter and shorter spokes.
- Lower center Gravity
26-inch wheels bring the bottom bracket closer to the ground and lower the center of gravity. The result is a bit more stability during tight turns.
Installing a 27-inch Fork On a 26-Inch Bike
It’s also possible to install a 27-inch fork on a bike with a 26-inch frame and wheels. The requirements for the installation are:
- The headtube and the fork steerer should be compatible. Currently, most bikes have a 1 1/8″ straight steerer or 1 1/8″ tapered steerer getting wider near the fork’s crown.
If the head tube is non-tapered and designed for 1 1/8″ steerers, you can install only forks with a 1/ 1/8″ steerer.
If the head tube is tapered, you can install tapered forks as well as non-tapered ones if you use a non-tapered to tapered headset adapter.
- The bike should also have disc brakes if one doesn’t want to get involved in the aforementioned measures required to re-align the rim brake bosses.
The installation of a 27-inch fork on a 26-inch bike will have the following effect on the bike’s geometry:
- A slacker headtube angle
27-inch forks are longer than 26-inch models to provide clearance for bigger wheels. As a consequence, a 27-inch fork would “slacken” the head tube angle of the bike and elevate the front end.
In some cases, the change may be considered beneficial because mountain bikes have been moving towards slack head tube angles for a long time. That said, a slacker head tube angle has a negative effect on climbing and tight maneuvering.
- Greater stack height + Shorter reach
Stack – the vertical distance between the middle of the bottom bracket and the center of the head tube’s upper part
Reach – the horizontal distance between the middle of the bottom bracket and the center of the head tube’s upper part.
A 27.5-inch fork would increase the stack and decrease the reach of the bicycle. Or in simpler terms, the handlebars will get higher and closer to the rider. Whether this will be a negative or a positive change depends on the particular frame and fork as well as on the rider’s anthropometry.
- Slacker/Shallower Seat Tube Angle
The elevation of the front wheel pushes the seat back and slackens the seat tube angle. This is considered a negative change because a slacker seat tube angle takes you away from the center of the bike and makes pedaling and climbing less efficient.
That said, the shift, in this case, isn’t big enough to be easily noticeable.
Note: The above changes to the geometry can be minimized by reducing the travel of the fork if that’s an option.
- Slightly Longer Wheelbase
27-inch forks tend to have a slightly longer offset than 26-inch models. The offset is the distance between the axle and a straight line through the head tube.
Consequently, a 27-inch fork would widen the wheelbase (the distance between the two axles) ever so slightly. A longer wheelbase increases stability but lowers maneuverability.
In most cases, however, the change is very small. Some fork manufacturers even offer the same offset for their 26-inch and 27-inch forks.
- Higher Bottom Bracket
The long axle-to-crown distance of the 27.5-inch fork will raise the bottom bracket too.
What Are The Benefits Of Installing a 27.5-in Fork On a 26-Inch Bike?
1. A slacker headtube angle and a bit better “roll-over-ability”.
In the world of MTB, 27.5-inch wheels are considered an upgrade. If you buy a 27.5-inch fork, you would be able to comfortably put it on a 27.5-inch frame in the future.
Summary: What You Need To Know
1. It’s possible to install a 26-inch wheel on a 27.5-inch fork. The process is much easier if the bike has disc brakes.
2. The smaller wheels will lower the bottom bracket by about 0.5in. In general, this isn’t a “game-ending change” and even has some benefits (a lower center of gravity).
3. 26-inch wheels have the following advantages – potentially stronger, more maneuverable, greater mud clearance, support of wider tires…etc.