Condensed answer: Some 27.5-inch forks have the clearance to accept a 29-inch wheel. However, the conversion makes the gap between the tire and fork’s arch noticeably smaller.
As a result, the tire starts throwing more small stones and debris against the arch. In the worst-case scenario, an external object will get stuck between the arch and fork and block the front wheel.
When that happens, a fall is close to inevitable, especially when riding fast.
Requirements to Run a 29-inch Wheel on a 27.5-inch Fork
The number one requirement to install a 29-inch wheel on a 27.5-inch fork is to have enough clearance on all sides.
If the wheel doesn’t fit by default, the only way to decrease its circumference and thickness is to run narrower slick tires.
Narrow slick tires are designed for asphalt and come without off-road treads. As a result, the tire doesn’t stick out as much.
If switching tires doesn’t work, the conversion will have to be abandoned.
A 27.5″ to 29″ conversion has the highest chance of working when the fork and frame are designed for 27.5″ plus wheels.
А 27.5″ plus wheel operates with 2.8″ or wider tires. As a consequence, the circumferences of 27.5″ plus wheels are really close to those of normal 29-inch wheels.
A 27.5″ inch wheel (650b) equipped with a 2.8″ wide tire has a rough circumference of 2281mm.
Meanwhile, a 29″ inch wheel (700c) with a 2.2″ wide tire has a circumference of about 2305.18mm. The difference between the two is 24mm/2.4cm/0.94in.
If you go down to 2.1″ wide tires, the circumference of the 29″ wheel drops to 2289mm.
However, the wheel’s circumference is not the most direct metric for making a conclusion in regards to clearance. The radius of the wheel gives a clearer picture because the fork’s arch is the only limiting factor height-wise.
The tables below compare the circumferences and radiuses of 27.5″ and 29″ wheels equipped with tires of different widths.
The difference column in the table shows how much bigger the radius of a 29″ wheel is.
In most cases, the radius difference is 19mm/1.9cm/0.74in.
If the distance between the fork’s arch and your current 27.5″ tire is notably larger than 19mm, there’s a greater chance that you’ll be able to fit a 29″ wheel.
Table 1: Regular 27.5″ Wheels and 29″ Wheels
|Tire Width||27.5″ Wheel Circumference||29″ Wheel Circumference||27.5″ Wheel Radius||29″ Wheel Radius||Difference|
Table 2: 27.5 Plus – Wheel Circumference and Radius
Table 3: 27.5″ Plus Compared To 29″
The next table compares the radius of 27.5″ plus wheels to that of regular 29″.
|27″ Plus Radius||29-inch Radius||Difference In MM|
|363.12mm (2.8″ tire)||377.04mm (2.6″ tire)||13.92mm|
|363.12mm (2.8″ tire)||374.49mm (2.5″ tire)||11.37mm|
|363.12mm (2.8″ tire)||371.95mm (2.4″ tire)||8.93mm|
|363.12mm (2.8″ tire)||369.4mm (2.3″ tire)||6.28mm|
|363.12mm (2.8″ tire)||366.88mm (2.2″ tire)||3.76mm|
|363.12mm (2.8″ tire)||364.34mm (2.1″ tire)||1.22mm|
|363.12mm (2.8″ tire)||361.8mm (2″ tire)||-1.32mm|
|363.12mm (2.8″ tire)||359.26mm (1.9″ tire)||-3.86mm|
The data reveals that a 29″ wheel can have a radius shorter than that of a 27″ plus wheel when the tire is slimmer (<2″)
In the last two cases above, the radius of the 29″ wheel is 1.32mm and 3.86mm shorter than that of a 27″ plus tire with a 2.8″ tire.
Conclusion: If you have a 27.5″ plus fork, it should be able to take a 29″ inch wheel with a slim tire provided that the rest of the conditions are met too.
2. Disc Brakes
A 27.5″ to 29″ inch conversion works best when the fork and the wheels are designed to operate with disc brakes.
If the fork itself can work only with rim brakes, the conversion becomes a lot more complicated because the brake pads will end up below the rim of the new wheel.
This problem could theoretically be circumvented via the following methods:
- Rim brake Adapters/Extenders
There are V-brake adapters that mount to the original V-brake bosses. The adapters have brake mounts themselves which end up a bit higher. As a result, a larger wheel can be used.
- V-brake Clamp-On Mounts
Another option is to install clamp-on V-brake mounts a bit higher than the original bosses to elevate the brakes and align the pads with the new rim.
The downside of the aforementioned solutions is that they are a bit finicky and arguably unsightly.
Since the conversion itself is also a bit cumbersome, it’s recommended to do it only if you have disc brakes because they allow you to swap wheels easily without worrying about the caliper’s position.
3. Axle/Hub Compatibility
The hub of the new wheel has to be compatible with that of the fork.
If the fork uses a thru-axle, the hub of the wheel has to be designed for a thru-axle.
If the fork relies on a quick-release skewer, the hub has to be using a quick-release skewer or a nutted axle.
The Advantages of Combining a 29-inch Wheel With a 27.5-inch Fork
The pros of 29″ wheels are:
1. Better Roll-over-ability
The larger the wheel, the less it’s bothered by the obstacles on the ground. Consequently, 29″ wheels roll over uneven terrain faster than their smaller rivals (26″, 27.5″). Hence why you see 29″ wheels on many race bikes.
2. Greater Inertia
A larger wheel is harder to get up to a high RPM (rotations per minute), but it has greater inertia and just keeps rolling.
3. A Larger Contact Patch
Theoretically, a 29″ wheel should offer a greater contact patch with the ground and thus provide more friction and stability. In practice, however, there are many parameters that come into play such as tire tread, tire width, air pressure…etc.
For example, a wide 27.5″ slick tire running at its lowest PSI settings will have a greater contact patch on flat roads than a 29″ MTB tire pumped to its maximum pressure.
Why? Because the slick tire doesn’t have knobby treads and subsequently more of its body touches the ground. When you lower the air pressure, the contact patch of the tire is increased even further.
4. A Slacker Head Tube Angle
Installing a 29″ wheel at the front of a 27″ bike will raise the front end and subsequently slacken the head tube angle.
The head tube angle (HTA) is the angle between the ground and the head tube. (image below)
A slack head tube angle is considered desirable in the world of mountain biking because it positions the front wheel further in front of the rider. As a result, the front wheel has an easier time rolling over obstacles. Also, the chance of falling over the handlebars is reduced.
That said, a slack head tube angle has a negative side too – the bike’s climbing ability and maneuverability at slow speeds deteriorate. Hence why XC bikes have a steeper HTA.
To find out how much the front end will rise, one has to play with the radiuses of the old and new wheel.
If the bike originally uses 2.8″ 27.5+ wheels, the wheel’s radius is roughly 363.12mm.
If you install a 2.4″ 29″ wheel, the new wheel radius will be 371.95mm or 8.82mm more.
Thus, the front end will rise by about 9mm or 0.9cm.
The Disadvantages of Combining a 29-inch Wheel With a 27.5-inch Fork
1. Reduced Clearance = Narrower Tire
A 29″ wheel on a 27.5″ fork will undoubtedly force the rider to use a narrower tire. If you ride primarily on heavy off-road terrain, the switch may hurt your performance.
2. A Slacker Seat Tube Angle
A larger tire at the front will slacken the seat tube angle too.
The seat tube angle (STA) is the angle between the seat tube and the ground.
A slacker head tube angle is considered undesirable because it positions the rider further away from the handlebars and makes it easier to lift the front of the bike without an intention to do so.
3. A Higher Bottom Bracket
The bottom bracket of the bicycle will get taller too.
The increased height is beneficial when riding over obstacles because the bike is taller.
However, the higher bottom bracket hurts the stability of the bike and makes it more difficult to reach the ground with your feet. You could lower your seat, but the change will make the seat height suboptimal.
An Alternative Cheaper Solution
Truth be told, it’s highly questionable whether a recreational rider would gain anything from doing a similar conversion.
A cheaper alternative would be to install a bigger 27.5″ tire which will bump up the radius of the wheel to a number closer to that of a narrower 29″ tire.
This solution is a lot cheaper and eliminates the compatibility problems associated with the conversion.
Summary: What You Need To Know
1. In most cases, one will need to switch to a narrower tire when converting to 29″ wheels. The reduced tire width can have a negative impact on off-road performance.
2. If the gap between the tire and the fork’s arch becomes exceptionally small (e.g., 3-4mm) upon switching to a 29″ wheel, it will be wiser to abort the conversion due to the chances of debris getting stuck and blocking the wheel.
3. A 27.5″ to 29″ wheel conversion has the highest chance of succeeding when the fork and frame are built for 27.5″ plus tires.
4. A 29″ wheel will increase the bike’s roll-over-ability and slacken the head tube angle, but the change will happen at the expense of a slacker seat tube angle and a higher bottom bracket.
5. A conversion from a 27.5″ to a 29″ wheel makes the most sense when the bike and wheels use disc brakes. If the bike relies on rim brakes, then the pads will not align with the new rim.
6. Overall, the conversion is of questionable benefit. One could simply run a wider 27.5″ tire which will render the circumference of the wheel exceptionally close to that of a regular 29″ wheel.