Condensed Answer: In some cases, it might be possible to install 29″ wheels on a 26 bike, but more often than not the fork and/or the frame will not have enough clearance for the new wheels, especially when combined with wide tires.
The larger wheels will raise the bottom bracket substantially and make the wheel less stable.
If the frame and fork use rim brakes, the brake arms will sit too low to grab the rim.
Ultimately, it’s recommended to stay away from this conversion.
The Problems With Installing 29″ Wheels On a 26″ Bike
1. Poor clearance
The rim diameter of a 26″ wheel is 559mm (279.5mm radius) whereas that of 29″ wheels is 622mm (311mm radius).
The extra clearance that the 26″ bike will have to provide is the radius difference between the two sizes.
Or in other words, the new wheels will take up at least 31.5mm/ 3.15cm/1.24in of extra space.
The reason for comparing the radiuses rather than the diameters is that only one point of the wheel is in great proximity to the frame or the fork’s arch.
Keep in mind that the final radius of the wheel depends on the tires too. If the new 29″ wheels have wider tires, the radius of the wheel will increase greatly. In different, by putting slimmer tires on the 29″ wheel, one will reduce the needed clearance.
FAQ: How much tire clearance does my bike really need?
In general, it’s recommended to have at least 3mm between the tire and the fork or frame. Anything less is problematic because even a small stone can get between the tire and the fork/frame and cause the wheel to jam. If that happens to the front wheel, a fall is close to guaranteed.
Of course, the needed clearance depends on the terrain too. An MTB will need more clearance since the tires accumulate mud and dirt. If the bike is used on paved roads, it can operate just fine with less clearance.
It’s also worth mentioning that besides safety, the lack of clearance could create discomfort by preventing the user from installing certain accessories such as full fenders.
Full fenders are much more effective than partials and are recommended for commuting bicycles. If you don’t have fenders on your commuter, riding in wet conditions becomes more uncomfortable than it has to be.
2. Changes To The Geometry
A larger wheel has the potential to make serious changes to the geometry.
Below are all the possibilities in terms of wheel placement and its effect on the bike:
a. Replace only the front wheel
If only the front wheel is replaced, the new unit will raise the front end of the bicycle and thus slacken the head tube angle (HTA). The head tube angle is the angle formed by the frame’s head tube and the ground.
A slacker headtube angle makes it easier to overcome road irregularities, but it increases drag (the rider’s back is more upright) and hurts handling at slow speeds. Also, climbing is harder because more of the rider’s weight is on the rear wheel, and it’s more difficult to lean forward (a technique that facilitates climbing.)
Another problem with the slacker HTA is the extra stress on the head tube. The slacker the head tube angle, the more stress the head tube has to deal with, especially during more aggressive riding including jumps and drops.
If the head tube is particularly weak, it may crack and render the frame obsolete. (Frame repair is often not worth the cost.)
Higher Bottom Bracket
If the front wheel is replaced while the rear remains the same, the bottom bracket of the bicycle will be raised slightly. A higher bottom bracket will improve the bike’s ground clearance but will also elevate the center of mass and thus make the bike less stable when cornering.
b. Replace only the rear wheel
Technically, it’s possible to replace the rear wheel only. However, this route could create serious issues because the head tube angle can get dangerously steep. By placing the new wheel at the back, you will raise the rear end and thus lower the front part of the bicycle.
If the frame already has a fairly steep head tube angle, the lowering could make the bike very uncomfortable because the rider’s back will have to become more horizontal. Additionally, the bike will be less capable on off-road terrain and will have a lower bottom bracket which may cause pedal strikes (the pedals hitting the ground when making a turn).
In some extreme cases, it may become easy for the rider to fall over the handlebars.
In general, if you have to replace only one wheel, it’s better to replace the front one. An extra slack head tube angle may not be optimal, but at least it’s not as sketchy as an unnecessarily steep HTA.
c. Replace both wheels
If both wheels are replaced the HTA angle will remain about the same. The only notable change would be the higher bottom bracket. (This option raises the bottom bracket the most.)
As already mentioned, a higher bottom bracket makes the bike less stable when cornering. The positive is the increased ground clearance.
3. Limited Tire Options
The user won’t be able to choose among many tire choices due to the lack of clearance. If you want to ride on off-road terrain with beefy, knobby tires, this conversion will not work because the wheels simply won’t fit.
4. Brake Incompatibility
If the bike uses rim brakes, this conversion will become even more inconvenient. Rim brakes have arms that grab the rim. The new wheels have a larger rim which will sit higher. As a result, the brake arms will be too low to grab the rim.
However, if the bike has disc brakes, the user will be able to install the new wheels without brake incompatibilities. Disc brakes operate by grabbing a rotor mounted to the hub. Thus, the rotor is always in the center of the wheel regardless of the wheel’s size.