Condensed Answer: Technically, it’s possible to install 26″ wheels on a 29″ bike if both are designed for disc brakes. However, 26″ wheels will cause drastic changes to the bike’s geometry (e.g., dangerously low bottom bracket) to the point where the conversion becomes pointless and dangerous.
A 26″ wheel uses a rim with a 22inch/559mm diameter
А 29″ wheel relies on a rim with a 24.5inch/622mm diameter.
The radius difference between 26″ and 29″ rims is 63mm/6.3cm/2.48inches.
Therefore, if a 26″ wheel and a 29″ wheel use the same tire model, the 26″ wheel will result in a 63mm reduction of the bike’s height.
The reason for comparing the radiuses rather than the diameters is that the wheel contacts only the ground and thus the radius decrease illustrates the discrepancy.
Option 1: A 26″ Wheel At The Front
If a 26″ wheel is installed at the front of a 29″ bike, it will create the following negative changes:
- A Very Steep Head Tube Angle
The head tube angle (HTA) is the angle between the head tube and the ground (image below)
A small 26″ wheel at the front of a 29″ bike will steepen the head tube angle (drop the front end of the bike) to dangerous levels.
The bike will have a hard time getting over obstacles, and the rider will be in greater danger of going over the handlebars.
Another change that will occur is the steeping of the seat tube angle. (The seat tube angle is the angle between the seat tube and a horizontal line passing through the bottom bracket.)
This is also a negative modification because the rider will be positioned further behind the pedals. The new position will result in inefficient pedaling and disbalance.
For the reasons above, it’s not recommended to put a “lonely” 26″ wheel at the front of a bike made to be a 29″-er.
Option 2: A 26″ Wheel At The Back
If a single 26″ wheel is installed at the back, it will create the following changes to the bike’s geometry:
- A Slacker Head Tube Angle
The smaller rear wheel will cause a dramatic drop of the rear end. Subsequently, the front end will rise, and the bike will obtain a slacker head tube angle.
The slacker head tube angle isn’t as detrimental as the steeper head tube angle because the chance of going over the handlebars will decrease.
Having said that, the extra slack head tube angle will have its downsides:
- The front wheel can lift unexpectedly because it will be supporting a lot less weight.
- The bike will lose maneuverability at low speeds.
- Climbing will become more difficult.
- The rider will be in a more upright but less aerodynamic position.
Since the radius difference between 26″ and 29″ wheels is notably large this modification is not recommended either.
Option 3: A 26″ Wheel At The Front and Back
Installing 26″ wheels at the front and back will have the smallest effect on the bike angles since the drop is equal on both sides.
However, the bottom bracket will drop to dangerous levels. If the bike is ridden on irregular terrain, obstacles will hit the frame and may even cause an accident.
Another downside that an excessively low bottom bracket can cause are pedal strikes (the pedal hitting the ground when turning).
Pedal strikes are quite unpleasant, especially when riding fast.
For the reasons above, installing two 26″ wheels is not recommended either.
Rim and Disc Brakes
Another problem that one may encounter when doing this conversion is brake incompatibility.
In this case, a rim brake cannot be used because the 26″ wheel is much smaller. As a result, the brake shoes will be far above the brake track of the rim.
This problem can be countered by using V-brake adaptors that lower the mounts or by re-welding/re-brazing the bosses. However, this solution is costly and makes little sense given the geometry problems that the conversion will cause anyway.
That said, disc brakes do not create this problem because the rotor is always located on the hub. Thus, even if the wheel is smaller, the rotor will end up where it should in relation to the brake pads.
The main reason why someone would want to install 26″ wheels on a 29″ bike is to increase the bike’s maneuverability.
To some degree, this can also be accomplished by switching to 27.5″ wheels front and back while using a very large tire. The big tire is needed to minimize the geometry changes explained here.
It’s also possible to install 27.5″ + tires since the 29″ fork and frame are large enough to accept them.
Summary: What You Need To Know
- A decrease from 29″ to 26″ wheels will lower the bottom bracket to the point where the bike becomes non-safe to ride.
- If the rider installs a 26″ wheel at the front, the head tube angle will become dangerously steep.
- If the rider installs a 26″ wheel at the back, the head tube angle will become too slack.
- A conversion to 27.5″ wheels with larger tires will negate some of the downsides that occur when converting from 29″ to 26″ wheels.