Condensed Answer: If the bike and the wheelset are designed for disc brakes, a 27.5” wheel will fit on a 29” frame and fork. Since 27.5” wheels are smaller, their diameter will change the geometry of the bicycle in a manner that the rider may not appreciate.
To find out, how much the wheel change will affect the bike, one needs to calculate the radius difference between the two wheelsets.
A 27.5” wheel relies on a rim with a 23in/585mm diameter.
Meanwhile, a 29” wheel uses a 24.5in/622mm diameter rim.
Thus, the radius difference is 18.5mm/1.85cm/0.72in. The reason for comparing the radiuses instead of the diameters is that the wheel contacts only the ground.
Therefore, if both wheelsets use the same tire, the 27.5” model will lower the bike by 1.85cm or 0.72 inches.
If the new wheels come with narrower tires, the drop will be even greater. And if the tire is much larger than the original 29” model, there could be a situation when the bike doesn’t drop at all.
Putting a 27.5” Wheel on a 29” Fork
If a 27.5” wheel is installed only at the front, it will steepen the head tube and seat tube angles.
The head tube angle is the angle formed by the head tube and the ground.
Currently, the MTB industry has accepted slack head tube angles as the way to go for aggressive bikes.
Slack head tube angles have the following benefits:
- The bike has an easier time passing over obstacles.
- The rider has a lower chance of going over the handlebars.
- The bike is more stable at high speeds when riding on irregular terrain.
- It’s easier to lift the front wheel (beneficial to people who do tricks with their MTBs).
Conversely, steep head tube angles are considered “retro” and less desirable because they don’t provide the aforementioned benefits.
At the same time, modern MTBs rely on steeper seat tube angles (the angle formed by the seat tube and a horizontal line passing through the bottom bracket).
A steep seat tube makes pedaling more efficient by positioning the rider over the cranks. It’s also needed to counter-balance the slack head tube angles and long top tubes common for modern MTBs.
However, an excessively steep seat tube angle will position the rider too far forward. This makes pedaling inefficient while also increasing the chances of losing balance and falling over the handlebars.
Ultimately, the geometrical changes that take place upon installing a 27.5” wheel only at the front of a 29” bike are considered negative.
A 27.5” Wheel Only At the Back
If the user installs a 27.5” wheel only at the back of the bike, then the changes to the geometry will not be as negative.
The smaller rear wheel will slacken the head and seat tube angle.
The effect of the slacker seat tube angle can be negated by sliding the saddle forward across its support rails.
Meanwhile, some people might enjoy the extra slack HTA created by this shift.
27.5” Wheels at the Front and Back
Installing two 27.5” wheels will cause the smallest changes to the bike’s geometry. The main modification will be the lower bottom bracket.
The downside of a lower bottom bracket are:
- Reduced Frame and Pedal Clearance
The lower bottom bracket will bring the bike frame closer to the ground. Consequently, the frame will be at a greater danger of hitting obstacles on the terrain. Also, the pedals will get dangerously close to the ground to the point where pedal strikes may occur – a pedal strike is a situation when the pedal hits the ground when turning if the rider is pedaling or going into a turn with one pedal in the bottom position.
That said, a lower bottom bracket has a positive too, namely:
- Sharper cornering
By lowering the bike’s center of gravity, the user will experience sharper cornering. Hence why some road bikes have notably low bottom brackets.
27.5” Plus Wheels On a 29″ Bike
27.5″ plus wheels rely on the so-called boost hubs. Boost hubs are much wider than standard hubs. For example, regular rear MTB hubs have an O.L.D. (over-the-locknut dimension) of 135mm whereas the boost version is 148mm.
The term O.L.D. refers to the distance between the locknuts on a hub. This is the usable part of the hub.
An MTB boost hub is too wide to fit on a bike designed for normal hubs.
That said, it’s possible to install 27.5″ plus tires on a 29″ bike, but the conversion can only work when using standard hubs.
27.5″ plus tires are quite wide and will therefore keep the new geometry of the bike similar to the original, although a bottom bracket drop is still expected.
Wheels swapping works only when the wheels, fork and frame are designed to operate with disc brakes. If the braking system is based on rim brakes, the change won’t work because the rim of 27.5” wheels is smaller and the brake track will end up below the brake shoes.
This problem has some solutions (e.g., V-brake adapters that lower the mounting bosses), but most are either expensive or a bit finicky.
Thankfully, 27.5” and 29” are modern MTB sizes and most bikes built for those wheels are engineered for disc brakes.
Summary: What You Need To Know
- A 27.5″ wheel can be installed on a 29″ bike.
- To minimize the negative changes to the geometry, it’s recommended to install two 27.5″ wheels.
- If you want/have to install only one wheel, it makes more sense to put it at the back while keeping the 29″ at the front. This will prevent an excessively steep head tube angle.
- A 27.5″ plus tire can also be installed on a 29″ bike as long as the user relies on standard hubs.
- The bigger the 27.5″ tire, the closer the new geometry will be to the original one.