Is it possible to fit a 26" fork to a 29er bike frame I want to buy a cheap frame and put parts from my old 26&quot; (wheel and fork) and 15&quot; (frame). Is this possible.
Update: Fixed bad calculation for the BB drop, thanks to @mattnz.
Physically, it may be possible to fit the fork alone. But the resulting bicycle will be dangerous to ride, for at least two reasons.

The wheels with 26&quot; rims vs 29&quot; rims  are at least (622 - 559)/2 = 31.5 mm or 1¼ inch lower, meaning everything will be lower, including the bottom bracket and the pedals in their lowest position. This means you will have much greater chance to scratch the road or trail when pedaling. Even if it won't touch the ground when you ride the straight line, at cornering the whole bike leans, and pedals go even lower. An unexpected pedal-to-ground contact, especially when cornering, can kick the rider out of the saddle.

What size of wheel will you put at the back? If it would be a 29&quot; one, the bike will look very odd, a &quot;reverse mullet&quot; of sorts. Besides the looks, a bigger wheel at the back will increase the steering angle, which in general corresponds to more sensitive steering. The bottom bracket will still become lower, although at lesser extent. The bike will be twitchy to control. If it would be a 26&quot; wheel at the back to match the front wheel, see the lower bottom bracket argument above. Plus, if you have a frame for rim brakes, they won't work because they are designed for a specific diameter of the rim.


Using 26&quot; wheels in a frame designed around 29&quot; ones is really not recommended unless you can run all the numbers yourself for the specific frame and are confident that it will work, and you understand how the resulting bicycle geometry changes affect ride characteristics. As an example, for frames designed around 27.5&quot; wheels, some people did use 26&quot; wheels with them. But then one needed extra wide/high tires to compensate for the rim diameter difference. Wider tires are less likely to fit into the frame/fork without rubbing, so there are limits to this as well.
The difference between 29&quot; and 26&quot; wheel is just too big for this trick to work on regular bicycle frames. Some fatbike frames allow even that, e.g. one of my bikes accepts both wheels with 26&quot;×4.8&quot; and 29&quot;×3&quot;, as confirmed by its manufacturer. But that is fairy unusual.