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I'm trying to figure out the feasibility of a 20" mini-velo for city use, which can then be converted to 24" MTB wheels for rougher terrain.

Assuming the wheels clear the fork and frame, would pedal strike be an issue (on the 20" wheels)? Would an eccentric bottom bracket help?

Has anyone done this before?

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    It's possible to do light (and even medium) trail riding with 20 inch wheels and the right tires. Swapping 20 inch street tires for trail tires will be easier than trying to swap different sized wheels. Depending on how rough your terrain is you might even be able to find a tire that will do both on and off road.
    – David D
    Jul 26 at 19:30
  • @DavidD My concern is riding particularly bombed-out trails (Eastern Europe) and friends with 26"+ full suspension bikes wanting to pick gnarlier routes because they can. I figured with 24" high volume tire I have a better chance of hanging on... instead of getting 2 bikes, which I want to avoid because of budget and space constraints.
    – Leeroy
    Jul 28 at 12:22
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    If you are riding with friends that have 26" full suspension bikes on trails specifically chosen to challenge those bikes you will need a bike like theirs. Even if you could install 24" wheels you won't be able to keep up.
    – David D
    Jul 28 at 12:48
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Pedal strike will absolutely be an issue. If you have the bike already, measure off a 20" wheel on your existing wheels, and visually check how close the pedal will be to the ground.

Most mini-velos have unusual frame shapes that place the axles lower, you can't just fit smaller wheels to a larger frame and make a minivelo. Are you able to do structural welding ?

Your other problem will be brakes - a disk brake would help a lot here, rim brakes will not work at all given the different rim positions.

Notice on this example that the rear axle is below the BB, so the chainstay slopes down toward the back. A larger bike has a horizontal chainstay, or full sized road bikes slope up from BB to the rear axle.

Side view of minivelo bike showing lower axles

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