I'd like to convert an old 26"-wheel mountain bike that i use as a commuter, to 20" wheels.

My initial thought is to make (or have made for me) a custom fork with a long segment between the fork crown and the lower headset bearing race. So the fork blades would be normal 20" size, then an extension, then the steerer tube. This fork would also adjust the geometry if necessary to compensate for the smaller wheel. Alternatively I could just move the brake posts lower on the existing 26" fork and put a 20" wheel in there, but that would not allow for any geometry adjustments.

For the rear, I'm thinking of an adapter that would clamp into the original dropouts and then have a new set of dropouts 3" below the existing ones. Think of something like an Xtracycle extension but going downward instead of rearword. I'm wondering if I would have major problems with my chain-line. Brakes i could figure out.

Does this seem doable? What geometry changes might i need to make?

Why? If it's to be different, you will be that. If it's to gain experience with fabracating stuff, you'll get that. If it's to gain some quality in the old bike then I'm not sure.

Of course it's doable and if done right won't result in any geo. changes. If done poorly then all bets are off. But that is due the skill of the one who does this project. Andy.

I think the chain would hit the chainstays.

Pedals would hit the street.

You'd Have to re make the frame to have BB rise , not drop..

Start with A BMX frame , and add superstructure to have the seat and bars up to your height.

See Minibike thread in Folding bike section.

The simplest way to do what you are suggesting is to add the wheels to the current frame, you do not need to build a new fork. All you need to do is arrange brakes. But the bottom bracket height is an issue. Best bet there would be either to make an adjustment to BB height, or to rear drop height. The latter will change quite a few things. So an option would be to consider the geometry of the current bike and see if there is any position you can scab another bb to. Probably hole sawing through the seat tube and inserting a BB into the seat tube, then adding a triangular brace to the DT would do it.

I have not even considered the chain line...

The OP suggested that he would lower the axle points so one might think he considered the BB height remaining the same. Andy.

To make your rear adapter strong enough you would have to triangulate it with new chainstays. And you could remove the old ones to clear the chain path.

However with that and the new fork you'd end up with an ugly mess... might as well build a proper mini-vélo from the start?

Or like MassiveD suggested, the simplest solution might be to put a new BB shell 3" up the ST and simply put the smaller wheels. You could even try a kiddy tandem crank that clamps on?

I think the kiddy tandem bb idea is the best, although the chainline might not work. Otherwise it's so much work that it makes more sense to build a new frame

hack saw out the bb and several inches of seat tube..
weld the new BB on the seat tube, then ,


heat and bend the chainstays and downtube up to meet the new higher BB,

seems like a monkey raping an NFL football ,. rather pointless..

Andy is right, the reason for the custom fork and rear adapter is to address BB height. Also, Tuz is on the right track talking about a triangulated arm on the rear adapter. This is what i was considering, something that would start at the new rear dropout and run up toward the BB and clamp to the existing chainstays somewhere around where the kickstand bracket is, maybe to that bracket itself. He added an idea I hadn't considered though, which is just to cut out the old chainstays at that point. I'd prefer not to, but chainline is a problem and that is an option to consider.

I'd like to dream up a custom fork and rear adapter that could be bolt on modifications to any basic old bike in order to convert it. The purpose would be to have sort of a double-ended cycle truck type bike. A ground-up new frame is simpler in many ways, no doubt, but i like the idea of a kit that would convert existing bikes, much like an xtracycle kit. Perhaps by the time you include new wheels and tires and all the monkey-motion, it may be no less expensive than just buying a new mini-velo frame/wheels and swapping the rest of the parts off the old bike onto the new frame.

The idea of just putting a new BB shell 3" up the seat tube is interesting. It seems like it may bring a whole host of other problems with it, but it's interesting at least. I'd need a super tall stem and seat post, but that's fairly do-able.

maybe it would be better to start with a sturdy bmx frame and build upward to get seat and handlebar heights at a comfortable level for commuting. I know NOTHING about BMX bikes. Would that be appropriate at all? Can cassettes and derailleurs be added to BMX bikes? Do any of them have canti/v-brake mounting studs?

I have a 406 wheel Bike Friday , they built it , I did not.
It was designed to partially fold,
and partially dismantle to fit a Travel bike into a Suitcase.
skirting the Bike Special handling fees of many air carriers.

Look also at the Mini-Velo sector .. a way to build a bike to fit in
smaller apartments in Jpn, ROK, ROC, and such
was to use smaller wheels .

The Shimano Caprio component lines are to serve that market.

You are in Bend,go look in some Bike Shops there.
BMX bikes are on the floor to look over

V brake is a common choice , in the builds of many BMX bikes.

Or , just drive down to Eugene Bike Friday and CAT,
have some locally made 20" wheel Bikes to Buy.
folding bikes ..