I found this forkless frame for next to nothing and am tempted to buy it. Supposedly it is designed for 650A/B wheels.
Since I'm trying to keep this cheap as possible and don't have a spare 650A/B fork lying around I am thinking about using a 1 inch threadless rigid mountain bike fork for 26 inch wheels. I'm curious exactly how bad of an idea this is?
I'm guessing most cantilever brakes on a 26 inch fork would reach 650B rims, but am I overlooking something?
I'm also guess a mountain bike fork would raise the front end of the bike up a little bit, but how badly would this mess with the geometry?
Has anybody done something like this?
Alternatively, would it be possible to run 26 inch wheels in this thing, or would the brakes hit the tires?
My plan for now is to throw some Bruce Gordon Rock n' Road 650b knobby tires on it and abuse it mostly off road.

Difference between 26" MTB and 650B rim diameter is only 25mm, 12.5 mm at the brake. Cantilevers should handle it. And 26" MTB tires are generally fatter than 650Bs, so the wheel diameter is not too different.
I don't think the geometry would be too messed up.

I'm sure you can run a different size wheel, depending on what tire size you chose, but the real challenge will be to get brakes to fit. This isn't a matter for guesswork; you would have to try the specific combination you have in mind and see what happens.

The same, more or less, goes for the fork. If you use a wheel with an overall diameter that's radically different from what was intended, you will mess with the geometry; but the overall diameter depends on the tire, not the rim. So, for example, a 559 x 56 tire has about the same overall diameter as a 622 x 25.

That's quite a bit of difference, and a lot of cantis probably won't handle it, though undoubtedly some will.

I just started a thread yesterday to try to gather measurements on people's successful wheel-size changes involving canti brakes.

In my mind - and I'm new at this, having only started thinking about it yesterday - the measurements that will determine success or failure for any given set of cantis include:

- distance from wheel axle to canti posts
- distance between posts
- rim width, since the pads are descending as they squeeze towards the rim (ie: the pads will be lower by the time they contact a narrower rim)

If you take on this project @hat and beard, and make it work, please post your results to that thread!

Some might but the vast majority will absolutely not. Only if the fork has pivot placement such that the pads are adjusted all the way in one direction, which most do not. The transition from 27" to 700C is only 4mm, but around half of the conversions I have seen attempted were not a simple conversion due to lack of brake adjustment. Also, the conversion from 26" to 650B is 12.5mm of pad adjustment away from the brake pivot, so the stopping power will be greatly decreased for given hand force at the lever.

What is that frame? It's probably ISO 559 (MTB 26") anyway. You can always measure it.

650B has only made a renaissance in the last few years. Before that it was an obscure french size that you heard about but never saw. IOW, if it is old, there's no way it is 650B. AFA EA3 (650A), that was the 3 speed size. Never seen one with cantilever brakes, so probably not that either.

26X1-3/8 was a very common size in decades past, and I have seen it on bikes with cantilever brakes - the most recent was a hybrid from the mid nineties made by Raleigh Canada. But you are right that it is a very uncommon combination so that is not likely the case with this bike.

+1. Do you have a set of canti brakes you can mount on the rear? Measuring from the rear dropouts to about where the brake pads should contact the rim, while inexact, should tell you whether the frame is meant for a 559 or a 584 wheel.

Just measured some cantis I had at hand, and I agree. They would work, but the shoes would be at an upward angle, and the stopping power will suffer due to different lever ratio.

I HIGHLY doubt the idea of a 650b wheel working in a 26" fork will work. HIghly. people have trouble getting the 4mm 27 to 700 to work nicely.

Zen and the Art of Bicycle Building: braze-on distances index

I would measure the boss center to axle center (mid slot I would think since these are horizontal) to see what the frame was truely made for. With such a huge difference from 26" to 650B you should be able to figure out which it is. I suspect this is a 26" MTB frame, but I could be wrong.

Thanks for the info everyone. Unfortunately I'm not able to measure the frame because it is on the Yahoo Auction Japan website. I'll try to ask the seller to measure again. I'm fairly confident that 650A or B is the correct size, though. Lots of Japanese touring bikes from the sixties on were made for that size, especially by Kawamura. Several (smaller) such frames have passed through my hands in the past.
Have a gander at the numbers on this Kawamura below, for instance. I admit the frame I'm interested in does like kind of look like a mountain bike, though, especially at the chain stays.



So, assuming the frame is 650 b-ish it will be a bit of a stretch for cantis on a 26 inch fork?
How about, still assuming the frame is 650b, i use 26 inch wheels front and rear? Is it likely cantis would hit the rear tire or I would have to angle them down at a bad angle? Should I just give up on this and wait until I happen upon a suitable fork?

From another angle. Weird chain stays, innit?

Some more possibly relevant info:

The dropouts are reportedly 126 mm.
Seatpost is 26 mm.
BB shell is 68 mm English / Japanese.

Oh, it's in Japan. That's a bit of a bombshell, isn't it... (I guess I should have noticed your location) I believe it could be 650A now since that size is still popular there. I still vote not likely on 650B, though I think the 650b revival has been going on longer there than here.

Anyhow, IMO just get a 26 x 1 3/8 fork for it (3 speed/650A size), as they are relatively common. You're unlikely to find one with canti bosses - though apparently they exist in Canada.

I don't think you'll be able to fit 559 26" wheels on there. It's like an inch of difference, despite being nominally 26.

Maybe call Velo Orange and/or SOMA? The could possibly have a leftover 650b fork with cantis from a warranty return or something.

Edit: maybe not practical. i'd just ask around some shops locally for a fork.

This thread needs more speculation.

After a bit more research I found this: The Velo ORANGE Blog: Converting From 26" to 650b

Velo Orange seem to think 26 inch to 650b works with Tektro CR720 brakes. I remain a little skeptical.
I'm going to look around in local bike shops for a suitable fork, but I'm not very confident I will find one with canti studs. Maybe it's time I learn how to braze?

Edit: I also found this: https://www.cyclingabout.com/converti...-touring-bike/

I'm starting to think I could make the 26 inch fork work with the Tektro brakes.

I ended up using a 26 x 2.1 tire on the front (with MTB fork) and 650b x 43 on the back. It works quite nicely on and off road.

Now that I have this built up and can clearly see the geometry it looks to me like this started out its life as a mountain bike. Really weird, though, that the cantilever studs are brazed on in 650b position. Maybe someone tried to make a 650b touring bike out of a mountain bike? Anybody have any guesses what this is? It has OS down tube, 28.6 seat tube, bowed chain stays and Shimano dropouts. The rear has plenty of clearance even for quite wide MTB tires.

Looks like you're running a 650B rear wheel with 26" mtb wheel in front?

Does that give a level toptube(?), and what is the resulting bb height?

How does she handle?

Kudos for trying to get this thing to work!