So, I have this unknown mtb frame that has been sitting around my collection (junk pile) for about 10 years. I'm going to add some braze-ons to it and make a monster crosser out of it. I am curious if anyone here has built a fork for use in this situation and if so what did you use for dimensions.
Any guesses as to what brand this is? The frame size is 20" and the rear spacing is 130mm. The serial number starts with M9J but it's not a Bridgestone, nor a Specialized Allez.

First thought of brand is Nishiki.

As to going to 650B, why? Will both wheels be that size? If so will the rear fit without frame mods? Won't the front fit the original fork? Do you still have it? Do you know what the original fork's geometry is/was?

I have to admit a certain skepticism about making major frame mods/changing steering geometry as well as the whole 650B thing. Why pursue a size that has so little options available, and spend so much effort doing so? For fun? For the experience?

I am curious to your reasons and expectations. Andy.

Welcome aboard Paul!

Hi Andrew - I have just about every other bike/wheel size, except a 650b bike. IMHO it's the same effort to build up any other bike. I have ****loads of parts laying around collecting dust that will work for this. Just need some 650b rims n spokes and I'm on my way. Both wheels will be 650b. I have a brand new 700c fork with disc mounts that may work. I will check the geometry and calculate what fork rake and trail should be. However, if I have to build a fork to fit and maintain good handling I can do that. It's more of an experiment I am willing to take on to see how it works out. :-)

From what I have read the existing canti boss placement in the rear will work with long reach brakes. Since I got this bike at a garage sale from a meth addict for free there was no fork, nor paint! IMHO there are plenty of options for a 650b bike. I went to NAHBS in Sacramento and there were quite a few 650b bikes on display.

Sounds like a fun project.

I always find this kind of reverse engineering of fork lengths to be tricky. I would think that you can probably go a little longer than the typical 26" fork. People are using slacker head angles now than was common when that bike was built.

VV- Sounds like you want another challenge/experience. Perhaps the best reasons to do this. Having worked the retail bike business and had the building experience I've had i get a bit cynical sometimes. Good luck. Andy.

I hear ya Andrew! I can only imagine the retail world. I've never worked in a shop but I've been in more shops that I can remember over the last 20 years and I am still amazed at people and their requests for service. Fortunately for me I have the experience and skills to pull this off without annoying anyone but myself when something goes wrong - LOL!!! I'll post pics as it progresses.

Typical MTB rims are 559, and 650b is 584. The difference in brake shoe position is 12.5 mm. If canti brakes (you have canti posts, I assume you are going to use them) can accommodate this mismatch, you probably don't need a new fork. I'm putting together my first canti-based bike, and I'm not sure at this point if they can be so flexible. Or you can remove the canti posts and go to a big brake like a vintage Mafac Raid or a Tektro Bigmouth. Those two are usually designed to clear big fenders as well as the big rubber donut.

If you do want the new fork, I had a new 700c low-trail fork made by a builder in Michigan, Ralph Ellis at Ypsilanti Cycles, for a very reasonable price. Well less than the US masters, such as Weigle, but I chose to use blades he had on hand rather than buy more expensive Kaisei blades. But the design is what I wanted, the shape is pretty, and it has brazeons where they needed to be.

There is no fork for this frame. I do have a brand new 700c disc brake only fork. I am going to get wheel started and see if the fork I have will work without making the HT angle to shallow. I also want to maintain a proper rake/trail with the fork so I have some work ahead of me.