J.Higgins 

I've been building up some old-school rigid mtb frames into touring bikes. One of the things that I really want to try is to convert them to disk brakes. I can easily buy the caliper mounts, and I'm an accomplished welder, however all I have in my home shop (I'm retired) is a Lincoln mig and an OA outfit. What would be my best course of action with adding the mounts? Is bronze brazing an acceptable method?

Thanks in advance for any and all advice and criticisms.

Andrew R Stewart 

A well done brazed joining should be every bit as good as a welded one, note the "well done" reference. MIG is not often used by frame builders, TIG is the vastly more common welding method. If you must use MIG do a few practice joinings on scrap and test after.


But my greater concerns are about the forks' ability to handle the very different force load paths and that the forks are likely not designed with this in mind. If you're doing this for yourself then all is good as long as you understand the possibility for fork bending under hard braking. But if you're considering doing this for any one else I would strongly suggest that you don't. Andy.

J.Higgins 

I was actually just going to add the mount to the left seat stay, and buy a fork with the caliper mount already attached. But for what its worth, they look to me like a just a simple MTB rigid steel fork with the mount welded on.

dsaul

Given the choices, brazing is the best option. Its probably not an issue on an old MTB, but you may want to check for disc rotor clearance at the chain and seat stays. Allowing clearance for a disc rotor wasn't a concern when building a frame for rim brakes, so sticking a disc wheel in the frame to check for clearance is a good idea.

J.Higgins 

This is good advice. My plan is to set up a rear hub with a disc in the rear dropouts. Then I'll clamp a caliper onto the disk with the caliper mount screwed to the caliper. Without any fancy jigs, this seems like it'd work long enough for me to tack the mount to the frame.

Any other thoughts? Does this sound like a viable method, or should I try to make a jig?

unterhausen

I'm sure that system has been used before. my suggestion is to just tack it on the ends and then finish up without the caliper or wheel.

Don't forget to add a brace between the stays so you don't break the seat stay.

I'm pretty sure that disc fork blades have gotten thicker since the early days of discs, because it's not hard to break a fork blade with a disc

duanedr 

I have used the hub/disc/caliper assembly as a positioning tool on a few with varying success. I then made my own jig with less success (tolerances need to be very tight and i'm not really a great machinist at this point). I just received an anvil post punk with dummy axles and expect my success rates to go up. they make really beautiful stuff.

Anyway, one problem with using the caliper as the tab holding device, aside from the inaccuracy, is the heat that goes into the caliper. with TIG the HAZ is smaller and faster so, maybe not as much a concern with TIG. I get that the calipers are designed to get hot during use.

Here's how I did it:
I used a fully functioning brake set I bought used at a local shop for $5. I installed the hub, disc and caliper on the disc. I then attached the brake lever to a bar and used a toe strap to squeeze the disc with the caliper in the right position on the disc. I tacked the mount to the stay keeping the flame as far from the caliper and disc as possible, then removed everything to stop the heat transfer. once it had cooled to the touch, I re-attached caliper/hub/disc to check alignment (make sure to clean out the hard flux so it all sits right). then uninstalled everything, added flux and finished brazing. It works but is slow and fiddly. I wouldn't use the caliper after it was used to braze the mounts. #ghettoframebuilding

Metal moves when it gets hot and if you hit the disc with the flame it could deform enough to throw this all off. The tolerances are tight - as in +-.1mm. One mount ended up off by enough that I had to machine tiny little spacers to make up the difference. Heaven help me if I ever need to rebuild that bike and lose the spacers!

J.Higgins 

I'm beginning to wonder if its actually worth the trouble to convert to disk.

duanedr 

Rim brakes have worked well for a hundred or so years. Are they perfect? No, but neither are current discs - especially as a retrofit.

J.Higgins 

I agree. It was a style issue, and a novelty issue. I have several old pre-war skip-tooth frames that I was thinking of adding disk brake and modern wheels/hubs. Novelty - not necessity. One of those, "just to say I did it," sort of things.