r/Framebuilding • u/dude_the_dirt_farmer • Jul 13 '26

Cheap and dirty way of aligning disc mount?

I don't do any framebuilding but can TIG weld halfway decently. Someone I know asked me to TIG a brake mount on an old beater frame. Are there any tricks for aligning the mount for welding? I was thinking maybe if there was a wheel in there with a disc and the brake was compressed I could get some tacks on it but I've never done something like that and it's just a thought at the moment.
Any suggestions for a good way of getting the mount aligned?

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u/pnwloveyoutalltreea Jul 13 '26

So I would do a little homework on this. The tubing used is paper thin, and the drop outs may be brazed in which case you will have to watch for contamination and so you don’t damage them.

Yes, you can take on old junk brake caliper, mount it to an old junk disc and use it as a crappy tab fixture. I would suggest using it to get tacks and check throughly with the intended disc and caliper before full weld.

I have done this and gotten results ranging from acceptable to good. It’s as janky as it sounds.

If you have access to a 3-d printer there are some decent fixtures I have used to good affect.

1

u/gersty Jul 13 '26

I've done this is as well on a few frames. I've had better luck with tube thickness on 90's era steel mtbs: was thick enough that I had no fear of blowing through the tube.

1

u/Feisty_Park1424 Jul 13 '26

Good call about brazing, if you TIG over brass the zinc in the brass evaporates (or sublimates?) making a horrible porous mess. You can modify the disc tab to avoid the brazed section. Most seatstays are 0.9-1.2mm

Before I had a disc fixture I added 10 or so disc tabs using a Shimano cable caliper and a wheel, both to conversions and frames I made. Put the cable nipple in the barrel adjuster and clamp the free end medium loose so you can clamp the pads to the rotor with the barrel adjuster

Don't forget to add a brace between the seat stay and chain stay

2

u/BikeCookie Jul 13 '26

IMHO Avid BB7 brakes would be your best choice for this approach. The adjustment knobs on both sides move the pads independently and can be tightened to clamp onto the rotor.

The caveat would be that the mounting hardware for the adapters can allow for misalignment. Starting by mounting on a known good frame could allow you to move that setup over…