Repairing rear brake bridge on bicycle

[ander]

Hi all, new here, new to welding. Been taking courses in MIG and TIG welding as a hobby over the past year, and loving it. Hoping you guys can give me some advice in a project I'd like to undertake. Not necessarily just welding advice, but also how to go about doing it.

A friend bought a used bike with a broken rear brake bridge (see photo). I was thinking I could remove the center square part with an angle grinder and replace it with a 12mm piece of steel pipe I've got, with a hole drilled in the center for the brake bolt to fit through. I'd probably TIG weld it, with a minimum of filler. As I understand it, it's not really a critical part of the frame, so there's some margin for amateurs like me, though it'd be nice if the brake stayed put while careening down a mountain pass...

I'm just wondering how to get a good fit for the new piece, as the angle grinder isn't exactly a very precise instrument, or how to keep such a small piece of pipe stay put while welding it. Or should I just get a new bridge, and weld that to the frame instead? Also, will the frame conduct well enough, since it's already painted? What about the heat built up during welding? Will it affect the paint job?

I'm new to metal work in general as well, so all these things that may seem old-hat to you have got me in a twist...

Thanks!

[gwiley]

You are probably already thinking this way, but…

Setup a few pieces of tube and practice welding on them first or you might find yourself buying them a new bike

Don't forget positioning, even with the tire removed that might get a little tight wrapping the joint. I like to go through the motions with the welder off to make sure I can get the electrode and my hands everywhere they will need to go.

I am constantly amazed at how badly things can go when I decide to take a short cut and not practice on non-critical materials to get the settings and position right before I start.

[GBM]

Have to wonder what caused that much damage in the first place....
You are dealing with lightweight tubing...so I would go about half way to the uprights and neatly cut the tubes... then find tube which would fit inside that one... and make two pieces as long as possible which meet in the opening... weld them... then take the same size tube and split it lengthwise....and put into opening over the inside tube.. weld those two seams and around the original tube also... grind everything down smooth....and find a tube which will fit over the original tube... split lenghwise and weld it over the whole thing at a position which will not allow the hole to touch the seam...
This may sound like a lot of stuff... but it is such light weight tubing... and will give you lots of tig practice.... and will almost surely last longer than the rest of the bicycle...
To sum up.. you have inside reinforcement, replacement of the tube, reinforcement over the original tube...whatever caused that ugly break should be covered by that...

[WeldingMachine]

cut the whole x brace out. be careful of the seat stays, they'll be thin.
Check out: http://www.cycle-frames.com/bicycle-frame-tubing/
Great folks to deal with, they'll have the exact part for you.
If they don't, ceeway might. http://www.framebuilding.com/
a dremel with a drum sander will work ok for something that small.

[WeldingMachine]

Have to wonder what caused that much damage in the first place....

a loose brake bolt.

[weregeek]

I would make that cut with something smaller than an angle grinder were it me. An electric rotary tool with a cut off wheel, or a die grinder would allow substantially better control in that tight space.

[fortyonethirty]

If it was mine to fix I think I would repair the bridge rather than replace it. I'd use tig and slowly add material to completely fill in the area that has been damaged. I'd stop frequently to cool the part with a squirt bottle, to avoid heating the brazed joint at the seat stays. When I had the whole thing built up solid I'd grind or file it back to the correct shape and re drill the hole.

[zank]

I use a 5/8" bi-metal hole saw to cope my brake bridges. But before I had a mill, I just used a 12" round file. Just use care to keep your miters in phase, tight, and the mount centered. The diameter of the file is about the same diameter as the seat stay. As noted by WeldingMachine above, pick up a new bridge from Nova. You can also get some reinforcements from Nova as well, which would be a good idea since you are reheating the stay. 45% silver is a good brazing alloy for this and allows you to get a nice little fillet. You will need to repaint the area. Or just get a coat of primer on the area and consider it "character".

I use this bridge:
http://www.cycle-frames.com/bicycle-...IDGE-80mm.html

And these reinforcements:
http://www.cycle-frames.com/bicycle-...EINFORCER.html

[zipzit]

You can just hand miter that yourself. Check out the tube miter program at:

http://www.cycle-frames.com/bicycle-...e-Notcher.html

You want to look at the last tab, tubular brace and gusset. Free program to create a full size template that wraps around the tube. Print out template, tape it to your tube, rough cut with hack saw, finish miter with rat tail file. Note, you can create a 'vise' insert for tubing using two pieces of 2x4 wood (with a small spacer in the middle) Clamp wood together tightly, drill hole down center, remove spacer, then clamp your work piece in a vise.

Oh. and full disclosure on the free tubenotcher printout thing. I'm the software programmer.

--zip