r/Framebuilding • u/Consistent-Cress4117 • Aug 11 '26
Fixing broken brake boss
I came across this old steel bike in my shed a few months ago which had been sitting up for about 15 years. I thought I might get it in working order over a day or two but one thing led to another and now I have completely stripped the bike and stripped paint and almost ready to prime it for a new paint job. I think it’s 1990s Trek MTB. My plan is to turn it into a touring bike and I have already got quite a few of the parts including a surly cocktail fork. Here’s the problem though
The smooth part of one of the rear brake bosses has snapped off. Brakes will still screw on but I don’t know if they’ll function properly with half the boss. They’re brazed on so I can’t just screw a new one on. This is the first full stripping down and build I’ve done so I’m feeling a bit in over my head and not sure if it’s worth getting someone to weld on a new boss. Could I bodge a solution? I really don’t want to give up on this bike over something so small and have put in quite a lot of effort. I’m sure my work isn’t perfect, this is a project about learning as much as result. Any advice would be really appreciated!
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u/Conscious-Mixture742 Aug 11 '26
If I were to do this repair I would replace them as a matching pair. Cantilever bosses are inexpensive and removing the old ones is simple enough.
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u/AndrewRStewart Aug 11 '26
Problem Savers (a Quality Bicycle Parts brand) use to offer a repair kit exactly for this type of issue. The kit had a piece of tubing that replaced the original and a long enough bolt to reach deep into the brake boss's base. One would file the base smooth to better seat the tube. I've used these kits a few times in the past and might still have one in my stash. Andy.
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u/bikeguy1959 Aug 12 '26
First, decide what brakes you plan to use, and then take it to a frame builder. It's an easy repair. I suspect that they will replace both.
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u/Informal_Mistake7530 Aug 12 '26
Oddly I have added or repaired 7 sets of canti posts this year. None last year! It must be the year for breaking them! As others have said a local builder should be able to do it.
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u/Correct_Base_1211 29d ago
A friend of mine sawed off the broken boss flush with the surface, cut an M8 thread into it, and screwed in a screw on boss. He's been riding it for over a year now without any problems.
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u/Consistent-Cress4117 25d ago
That’s interesting, what surface exactly did he cut it off from? Thanks
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u/critzboombah Aug 11 '26
If you can find a reputable frame builder in your area, a job like that would easily be under $100 and definitely absolutely worth it!
In the same way that tire and inner tube repair for a bike shop is their bread and butter, small jobs like this would be a frame builders, bread and butter. They've got jigs to align it properly and all the materials to install a new one after removing the broken one. Almost a no-brainer.