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Old 09-15-08, 01:13 PM   #1
Joshua A.C. New
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Drilling seat stay bridges for brakes

(X-posted with the Mechanics forum)

I just came across a nice, light steel frame yesterday and I'd like to use it for a project. The catch is, it's got canti bosses and they're both totally fuxx0rd.

On the one hand, this is good, cuz I think I want to use dual pivot calipers anyway. On the other hand, it's not drilled for calipers.

So what I'd like to do is drill for calipers and grind off the messed up canti bosses. The question is, how do I figure out if the seatstay bridge is in the right place? I could probably fab up a canti mount, if I had to, out of carbon fiber or something (I don't have the facility to weld or braze. That comes later.) but I need to know where it belongs first anyway.

I also need to know if there's any reason that the bridge wouldn't be able to take the drilling.
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Old 09-15-08, 01:51 PM   #2
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Wouldn't the manufacturer have the reach specified on their site, then you can check your frame. Canti wise all four can't be messed, a bird in the hand...
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Old 09-15-08, 02:22 PM   #3
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Only two mounts. Even if I could find the fork, I've got another I plan on using. One's wiggly and cracked, the other has had some sort of bull**** "repair" done to it.

I don't know what the manufacturer is. It's short like a road racer, but has canti bosses. It's lighter than necessary for cyclocross, too.

It came out of a dumpster into someone else's hands, they got flummoxed by the busted bosses, and now I'm falling into the same trap.
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Old 09-16-08, 05:46 AM   #4
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I used to know this number, but it's been a while since I was building frames.
You need to borrow a measuring tape, a rear hub axle, and a road frame designed for caliper brakes. Stick the axle into the rear dropouts and measure from the center of that to the center of the caliper mount hole in the seatstay bridge. Do the same to your frame and if it matches, then get out the drill.
Before drilling, be sure to measure, mark, and centerpunch the bridge carefully. you need that hole to be perfectly centered and square.
Only trouble I can forsee is if the bridge is made of too thin steel. Then you may need to replace it with something else.
Most cantilever bosses are brazed onto frames rather than welded. If this is the case in yours, there's an easier and safer way to get them off without grinding. Heat the bosses up with a torch until the braze flows, and they should pretty much fall off. Then you just have to file/sand off the remnants of braze on the tube.
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Old 09-16-08, 09:19 AM   #5
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Put a wheel in the frame, measure from the center of the rim's braking surface to the center of the brake bridge where you would drill the mounting hole- if this measurement is 50mm or a little less just about any dual pivot caliper will work. If it's over 60mm it will fall into the mid-long reach brakes category.
Other than the bridge being too thin, the other problem I see is that modern calipers are made to bolt to a flat surface and your bridge is probably round. The easy way around that is to find some old calipers that come with the concave spacers and as a bonus, they usually have longer reach than newer ones.
All that said, I think the easier and better fix would be to replace the canti studs.
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Old 09-16-08, 10:25 PM   #6
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Most common bridges are not strong enough for a brake. Typically the builder takes a piece of the seat stays and brazes it in place. A common brake bridge is at least twice as thick. Do what you want but I wouldn't mess around with something like this unless you know for a fact what the bridge is made out of.
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Old 09-17-08, 10:50 AM   #7
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All very good information, folks. I think I have to let this frame go.

... hmmm... unless it could become a fixie... then I could put on a fork that can take a brake...
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