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Rear brake bridge drilled too large -- brake is not stable

Old 01-18-12, 10:34 AM
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TurbineBlade
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Rear brake bridge drilled too large -- brake is not stable

Hi -- I have an issue with my rear brake constantly shifting forward (with the direction of the wheel during braking) that I can't seem to fix. Tightening the brake nut (with those proper curved washers on both sides of course) doesn't hold it in place and it always shifts, throwing the pads off of the rim.

Any ideas about how to stabilize the brake bolt when the bridge is drilled too large to stabilize it properly?

Also - I have recessed brakes and am willing to do some drilling if necessary to get a stable rear brake. The front brake is just fine.

I thought about trying to jam something in the bridge (which is the tubular kind BTW -- not the square kind) to shim it and keep the bolt in place, but I thought I'd get opinions before i do somthing stupid.

Thanks,
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Old 01-18-12, 11:00 AM
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Check both sided of the brake bridge for hole size. The rear hole should be 6 mm and the front hole either 6 mm for an external nutted brakes or 8 mm (5/16" is nearly identical) for a recessed nut brake. If both are 8 mm someone drilled it incorrectly and you will have to shim both sides to use a nutted brake or shim the rear hole to use a recessed nut brake. If the rear hole is 6 mm and the front 8 mm you can use any recessed nutted brake directly.
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Old 01-18-12, 11:09 AM
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It sounds like you are using hardware from a nutted brake. The correct part is a tubular nut that fits the hole and a concave washer with a larger hole that fits the nut. You can sort of see it in the picture below of a '73 Raleigh Competition that I drilled for recessed brakes. I got the parts from Bike Parts Etc.

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Old 01-18-12, 11:17 AM
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From your description I assume you have a plain tubular bridge that lacks a brazed on flat surface for the brake bolt to rest on. (see the photo in the prior post)

This is a lousy system which depends on two half round washers to provide a mounting surface, and also avoid crushing the tube hen tightened. Rotation under brake load resisted by friction alone and can be inadequate to the task. Trying to make a tighter fit of the bolt in the hole won't help because the torque is too high.

Here's an easy cure. Use coarse lapping compound - grit in grease - on the half round to tube mating surface. When the bolt is tightened the grit will bite into both then bridge and washer making a secure mechanical interlock to keep the assembly stable. Lapping compound isn't cheap, but you only need a dab, so beg some from the local auto engine rebuilder, paying with a cold beer.

Alternately, you can glue adhesive sandpaper, or emery cloth to the mating area of the half round washers to do the same thing. Or you can use super glue or a Loctite bearing retaining compound to bond the washers in place on the tube (you should sand away the paint first for a good bond). Be sure to position, tighten and allow to cure before riding, and do not get any adhesive on the brake bolt itself.

Lastly you could use JB Weld and a stainless steel washer to build up a flat plate similar to what's on better bikes for a permanent job.
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Old 01-18-12, 11:25 AM
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I dunno. If the mounting holes are the right size, even a tubular brake bridge shouldn't let the brake move appreciably.
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Old 01-18-12, 11:31 AM
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Or:
Measure size of hole at both ends precisely (a digital caliper works well for this)
Measure thickness of brake bridge
Go to machine shop and have them cut you a tubular shim with the ID that's the same as the OD of your brake bolt, an OD that's the same size as the hole in your brake bridge, and length the thickness of your bridge
Place shim over brake bolt and install using hardware described above (BTW, Loose Screws also carries this part).
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Old 01-18-12, 11:36 AM
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Originally Posted by HillRider View Post
I dunno. If the mounting holes are the right size, even a tubular brake bridge shouldn't let the brake move appreciably.
We worked on many crappy bikes with this arrangement back in the sixties and seventies. Brakes that held for small riders slipped with heavier ones. Also today's brake shoes have higher friction that what we had back then so the torque range is higher.

Back then, whenever we saw the problem we fixed it with homemade gluegrit (grit/rubber cement mix), and never had to do the same job twice.
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Old 01-18-12, 11:49 AM
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Footnote, I failed to mention earlier because it was too obvious.

before doing anything make sure that the two curved washers do not touch, since then they would be tightening against each other not against the tube. (the photo is how they should look, though any gap both top and bottom when tightened is enough)
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Old 01-18-12, 12:49 PM
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It sounds like you are using hardware from a nutted brake. The correct part is a tubular nut that fits the hole and a concave washer with a larger hole that fits the nut. You can sort of see it in the picture below of a '73 Raleigh Competition that I drilled for recessed brakes. I got the parts from Bike Parts Etc.
No -- my brakes are the old school nutted type -- it's just that the rear-facing hold in the brake bridge seems too large to secure the brake bolt and I get slippage under hard braking. It's lamost like someone drilled this particular hole for a recessed nut instead of the brakes that are supplied with the bike.

And yes, it's the same bridge as the red one in the picture shows. I'm pretty sure the 2 circular washers do not touch, but I'll double check tonight.

Great ideas here! I can visualize the lapping compound job -- but how does the JB Weld and stainless washer work? Would you basically glue a flat washer onto the rear-facing hole to make a flat platform?

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Old 01-18-12, 07:52 PM
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Yep, the washers don't touch -- so no problems there. I can tighten the brake on there in the correct alignment and then literally take my hand and pull hard enough in the direction that braking would deflect the pads, making the whole brake move out of line.......yep, poor design for the rear bridge. I'm going to try the lapping compound job since that will allow me to theoretically use the brake I already have on there. If for whatever reason that doesn't work, I'll move to the other options.

Thank you very much for the help!
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Old 01-18-12, 08:04 PM
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are you running out of thread when tightening? the ramp up in resistance is the big teller
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Old 01-22-12, 07:39 PM
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Update: I got some JB-Weld and used a small amount on the contact faces of both curved washers, as well as the contact surfaces on both sides of the brake bridge. I "roughed up" the faces of the washers and sanded the contact area of the brake bridge to remove paint before hand. Then I installed the brake in the proper position and tightened it like normal and let it set for 36 hours.

The weld appears to have held just fine, and I took the bike out for a while to purposely ride fast down a couple of hills and repeatedly locking up the rear brake to try to break that sucker loose. As far as I can tell it holds -- so thanks for the advice all! I realize that this was the more permanent solution, but I can still remove the bolt and use a different type of brake with the same washers later if I choose, so I don't care.
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Old 01-22-12, 10:16 PM
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Originally Posted by TurbineBlade View Post
No -- my brakes are the old school nutted type -- it's just that the rear-facing hold in the brake bridge seems too large to secure the brake bolt and I get slippage under hard braking. It's almost like someone drilled this particular hole for a recessed nut instead of the brakes that are supplied with the bike.
If your fix doesn't continue to work for you there is another solution. I once owned a fork that had a short recessed nut that was drilled out and mounted in the rear hole; this allowed the use of a nutted brake, or if it was removed allowed the use of a recessed brake. If indeed someone drilled out the hole hoping to use a recessed brake, you can pick up a couple of recessed nuts, drill them out and then grind them shorter enough so if you insert them into the holes front and rear, they will not touch. Now epoxy the drilled out recessed nuts into place and mount your brakes. This will give you two flat surfaces to mount your brake; the only reason for the epoxy is to help stop the nuts from turning. FBinNY, does this qualify as using my little gray cells
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Old 01-22-12, 10:30 PM
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Originally Posted by onespeedbiker View Post
FBinNY, does this qualify as using my little gray cells
Yes, and it counts even more if you happen to have a bunch of extra nuts sitting around in a parts box.
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Old 01-22-12, 11:03 PM
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I do and one is already drilled..
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