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Old 01-18-09, 02:13 AM   #1
mr.smith.pdx
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Brake bridge crack in 7sp Al frame

So I cracked my frame.
See pictures:
http://www.bikeforums.net/attachment...1&d=1232265762

I know that this is all my fault because:
a) I jammed a 135mmhub into a 126mm dropout
b) I weigh 220lbs.
III) I am a bit of a masher

So my question is: Will it crack further and dump me on the road at high speed requiring cosmetic surgery? ( on me, not the bike)

After vigorous visual inspection I have been unable to locate any other cracks in the rear triangle.

I looked at this thread to learn about brake bridge cracks: Help with possible cracks in brake bridge
This thread gave me hope despite the fact that it was on a fixie with no rear brake attached.

My main question is:

Am I taking my life in my hands by riding on this frame, or can I spend the next couple weeks tracking down a new (to me anyway) frame? If it helps, this is an old 6061 Al frame.

Any insight would be GREATLY appreciated.
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Old 01-18-09, 09:38 AM   #2
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It's not going to dump you on the ground but I wouldn't use the rear brake much (if at all).

BTW, you sure that's an aluminum frame? Weld on the stay doesn't look like the typical aluminum weld.
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Old 01-18-09, 09:58 AM   #3
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Time for the magnet test.
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Old 01-18-09, 10:39 AM   #4
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This was my first clue that the frame was aluminum:
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Old 01-18-09, 11:23 AM   #5
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Quote:
Originally Posted by mr.smith.pdx View Post
This was my first clue that the frame was aluminum:
Good clue!
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Old 01-18-09, 02:51 PM   #6
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I agree you mostly have a braking problem there, not a frame problem. That part is not usually structurally required by the frame. That style of bridge has a lot of bearing, and it might be possible to epoxy it (won't begin to replace weld strength, but will bed it). Then wrap it with some tow. Pieces of fiberglass roving, or carbon or kevlar tow from aircraft spruce etc.. Spectra line used in fishing or archery, bind it, and glue that binding with epoxy. Ideally you would remove the paint first. Entomb the full Y shape. use some tow strands parallel to the tubing, and be sure your epoxy is built in as you go, or has the viscosity to penetrate right through. Pretend you are one of those guys wanting to build using bamboo tube or carbon tubes joined with composites.
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Old 01-20-09, 01:39 AM   #7
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Thank you for the possible solutions and reassurance that I am not going to fall on the road.

The plot thickens. I was working on my bike (again) and noticed that there appears to be two bolts running through the brake bridge, one into each seat stay.

I Didn't get a picture, but I will investigate further and post one soon. Is it possible that this frame has a bolted in brake bridge and all I have done is crack the paint?
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Old 01-20-09, 08:00 PM   #8
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That sounds really unlikely, but... who knows. I gather you are seeing something.
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Old 01-20-09, 08:36 PM   #9
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Makes sense to me that they are using some sort of mechanical fasteners to hold the bridge in place. If that is the case, those cracks are most likely just cracks in the paint and nothing to worry about.
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Old 01-22-09, 01:44 AM   #10
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So here are some pictures of the underside of the brake bridge.

There is definitely a rivet running through the brake bridge into the seat stay. I think that I have just cracked the paint...so far. I am a bit concerned that I will eventually shear the rivet.

This is consistent with the earlier post that the welds did not look like aluminum.

Thanks for the help guys, I really appreciate it.

I do have a follow up question. I've had trouble indexing the rear derailleur on this bike. By wedging a 135mm rear hub into 126mm dropouts have I misaligned the RD enough that it won't shift correctly? Kind of like if I had a bent hanger? Can I bend the hanger to correct for it? Could I get away with a 130mm hub?

Or is this a better question for the mechanics forum?
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Old 01-22-09, 08:28 AM   #11
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Looks like that bridge isn't going anywhere. I'd use the bike but first do a sanity check by grabbing the brake caliper with your hand and tweak it around firmly and watching to see if that crack opens up at all.

Regarding your shifting, I don't like speculating on this kind of thing and suggest you measure the derailleur hanger alignment with the appropriate tool. To measure is to know.
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