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Old 02-08-08, 07:06 PM   #1
mssurfn1
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Frame question

My Cannondale CAAD 8 aluminum frame was slightly damaged during shipment. The left seat stay was slightly bent inward reducing the spacing for the rear wheel to 128mm. Can I pull the stays apart and insert a wheel without causing any damage to the frame and / or decreasing the structural integrity of the frame?
Thanks for your help.
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Old 02-08-08, 09:00 PM   #2
rhenning
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I personally wouldn't trust a bent and then straightened aluminum frame. There is no way to know if it will last 100 years or 5 minutes. Roger

Last edited by rhenning; 02-09-08 at 11:58 AM.
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Old 02-08-08, 11:23 PM   #3
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2mm is no big deal. Relax. Just spring the frame out that small amount and insert the wheel.
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Old 02-08-08, 11:30 PM   #4
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I doubt it is signfiicant enough to worry about - 2mm isn't much - but OTOH, if it's a brand new frame, 2mm is enough that it is probably out of spec and I would think Cannondale or your dealer would make it right by you.

I doubt there is any structural integrity or longevity issue, it's really a matter of how much your rear wheel is going to be misaligned after assembly and whether this is going to affect handling. I'd be more worried about the axle not being perpendicular to the frame centerline than a simple lateral translation.

Out of curiosity, how can you tell that the left side is the one that is bent?

- Mark
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Old 02-09-08, 11:18 AM   #5
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I measured the spacing between the drop outs and the left seatstay is slightly "off" and there is a slight dent just below rear brake mount bar. The frame is brand new never built up. This frame was given to me by my team at the end of the season to get rid of "old stock".
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Old 02-09-08, 01:52 PM   #6
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Originally Posted by mssurfn1 View Post
I measured the spacing between the drop outs and the left seatstay is slightly "off" and there is a slight dent just below rear brake mount bar. The frame is brand new never built up. This frame was given to me by my team at the end of the season to get rid of "old stock".
Without an alignment jig, I would think it would be difficult to tell much about where the 2mm is lost. There are some home-brew alignment techniques involving running a string between the rear dropouts and the head tube and then carefully measuring the distance between the string and the seat tube. You might try this. But certainly the dent would indicate the most likely source of pressure.

Since it's a freebie, I would just build it up and ride. See how it handles.

- Mark
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Old 02-10-08, 03:47 AM   #7
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When you bend aluminum you will initiate stress risers and eventually the frame will crack. No telling when. You can probably ride it safely but eventually it will fail sooner than an unstressed bike. When you bend it out it will not cold set like steel. Good luck.
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Old 02-10-08, 01:48 PM   #8
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When you bend aluminum you will initiate stress risers and eventually the frame will crack. No telling when. You can probably ride it safely but eventually it will fail sooner than an unstressed bike. When you bend it out it will not cold set like steel. Good luck.
While what you are saying it true at the limit, a tiny bend in an alum structure (2mm over a 60cm distance) is unlikely to be significant. A nice property of alum is that it is malleable and alum structures are built all the time which are formed into shape by bending aluminum - for example airliners. I'm sure the bending does cause some ultimate reduction in fatigue strength but unless a crack was formed and unless the material is repeatedly bent to deformation, I doubt the reduction in frame life is significant.

As a further example, people crash their alum frame bikes all the time and I'm sure they tweak them now and then by small amounts. If what you were saying is true, any alum frame bike that has been crashed either needs to put on a factory jig and seen if the alignment changed or immediately thrown away.

I do agree that you don't want to attempt to bend it back like a steel frame.

But I'm not a framebuilder or a metallurgist; this is just a common sense approach to the issue.

- Mark
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