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  1. #1
    Rouleur gattm99's Avatar
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    repairing broken aluminum frame

    I've got a Schwinn Homegrown frame with totally broken top tube. I was thinking the other day about throwing it away, but I wonder about wrapping the damaged tube section in Carbon Fiber. If the tube was stripped, cleaned, sanded, maybe had a few holes drilled in it, then wrapped in carbon and cured would it adhere?

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    Ther have been several threads about cracked or broken Al frames here lately. A search should turn them up. The general concensus is: Theoretically it is possible to repair this kind of damage but the cost and expertise needed to do it right make the project very unattractive. Discard it and get a new frame.

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    Quote Originally Posted by HillRider View Post
    Ther have been several threads about cracked or broken Al frames here lately. A search should turn them up. The general concensus is: Theoretically it is possible to repair this kind of damage but the cost and expertise needed to do it right make the project very unattractive. Discard it and get a new frame.
    +1, while minor damage can be fixed, anything else is more costly and short lived than a replacement.

    But do not discard an aluminum frame. Aluminum is highly recyclable so it doesn't make sense to send it to the landfill. There are recycling centers all over who'll take it, and might even pay you for it.

    If it's not worth your time, call your local boy scout troop as many collect scrap to raise money. The same applies to things line crank arms which are highly prized by recyclers, along with alloy handlebars, stems, seatposts and rims, as long as they don't have eyelets.
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    Quote Originally Posted by gattm99 View Post
    I've got a Schwinn Homegrown frame with totally broken top tube. I was thinking the other day about throwing it away, but I wonder about wrapping the damaged tube section in Carbon Fiber. If the tube was stripped, cleaned, sanded, maybe had a few holes drilled in it, then wrapped in carbon and cured would it adhere?
    Not saying it's a worthwhile effort, but if you want it to have the best chance of getting good adhesion there's one more thing you can try - wet sanding, but with resin. Apply a small amount of resin to the area, then lightly sand it with a medium/fine grit paper until the whole surface is covered in a grayish slush. This allows the resin to make direct contact with the pure aluminum, and not the sometimes pesky, sometimes useful oxide skin. Then wrap, add more resin, compress etc as according to regular repair methods.

    If the break is a decent distance (maybe 2") away from head tube/seat tube joints I wouldn't have any worries about performing the repair and using the bike later, but if it's a break involving a joint it becomes more complicated.

    YMMV though, depending on your skill in working with laminates.

  5. #5
    bike whisperer Kimmo's Avatar
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    Cut the top and down tubes off the seat tube, and you have a wheel truing stand.

    Quote Originally Posted by dabac View Post
    Not saying it's a worthwhile effort, but if you want it to have the best chance of getting good adhesion there's one more thing you can try - wet sanding, but with resin.
    Hey, that's a nifty tip.

  6. #6
    Senior Member BCRider's Avatar
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    As dabac said if the break is out well away from a weld joint then you could likely do it decently well IF the ends are a good butt fit at the moment. But if the tube broke off right at the weld the difficulty with wrapping cloth around the top and head tube to achieve a supportive wrap would be extremely hard to the point that I would suggest not even trying. It's not enough to just make it pretty, you need to have the fibers running in the right directions and under some tension to take the loads.
    Model airplanes are cool too!.....

  7. #7
    Ride, Wrench, Swap, Race dddd's Avatar
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    +1 on all posts emphasizing the difficulty of assuring a strong repair.

    Any repair to an aluminum frame puts you in the driver's seat in terms of DESIGNING a repair. It's not really a simple matter, as stress-concentration comes into play as well as the known tendency for bonded overlap-type joints to fail beginning at one end of the repair where the overlap begins. From there, the bond often undergoes a creeping failure until the entire bond has failed.
    Remember, these are highly-stressed, i.e. lightweight "hi-performance" structures, which makes the quality of material and repair design every bit as critical as the design that went into the frame in the first place.
    So, the questions are:
    1) Are you qualified in composite structure design?
    2) Would it be worth your considerable effort and material costs to design and effect a competent repair?

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