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Seatstay repair on aluminium bike

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Seatstay repair on aluminium bike

Old 09-18-14, 07:21 AM
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dabac
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Seatstay repair on aluminium bike

Hi All,
I've been given a 7005 alu alloy frame which, probably due to a botched theft attempt, has a seatstay that's been cut by a hacksaw. And I'm pondering repair options. Replacing the stay entirely is beyond me, not that I've ever heard about it being done on an aluminium frame anyhow.
But I am reasonably skilled at working laminates, so I could simply sleeve the cut in carbon fiber. A bit of crude maths indicate that it'd be entirely possible to create a repair whose strength would be comparable to the stay itself.
Or I could make an aluminium sleeve, and use a low-temperature aluminium solder(alutite etc) to set it in place.
Or a longer sleeve and bond it in place using a bushing assembly compound.
Which, if any, method do you think would be most likely to succeed?
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Old 09-18-14, 10:38 AM
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round tube? you can sleeve inside the stay down a ways with a shoulder tube outside of it .
that would take compression loads the inner sleeve doubling the wall thickness..

the inner tube sections, probably Epoxied in the frame ,+, maybe a split tube and then wet wrap it with more cloth and resin..

Consider .. More than just butt soldering in a section of Al tube ..
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Old 09-18-14, 11:57 AM
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Originally Posted by fietsbob View Post
round tube?
As far as I can tell with the naked eye, yes.

Originally Posted by fietsbob View Post
.... you can sleeve inside the stay ..
I'd prefer not to. The chain stay is still attached, so I'd need to do a fair bit of bending to get an inside sleeve into position.
Originally Posted by fietsbob View Post
... the inner tube sections, probably Epoxied in the frame ,+, maybe a split tube and then wet wrap it with more cloth and resin.. ..
I've thought about a small nub inside to maintain alignment, then wrap carbon fiber + epoxy to 1.5 -2 mm thickness and 20-30 mm overlap on both sides of the cut.
Originally Posted by fietsbob View Post
Consider .. More than just butt soldering in a section of Al tube ..
The stay is cut, but it's all there. Don't need to add anything lengthwise. Wasn't planning a butt joint at all. Possibly a soldered external sleeve. Piece of tubing, no halves. Pull the stay sideways, slide the sleeve on, close the stay, slide the sleeve down over the cut. Solder in place.
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Old 09-18-14, 01:11 PM
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You need to fiberglass it first and then put the carbon on top of the glass. It's totally feasible although it won't look great.
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Old 10-02-14, 09:54 AM
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Originally Posted by Canaboo View Post
You need to fiberglass it first and then put the carbon on top of the glass.
Yeah, I've read about the possible galvanic/corrosion issues of CF straight onto Al.
One thought-what if I keep the Powdercoat on as primer, just scuff the surface as prep and then apply the CF on top of that? How do you think the paint would do in terms of adhesion?
Originally Posted by Canaboo View Post
It's totally feasible although it won't look great.
Well, either I can touch the paint up, which would make it fairly discrete. Or I can finish with a cosmetic wrap of the whole stays, make it look intentional, like the bike has a CF rear triangle.
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Old 10-02-14, 12:44 PM
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anyhow .. 7000 series has Zn in it Add C and you have a Battery.
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Old 10-02-14, 02:36 PM
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Prime it with epoxy. There are lots of products that have been around for a long time, that use graphite over aluminum. Arrows, climbing gear, come to mind immediately. I think some ***** barrels. What about aerospace? I think it is a myth, it might happen, but it can't always happen. Typically structures had aluminum, carbon, then glass, the rationale being the aluminum for hardware bonding, and shape; the graphite for stiffness and strength; and the glass because carbon is easily degraded through damage.

The powder coat might be OK, but it is not heavily bonded, itself. Still, with a sleeve that shouldn't mater. Just look at what some of the bamboo bikes are cobbled together with, hemp. You can get linear glass which is very strong itself. Carbon is a reflex, but often leads to as much trouble as not. With glass the repair may not be visible at first glance.
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Old 10-02-14, 02:42 PM
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What I would probably do is offset it enough you can insert a cork plug, or something. The realign. Enlarge a vent hole or drill one on the wheel side. Wrap with tape, use some thing to hold the spacing, like some wire as a bridge. Then inject epoxy so it fills up from the cork across the joint. You will have a solid plug of epoxy. It won't weight anything much. You could fill the tube with some kevlar pulp, which would give the internal plug a lot of tensile strength. Then splint with linear glass when it is dry.
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