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tomBIKE
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Hi all, I purchased a lovely bike 2nd hand, and it has developed (or was sold with) a crack on one of the seat stay welds. I have no chance of a warranty, and ebay cant help. I spoke in person to my very reliable local bike shop who offered some hope. Basically I don't want to scrap the frame, I like fixing things and quite frankly I cant afford a new frame. They suggested that 1. the crack isnt too bad, and many people would ride on it no problems..I'm not one of those people. This is paraphrasing a long conversation, with a frame builder. 2. Frame builder suggested that a carbon support/'lug' could be created using Carbon Fibre Tow and epoxy of some sort. He used the examples of many bamboo bikes being created using similar materials. The pic below gives some idea of what the crack looks like - but it hasnt reproduced that well here. lower section of left weld area. I think what Im trying to achieve is not a full repair like a weld might provide (probably unachievable in aluminium) but a bit of a safety net in terms of strength support, and if the tube was to shear, some sort of control of that - i.e. not stabbing into a thigh or similar! Does anybody here have any experience of a similar repair, i.e. a fairly complex shape, aluminium. If so - what materials might be best, in terms of weight of Tow and epoxy type, and or -is it even worth a go? 
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Lester Populaire
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Hello tomBIKE
I'm an avid biker myself and recently built a frame as well. In fact I'm sitting in the train home after the first really long ride with it!
In my opinion it is unfortunately a little more complicated than you probably envision. there is a number of issues:
Carbon fibers composites are a lot stiffer than aluminium, which means that when you deform a hybrid part with aluminum and carbon the carbon will take the majority of the load. This is especially problematic as carbon has a lower elongation at failure than the aluminum. This difference in stiffness creates big shear stresses at the interface.
Carbon and aluminum are quite hard to bond. A halfway decent bond can be achieved quite easily with proper roughing and scoring of the aluminum, but to get a really good bond you need to acid etch the metal which is nasty.
And last but not least the area of the crack is really tight and achieving proper fiber alignment will be hard. It certainly was the area i struggled the most on my frame.
Oh and one more point actually once covered in carbon you do not know what is going on underneath. So the crack might be growing and delaminating from the composite layer without you knowing until it fails.
So to sum it up: a repair is certainly possible but i would not recommend it as a first project. I personally would drill a 1.5mm hole at either end of the crack (and make sure it really is the end of the crack...) to get rid of the stress concentration and then ride it and keep an eye on it to see if it keeps spreading. If it spreads i would repair it.
If you want to attempt that, give us a heads up and i and the others will talk you through. Now you know the problems that come with such a repair and hopefully can make a more informed decision...
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tomBIKE
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Group: Forum Members
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+xHello tomBIKE I'm an avid biker myself and recently built a frame as well. In fact I'm sitting in the train home after the first really long ride with it! In my opinion it is unfortunately a little more complicated than you probably envision. there is a number of issues: Carbon fibers composites are a lot stiffer than aluminium, which means that when you deform a hybrid part with aluminum and carbon the carbon will take the majority of the load. This is especially problematic as carbon has a lower elongation at failure than the aluminum. This difference in stiffness creates big shear stresses at the interface. Carbon and aluminum are quite hard to bond. A halfway decent bond can be achieved quite easily with proper roughing and scoring of the aluminum, but to get a really good bond you need to acid etch the metal which is nasty. And last but not least the area of the crack is really tight and achieving proper fiber alignment will be hard. It certainly was the area i struggled the most on my frame. Oh and one more point actually once covered in carbon you do not know what is going on underneath. So the crack might be growing and delaminating from the composite layer without you knowing until it fails. So to sum it up: a repair is certainly possible but i would not recommend it as a first project. I personally would drill a 1.5mm hole at either end of the crack (and make sure it really is the end of the crack...) to get rid of the stress concentration and then ride it and keep an eye on it to see if it keeps spreading. If it spreads i would repair it. If you want to attempt that, give us a heads up and i and the others will talk you through. Now you know the problems that come with such a repair and hopefully can make a more informed decision... That is brilliant information - many thanks. Very informative. I dremmeld an area of paint off the frame today, and took it for a short spin - the crack ( initially not visible) manifested itself again after a smoothish 6 miles. I'm not sure I'm going to be able to live with the frame as it is, drilled or otherwise, although I do see the logic in what you are saying. Time to strip the parts off and see if anyone on ebay has a use for the nice forks and a possible repair project. Genuine thanks for the informed response. Tom
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Steve Broad
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Why not simply get someone who can TIG weld to repair it properly?
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Lester Populaire
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Group: Forum Members
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+xWhy not simply get someone who can TIG weld to repair it properly? Bike frames are usually made from 6000 series aluminum that needs heat treatment after welding.
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Steve Broad
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Group: Forum Members
Posts: 408,
Visits: 4.1K
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+x+xWhy not simply get someone who can TIG weld to repair it properly? Bike frames are usually made from 6000 series aluminum that needs heat treatment after welding. If the frame is made from a T6000 series aluminium, I agree that ideally it should be heat treated after welding. However, the frame has already cracked so isn't exactly a brilliant or strong design to start with so welding up the crack and reinforcing with a welded on sleeve will do a better job than a bodge with carbon fibre. IMO
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Lester Populaire
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Group: Forum Members
Posts: 311,
Visits: 13K
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+x+x+xWhy not simply get someone who can TIG weld to repair it properly? Bike frames are usually made from 6000 series aluminum that needs heat treatment after welding. If the frame is made from a T6000 series aluminium, I agree that ideally it should be heat treated after welding. However, the frame has already cracked so isn't exactly a brilliant or strong design to start with so welding up the crack and reinforcing with a welded on sleeve will do a better job than a bodge with carbon fibre. IMO I have seen a lot of welded frames and never anything that worked. Especially at the transition there you can't put a sleeve over it either...
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Steve Broad
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Group: Forum Members
Posts: 408,
Visits: 4.1K
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+x+x+x+xWhy not simply get someone who can TIG weld to repair it properly? Bike frames are usually made from 6000 series aluminum that needs heat treatment after welding. If the frame is made from a T6000 series aluminium, I agree that ideally it should be heat treated after welding. However, the frame has already cracked so isn't exactly a brilliant or strong design to start with so welding up the crack and reinforcing with a welded on sleeve will do a better job than a bodge with carbon fibre. IMO I have seen a lot of welded frames and never anything that worked. Especially at the transition there you can't put a sleeve over it either... But I can't see a carbon repair working either, bearing in mind the stresses involved at this joint (they cracked the aluminium). Yes, I know that bikes are made from carbon, but that is a completely different ballgame to wrapping a cracked joint with the stuff. I don't think that I would feel save riding a bike repaired that way.
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tomBIKE
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Group: Forum Members
Posts: 4,
Visits: 12
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+x+x+x+x+xWhy not simply get someone who can TIG weld to repair it properly? Bike frames are usually made from 6000 series aluminum that needs heat treatment after welding. If the frame is made from a T6000 series aluminium, I agree that ideally it should be heat treated after welding. However, the frame has already cracked so isn't exactly a brilliant or strong design to start with so welding up the crack and reinforcing with a welded on sleeve will do a better job than a bodge with carbon fibre. IMO I have seen a lot of welded frames and never anything that worked. Especially at the transition there you can't put a sleeve over it either... But I can't see a carbon repair working either, bearing in mind the stresses involved at this joint (they cracked the aluminium). Yes, I know that bikes are made from carbon, but that is a completely different ballgame to wrapping a cracked joint with the stuff. I don't think that I would feel save riding a bike repaired that way. Yep - agree with both on this - think the frame is a really tricky repair either way, and I'm not sure either way is going to be worth the time/money and eventual probable risk. i.e. its broken! Really appreciate the input from both points of view - exactly what I was looking for. Its been a stressy couple of days since the discovery of the issue. But I have recovered and currently looking for a new frame with a warranty! 
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Lester Populaire
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Group: Forum Members
Posts: 311,
Visits: 13K
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+x+x+x+x+x+xWhy not simply get someone who can TIG weld to repair it properly? Bike frames are usually made from 6000 series aluminum that needs heat treatment after welding. If the frame is made from a T6000 series aluminium, I agree that ideally it should be heat treated after welding. However, the frame has already cracked so isn't exactly a brilliant or strong design to start with so welding up the crack and reinforcing with a welded on sleeve will do a better job than a bodge with carbon fibre. IMO I have seen a lot of welded frames and never anything that worked. Especially at the transition there you can't put a sleeve over it either... But I can't see a carbon repair working either, bearing in mind the stresses involved at this joint (they cracked the aluminium). Yes, I know that bikes are made from carbon, but that is a completely different ballgame to wrapping a cracked joint with the stuff. I don't think that I would feel save riding a bike repaired that way. Yep - agree with both on this - think the frame is a really tricky repair either way, and I'm not sure either way is going to be worth the time/money and eventual probable risk. i.e. its broken! Really appreciate the input from both points of view - exactly what I was looking for. Its been a stressy couple of days since the discovery of the issue. But I have recovered and currently looking for a new frame with a warranty! If the warranty doesn't lead anywhere have a look at commencal. they usually have really good deals on frames from last season. you should get a very nice frame for relatively little money. Just make sure you can transfer your parts over (same frame standards).
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