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 Post subject: aluminium frame repair
PostPosted: Sat Mar 28, 2020 10:47 pm 
Dirt Disciple
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Joined: Tue Dec 28, 2010 9:05 pm
Posts: 58
Location: Derbyshire
The front derailleur mount on my GT Edge Aero has been damaged beyond the point where the derailleur will correctly align. The mounting face is indented from the clamp and the slot distorted due to abuse over the years, so I think the mount needs to be removed and re-welded. Any ideas if this is do-able / how much it might cost?


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 Post subject: Re:
PostPosted: Sat Mar 28, 2020 11:12 pm 
Retro Guru

Joined: Wed Dec 10, 2008 9:19 am
Posts: 2576
Location: Sheffield, top city
Depending how nice the frame is, you could saw, file, sand it off and replace with a band-on mount


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 Post subject: Re:
PostPosted: Sat Mar 28, 2020 11:21 pm 
Retro Guru

Joined: Tue Aug 21, 2018 3:43 pm
Posts: 755
Location: Cambridgeshire - flatlands (the horror, the horror)
Was just writing this and the previous post anticipated option 1 below....

Regret that there’s a bit of a clue on the frame sticker adjacent to the mount - ie ‘heat treated’. It’s difficult to work on some alloy frames if they are a post -fabrication heat-treated frame. I have pushed it quite a lot (retracking rear ends for example, which is a quite brutal affair with jacks and scaffolding poles) and theoretically it is totally unadvisable. But so far I have got away with it. You may too, but the warning is that the temper of the tubing changes with welding, and needs heat treating again to restore the ductility of the frame.

Ok....options....

I would check out whether you could file off the mount and use a banded front mech. Shimano do endless-band versions, so they could cope with having a bit of the base of the mount (1mm or so) still proud of the tube. That would preserve the qualities of the frame. You need to check whether the mech can be mounted in the right position relative to the cage of your existing mech. But the endless band ones have got quite a lot of throw adjustment. You’d need to cut the mount off with a hacksaw and then file very carefully.

Second option is to risk welding, ask a frame repairer - and there are some very very good ones who work with alloy - to fabricate a new mount and then weld it to the remains of the existing one. He or she will evaluate whether they can weld to the weld - which can be a problem. The fortunate thing is that this part of the frame is not heavily torsionally stressed, so a loss of ductility may not be an issue. A frame repairer would be able to judge this.

Third option is a variant of the first, which is to file off the old bracket but get an engineer to make up a new bracket in two halves, which can be bolted around the seat tube. This would consistent of two identical pieces, bolted on the opposite side of the mech, and with the mech bolt holding the mech side of the bracket together. This is quite a simple bit of design and build - essentially a thinnish flat plate with a slot in it, bent into a radius the same as the outer of the seattube. Any jobbing engineer could fabricate it. I have made such things myself - do it in paper or cardboard first, then in sheet aluminium.

I am not sure about how much each would cost - the first you could do entirely by yourself, as long as the mech would be in the right alignment, which you could comprehensively check before you do anything.

You’d need to get estimates for the second and third options.


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 Post subject: Re:
PostPosted: Sat Mar 28, 2020 11:26 pm 
Retro Guru

Joined: Sun Apr 25, 2010 10:07 pm
Posts: 2261
Its being heat treated to T6 hardness,dont weld it.I`d just carefully cut it off and file smooth and fit and endless band mech.


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PostPosted: Sun Mar 29, 2020 9:00 am 
King of the Skip Monkeys
King of the Skip Monkeys
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Joined: Wed Nov 07, 2007 4:34 pm
Posts: 32237
Location: Bournemouth's most wanted
Two steel shaped plates would sit over that nicely, front to back, heck, water cut titanium would do.

Steel would be easy enough to make from a dead frame tube.


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 Post subject: Re:
PostPosted: Sun Mar 29, 2020 9:30 am 
Retro Guru

Joined: Tue Aug 21, 2018 3:43 pm
Posts: 755
Location: Cambridgeshire - flatlands (the horror, the horror)
fromage....oooh oooh water-cut ti - now you're talking....

But...I hadn't thought of a slightly larger diameter bit of steel or alloy tubing, cut in half and then with brackets welded L and R, one side to carry the mech and the other to bolt together, Nice solution. Elegant and quick. Nice in alloy actually, could polish up nicely.

BUT .... always better in Ti.....now, where did I put that 1990s Glasnost Ti Russian tubing from the MIG factory....


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PostPosted: Sun Mar 29, 2020 10:01 am 
Retro Guru

Joined: Tue Feb 02, 2010 6:11 pm
Posts: 2212
Location: wellingborough
Get in touch with Woodrup cycles as they did a similar repair for me a while back


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PostPosted: Sun Mar 29, 2020 4:12 pm 
Retro Guru
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Joined: Mon Mar 02, 2009 4:22 pm
Posts: 1752
Initially, I might be inclined to tap that back into shape with a small hammer and dress it with a fine file. It might work fine after that


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PostPosted: Sun Mar 29, 2020 5:50 pm 
Dirt Disciple
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Joined: Tue Dec 28, 2010 9:05 pm
Posts: 58
Location: Derbyshire
I forgot to say that the seat post is teardrop-shaped so I'd discounted the idea of using a banded adapter, but I'll have to have a look at the endless adapter type.

I contacted my local frame repairer, but they only do steel so I'll have to look further afield to find one that does aluminium repairs.

Thanks for the comments


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PostPosted: Mon Mar 30, 2020 8:26 am 
Old School Grand Master

Joined: Tue Oct 09, 2007 1:55 pm
Posts: 10024
Location: New Forest, UK
One other thought is a riveted-on boss They are common enough on carbon frames.


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