Welding a mech
Discussion
My chain snapped and promptly caught the front mech, dragging it into the front chainring and breaking the mech cage (outer side of the cage, fairly central). Is there any reason why I shouldn't try to get the mech cage welded rather than replacing the whole mech? It's an ultegra di2 mech (£120 for a replacement) and there's no way of just replacing the mech cage (which is a stupid design). I've spoken to one bike shop who advised I replaced the mech at a cost of £170 with them and that it shouldn't be welded because it's thin, a weld would need to be smooth and welding it would be expensive...I suspect this is because they want me to buy a new mech from them but thought I'd ask for a few more opinions first 
If anybody knows anywhere that would do this in SW London then feel free to provide suggestions please.
If anybody knows anywhere that would do this in SW London then feel free to provide suggestions please.Could it be possible to have a reinforcing plate bonded to the outside face of the mech? You'd then be less reliant on the strength of single weld over a small area, with the (chemical?/mechanical?) bond, and the forces acting upon it, spread over the full length of the outer plate. It might be something a small engineering firm could take on, but again, there's the question of 'is it worth their while'. Whatever happens, it's probably worth asking outside of the bike industry, as it's far too convenient for insiders to simply 'order up' a new mech and get it fitted relatively quickly.
The other issue to overcome is that of passing electricity through a component with potentially sensitive electronics built into it, but if it's buggered anyway, it may be worth the risk...
The other issue to overcome is that of passing electricity through a component with potentially sensitive electronics built into it, but if it's buggered anyway, it may be worth the risk...
yellowjack said:
Could it be possible to have a reinforcing plate bonded to the outside face of the mech? You'd then be less reliant on the strength of single weld over a small area, with the (chemical?/mechanical?) bond, and the forces acting upon it, spread over the full length of the outer plate. It might be something a small engineering firm could take on, but again, there's the question of 'is it worth their while'. Whatever happens, it's probably worth asking outside of the bike industry, as it's far too convenient for insiders to simply 'order up' a new mech and get it fitted relatively quickly.
The other issue to overcome is that of passing electricity through a component with potentially sensitive electronics built into it, but if it's buggered anyway, it may be worth the risk...
I'm not convinced that the forces of shifting a chain sideways is that great anyway so not sure whether a plate would be necessary? Especially since it's only contact with the chain is off the big ring to the small. Like you say, I'd considered the electronics but figured it was probably worth a £10 attempt at mending it before forking out for a new one.The other issue to overcome is that of passing electricity through a component with potentially sensitive electronics built into it, but if it's buggered anyway, it may be worth the risk...
I'd give it a go. The forces going from big to small ring aren't that great and I'm sure it will be fine. All Shimano kit tends to be officially bin/replace rather than repairable. A Google might show up otherwise. e.g. it might be possible to swap out the cage for one off a none electronic mech you could pick up for £15 on Ebay.
Plain old electric arc almost certainly won't work - tends to burn holes in small/light metal. Something more subtle like mig/tig might work, not sure about frying the components; potentially you're sinking a lot of heat in there. Person to ask would be a specialist welder (obviously), bike shop's unlikely to know the first thing..
How broken is it?
The cage on the front mech on mine broke when the chain jammed between the 34 and the 50 for no real reason that I could find. The outside plate of the cage came away from one of the two little beams that go across from the inside plate.
Once I’d worked out what had happened I just bent it back roughly in place and it works fine, even when I’ve forgotten about it and not taken care to shift gently.
I’ll admit though that this isn’t really in the spirit of having high-end electric shifting.
The cage on the front mech on mine broke when the chain jammed between the 34 and the 50 for no real reason that I could find. The outside plate of the cage came away from one of the two little beams that go across from the inside plate.
Once I’d worked out what had happened I just bent it back roughly in place and it works fine, even when I’ve forgotten about it and not taken care to shift gently.
I’ll admit though that this isn’t really in the spirit of having high-end electric shifting.
I will TIG weld it for you or at least have a go, if the metal is cheap it will not weld anyway.
Post me a picture of the damage so I can see if it worth a go
Cost will be £25 inc p+p back to you or £10 if it is a fail just to cover the cost and p+p
You stand the risk of the TIG set frying the electric bits I will not know until you try it (I have Di2 but a damaged mech could short my unit out, it is a risk)
I will put a meter across it to check the resistance before and after welding to see if it has damaged only real test i can do for you.
It's your choice.
Paul 07757673303 DN350SN
Post me a picture of the damage so I can see if it worth a go
Cost will be £25 inc p+p back to you or £10 if it is a fail just to cover the cost and p+p
You stand the risk of the TIG set frying the electric bits I will not know until you try it (I have Di2 but a damaged mech could short my unit out, it is a risk)
I will put a meter across it to check the resistance before and after welding to see if it has damaged only real test i can do for you.
It's your choice.
Paul 07757673303 DN350SN
I will TIG weld it for you or at least have a go, if the metal is cheap it will not weld anyway.
Post me a picture of the damage so I can see if it worth a go
Cost will be £25 inc p+p back to you or £10 if it is a fail just to cover the cost and p+p
You stand the risk of the TIG set frying the electric bits I will not know until you try it (I have Di2 but a damaged mech could short my unit out, it is a risk)
I will put a meter across it to check the resistance before and after welding to see if it has damaged only real test i can do for you.
It's your choice.
Paul 07757673303 DN350SN
Post me a picture of the damage so I can see if it worth a go
Cost will be £25 inc p+p back to you or £10 if it is a fail just to cover the cost and p+p
You stand the risk of the TIG set frying the electric bits I will not know until you try it (I have Di2 but a damaged mech could short my unit out, it is a risk)
I will put a meter across it to check the resistance before and after welding to see if it has damaged only real test i can do for you.
It's your choice.
Paul 07757673303 DN350SN
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