Extending Braze-On FD Tab Slot, Dangerous?

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dvincere
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by dvincere

I have a frame where the braze-on tab is mounted a bit too low to accommodate the ring size I want. It looks to need about 2 mm more of travel up than it has. Given the amount of material present at the top, and bottom, and that this doesn't seem to be a component bearing a lot of load I suspected I can gently dremel out the material and attach the derailleur. This won't affect shifting because I weakened the tab too much, I assume.

Is there any reason to think that this is a bad idea? Has anyone had/observed good or bad experiences doing this?

em3
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by em3

....dremel-away. As long as you leave enough material (approx 4-5mm) on the end you should be fine. This was a common fix needed for many mass produced steel frames from the 80s, with braze on front mech tabs. EM3
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dvincere
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by dvincere

It's at 5mm of material right now exactly. I was thinking of taking 2mm off bringing it down to 3m at the thinnest point.

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Calnago
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by Calnago

I wouldn't. The front dérailleur tab takes a lot of stress, no idea why you don't think it takes a big load. Especially if you are working with an electric system or the new front dérailleurs with longer shift arms and more leverage. I've had to repair a broken one on a Cannondale Evo. Cracked right through where you're talking about. It was not steel however. If your dérailleur tab is steel I'd probably be less concerned. But if you mess it up, it's not the easiest repair in the world to do successfully.

Image

Broken tab on left, the result of using the support screw on DA9000. Tab was not really designed for that. They updated the tab (beefed it up and redesigned it) the following year to accommodate the new DA9000 derailleur better. New tab had both more material and a proper flat spot for the support screw to butt against.
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ms6073
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by ms6073

A friend had a Giant TCR Advanced SL frameset which he took to a local shop he frequented and had it built with Campy Super Record with an Oval chain ring, only to find the bolt on derailleur hanger did not allow the derailleur to be mounted high enough to clear the teeth of the chainring teeth through the full crank revolution. Not to be deterred, the mechanic decided to take a Dremel tool and round file to the mount until enough material had been removed to allow for proper derailleur height. Problem was that the mechanic did all the work with the derailleur hanger bolted on the frame for which the vibration of the Dremel tool which apparently caused the threaded inserts bonded into the frame to weaken. Once out on the road, it was not longer before the stresses of shifting from small to large chainring caused first the upper threaded insert came loose inside the frame, then the hanger cracked at the top allowing the derailleur to slide down and rub the chainring. To make matters worse, since neither the bike owner nor the mechanic bothered to contact Giant to inquire as to how to resolve said compatibility issues prior to making the alterations, Giant did not provide a warranty replacement for the frame.
- Michael
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dvincere
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by dvincere

Ok these stories are exactly what I needed to hear. It's a carbon fiber frame so I don't want to vibrate the rivets out of it by accident. And if it's already capable of cracking at the exact point I'm looking at reducing the material on, then I definitely shouldn't. It's a soft aluminum tab and will have a 6800 front derailleur on it. I noticed Rotor sells shims for their Oval/QXL rings, so I'm buying one of those instead.

Thanks for potentially saving my frame!

Svetty
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by Svetty

Calnago wrote:I wouldn't. The front dérailleur tab takes a lot of stress, no idea why you don't think it takes a big load. Especially if you are working with an electric system or the new front dérailleurs with longer shift arms and more leverage. I've had to repair a broken one on a Cannondale Evo. Cracked right through where you're talking about. It was not steel however. If your dérailleur tab is steel I'd probably be less concerned. But if you mess it up, it's not the easiest repair in the world to do successfully.

Image

Broken tab on left, the result of using the support screw on DA9000. Tab was not really designed for that. They updated the tab (beefed it up and redesigned it) the following year to accommodate the new DA9000 derailleur better. New tab had both more material and a proper flat spot for the support screw to butt against.


Are you suggesting this can happen with correct installation of DA9000 or improper adjustment? Doesn't the broken mounting imply excessive torque load being applied as a result of over-tightening the support screw? As an Evo owner with DA9000 you've got me worried now! :D

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Calnago
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by Calnago

@Svetty: what year is your Evo? Does it have the tab that looks like the broken one on the left (2012) or the revised one on the right (2013)? In any case, the broken one was indeed as you say caused more by turning in the support screw too much. You are probably just fine, but if you have the type on the left you will find the support screw hits it at quite an angle and can put a lot of outward and angular pressure on the tab. So don't overdo it with that screw. Just make it so it touches and can provide some support if it needs it. The revised tab is both beefier with more material and has a flat spot now for the support screw to be able to butt against it more directly.
Last edited by Calnago on Thu Jun 11, 2015 1:45 pm, edited 1 time in total.
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wheelsONfire
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by wheelsONfire

Bummer! I was worried to when i mounted Rotor 3D+ MAS with 52 ring.
But it was no problem on my frame.
The shims you talk of, they are mainly to move RD towards rear wheel and not to get it higher up.
I have the shims kit at home.
Couldn't Giant come up with any ideas if you correspond with them directly (technical issues) over email?
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Ax Lightness Vial EVO Race (2019.01.03)
Open *UP* (2016.04.14)
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ghostinthemachine
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by ghostinthemachine

If you move the mech backwards, it also moves it away from the centre of the bottom bracket..... Slightly.
Last edited by ghostinthemachine on Thu Jun 11, 2015 5:57 pm, edited 1 time in total.

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Kayrehn
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by Kayrehn

Image

Perhaps using a spacer like this would help? Tillquist sent me instructions to drill a lower hole on the spacer for attachment to the tab so that the front derailleur and the spacer can both sit higher. PM me if you need help.

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Svetty
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by Svetty

Calnago wrote:@Svetty: what year is your Evo? Does it have the tab that looks like the broken one on the left (2012) or the revised one on the right (2013)? In any case, the broken one was indeed as you say caused more by turning in the support screw too much. You are probably just fine, but if you have the type on the left you will find the support screw hits it at quite an angle and can put a lot of outward and angular pressure on the tab. So don't overdo it with that screw. Just make it so it touches and can provide some support if it needs it. The revised tab is both beefier with more material and has a flat spot now for the support screw to be able to butt against it more directly.


2012 Hi-Mod so yes it's the older style. No issues at present but I was careful to ensure that the support screw was just that - a 'support' not a 'turning moment inducer' :)

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Calnago
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by Calnago

Good plan @Svetty. Interestingly, if you read the Shimano tech instructions carefully they actually tell you to install the front dérailleur so that the rear is angled ever so slightly inward and then use the support screw to get it perfectly parallel with the chainring. Not a good procedure to follow to the letter in this case.
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