What's This Tab For?

[Mick F]

Brand new Deore XT Rear Mech Medium Cage.

Out a 20mile ride today early this morning, and I could hear a "tinkling" buzzy sort sound, sort of like the indexing wasn't quite spot on, but not .......... if you see what I mean. It was a sort of ticking rattle but at a high rate. It came and went but only at a reasonable cadence.

I stopped a few times to see what it was and to alter the indexing. All I managed to do was to put the indexing out.
Getting it spot on later, the noise was still there. No symptoms whatsoever, other than the noise.

Putting the bike on the rack, I got it into a medium gear and turned the cranks whist listening and looking closely, and found the guilty party.

You can see where the chain is catching ever-so-gently at speed. Pedalling slowly, there isn't a problem. It's caused by the chain setting up vibrations as it traverses the jockey wheels and "buzzing" on the tab as it goes through the cage.

What is this tab for?
It's a bit thick for trying to bend it as I'd be in danger of bending the cage itself.
I could grind it down a bit thinner perhaps.

Any ideas?

[Tompsk]

I had something vaguely similar once, are you 100% sure you have threaded the chain through the cage correctly? - I missed this once, fortunately I spotted it before I took the bike out.

[Mick F]

Absolutely 100% sure it's laced correctly.

It could be that Mr Shimano doesn't like me using Mr Campag's chain. Perhaps the Campag chain is a tiny bit "taller" than a Shimano one, so it catches.

[Mick F]

This is it back in situ.
I can't actually see what it's for.

What's the problem in just cutting it off?

[robgul]

This is it back in situ.
I can't actually see what it's for.

What's the problem in just cutting it off?

IMG_0294.JPG

Post a picture of a side on view (on the drive side!) with the derailleur in the lowest gear (biggest sprocket) so that the chain path is visible - that may make it easier to spot the issue, if there is one

My understanding is that the tab is to stop/reduce the risk of the chain jumping off the jockey wheels - especially if you get some chain-suck.

Rob

[Mick F]

Two photos.
First is small/small.
Second in top gear - big front/small rear.

....................but I've sorted it.
The tab was 2.6mm thick, and I've put the inner cage in the vice and taken a smooth file to it and taken the tab down to 2.1mm.
Reassembled, and all is well. The tab is now 0.5mm away from the chain.

Never had a rear mech with a tab there before. Top and bottom, yes. I've had rear mechs with no tabs at all, and the Tiagra mech I've discarded due to top pivot being worn, I removed the tabs top and bottom.

[Bmblbzzz]

Are you sure your chain isn't a bit too long? The small-small photo looks very scrunched up. I guess it depends to an extent on the ratios though; and I don't see how this would cause your problem in any case. And no, I'm not sure what the tab is for either, but robgul's suggestion sounds likely.

[robgul]

Two photos.
First is small/small.
Second in top gear - big front/small rear.
IMG_0296.JPGIMG_0295.JPG

....................but I've sorted it.
The tab was 2.6mm thick, and I've put the inner cage in the vice and taken a smooth file to it and taken the tab down to 2.1mm.
Reassembled, and all is well. The tab is now 0.5mm away from the chain.

Never had a rear mech with a tab there before. Top and bottom, yes. I've had rear mechs with no tabs at all, and the Tiagra mech I've discarded due to top pivot being worn, I removed the tabs top and bottom.

A pic as I suggested in lowest rear sprocket (i.e. the big one) is required - and the chain does look as if it might be too long [Cue a whole new thread about how to measure chain lengths ]

It will show the chain line and also whether the B-screw might need adjustment for the upper jockey wheel/chain to clear the sprocket. And another thought - if you you've filed as little as you have off the tab it MIGHT be that the jockey wheels are slightly worn (i.e. smaller diameter at the base of the teeth) affecting the chain alignment.

Rob

[igauk]

Should the chain not go the other side of the tab? Looking at the first picture you posted it looks like chain is running out the back of the cage.

[MikeDee]

Mick F, I think this is the first anyone has complained about this derailleur feature. I think all my Shimano derailleurs going years back have this tab. I have screwed up a few times by routing my chain over rather than under this tab, but it's immediately obvious from the noise of the chain rubbing on the tab. Like other posters have stated, I think your chain length is too long.

[andrew_s]

Have the jockey wheels ever been changed?
If the upper is larger than original, it could be a cause.

[meic]

Same size pulleys top and bottom on that particular model derailleur.

[keyboardmonkey]

Should the chain not go the other side of the tab? Looking at the first picture you posted it looks like chain is running out the back of the cage.

There may some other problem, but the chain is on the correct side of the tab.

[Brucey]

IIRC Mick is running something that by rights ought to have an SGS mech making it go, but such a mech may be too long to fit onto a moulton. The result is that the chain runs slack on small-small.

I have rarely seen a mech of that sort where there are no chain marks at all on that tab, but then again I have not heard one that makes contact all the time (except when routed wrongly, which it isn't) either.

The tab is there to control the chain when the going gets rough, I think.

cheers

[Mick F]

IIRC Mick is running something that by rights ought to have an SGS mech making it go, but such a mech may be too long to fit onto a moulton. The result is that the chain runs slack on small-small.

I have rarely seen a mech of that sort where there are no chain marks at all on that tab, but then again I have not heard one that makes contact all the time (except when routed wrongly, which it isn't) either.

The tab is there to control the chain when the going gets rough, I think.

cheers

All spot on there Brucey .................. but not quite right.
The chain "vibrates" but only on higher cadences.
The chain does not touch the tab as it goes through, but "vibrates" as it "resonates" and touches as it goes through when it goes through quickly.

I've taken 0.5mm off the tab by file, but checking again and really spinning the cranks, the tab still gets touched a little bit. It's not an issue now I reckon as now I know what is causing the the sound, it concerns me not so much. This morning, I was concerned, and now I've sorted to a major degree and now it still makes a slight noise and I know what's causing it, it's fine. I could file another 0.25mm off, and that would silence the thing for good.

Not sure if it's anything to do with the long cage/medium cage issue, but more to do with the Campag/Shimano issue as it's there when the cage is straight and isn't on an extended angle.