What's This Tab For?

[old_windbag]

In a similar vein I have a deda dog fang on my audax( front mech ) with campy triple setup to avoid unshipping onto BB. This happened several times when doing a double shift( front/rear together very unprofessional! ) and lots of slack and twanging around. The dog fang works brilliantly. I don't like long cage rear mechs much.

[meic]

I don't like long cage rear mechs much.

A necessary evil, in my case. Though I think the 772 shadows which I bought were taking it too far.
I will not be using a 36 rear sprocket cassette and would have been better off with the shorter SGS cages on non-shadow MTB derailleurs.

[Brucey]

eg you change from a large to a small scrocket and for a moment get lots of loose chain flopping around?

or any one of a hundred other happenstances that might never occur during average riding on smooth roads, but are not unusual otherwise.

BTW I have often wondered if the chain span between the pulleys can be made to resonate (as I think MIck suspects might be happening) if the centre to centre distance of the pulleys is set in a particular way. I think that if the centre distance is an exact multiple of 1/4" but not 1/2" then the whole chain span flaps from side to side at one time (antinode in the span centre), and if it is an exact multiple of 1/2" then the chain span flaps cross-wise (node in the span centre) or something. If this resonance gets into phase with others (e.g. arising from the chordal action of the chainrings and/or sprockets) then may be it can become exaggerated.

cheers

[Mick F]

eg you change from a large to a small scrocket and for a moment get lots of loose chain flopping around?

or any one of a hundred other happenstances that might never occur during average riding on smooth roads, but are not unusual otherwise.

BTW I have often wondered if the chain span between the pulleys can be made to resonate (as I think MIck suspects might be happening) if the centre to centre distance of the pulleys is set in a particular way. I think that if the centre distance is an exact multiple of 1/4" but not 1/2" then the whole chain span flaps from side to side at one time (antinode in the span centre), and if it is an exact multiple of 1/2" then the chain span flaps cross-wise (node in the span centre) or something. If this resonance gets into phase with others (e.g. arising from the chordal action of the chainrings and/or sprockets) then may be it can become exaggerated.

cheers

Sounds right to me.
Any road up, it's sorted now as the tab is far enough away not to catch normally. No doubt it would catch if it needs to, but I don't expect it ever to be needed on the road.

My Campag Comp is long cage and longer than the XT, yet doesn't have a centre tab.

As for the loose chain in small/small, I have an idea .....................
The SA has chunky non-turn washers and the lugs are forward. If I were to reverse them, the wheel would be a quarter of an inch(?) nearer to the chainwheel. It could be possible, that if I were to reverse them, and then remove a pair of links from the chain, I may be still be able to achieve big/big, but not so loose in small/small.

I'll experiment later.

[Brucey]

it won't have escaped your notice that there is an adjustment screw where the B tension screw is on other shimano rear mechs; this adjusts the clearance onto the sprockets but it also adjusts the effective chain length slightly (by an amount that varies with the chain wrap). It might be that this screw offers a means of optimising the chain length adjustment.

BTW the pulley centres are about 79mm on that mech, aren't they? If so that length isn't an exact multiple of 1/4", and maybe that exact thing isn't causing a bad resonance in the chain span between them.

cheers

[Mick F]

I've adjusted the B screw, but the best place for it is fully out.
It would be in, if the cassette were taller, and as mine is only an 11-28, the B screw isn't needed.
Same as on the Tiagra. I took it right out and disposed of it.
I could screw it fully in, but it doesn't make much difference to chain-lenght.

I'll be having a tinker shortly, and one thing I'll do is play again with the B screw.

Checking the distance between the largest sprocket and the guide pulley.
Set the rear derailleur to the lowest gear position, stop the wheel from turning, and then check that the distance from the edge of the guide pulley to the edge of the largest sprocket is within the range of 5 - 6 mm. Turn the crank arm to shift gears and check that there is no roughness in the feel. If the number of teeth for the cassette sprocket is changed, carry out this setting again.

With the B screw FULLY out, the guide pulley is more like 15mm away. Screwing it in, moves it further away.

[Brucey]

...With the B screw FULLY out, the guide pulley is more like 15mm away. Screwing it in, moves it further away.

I guess that is kind of inevitable given that you are using a 28T sprocket instead of a larger one that is meant to go with that RD. However provided the clearance onto the small sprockets or the chainstay etc isn't too bad, it may be possible to file a little off the hanger adaptor (where the screw bears against it) to give you a little more useful adjustment. Take it too far and parts of the mech will start hitting something though.

cheers

[Mick F]

My hunch was correct.
Reversing the non-turn washers has indeed moved the wheel forward enough to allow the chain to be reduced from 118links to 116links.
This makes the chain on small/small less loose and at the same time allow for a big/big combination ............... just.
Also, by doing this, the B adjustment gets the guide pulley about 6mm from the big sprocket ie within spec.

I can wind in the B screw so the chain isn't loose at all in small/small, but that takes the guide pulley back to 15mm from the big sprocket.
What I've done, is get the B screw half way between the two.
This gets the guide pulley about 9mm from the big sprocket and the chain only slightly loose in small/small.
Maybe a compromise.

As for the tab catching on the chain, it doesn't at all statically or when in gentle cadence. It's only when the chain speed increases to quite high, that it tinkles. Therefore the chain must "waggle" or "resonate". The distance statically between the tab and the chain is 2mm, so the chain must resonate 2mm at least. Remember. I've taken 0.5mm off it, but it's still catching, but not very much now, and ignorable perhaps.

[roger]

Do Shimano ( or others when applicable) read these forums?

Roger.

[Brucey]

the funny thing is that this is a popular style of mech (esp in SGS form) and folk don't mention the noise, despite the fact that the clearance to the tab is about the same in all models, and flapping ought perhaps to be worse in an SGS mech than in a GS mech (like Mick's).

Examination of used mechs of this type shows clear signs of occasional contact, not persistent contact. So I'd have to say that I think that 'something is different' about Mick's setup.

It may be that the chain speed through the mech is higher than normal (those big chainrings on the Moulton) but there could be another cause; sticky chain lube may work a bit like having a load of stiff links, and IIRC Mick has a KMC QL in the chain, which might be a tight fit?

Anyway to double-check for stiff links , hold the rear mech forwards so that the chain isn't tensioned fully, and turn the cranks backwards; any significantly stiff links will appear in the lower run as it emerges from the rear mech. Stiff links will of course clout the mystery tab as they pass through the mech in the normal way (pedalling forwards).

cheers

[meic]

The tab on my 772 SGS is much closer to the chain than on any of my other derailleurs. The tab always gets a couple of oily black "tram lines" deposited on it by the sideplates, within a reasonably short riding mileage. There is also slight grooving on the tab showing there is frequent enough contact for a tiny bit of wear to have occurred.

[Mick F]

Thanks Brucey.
Can't do or test anything at the mo as we're not home but on holiday abroad. However when I can check this, I will.

Sent on our iPad courtessy of the hotel wifi.

[crazydave789]

as I understand it (and my xt mech is set up that way) the chain is supposed to stay within the cage shape so inside the tab not outside.

I threaded mine that way until I noticed it rattled and didn't index properly so I put it the right way, much better.

[Mick F]

Putting the chain inside the tab is completely wrong. It goes outside.

[Brucey]

it does depend what you mean by 'inside' and 'outside' though....

cheers