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XT M772 rear mech cage tab

clydefrogclydefrog Posts: 162
edited February 2016 in MTB workshop & tech
Anybody here who has this derailleur, do you have the model where there is a tab on the rear side of the cage as in this picture (not my pic, it's from a thread on another forum where somebody a rubbing issue with this tab)?

derailleur_grind.jpg

I've not installed it yet but just put the chain on my chain whip through it tensioned and it touches the tab, I can't understand this at all, do any of you have issues with it? Is the cage length going to be a factor on whether my chain rubs on it or not? I'm guessing not as the chain is still going to be at the same angle and parallel to the tab, just there'll be a slightly longer section of it. I'm not going to fit it yet as I can't return it if I do that so I want to know if it is fine before fitting. I've not actually read one review that mentioned this as an issue and there are only about 2 threads anywhere on the internet where people mention it, but from just trying that with the chain through it seems like it's going to be one.


Another thing I noticed is the guide pulley (the upper one) doesn't move as freely as the lower pulley and you can hear and feel resistance as you turn it, is that normal (it is a bushing rather than a bearing like the lower pulley)?

Posts

  • nicklousenicklouse Posts: 81,520 Lives Here
    the problem most people have is routing the chain on the wrong side of the tab.

    nothing to do with the mech just the fitter.

    yes bushes tend to have more resistance than bearings.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
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  • The RookieThe Rookie Posts: 27,747
    edited February 2016
    All the rear mechs I have had have had that tab and they don't get in the way if the chain is routed correctly, the one in the photo is, as is obvious, not routed correctly somewhere.

    Easy enough to remove the jockey wheels, clean the bushing, grease and refit.
  • No, that is the correct way. It's impossible to route it under the tab as the chain doesn't fit though the gap on the medium cage model, I've tried it. And if it did fit through it would rub even worse surely. If you're right in what you say then mine must be a manufacturing defect as it is impossible to route the chain any other way, and I doubt that as for a start the tab couldn't be any further up the cage as it is on the edge of the straight part before the angle changes the same as in the picture.
  • oodboooodboo Posts: 2,177
    I made the same mistake fitting the same rear mech. Wondered what all the noise was. Didn't take long to realise and re-route the chain. It's an easy mistake to make though.
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  • nicklousenicklouse Posts: 81,520 Lives Here
    clydefrog wrote:
    No, that is the correct way. It's impossible to route it under the tab as the chain doesn't fit though the gap on the medium cage model, I've tried it. And if it did fit through it would rub even worse surely. If you're right in what you say then mine must be a manufacturing defect as it is impossible to route the chain any other way, and I doubt that as for a start the tab couldn't be any further up the cage as it is on the edge of the straight part before the angle changes the same as in the picture.
    and as normal without seeing what you actually have we can only go on what people have done before.

    it may be faulty or in may be fitter error.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • It is identical to this photo:

    821749d1375564173-barrel-adjuster-xt-m772-shadow-9-speed-rear-mech-gs-img_0618_xt-m772-shadow-9-speed-rear-mech-gs_-circled-snipped-mtbr.jpg

    You can see the tab is right next to the lower jockey wheel and it is impossible to run the chain under that tab.
  • The RookieThe Rookie Posts: 27,747
    edited February 2016
    OK, to put it in perspective, either Shimano have made 1000's of useless mechs and no-one is yelling about it but a few or you've got something wrong....I now where I'm putting my money.

    Photo's of the condition, not a random library photo that shows nothing.
  • nicklousenicklouse Posts: 81,520 Lives Here
    still does not show YOUR problem.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • The Rookie wrote:
    OK, to put it in perspective, either Shimano have made 1000's of useless mechs and no-one is yelling about it but a few or you've got something wrong....I now where I'm putting my money.

    Photo's of the condition, not a random library photo that shows nothing.

    Oh and in that photo it is routed differently to every other Shimano mech I have had that never caused an issue, it's wrong.

    N1B5UyG.jpg

    rOvdJFS.jpg

    There you go, it doesn't fit under the tab. And it's the GS so Shimano long. By the way if you google images of this derailleur you will see pretty much every single one of them that shows it on a bike has the chain routed over the tab and not under it. For your derailleurs it may be the correct way to route it for it to go under the tab but not for this one, unless like you say Shimano have made thousands of them with a design fault...

    https://www.google.co.uk/search?q=xt+m7 ... AQ#imgrc=_
  • The RookieThe Rookie Posts: 27,747
    Sorry, brain fart (old age) on my part, it goes the side shown in the top photo, but I've still never had one rub suggesting another issue.
  • Yeah np, well it's not definitely going to happen but when putting a chain through the cage it really does seem like if it's a bit slack it's just going to drag on that tab :?

    I went with the GS for a 3x9 set up because I know it will be fine if I don't cross-chain especially with the smaller gears where there is more chain slack, but apart from that the shorter cage isn't going to give me slack in "normal" gear combinations is it? My chain is properly measured (big cog and big chainwheel plus 2 links). The bike actually came with a medium cage SRAM X5 on it too which was never an issue so I decided to stick with that size cage (Shimano GS is just a few mm longer than a SRAM medium, funnily enough though Shimano state their GS chain capacity as being less than SRAMs medium - 33T vs 37T).

    i just can't see why they'd choose to put that there rather than on the other side where it's out of the way, I'm sure there's a reason for it, probably to do with strength, but it's strange.
  • The RookieThe Rookie Posts: 27,747
    A few mm is all you need! A miss is as good as a mile, so when you said it touches the tab, you meant 'it's a bit close'?
  • The Rookie wrote:
    A few mm is all you need! A miss is as good as a mile, so when you said it touches the tab, you meant 'it's a bit close'?

    No if there isn't a lot of tension on the chain it literally touches it.
  • The RookieThe Rookie Posts: 27,747
    But the mech is there to tension it...all mine are close, never had an issue, and I've run that exact mech before going to a short cage XTR (which actually runs it even closer) I cleaned up my XT mech before selling it and I can't say I spotted any marking on the tab from chain contact, in fact overall it looked like new. Over any rough road the weight of the chain acting between the front and rear will pull that section even tighter.

    As I said above I'm sure Shimano know how there mechs work better than you or I!
  • Haha yeah, like I said this has never been mentioned in reviews afaik and I've only seen it brought up on about 2 forums, maybe their chains were too long? I don't know.

    So then me having a shorter cage than is recommended for my setup shouldn't cause any issue with the chain running slack inside the cage and rubbing it either? I know the recommendations are quite precautionary and plenty of people run medium size cages on triple chainsets but I guess they won't usually have to take into account a little bit of metal sticking out in their cage.
  • cooldadcooldad Posts: 32,904
    Go back to the first picture, which is correct, and your's and it looks like you're putting the chain on the wrong side of the tab.
    I don't do smileys.

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  • nicklousenicklouse Posts: 81,520 Lives Here
    clydefrog wrote:
    Haha yeah, like I said this has never been mentioned in reviews afaik and I've only seen it brought up on about 2 forums, maybe their chains were too long? I don't know.

    So then me having a shorter cage than is recommended for my setup shouldn't cause any issue with the chain running slack inside the cage and rubbing it either? I know the recommendations are quite precautionary and plenty of people run medium size cages on triple chainsets but I guess they won't usually have to take into account a little bit of metal sticking out in their cage.
    If the cage is too short you will either have a slack chain in some gears or you could rip the mech of in other gears.

    And the images of you with a small length of chain and your mech bring up the issue of the person doing the work.

    Looks like when it is fitted correctly you will have no issues.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • I was doing it the correct way (the same way that first photo shows) and got told I wasn't by people in this thread so I posted those two photos of me trying to push it under the tab (the wrong way) because The Rookie was adamant it was meant to go under it and not over it, and I was showing him that it isn't even possible to put it under it. I know it is meant to go above the tab as in the first photo, but people said that was the wrong way to do it and wanted photo evidence off me to prove otherwise, which I provided. I even posted under those photos that what I'm doing in them is obviously not the correct way to route it.

    For example (re: that photo I linked in the first post):
    The Rookie wrote:
    All the rear mechs I have had have had that tab and they don't get in the way if the chain is routed correctly, the one in the photo is, as is obvious, not routed correctly somewhere.
    The Rookie wrote:
    Oh and in that photo it is routed differently to every other Shimano mech I have had that never caused an issue, it's wrong.

    He did eventually realise he was wrong about it though and that the chain should run over the tab and not under it.


    I love this, I make the thread showing how it's correctly routed but saying it runs very close to that tab, then get told that's the wrong way and have to post photos to prove that it can't be done any other way, then people are still telling me I'm doing it wrong because it seems like they just looked at the pictures and didn't read. lol

    @ nicklouse: Hopefully ripping the mech off is impossible because my chain is sized correctly.
  • cooldadcooldad Posts: 32,904
    I can't decide if this is the most confusing or most pointless thread ever?
    I don't do smileys.

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  • Confusing, a bit pointless because I'm probably worrying about nothing but yes if you don't read it properly and think those 2 photos I posted further down the last page are me thinking that is the correct way to do it rather than proving it's the wrong way, then yes it will be confusing.
  • cooldadcooldad Posts: 32,904
    Just fit the blasted thing. Millions of people have fitted millions of Shimano mechs without having the issues you are imagining.
    I don't do smileys.

    There is no secret ingredient - Kung Fu Panda

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  • Just to add regarding my question about the guide pulley having more resistance than the tension pulley, it's apparently completely normal that it is like this on some Shimano mechs using a bushing for it, and it is actually caused by the rubber seals. Thought it would be helpful to add in case anybody finds this thread from googling for that non-issue.
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