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Old 08-02-05, 02:22 AM   #1
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Upper pulley rubbing against largest sprocket

The derailer is a Shimano XT M750 SGS, which is supposed to handle a 34T sprocket, which is what my largest one is. But still, the pulley and the sprocket rub against each other. The tips of the teeth just touch and make a rattling noise.

I always thought the angle adjustment screw was supposed to take care of this problem, but the only thing that screw does is changing the limit for the entire derailer arm's swing. It does absolutely nothing to increase or decrease the distance between pulley and sprocket.

This isn't a major problem, because the worst thing that will happen is my upper pulley wearing out faster (since it's plastic), but the noise is annoying, so I'd like it fixed, if possible.

Any suggestions?
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Old 08-02-05, 02:58 AM   #2
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I should add that shifting is smooth and quick at all times, so all other settings are fine.
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Old 08-02-05, 03:11 AM   #3
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On Campa derailleurs there is a screw at the top of the cage that is used to adjust this distance. I assume that there is also one on a Shimano der. It should be in the instructions, which I know you read.
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Old 08-02-05, 03:38 AM   #4
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As I said, that screw does nothing like that. It only rotates a limiting piece of metal that a protrusion on the swing-arm body hits against when the derailer arm is stretched out. The pulley-sprocket distance is unchanged throughout that screw's full adjustment range.
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Old 08-02-05, 03:42 AM   #5
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Another thing...
It didn't rub on my previous bike. I first bought both the chain and cassette for use on my first bike. Replaced some Acera-level components on that one.
Recently, I bought a frame and a lot of other stuff to build up a new bike from scratch, and now this happens.

The only thing I can think of is that the derailer hanger is too short. But, obviously, that's not something I can change...
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Old 08-02-05, 04:18 AM   #6
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Well, it looks a lot like a Campa to me. Three screws - two limits (#s 4) and a cage distance (#3) unless I don't understand what I am looking at.

http://www.paul-lange.de/ftp/SHIMANO...sz/ST-M750.pdf
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Old 08-02-05, 04:32 AM   #7
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You need to adjust the angle adjustment screw (I think it is called the B-limit screw)

it is not a good idea to have the pulley running on the cogs as I broke a chain this way once when the shift went wrong and it just pulled the chain apart.
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Old 08-02-05, 04:35 AM   #8
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Forgive my presumptions, but are you sure you know what screw is which? There are two limit screws, and a b-screw which should be tightened to give you more clearance.

Shimano derailleurs are pretty straight-forward with regards to adjustments.

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Old 08-02-05, 06:27 AM   #9
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Yes, I know which one is which. There are only three...

And again, that screw does NOT do anything. There's nothing wrong with it. It screws in and out as it should, it's just that it's pretty obvious that there's nothing for it to do, which you can see for yourself if you pivot the arm manually.

Screwing it all the way in and all the way out produces absolutely no change in clearance.

The only thing I can think of, other than the hanger being too short, is that the chain is longer than the derailer can handle. Of course, with proper construction of the mechanism, that wouldn't be an issue, but maybe Shimano's RD guys only work with the simplest, cheapest solutions, and not the best...

And shortening the chain is not an option, as it's the correct length.

(BTW, that link leads to a PDF with an exploded view of a Shimano shift lever... )
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Old 08-02-05, 07:10 AM   #10
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If the screw isn't doing anything, then there may be some compatability issues with the derailleur hanger you've got it on. The b-limit screw's end should reach down and touch the hanger, thus when you screw it in, pushing the derailleur back away from the hub/hanger. If you can get the bike in a position where you can see what is limiting the forward swing of the derailleur ie: what's keeping the thing from being pulled towards the crank by the return chain line - then you might find your answer. I recently installed a new Shimano 105 on my road bike and had to play with this screw quite a bit to take out the slack in the new chain and get the proper distance from the freewheel on the largest rings. Hope this helps.
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Old 08-02-05, 07:16 AM   #11
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Quote:
Originally Posted by CdCf
Yes, I know which one is which. There are only three...

And again, that screw does NOT do anything. There's nothing wrong with it. It screws in and out as it should, it's just that it's pretty obvious that there's nothing for it to do, which you can see for yourself if you pivot the arm manually.

Screwing it all the way in and all the way out produces absolutely no change in clearance.

The only thing I can think of, other than the hanger being too short, is that the chain is longer than the derailer can handle. Of course, with proper construction of the mechanism, that wouldn't be an issue, but maybe Shimano's RD guys only work with the simplest, cheapest solutions, and not the best...

And shortening the chain is not an option, as it's the correct length.

(BTW, that link leads to a PDF with an exploded view of a Shimano shift lever... )
Is this the same frame thet had the FD/BB spindle length issue? If so, maybe it's time to chuck it in the dumpster and walk away or reexamine what you are doing. I've yet t run into all these 'problems'.
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Old 08-02-05, 08:32 AM   #12
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Some hanger/derailleur combo's just do not allow enough adjustment for the B screw to work with the 34 T Cassette. The hangers on the 3D link Rockys have this problem. As a dealer it drove me crazy. I resolved the issue by using a longer screw, bent slightly to engage the stop point on the hanger cleanly. Another possibility is your chain is too long. Put it into the Big/Big combo and see if it is possible to remove one full link. Sometimes removing a full link will create more tension on the derailleur and move the pulley away from the cog.

And as someone else pointed out, this rubbing you are getting is potentially hazardous to your drive train health. I would address it sooner than later.
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Old 08-02-05, 09:38 AM   #13
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Quote:
Originally Posted by CdCf
(BTW, that link leads to a PDF with an exploded view of a Shimano shift lever... )
Picky, picky ...


http://www.paul-lange.de/ftp/SHIMAN...nsz/RD-M750.pdf
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Old 08-02-05, 09:48 AM   #14
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Some hanger/derailleur combo's just do not allow enough adjustment for the B screw to work with the 34 T Cassette. The hangers on the 3D link Rockys have this problem. As a dealer it drove me crazy. I resolved the issue by using a longer screw, bent slightly to engage the stop point on the hanger cleanly. Another possibility is your chain is too long. Put it into the Big/Big combo and see if it is possible to remove one full link. Sometimes removing a full link will create more tension on the derailleur and move the pulley away from the cog.

And as someone else pointed out, this rubbing you are getting is potentially hazardous to your drive train health. I would address it sooner than later.
No, it's not hazardous, and there's nothing I can do about it, it seems, unless the trick with a longer screw works. I'll try that later.

Otherwise, I'll just have to live with it...
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Old 08-02-05, 09:49 AM   #15
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Another possibility is your chain is too long. Put it into the Big/Big combo and see if it is possible to remove one full link. Sometimes removing a full link will create more tension on the derailleur and move the pulley away from the cog.
The chain is exactly the minimum length needed for my setup. 116 links.
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Old 08-02-05, 09:49 AM   #16
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And shortening the chain is not an option, as it's the correct length.
I looked up the largest rear cog limit for the SGS derailleur and it is 34T. Your derailleur should be able to handle it. As to your chain being the correct length, well, it may be. But checking the big/big combo would still be a good idea. I have found that the arbitrary rules governing chain length do not take into account chain stay length. With all the new Dual suspension rides out there, finding the correct length is often a hunt and peck situation. Using the Shimano method of chain sizing will get you in the ball park, but oftentimes not nail it cleanly.
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Old 08-02-05, 09:52 AM   #17
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The chain IS the correct length. I had to add one link (1.5") to make it wrap around the big/big combo. That's when I ended up with the 116 links. Which also happens to be exactly what the formula at Park Tool's site told me.
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Old 08-02-05, 09:58 AM   #18
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The chain is exactly the minimum length needed for my setup. 116 links.
Hmm. Until I know what frame you are riding, I would have to assume your 116 links is way too long. That 116 indicates to me that what you are using is a Shimano chain, as they are sold in lengths of 116 links. I have never used the full length of a chain on any bike except pn a tandem or some oddball cruiser. As a matter of fact almost all other chain mfg sell their chains in 114 link lengths. And even with them I remove several full links when installing. Chains are purposefully sold long so that the right length can be acheived through removal of a few. When your chain is in the granny up front and the smallest cog in the rear, does it droop dramatically? Most chain lengths on multisped bikes end up around 110 links or so.

Edit note - I see from your signature it is a Surly "Long Haul". I stand corrected. That bike has a very long chain stay length. It is definitely not inside the norm for other multi speed bikes. Sorry for that.

I would still check the BIg/Big combo. You will know that you can remove a full link if when you push up on the lower cage, the chain droops a tad. This bike definitely falls outside the parameters when rules of thumb are the absolute.

I would suggest that you contact Surly Bikes . They are a bunch of great guys and can possibly suggest some answers.
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Old 08-02-05, 10:04 AM   #19
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The last time I had trouble with my upper pulley hitting the large cog, adjusting the b-screw didn't seem to do anything until I got my cable tension sorted out. Maybe that's something you can look at?
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Old 08-02-05, 10:08 AM   #20
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Also, I thought the large/large chain length formula was seeing where they could be joined and then adding two links?

http://www.parktool.com/repair_help/...inlength.shtml
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Old 08-02-05, 10:18 AM   #21
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Also, I thought the large/large chain length formula was seeing where they could be joined and then adding two links?

http://www.parktool.com/repair_help/...inlength.shtml
Yes, that's what we used to call the Shimano method. It works fine in most cases. But with the newer bikes and extreme gear combos that are available now, it is only a starting point sometimes. I always start with it and then trim the chain down if needed. What I am referring to is after this method has been used. The ideal IMO, is to make the chain as short as possible and still shift into the Big/Big combo.
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Old 08-02-05, 11:09 AM   #22
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Yes, that's what we used to call the Shimano method...
Ahh - thanks for the clarification.
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Old 08-02-05, 03:19 PM   #23
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And again, that screw does NOT do anything. There's nothing wrong with it. It screws in and out as it should, it's just that it's pretty obvious that there's nothing for it to do
You can remove the screw and enjoy the reduction in weight.
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Old 08-02-05, 03:23 PM   #24
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I use a SRAM PC-950 chain. They're 114 links out of the box.

I put that on and (pedalling slowly by hand) shifted into the large/large combo.
That was a bit too strained for my comfort, and I never fully engaged the chain on the chainring. So, I got out a piece of the same model chain that I took off from my old bike, which only had 108 or 110 links, and used a second "quick link" to get it on there. The chain is now 116 links total, and can safely shift into large/large.

I know I've exceeded the stated range of the rear derailer, so I can't shift into the 11-13-15 sprockets on the cassette without getting a slack chain if I'm in the smallest front chainring. Not a problem, though, as those combos aren't necessary.

Cable tension is good. Shifting is nice and smooth throughout.

Anyway, in reply to sydney, I don't think I've had any real problems.

I ordered the wrong BB length. My mistake. Couldn't find the info beforehand. Nothing wrong with the hardware.

Here, I know I've exceeded the recommended range for the gearing, which is something I did because I wanted a specific gear range. It was a gamble, and except for this minor issue, the gamble paid off.
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Old 08-02-05, 03:27 PM   #25
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You can remove the screw and enjoy the reduction in weight.
Now that I think about it, I only tried it when the chain was on the 34T sprocket. If the pulley is all the way up against it, the screw wouldn't appear to be doing anything. But I suppose it will seem to work in other gears.
A longer screw will probably help, but that remains to be seen. Tomorrow...
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