Rear derailleur makes chain rub against sprockets

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raymondliu
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Rear derailleur makes chain rub against sprockets

Postby raymondliu » Thu Jan 02, 2014 10:17 am

Hi all I am building up a bike myself for the first time and finding myself lost in terms of how to set up the rear derailleur. I am running full Ultegra group with 53/39 chainset and 11-28 cassette with a short cage derailleur. In addition to the fact that the top jockey wheel rubs against the sprockets on every sprocket, I am also unable to get the chain onto the smallest two sprockets and also the chain goes completely slack if I put it on the small chainring. I have tried adjusting the H and L limit screws and also the B screw, and even followed tips on other forums of inserting the B screw backwards to give the rear derailleur more tension.

2 photos:
http://www.flickr.com/photos/rliu/11705337443/lightbox/
http://www.flickr.com/photos/rliu/11705117155/lightbox/

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Me-109
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Re: Rear derailleur makes chain rub against sprockets

Postby Me-109 » Thu Jan 02, 2014 10:59 am

Sounds like a combination of B-screw and chain length issues. Can't see the pics without signing in - maybe you have them in a private folder?

raymondliu
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Re: Rear derailleur makes chain rub against sprockets

Postby raymondliu » Thu Jan 02, 2014 11:25 am

Thanks for reply, I have re-linked the photos: http://www.flickr.com/photos/rliu/sets/ ... 306453493/
I had used a chain length calculator which said 55 links and that's what I have at the moment. I did have it at 56 initially. Removing the extra link did not make a difference.

DesWeller
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Re: Rear derailleur makes chain rub against sprockets

Postby DesWeller » Thu Jan 02, 2014 11:39 am

Your chain is way too long.

Use this guide to set your chain length.

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Me-109
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Re: Rear derailleur makes chain rub against sprockets

Postby Me-109 » Thu Jan 02, 2014 11:44 am

I count links individually rather than paired, but have 106 (I think) using a compact chainset, so you are probably not far off the mark. You could maybe take a pair out looking at the first photo, but upside down it it hard to judge whether it is hanging vertically. It's definitely not taking up the slack though, so if it is not just the tension screw, then I'd consider it to be a broken spring in the mech. Does it work ok through the gears? Does the Ultegra mech have an adjustable return spring that can be tensioned by repositioning one end?

raymondliu
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Re: Rear derailleur makes chain rub against sprockets

Postby raymondliu » Thu Jan 02, 2014 11:49 am

Thanks for replies. I am starting to suspect a broken derailleur but it was brand new and does shift through the sprockets. The cage does also extend if I pull on it vertically.
The slack in the 2nd photo is from the fact the rear derailleur does not extend itself when I shift onto the small ring which should happen if working properly to take up the slack right?

keef66
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Re: Rear derailleur makes chain rub against sprockets

Postby keef66 » Thu Jan 02, 2014 12:19 pm

Looks like the chain is far too long to me. You should be able to use the small chainring / smallest sprocket combo with reasonable tension in the chain.

I use the following:

Route the chain round the big chainring and biggest sprocket but not through the rear derailleur, then add two whole links (2 inners and 2 outers). Cut the chain at this point, then thread through the rear mech before joining.

raymondliu
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Re: Rear derailleur makes chain rub against sprockets

Postby raymondliu » Thu Jan 02, 2014 12:31 pm

keef66 wrote:Looks like the chain is far too long to me. You should be able to use the small chainring / smallest sprocket combo with reasonable tension in the chain.

I use the following:

Route the chain round the big chainring and biggest sprocket but not through the rear derailleur, then add two whole links (2 inners and 2 outers). Cut the chain at this point, then thread through the rear mech before joining.


The main issue is that when you're in the small ring the rear derailleur should extend itself to take up the slack, as you can see there isn't that much slack on the big ring. I don't think going to less than 55 links is a wise thing to do when big-big is 53/28.

shmooster
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Re: Rear derailleur makes chain rub against sprockets

Postby shmooster » Thu Jan 02, 2014 12:37 pm

The Derailleur is sitting too far forward, is it pivoting OK where it's fixed onto the frame? Have a look at this Shimano doc on adjusting the B screw, but it's the Derailleur placement rather than the chain length you need to fix first.

http://techdocs.shimano.com/media/techd ... 702204.PDF

shmooster
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Re: Rear derailleur makes chain rub against sprockets

Postby shmooster » Thu Jan 02, 2014 12:51 pm

Looking at an old Derailleur, my guess is you have the B-screw on the wrong side of the dropout tab (in front instead of behind), which has pushed it right forward preventing it from taking up the tension and causing it to hit the cassette sprockets. Have a look at this video showing how to fit a derailleur

http://www.madegood.org/bikes/repair/fi ... erailleur/

keef66
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Re: Rear derailleur makes chain rub against sprockets

Postby keef66 » Thu Jan 02, 2014 12:58 pm

Good point, I hadn't considered that the mech could be mounted wrongly. I've always found the B screw fouls on some part of the hanger if it's not properly aligned, so it's obvious how it should sit.

raymondliu
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Re: Rear derailleur makes chain rub against sprockets

Postby raymondliu » Thu Jan 02, 2014 13:36 pm

Just moved the rear derailleur round, just wondered if this looks more like it: http://www.flickr.com/photos/rliu/11707 ... 9306453493
(Will obviously have to adjust the gear cable length as well)

keef66
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Re: Rear derailleur makes chain rub against sprockets

Postby keef66 » Thu Jan 02, 2014 13:49 pm

Can't see it. Yahoo is wanting me to log in??


shmooster
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Re: Rear derailleur makes chain rub against sprockets

Postby shmooster » Thu Jan 02, 2014 14:04 pm

raymondliu wrote:Just moved the rear derailleur round, just wondered if this looks more like it: http://www.flickr.com/photos/rliu/11707 ... 9306453493
(Will obviously have to adjust the gear cable length as well)


Looks better, could maybe go further but I'm not an expert :-) Mines about 30 degrees more than that with the chain off. Has it sorted the chain tension and the shifting?

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Me-109
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Re: Rear derailleur makes chain rub against sprockets

Postby Me-109 » Thu Jan 02, 2014 14:14 pm

Looks ok. Sounds like shmooster's description may have been right if you have removed and refitted. Mech body should be able to be rotated backwards away from the stop but not forwards. If you have fitted it correctly, try pulling the bottom of the mech cage forwards and down - the mech body should stay in position and you will feel the spring tension in the cage by way of some resistance. You did remember to refit the B-screw the right way round, didn't you (head to the outside, threaded down through the mech so the threaded end is touching against the hanger)?

keef66
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Re: Rear derailleur makes chain rub against sprockets

Postby keef66 » Thu Jan 02, 2014 15:02 pm

Looks normal now. Is the end of the B screw sitting against the step in the mech hanger? Next need to see it with the chain on.

raymondliu
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Re: Rear derailleur makes chain rub against sprockets

Postby raymondliu » Fri Jan 03, 2014 11:10 am

Thanks all for looking over this for me, works much better now. Shame that I screwed B screw so tight I can't loosen it again so I can't get on the biggest cog.

keef66
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Re: Rear derailleur makes chain rub against sprockets

Postby keef66 » Fri Jan 03, 2014 11:30 am

raymondliu wrote:Thanks all for looking over this for me, works much better now. Shame that I screwed B screw so tight I can't loosen it again so I can't get on the biggest cog.


If the B screw is screwed all the way in that should be giving you maximum clearance, no? Or have you got it back to front?

Semantik
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Re: Rear derailleur makes chain rub against sprockets

Postby Semantik » Sun Jan 05, 2014 00:29 am

The OP is clueless and would be advised to take it to a bike shop.
He's asked the same question here
http://www.cyclechat.net/threads/rear-d ... ts.146587/
and still not got it fixed by the sounds of it.


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