Derailleur hanger, B-screw, chain tension...?

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esuhl
Posts: 114
Joined: 22 Mar 2017, 3:20am

Derailleur hanger, B-screw, chain tension...?

Postby esuhl » 17 Apr 2019, 9:58pm

Today I used an "alignment tool" to make a small adjustment to the rear hanger.

I got the shifting *perfect*... except when using the smaller gears. I shifted onto the very smallest pair, and saw the chain was slack, resting on the chainstay.

I tried adjusting the B-screw on the rear mech. I had to screw it almost fully in to get rid of the slack, but then the gears didn't shift as well. I noticed that the B-screw had worn away part of the tab on the hanger it was supposed to press against.

I replaced the hanger with one that was slightly bent, but had an intact tab, and the B-screw still only made partial contact with it, and still needed to be almost fully screwed-in to get rid of chain slack.

So... I'm not sure what to do now... :?

1. Could my chain be too long? I replaced it years ago, but made sure to use the same length chain as the bike was sold with (by removing links). And I haven't noticed any problems before today... :-/

2. Should I put some sort of nut on the tip of the B-screw to give better contact with the hanger?

Has anyone come across anything like this before...? Help! :oops:

Brucey
Posts: 35617
Joined: 4 Jan 2012, 6:25pm

Re: Derailleur hanger, B-screw, chain tension...?

Postby Brucey » 17 Apr 2019, 10:32pm

your chain might well be too long. If you do shorten it, be sure that you have enough length to access big-big safely. Your chain might be too long now because either it is itself worn, or the one that was 'the same length' before was worn. Or it might never have been the right length to start with. There is also the possibility that there is another fault with the RD.

If the top run of the chain is slack when freewheeling only, this suggest that the freewheel is draggy. If you have a Mavic hub this is a common fault.

FWIW I wouldn't worry too much if the chain is a bit slack in small-small; you shouldn't use this gear anyway.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

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CJ
Posts: 2995
Joined: 15 Jan 2007, 9:55pm

Re: Derailleur hanger, B-screw, chain tension...?

Postby CJ » 18 Apr 2019, 7:38pm

The B-screw is not for tensioning the chain, it is for adjusting the clearance between the mech's guide (top) pulley and the biggest sprocket. Too close (ie fewer than four free links of chain between pulley and sprocket) and the chain might not be able to escape the clutches of the big sprocket's teeth and remain stuck in that gear (especially when big ring is also engaged) when you try to shift up a sprocket to next-to-biggest. Much further away and you'll find - as you've found already - that shifting gets vague at the small end of the cassette, due to the guide pulley now being too far away from those teeth to do it's rather obvious job - of guiding the chain during shifts.

If the chain slaps on the chainstay it's either too long, or your mech does not have enough chain wrap capacity to cope with the spread of gears on your bike (front tooth difference plus rear tooth difference). In that case it is MUCH better to have a chain that kinda dangles (like yours) in little and little - which is a gear you'll never really need since it's duplicated by other gears on bigger rings - than a chain too short to allow selection of the similarly unnecessary big and big. Because one day you WILL attempt to select big and big by accident, and then you'll break something! So if you can't get a compatible mech with more chain wrap capacity, adjust your chain length so it'll just wrap around big and big with at least one link to spare, and accept that there will be some flapping and rattling when you inadvertently select little and little.
Chris Juden
One lady owner, never raced or jumped.

gregoryoftours
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Joined: 22 May 2011, 7:14pm

Re: Derailleur hanger, B-screw, chain tension...?

Postby gregoryoftours » 18 Apr 2019, 10:22pm

Check that the b tension pivot isn't seized or partially seized too, this is the pivot where the derailleur mounting bolt is attached to the frame. If it's a bit stiff it is probably not allowing the mech to tension the chain properly. Can be stripped down and the bushing and spring cleaned easily. Re assembly can be simple or a little troublesome on occasion.

Samuel D
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Joined: 8 Mar 2015, 11:05pm
Location: Paris

Re: Derailleur hanger, B-screw, chain tension...?

Postby Samuel D » 18 Apr 2019, 11:08pm

One thing you should triple-check before spending too much time with other adjustments is that the tab on the cable clamp of the rear derailleur is properly orientated. If this is wrong – and the correct orientation is 90 degrees off what you might expect at first glance – the shifting on small sprockets (maybe your “smaller gears”?) is affected.

I’ve just added this to my list of mechanical blunders. After footering around with the B-tension screw and other things, I eventually studied the si.shimano.com documents for my derailleur carefully and rotated the tab 90 degrees. The shifting cured itself instantly. Note that shifting on the largest five or six sprockets was perfect with the tab the wrong way. It was only the smallest sprockets that were affected.

esuhl
Posts: 114
Joined: 22 Mar 2017, 3:20am

Re: Derailleur hanger, B-screw, chain tension...?

Postby esuhl » 22 Apr 2019, 3:21am

Thanks all, and apologies for the delay in responding. I appreciate all your helpful responses. :D

I measured the chain, and it's well overdue for replacement! I thought I'd been checking regularly, but it's gone from "barely worn" to "off the scale"! :shock:

As for the B-tension screw not making proper contact with the tab/protrusion on the mech hanger... a 5mm nut on the tip just fits, and totally fixes the problem! :D

esuhl
Posts: 114
Joined: 22 Mar 2017, 3:20am

Re: Derailleur hanger, B-screw, chain tension...?

Postby esuhl » 7 May 2019, 1:46am

esuhl wrote:I noticed that the B-screw had worn away part of the tab on the hanger it was supposed to press against.

I replaced the hanger with one that was slightly bent, but had an intact tab, and the B-screw still only made partial contact with it...


esuhl wrote:As for the B-tension screw not making proper contact with the tab/protrusion on the mech hanger... a 5mm nut on the tip just fits, and totally fixes the problem! :D


Ahh... I had a closer look at the rear-mech, and there's a kind-of rotatable washer on the mounting bolt, which has a tab on it. Rotate it to the right position, and the B-screw presses against the tab on the "washer", which then presses against the tab on the hanger. So I don't need the nut after all! :oops:

nigelnightmare
Posts: 533
Joined: 19 Sep 2016, 10:33pm

Re: Derailleur hanger, B-screw, chain tension...?

Postby nigelnightmare » 9 May 2019, 9:47pm

Larger jockey wheels will increase chain wrap capacity of the derailleur.