Lost all tension in rear derailleur

[ChrisEyles]

I am having a bad day in the shed today

Started off trying to tweak straight a slightly bent derailleur cage (after checking the hanger was OK). Thought I'd take the derailleur off to pop the cage in a vice and do it "properly". This worked fine, as far as I can tell.

However, I have just popped the mech back on the frame, and it is now failing to take up any of the chain slack. It's a bit like when the B-limit screw is too loose, only 100x worse, so the jockey wheels are skating over the sprockets in all gears, with a big swag of chain hanging down. My guess is that the spring where the mech bolts onto the frame/hanger has unwound itself so the mech is no longer pivoting back to take up the chain slack. I've tried to turn the spring through 360 degrees to retension, but this seems too much (more than I can do with my fingers), and I can't hold it in place while I screw the mech back into the frame.

An hour or so of swearing in the garage has made no positive difference so far, so I thought I'd see if anyone smarter than me about these things has any tips before driving myself mad!

It's an old (ca 1997) Shimano Acera mech, if that makes any difference, and is bolted directly onto the frame (no removable hanger).

Thanks in advance for any help!

 

[screenman]

My bet would be that you have not hung it back on the hanger correctly or have run the chain wrong. A picture would help though.

As for checking hanger, how did you do that ?

 

[ChrisEyles]

Chain looks fine to me, and the derailleur does shift through all the gears. The jockey wheels are riding over the sprockets due to the lack of tension in the derailleur arm though.

Here's a pic, but there's not a huge amount to see I'm afraid. The body of the mech should be rotated in the clockwise direction to bring the upper (lower in the pic as bike is upside down) pulley away from the sprockets, but there is no tension in the spring (located where the derailleur bolts onto the frame) to do this.

The hanger I simply eyeballed to see that it was parallel to the sprockets. It's integrated into the frame and made of pretty thick steel, so it would take a hefty blow to knock it out of true I think. Plus the mech was working perfectly before I took it off, so I think the hanger is OK.

Bit of a mystery, and just when I thought I was getting the hang of working with them too!

 

[steve50]

derailleur repair guide

View: https://www.youtube.com/watch?v=bgiNuJNynMc

 

[ChrisEyles]

Thanks for the vid! It is indeed that spring that's causing the issues (only it's too loose instead of stiff).

The plate (below the surclip) on my mech seems to have a slightly bent tab on it, which isn't engaging properly with the derailleur body, so it's not possible to re-tension the spring as shown above. I like the trick with the vice and the allen key though, that would certainly have saved my fingers this afternoon.

Looks like it is going to be a trip to the LBS...

 

[Levo-Lon]

Just try a refit first

Take chain off front ..undo mec, refit at 90deg back rotation and try again..
as screenman says by the look of it..you have fitted it wrongly , i mean incorrectly looking at the pic.

 

[screenman]

You have it the wrong side of the stop, do as meta says.

 

[Levo-Lon]

You have it the wrong side of the stop, do as meta says.

i should edit Wrongly..

 

[ChrisEyles]

Which part am I trying to rotate if I re-fit? I've tried attaching with the whole thing rotated further around earlier on (with the mech rotated clockwise as you're looking at it in the picture), and it just kept sliding back to the same position pictured, only stopping when the B-screw part butts up against the drop out.

My idea was that the B-screw part (the plate beneath the surclip) may need to be rotated around 360 degrees to tension up the spring, and that this would then provide enough tension to pull the mech into the correct place. It is pretty fiddly to do though!

I'll give it a shot tomorrow night though, would be great to be able to sort this myself (with a little help from my cyclechat friends of course!). Any other suggetions more than welcome!

 

[Levo-Lon]

Note position of the mec in relation to the stop and B tension screw

View: http://m.youtube.com/watch?v=CPLiW-0zLa4

 

[ChrisEyles]

Pretty sure my b-tension screw is in the correct place relative to the stop (to the right of the dropouts in the pic above, with the threaded end of the screw butting up against the stop), though I agree it cause the same problem as I am having if it was on the wrong side.

I still think I probably need to retension the spring on the hanging bolt like this:

It's a fiddly job though, and the little tab on the plate isn't engaging properly with the corresponding stop on the derailleur body as it should, so as soon as I let go the spring whizzes around again and de-tensions. Might be able to bend it back in the right place with a pair of pliers or vice but not sure if this is a job best left to the pros.

 

[jonny jeez]

I.

proof that even zombies like to riiiiide!!

 

[ChrisEyles]

Blimey that was definitely one of the trickiest to diagnose and fiddliest to carry out fixes I've done yet! But it's working again - I won't say fixed, since I did in fact break a bit of it, but working is better than not working Thanks for the helpful links and sorry for the poor pic which maybe didn't show what was going on as clearly as it could have.

The problem was indeed that the tension spring had come unravelled when I first removed the derailleur. This happened because the little pin circled in red here was bent, and was not engaging with the stop on the derailleur body (which should have prevented it from de-tensioning on removal).

I had to take off the plate and bend this catch back into shape with a vice and pliers in order to re-tension the spring, then re-installed the mech - and to my pleasant suprise it now works as it should, correctly picking up the chain tension.

Unfortunately (re)bending this catch fatigued the metal, and it fell off as soon as I started shifting. I guess this means I'll never be able to take the mech off again (the spring would unwind and I'd have no way of retensioning it without sourcing a new back plate, which I guess is impossible). This is OK though, hopefully I won't have to remove it again unless I damage the cage badly, in which case I may as well replace with a new one.

But does anyone know if this catch serves any other purpose in the operation of the mech? If not I'll run it 'til it dies, but I am now a touch worried I might have turned it into a ticking time bomb.... can't think of any danger its absence might pose, but I really don't want to find out when my mech catapults itself into the rear wheel at speed! If anyone knows either way I'd be very grateful for a heads up!