Author Topic: Straighten a bent derailleur cage  (Read 1728 times)

Straighten a bent derailleur cage
« on: 28 July, 2021, 03:31:57 pm »
My chain keeps coming off the lower jockey wheel and running through the cage beside it rather than on it. 

I noticed this a while ago and replaced the jockey wheel, which was very worn, and assume that it would fix it.  But it became clear the other day that it is still happening.

So I changed the hanger.  And it is still happening.  Within a couple of minutes of me putting the chain back on the jockey wheel it will have slipped off, especially if I shift a couple of times. 

It's not clear why it is happening.  Nothing looks massively out of alignment but it must be or it wouldn't keep happening.  The only other thing that I can think of is that the cage must have got bent.  I don't know how as I don't recall any incident, but I can't think what else it could be. 

Any tips on how to straighten a cage?  Can I just yank it, or does the mech need to come to pieces?  Does it need a new cage?

Or anything else that could be causing it?

zigzag

  • unfuckwithable
Re: Straighten a bent derailleur cage
« Reply #1 on: 28 July, 2021, 04:48:04 pm »
if the mech hanger is straight, then the most likely culprit is the mech cage which might be slightly bent or twisted. difficult to tell without a close-up examination. my clubmate had this happening on a newly purchased bike with sram etap - the shop replaced rear mech under warranty, without trying to fix it.

JJ

Re: Straighten a bent derailleur cage
« Reply #2 on: 28 July, 2021, 05:43:08 pm »
I'd also check that the two side-plates of the tensioner are properly aligned with each other and the axle/screw of the bottom jockey is nicely perpendicular/not cross-threaded.

Paul

  • L'enfer, c'est les autos.
Re: Straighten a bent derailleur cage
« Reply #3 on: 28 July, 2021, 05:54:47 pm »
Have you threaded the chain correctly through the mech? It is very easy to muck this up!
Eta:
Okay, I’ve checked a few of my rear mechs, and my suggestion is extremely unlikely.
What's so funny about peace, love and understanding?

Re: Straighten a bent derailleur cage
« Reply #4 on: 28 July, 2021, 06:00:28 pm »
Frank, I'm sure you will have checked this but I wonder if possibly a pivot in the cage has snapped?  Never come across it myself but I don't ride your distances.  (These days I hardly ride my distances!)

BFC

  • ACME Wheelwright and Bike Fettler
Re: Straighten a bent derailleur cage
« Reply #5 on: 29 July, 2021, 07:13:58 pm »
Are the jockey wheels swapped over or wrong way round - they are different on most designs and some have direction of rotation markings?

Re: Straighten a bent derailleur cage
« Reply #6 on: 29 July, 2021, 07:16:58 pm »
If you've ever taken the jockey wheels off check you haven't mixed them up. IIRC sometimes one of them are longer, which would space the cage side plates further appart.

Re: Straighten a bent derailleur cage
« Reply #7 on: 30 July, 2021, 08:29:51 am »
Thanks for comments.

I think what I will do first is buy another set of new jockey wheels.  That will be the next simplest and cheapest thing to check, before I start messing around with the cage.

I don't recall there having been a difference between the two wheels when I changed them but maybe there was.  It's possible that excessive side to side play in the lower one might be my problem.

Re: Straighten a bent derailleur cage
« Reply #8 on: 30 July, 2021, 10:44:23 am »
It’s the upper jockey wheel that’s meant to have play to allow for indexing tolerances. So it may just be you have the wheels the wrong way round.

BFC

  • ACME Wheelwright and Bike Fettler
Re: Straighten a bent derailleur cage
« Reply #9 on: 30 July, 2021, 11:45:31 am »
In many designs the top jockey wheel has side float to enable the chain to find a natural alignment with the sprocket, the teeth on this wheel have a profile that assumes the chain is coming in from a very narrow angle.
This can be marked as "top" on some makes, Shimano mark them as Guide.
The other one is sometimes called "bottom", Shimano mark them as Tension, this has a tooth profile that is tolerant of the wider angles of approach seen as the chain comes in from the chainring. These often have direction of rotation markings.
Aftermarket pulley sets sometimes come with a set of pick and mix spacers that make a set of moulded jockey wheels do many different versions, you really need to know what version of rear mech you are running if you use these sets.

Re: Straighten a bent derailleur cage
« Reply #10 on: 30 July, 2021, 06:08:59 pm »
Interesting.  It seems possble that I've got the wheels the wrong way round.  However, the problem was there before I put the new wheels on.  It could have been that the old lower wheel being worn caused the original issue.

I'll investigate.

BFC

  • ACME Wheelwright and Bike Fettler
Re: Straighten a bent derailleur cage
« Reply #11 on: 30 July, 2021, 07:49:32 pm »
If the lower wheel becomes resistant to turning it will wear the teeth down pretty quickly, once the teeth are worn it will not function correctly. Same applies to the upper wheel. It can be spotted by increased pedal drag but is easily missed.

Possible causes for a jockey wheel siezing are:
Water ingress into sealed bearings.
Steel bushes going rusty.
Grit getting into the moving parts (between the caps and the wheels).

Some jockey wheels have ceramic bushings with a ceramic center moulded into the wheel - I have never had a failure on these, use ceramic or lithium grease.
Some have sealed bearings which are overmoulded - these work fine 'til the water gets past the seals, can be rescued if spotted before the teeth wear.
Many have basic metal bushes and some really low series run plastic wheels straight onto a steel bush. Bin and replace, preferably with an upgrade!

Re: Straighten a bent derailleur cage
« Reply #12 on: 31 July, 2021, 06:04:30 am »
Interesting.  It seems possble that I've got the wheels the wrong way round.  However, the problem was there before I put the new wheels on.  It could have been that the old lower wheel being worn caused the original issue.

I'll investigate.

I've had a look at it and I had the right wheel at the bottom, but it was the wrong way round.  I've reversed it and it looks like it has fixed the problem.  I'll know for sure when I've done a ride on Sunday.