Rear Derailleur

fastercyclist

Hey guys.

Tried to go out for a ride today. Randomly, pretty much out of nowhere - I topped out a small slope and my rear derailleur kept dropping the chain on one of its chain rings. Forcing me to halt and put it back on. However it kept falling off within seconds of trying to start again.

In particular I noticed it had a much higher chance of falling off if my pedals went the "wrong way" around. In fact it guaranteed it fell off immediately.

If you follow the chain from the underside from my front chain rings towards the rear derailleur it was the first chain ring the chain goes through that it kept falling off. I tried to cycle back very carefully after changing the gears a lot and finding that front chaining small and back cassette big "aligned" the two chainrings on the derailleur enough that I could go 2-3 minutes before it fell off again. I tried to change gear (front chainring from small->big) at one point and the chain went completely slack and began rattling against my spokes! Also when you change the front gears the derailleur gets "pulled around" to its "stretched out" position, with the chain rings vertically under the cassette rather then its "hunched" position where they slope away from it slightly - the movement of the derailleur between these two positions seemed really random and sometimes switched even without gear changes. Utterly bizzare!

Any experienced guys able to shed light on this?

Not keen on taking it to an LBS unless it sounds necessary, have no money. Shimano ultegra groupset.

skyd0g

Check through the settings for the derallieur as listed here: http://www.parktool.com/repair/readhowto.asp?id=64

It may just be the cable tension, but from your description it sounds as if it could be a combination of things. It could even be the tension spring/s that have failed. A pic may help shed some light on your prob.

rhnb

Check it's not a stuck freehub. If you stick the bike on a stand, or hold the back wheel off the ground and slowly turn the cranks backwards, does the chain just go slack (ie the cassette doesn't move)? Also try pedallig forward slowly and freewheel, does the chain go slack?

fastercyclist

I had a close look at it today and the bottom pulley of the derailleur is very stiff, I thought it was grime so I set about cleaning it. However when I was cleaning it I started to suspect a part next to the pulley was missing.



If you imagine that bolted between two pieces of metal. On one side there is no gap between the pulley and the metal and I think this is causing it to rub and hence making it very hard to turn around. I believe the pulley in question is called the tension one...it's the bottom one of the two as described above. The other pulley on the dérailleur moves fine and is in a more central position with a small space on each side of the pulley, rather then rubbing against the metal.

I'm not 100% sure if this theory is correct because I tried to prise open the two bits of metal to see if there was an obvious gap there but they didn't budge.



If you focus on the pulley in that picture you can see it has a screw in place in the middle, I don't have one on mine, or at least the cap isn't there. Right beneath where it says "bear" on the pulley you can see a glimmer of white - the bit that gives it space from the metal casing. I am missing that on one side of my pulley. Is anyone wised up to know if this is normal for shimano ultegra or if I do indeed have a part missing?

giant_man

A jockey wheel consists of the wheel itself, a cupped washer on either side and the bushing sleeve in the middle, with the allen bolt holding it all in place. This may help you

http://bicycletutor.com/jockey-wheel/

John.T

http://techdocs.shimano.com/media/techdocs/content/cycle/EV/bikecomponents/RD/EV-RD-6600-2358A_v1_m56577569830621055.pdf
This shows what you should have. The tension pulley has a ball race in the centre while the top pulley has a bushing.
The strange behavior of the mech could be caused by a seized 'B' pivot. The top one at the hanger end. Check both pivots.

fastercyclist

We've noticed that the jockey wheel is actually bent. Ordered an ultegra 6500 one, should be fine on my ultegra groupset. Made sure it was the lower one. Hopefully replacing that part will fix the problem. If it doesn't work I'll look into adjusting the derailleur but the set up was fine until Sunday.