Issue with Chain Coming Off Lower Jockey Wheel

Tim76

Hi all,

I’m having an issue where the chain has started to jump off the lower jockey wheel when going over any type of rough terrain. It seems to only happen when travelling with a bit of pace (i.e. 25km/h +) and when the chain is in the small chain ring at the front and the lower half of the cassette at the back. 

I’ve got off the bike during a recent occurrence to have a look/take some pics and the chain is getting jammed between the cage and the inside face of the jockey wheel. When riding, back pedalling usually frees it and it slips back onto the lower jockey and will operate perfectly for the rest of the ride unless I’m in the same gear combination over bumpy ground.

The derailleur is a Shimano Ultegra RD-6800 and the chain is a KMC x11. All parts of the drivetrain, excluding the chain and lower jockey wheel, are over 5 years old and have about 10,000km on the clock.

So far, I have tried the following:

1. Full clean and lube of the drivetrain;

2. Replacement of lower jockey wheel. Last one was very rounded but the upper jockey wheel was perfect;

3. Tightening of cable to rear derailleur;

4. Wear check on chain – not showing 0.5% yet according to Parktool CC-3.2;

5. Checked for stiff link on the chain – all links seem fine;

6. Checked derailleur cage and derailleur hanger alignment – both seem ok but can’t be 100% sure without some kind of tool I’m guessing.

Thanks for any advice!

randomname2005

Without knowing the exact derailleur I don't want to generalise but I don't know of any that would normally have space for a chain between the jockey wheel and the cage holding the wheels. In my head the cage is usually just wider than the chain leaving next to no room for the chain to hop off without slipping back, but would need to double check that on the bike. Is the cage tightly screwed together?

Could the chain be too long, as in have too many links, and hence becoming slack in the gear combinations?

Tim76

Thanks for the response. Some pics attached above. The derailleur is a Shimano Ultegra RD-6800

Tim76

I also checked the chain length using a few online calculators and the amount of links corresponds to the chain stay length, amount of teeth on largest chainring/sprocket etc

Tim76

I will double check the cage though to make sure it's securely fixed together.

Idleater

Do the pulleys wobble side to side if you push them?

I had something similar happen due to bearings failed in the pulleys.

Tim76

https://www.boards.ie/discussion/comment/117791988#Comment_117791988

Replaced the lower pulley a couple of weeks ago so the bearings should be fine. I didn't replace the upper one as it still seemed ok but i think there is meant to be a bit of play in that one regardless.

koutoubia

Bent hanger would be my first port of call here.

And then having a bent cage.

From some of those pics iy looks like the alignment if one or the other is off!

10-10-20

A bit of play, yes, but not to the point where the chain slips as it is doing. Try test the amount of play against a new derailleur, I'm going to guess that it's less.

Also, was the width of the new wheel the same as the one removed? Is there a chance that the old one was a wider fit (hence allowing lateral movement)? If the wheel and bearing were replaced, I'd guess it's worn pin/bolt.

Tim76

https://www.boards.ie/discussion/comment/117792220#Comment_117792220

The chain was slipping before I replaced the jockey wheel and the wheel was a like for like replacement, installed in the correct direction also. I was hoping a new lower jockey would have sorted the problem but alas no.

Could possibly be a worn pin/bolt - is there any way of checking for sure?

10-10-20

Take the bolt out, clean it and use a micrometer to measure the shank at various locations.

Mercian Pro

I had the exact same problem with my Ultegra 6800 so tried various adjustments and new jockey wheels. Like you, it only happened when the chain was fairly slack in the samall chainring and sprockets. I thought it might be related to chain tension so dismantled and greased both sets of pivots but no joy. Eventually a clubmate diagnosed a bent hanger when cycling behind me and fortunately had the right tool to measure (it was well off) and adjust it. Happy days!

Tim76

https://www.boards.ie/discussion/comment/117794060#Comment_117794060

Took it to a bike mechanic buddy of mine last night, we went through everything and he reckoned it was the hanger. He measured it and sure enough it was off. It's now been adjusted but I'll need to get back out on the rough Irish roads to fully test it!

Thanks to everyone for their input 👍️