There I am, enjoying a mild and sunny December on my mountain bike today and I begin accelerating for maximum speed, clicking up the rear gears, and then a problem. It’s suddenly got awfully hard to pedal, and I try going down a gear but nothing happens. In dread, remembering the time when my Shimano RD-M980 rear derailleur had ended up in the spokes of my wheel this year, I stopped the bike and had a look at the rear derailleur (RD-M986).
The chain had become unseated from the jockey-wheel and was jammed between the cage and the outside of the jockey-wheel. Also, the Bionicon C.Guide had rotated 180 degrees so that it was now sitting above the chainstay, and one of the C.Guide barrel legs had snapped; the three remaining functional cable-ties meant that the C.Guide was probably still functional though.
This all looked fairly easy to fix, but when I managed to reseat the chain onto the jockey-wheel, the jockey-wheel would not rotate easily. To compound it all, I had neglected to bring any tools (this was only meant to be a short fun ride!). The hub was still freewheeling, so I walked the bike home.
The first thing I did when I got home was to remove the jockey-wheel. Unfortunately, the jockey-wheel bolt/screw seemed to be stuck so the Allen hex tool just ended up rounding out the head on the bolt. Oops. I needed some help now but we managed to drill the bolt and remove the tapered head, which then allowed the cage to be removed, and then the bolt could be removed with pliers. Whew!
I could see why the jockey-wheel wasn’t turning. The bushing had been displaced by a couple of millimetres so that the jockey-wheel was off-centre and rubbing against the rear derailleur cage. This probably happened when the chain became unseated and had applied a force against the side of the jockey-wheel when I was pedalling (the jockey-wheel was heavily lacerated by the chain but still useable). Judicious use of a vice soon had the bushing centred correctly in the jockey-wheel. Of course I had sacrificed the bolt, but luckily I was able to scavenge a bolt from my older broken RD-M980 rear derailleur which fit perfectly. Result!
I soon had everything reassembled. But I decided to completely remove the C.Guide as I wasn’t convinced that it was really needed on a bike with Shadow Plus technology. Shadow Plus had arrived on my bike in the guise of the RD-M986 which had replaced my broken RD-M980 earlier this year. Anyway, I should really see how Shadow Plus works on its own, and I have a sneaky suspicion that the C.Guide had ended up on top of the chainstay as a result of it getting caught in my Five10 shoes whilst pedalling furiously, and that this had then unseated the chain from the jockey-wheel. It’s just a suspicion.