17-August La Costa Technical Fiasco

It started off like a great ride. I got to the trailhead at about 4pm and was on my bike by 4:20 (Oh how I wish...) Anyway, video camera was rolling, the ipod was jamming and I was zipping up the hill with all my protective gear in place.

I made some of the switchbacks that usually have me pushing the bike and some of the steep loose climbs were easy with my Mojo SL. I had just made it around one of those corners and up a steep grade when I noticed I had some chain suck. I back pedaled and the chain fell out from between the cranks and the chainstay, but then I noticed that the chain was dragging on the ground. I looked back and saw that the lower jockey wheel was completely forward. I handled the Shimano XTR Shadow derailleur hoping that something would snap back into place or something, but there was absolutely no tension on the chain. A fellow MtB rider came by and offered about the same amount of assistance I was able to provide--pushing the jockey wheel cage backwards only to watch it spring forward.

Fortunately, the return to my car was about 98% down hill, so I coasted back, only having to push a few times. I drove home, put the bike on the stand and grabbed my favorite bike repair manual "Zinn and the Art of Mountain Bike Maintenance." I quickly found the page and discovered that there is a part in the derailleur called a Pivot Spring or P-Spring that puts tension on the jockey wheel cage, and thus, pulls the chain taught. This P-Spring was the source of my issues. I quickly disassembled the derailleur and found the P-Spring, put it into place and tightened everything up, but still no tension. So, it was time to read the manual a bit more closely. It turned out I needed to twist the jockey wheel cage around. But to do that I needed to remove a set screw that prevents the jockey wheel cage from spinning all the way around. I removed the screw, but discovered, to my frustration, that I needed to break the chain in order for everything to work right. I guess it was a good time to replace the chain, even though it was almost brand new.

The original chain was an XTR, which while a good chain, caused me some consternation in that I have been using SRAM 971 chains for some years and don't have any XTR links in the even that I would need to replace a few on the trail. So, with the added benefit of some over-zealous chain breaking, where I knocked the pin completely out of the link, it seemed like a good time to just put on the SRAM chain that I had sitting in front of me in the toolbox.

After breaking the chain, I once again removed the set screw from the jockey wheel housing, twisted the jockey wheel housing around, replaced the set screw, and voila, I had a derailleur that would tension the chain once again. I put on the new chain; tested the shifting, which worked flawlessly, and went about cleaning up my mess.

I'm questioning whether I should take the bike in to the shop to have them take a look at it, or whether I should just trust Mr. Zinn's technical explanation and give the bike a ride. I'll be leading two rides this coming weekend for the California Men's Gathering's camping trip to Mt. Laguna. I think I'll bring along another derailleur though, just in case.