So I manged to have an odd mechanical problem during an Oly this weekend.
During an early part of the race, I shifted from the little to the big ring and manged to drop the chain to the outside (might have hit a bump or shifted while pedaling too hard). I gently moved the crank back and forth a little while riding so I could reach down and put the chain back on the ring and kept riding. Immediately, I started hearing noise from the drive train and figured that it was the chain rubbing on the FD, so I shifted up and down a few times with no luck. The noise continued, and I could see the chain pressing to the inside of the FD when it was both in the big and small ring.
I kept riding, and fiddling with the shifting and searched around for some gears where the noise was minimal, but sat there for the next 20 miles thinking about how many watts I lost to the rubbing. I briefly considered adjusting the Di2 on the fly, but thought better of that.
When I got home last night and put the bike on the stand, I found that the chain was to the OUTSIDE of the rear pin on the RD (9070 short cage)! I took off the quick link and put the chain back on the cog, and found that the chain had been rubbing against the pin and had dug deep grooves into the metal - the source of all that noise. It looks like the rear pin holding the cage plates together had separated from one side, and when the chain was loose, my attempts at getting it back on the big ring might have forced it through that separation. I could not get the chain back through that gap, and had to remove the quick link, so it is not as if the RD is falling apart.
I’ll put on a new chain, and have my LBS take a look, but wanted to share as this was one of the odder problems I’ve ever had on during a race!
