[Ed – The information contained in this post is solid, though digging deeper into the problem I eventually came to find that the main culprit to my tale of woe was a bad chainring.]

When we last left this sordid tale, I’d added a 1 mm shim behind my cassette, that’s 1 mm on top of the shim that came with the wheelset to fit a 10-speed cassette on an 11-speed cassette body.

I added the extra shim because I had a bit of a chain line problem that caused my chain to drop into the bottom bracket under intense power (500 + watts). This isn’t a problem under normal riding conditions, but when I’m hammering up a hill to hold a wheel, keeping the chain on the drivetrain becomes quite important!

We’re here because the issue wasn’t quite fixed. It was vastly improved and very close to fixed, even “good enough for government work”, but I could still make it skip (though not drop) in the smaller cogs out of the saddle in the little ring up front.

I assumed, when I skipped the chain last evening, that I needed to tune the rear derailleur by dialing the barrel adjuster clockwise… same as last time. So I did.

I hopped off the bike and gave it half a turn to the right. Shifting was slightly improved. I gave it another half clockwise a little while later… Every gear hit perfectly. Then I heard a skip. And another. And another. The barrel adjustment wanted to go counterclockwise to stop the skip this time, but it wanted clockwise to shift better. This is opposite what I originally started with.

I needed a thinner shim. A half-millimeter. See, I initially went with the full millimeter because “go big or go home”, figuring a half wouldn’t be enough to make a difference.

Dialing in the rear derailleur was easier than with the .5 mm shim and shifts were crisp and precise. No skips were perceptible in any gear, up and down the cassette, big and little chainrings, front or backwards with the pedals.

I’m hopeful it’s even better this time around. The best part is, I’ll have to put in a bunch of miles on the Trek to test the theory. I think I shall start with the experimenting right away…