So, I’ve come a long way to find out my problem wasn’t so much in the chain line, though that was definitely some of the issue with the shifting on my Trek. It’s a lot more complex than that, though.
I’ve had to re-write this post three times… because every time I thought I found a solution, the bastard would skip on me in the little ring once I got to laying down real power to it.
I thought it was the chain line… shimming the cassette did help, but that was to cover a different, smaller, harder to find problem. My low limit set screw wasn’t set right so the pulley wheel wasn’t… erm… low enough. Fixing the rear derailleur set screw took out the need for the extra shim behind the cassette. The fix also helped considerably with the skipping, but it didn’t completely remedy the situation, either.
Whilst on the third day of DALMAC and conversing with the owner of the LBS, he put it succinctly, “Let me get this straight. This only happens in the little ring? You just need a new chainring.”
I wanted it to be a mistake I’d made. That would be simpler. Clean. Easier. Just fix what I’d done wrong, and bam! Good to go.
That night I ordered a big and little 50/34 chainring. I didn’t mess around, either. I bought Shimano 105 chainrings. Now my drivetrain is complete Shimano from stem to stern. 105 everything except the chain and cassette which are Ultegra.
And oh my, is it beautiful.
I took the bike out for a test last evening just before dusk (and I mean just) and I could tell within a pedal stroke, the bike was, if not completely fixed, vastly improved. The 105 chainrings are smooth when contrasted against the aftermarket SRAM chainrings I had on the Trek previously.
Now, I don’t have a decent hill within 15 miles of my house so I’ve had to make due when I test the tenacity of my drivetrain… I don my cycling shoes, wheel my bike out to the front yard, get the bike into the small ring in the driveway and turn onto the grass. Then I pull the brakes. I can simulate a 20% grade easy by turning the crank with the brakes pulled (carefully modulated so I don’t stop dead and fall over, which is why this was done on the grass in the first place, just in case… that the grass provides a little more resistance was just a bonus).
Not a skip, and I did pull the brakes hard enough I almost fell over. It took me a month of farting around to nail this down, but in the end it really just came down to needing a new small chainring. Now, in fairness, as little as I use the baby ring, I couldn’t believe I’d need a new one already. And then there was missing the setup on the low limit screw slightly. And a barrel adjustment problem in transit on the car rack… but whatever, it’s done! It’s paradise in paradise, now!
Fit Recovery 
Persistence pays off. Glad you could finally nail it down.
Did you remove the cassette shim also?
Yes.
And after today’s ride, I put it right back on! The new big chainring did not play well at all with the big cog on the back… I think it sits a little outboard of the old ring. I’ll have to delve a little deeper into this. In the meantime, the extra half-millimeter shim stays.
Ahh the head scratching niggles of matching modern kit onto retro bikes! 🙂
Exactly! Fun stuff.
The cassette, chain and chainrings all wear together. On an MTB we get a lot of wear and I’d never replace just one component because I’d surely get the kind of skipping you’ve experienced. The exception is the chain but you need to measure for stretch and if it’s gone too far you’re too late to replace it. You might be best to get through the winter then go for a complete new setup for next year which would give you years of perfection. My own Trek, a Fuel EX8, started skipping in the spring this year so I need to replace the lot for this winter.
I’m a maintenance hound. The chainrings last a decade or more under normal circumstances on a road bike. I have a Park Tool chain checker and check/replace chains as needed. Cassettes typically last the life of two chains (usually I get a season out of a chain because I keep them up so well… and I split time between two bikes). 👍