Originally Posted by
2left
I'm having a problem that I'd like to diagnose before I bring it into the shop. I don't trust the guys by me and if I can fix it, all the better. Here's the deal:
Bike is shifting to another gear with a big "Chunk" mostly when I stand or put a heavy load. I can't reproduce it on the bike stand. It happens in big and little ring, various cogs in the back, but smallest cog in back in particular. I noticed that when I PULL on the rear derailleur cable, the right shifter clicks into the next gear. This doesn't happen on my other bike, which leads me to believe that it is a problem with the shifter, but I'm not sure. Otherwise, the shifter clicks just fine up and down. I'm guessing this shifter has about 7000 - 10000 miles on it since it was overhauled (I got this bike used, I put around 6000 of those miles on it).
1) That's not out of line with the mileage I used to get out of G-springs (I shift like I have ADHD)
2) The right G-spring carrier fatigues and breaks too with similar effects. Mine always seemed to last through about two G-spring sets. I'd suspect that if it wasn't replaced when rebuilding the shifter.
To speculate on how this leads to load dependent shifting: maybe your rear triangle moves a bit under heavy load and the derailleur spring is stiffer than the weakened shifter detent mechanism so the shifter is moving a cog smaller instead of the derailleur briefly pushing the chain towards a larger cog.