runnergoneridin

It's a Cannondale R600 CAAD4, running a Shimano 105 9-speed rear cassette.

Crazy thing is my shifter seems out of sync. It upshifts perfectly, but there's one gear that when I go to downshift from, it won't. When downshifting, I'll go click (brake lever pushed inward towards center of bike), good, then click, good, then nothing - just no tension. So the remedy is to upshift again, then downshift - and it'll double shift for some reason over that gear. Sometimes it'll function without issue, but not often.

Bad shifter, or maybe just needs to be taken down and lubed or something?

dsbrantjr

I suspect that you may have a cable friction issue.

runnergoneridin

Inside of the shifter?

It downshifts all gears somewhat fine except for that one gear, then once it gets past that it downshifts clean again. It's almost like there's nothing - no tension, not catching/clicking - when the problem gear is reached.

RoCou

It's either the shifter, the cable run to the RD, or the RD itself. Easiest to eliminate is the latter - place the bike on a workstand, remove the rear wheel, shift the bike to the lowest gear (largest cog), and then pull out on the derailleur mechanism to move it towards the smallest cog position. You will immediately feel if there is resistance going up or down. Second, the shifter. The easiest is to disconnect the cable at the RD; hold the cable tension by hand, and go through all the gears on the shifter while maintaining cable tension in your hand. You will also immediately feel if the brifter is sticking along the way. Anything left after that is in the cable run.

KCT1986

Based on the symptoms, seems that there is sticky pawl and/or hardened grease clogging one of the valleys between the teeth. Doing a double-shift uses a different teeth that engages properly.

My suggestion is to try a bit of high solvent content lube like WD-40. Flip the bike so the name plate of the shifter is down, brake lever up. Wrap a rag around the bracket and rubber hood to protect it. Shift to larger cogs. Activate B lever, (upshift) and hold. Insert straw of lube can into gap created by moving B lever and give it a quick spray. Let flow/soak for a moment and try gears. Repeat if needed, but try not to flood as this can make a unneeded mess.

If it start to shift properly, flip bike and let the extra solvent lube drain out. If shifting seems dry, a medium lube can be used.

If this doesn't work a more complete tear-down may be needed.

Good luck.

srode1

So for clarification - when you say up shift you mean going to a bigger or smaller cog? Sounds like a frayed cable at the shifter to me if it happens when you are going to a smaller cog when you have problems.

billyymc

I'd agree with this based on my experience. Especially if your cables haven't been changed in a while. The most likely culprit is the piece of cable that goes into the derailler...it has a tendency to get mucked up because of its location. Temporary and quick solution is to spray a little lube into that section of cable housing if possible. It'll work for a short time.

Quick and slightly better solution is to replace that little loop of cable housing if possible.

Best solution is to replace the cable and all the associated cable housing for that cable (probably two pieces). It's an easy job that you can do yourself, or a quick and relatively cheap repair if you have to have a shop do it.

dedhed

Common problem on Shimano 10s brifters. I replace my ultegra ones yearly whether they need it or not.

https://www.bikeforums.net/bicycle-m...5-shifter.html