My Shimano Thumb Shifter has excessive play when I press down with my thumb (rear derailler). It shifts fine, but I have to push it all the way down once, release it and press it again. On the second press it is pulling on the cable immediately. Is there a way to adjust this?
P.S. It doesn't do this all the time. It seems to happen on every other shift change.
I have exactly the opposite problem on my front derailleur. Now it micro-shifts: I need about a fifth of a push to bring it into a larger chainring. It started doing this after I messed with the outboard adjustment screw. I realize that this makes no sense, that the adjustment screw should only affect how far out the derailleur can travel, but it's the only reason I can think of for it doing this.
OK, I was feeling industrious this morning with a day off work so I went out intent on solving this vexing problem on my bike. I began by disconnecting the whole cable at both ends--at the derailleur and at the shifter. I have partial housing at the front and rear, so I then slid those down and mostly off, and wiped the cable down, sprayed some silicon lubricant on, and wiped it off, then repeated. After it was nice and lubed, I reconnected everything and tightened the cable up really tight. Then some minor derailleur adjustments to fix some minor missed shifts I've been having, and I finished up. I rode it around the parking lot for about 5-10 minutes and I didn't have any problems (at least with the shifter anyways). If it stops raining I'll take it out tonight and see how it works on a longer ride. So far, at least, it seems to have cured the play in the thumb shifter problem. I don't know if our problems are the same, but they do sound alike. You may try what I did and see how it works for you. It couldn't hurt in any case.
If your rapidfires get used enough the gunk in the ratchet mechanism can make shifting problematic as described. Personally when that happens I tend to use it an excuse to upgrade or replace; I've got a couple sets of xt rapidfires that I've got in a box waiting for the day I want to screw around with them but I got extremely long use out of each before the shifting degraded that much. There are threads on how to clean and relube the internal mechanism if you can't improve through proper setup/adjustment.
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