hi, I have a used bike I just purchased, it has a rapid fire shifter on it that wont shift all the way down, at first I thought the problem was in the rear but I'm beginning to believe it's in the shifter. Can someone please tell [/font]me how to determine which it is, the shifter or derailleur?
disconnect the cable from the derailleur; move the derailleur by hand across its stroke while turning the pedals (or remove the chain). The derailleur should spring to the H limit (little cog), but without too much force you should be able to move it to the L limit (big cog). Then make sure the limit screws are adjusted correctly.
The problem could also be a frayed cable - I'd replace all of the cables as a matter of course.
what is the model of your shifter? how old is it? lets see a pic of it. it could have solidified grease causing it not to work right
There are two kinds of people in the world, "Those who help themselves to people, and those who help people!"
Also, when you disconnect the cable, try holding the cable in your hand with light pressure and shift through the gears on the shifter. Count how many distinct positions you get. (Note that a 7 speed shifter will have 7 positions, but only 6 "clicks"). You should feel the cable move a couple mm on each click. If the # of positions doesn't match how many gears you have in the back, it could be a problem with the shifter or that someone put the wrong shifter on the bike.