On my mountain bike, the gears on the back wheel will not go to first gear, i tried to tighten the cable but then it wont move out of 6th, so i tried loosening it and it still wont budge! any idea?
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Try http://www.sheldonbrown.com for solid, easy to understand instructions to solve most bike problems. Yours probably is related to either lack of lubrication, maladjustment or seized cable. How long since the cable and housing have been replaced?
You might need to adjust your rear derailleur.
See here for instructions;
http://www.youtube.com/watch?v=2j1gSwhsVN8
http://www.youtube.com/watch?v=D0Xt_QCHD1U
Chris is probably right. I would add that you might try this site:
http://www.parktool.com
They have some good online how to pages. You just need to adjust the + and – screws on your derailier.
Luck
Did you crash recently? or maybe drop the bike? Sounds like a bent derailleur hanger (or even the derailleur itself). No amount of derailleur adjustment, lubrication, or faerie dust will solve a bent hanger; take it to a shop for straightening or hanger replacement.
Could be the rear derailleur is out of adjustment. You also may have an issue with your cassette (any ghost shifting / double shifting) going on?
Also there are adjustments and the shifter you can try adjusting and replacing the cable completely may in store.
There are a lot of angles to adjusting a rear derailleur, but to oversimplify it for beginners, I’ll tell you that the top screw adjusts the high gears while the bottom screw adjusts the low gears. the barrel adjustment can be played with to get the middle good after you have adjusted the high and low. The tension adjustment only adjust the placement of the derailleur, and Is a bit more difficult than the basic two screws.
There are limit screws on all derailleurs. You may have to adjust them. Get a book or DVD on maintenance. Basic adjustments are actually very easy and require only basic tools. If it’s over your head, take it to a shop and get a tune up for 40 bucks and they will adjust everything.