I'm using a 21 speed bicycle,the problem is I'm unable to shift the chain to the smallest derailleur(rear)...the shifters are thumb press type there's no problem in pressing the shifter...when shifting to the toughest gear the chain shifts up to second smallest derailleur(rear)..when i press the shifter it's moving but the chain is unable to catch the smallest one...can't figure out the problem :(
If the chain won't move to the smallest sprocket, there are two likely causes:
- too much tension on the Bowden cable (usually a result of re-tightening it too much – during operation it is more likely to lose tension)
- outer limiter screw on the derailleur too tight
Of course there's also the possibility of a bent derailleur, but I'll leave that out for now.
To find out:
While turning the wheels, shift to the lowest gear (so the chain is on the second-last sprocket).
Slightly increase the tension on the Bowden cable, either by pulling at an exposed section of the cable or by gently moving the shifter towards a lower gear. Watch if the derailleur moves inward as you do so.
If it does, the Bowden cable is likely to be too tight. If you have a barrel adjuster (on your handle bars or the derailleur itself), try screwing it a little further inwards. If it it already at its limit, you may need to loosen the screw that holds the Bowden cable in place at the rear derailleur only far enough for the cable to slip out a bit, then re-tighten it.
If the derailleur did not move in the previous test, or you had to tighten the cable by a significant amount to get it to move, it may be the limiter screw. You will find two screws labeled H and L on your derailleur. Make sure you have shifted all the way to the smallest sprocket, then slightly loosen the L screw by turning it counterclockwise. You should notice the derailleur moving outward by a small amount. Don't go too far as this might cause your chain to derail – better to move forward in small steps and test in between, then adjust further if necessary.
Now try shifting to one of the inner sprockets and back to the smallest one. If you succeed – good, if not, repeat the above steps.
Once you can shift to the smallest sprocket, shift to the biggest sprocket. Then, shift to the next smaller sprocket. Repeat until you reach the smallest one. Then shift back, sprocket by sprocket, until you are back at the biggest one. You should reach each sprocket with no rattling from the derailleur.
If you hear a rattle on one of the middle sprockets, or you cannot reach some of them, you will need to tweak the tension of your Bowden cable. Shift to a position where there is a rattle, and turn the barrel adjuster so that the rattle goes away (again, turn a bit and listen to find out the right direction and the right amount.