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 :(
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It sounds like you need to adjust your rear derailleur - the cable may have slipped a bit and have insufficient cable tension, or if you recently had a crash, you may have bent your derailleur.
I'd recommend reading this to learn how to adjust your derailleur.
If the chain won't move to the smallest sprocket, there are two likely causes:
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.
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.