I bought a 21-speed bike. The rear gear at 1st position puts the chain at 2nd toothed-wheel. Likewise 6th position puts at 7th wheel. When at 7th position, it goes out of the assembly. This problem started after a few rides. Can anybody help me out?
Your rear derailleur is out of adjustment, possibly because it was knocked against something. There are already many good guides to derailleur adjustment on the internet, so I'd rather link to one from one from Park Tool than write another one. If your derailleur has been knocked badly out of alignment, you might also need to realign the hanger.
Brand new bike? The silvery-inner cable for the gears has settled and effectively stretched. This is normal - all decent bike shops will offer a tune-up after a month or 6 weeks.
If you bought it on-line then you'll have to fix it yourself or pay a LBS to do it.
If your bike was used, then it could still be stretched cable, or the cable may have slipped in the clamping mechanism at the rear. Fix is the same.
In short, I'd fix it by
- set the bike so the chain is on the second-smallest cog.
- Then click the right-hand shifter so it would move to the highest gear/smallest cog if you pedal, but don't pedal.
- Use a ring spanner or similar to release the pinch bolt clamping the cable in the rear derailler mech.
- Pull the inner cable snug, making sure all the outers are seated in the frame bosses. Mind out for fly strands in the inner cable, they pierce skin easily.
- With the cable snug but not moving the rear derailleur, tighten the pinch bolt onto the wire.
- Minor tweaking from there can be done using the barrel adjuster which is on the rear of the rear derailleur where outer cable housing loops around.
To test and tweak, you want to hang the bike with the rear wheel off the ground. Push the right pedals with your left hand, use your right hand on the handlebar gear control, and watch the cassette with your eyes.
This would take a shop 2 minutes, you could fiddle about for 30+ doing the same.