How best to stop ';crunching of gears'; on bike?

When I put my full weight on the pedals, (such as when I start pedalling at a red to green light) the chain always changes gears of its own accord.



How can I stop it?How best to stop ';crunching of gears'; on bike?
something might be faulty with the wire

if you have a bike shop nearby

send in your bike for a check up

otherwise

check your wire, the one that controls gears

also fiddle around with the screws on your gears (they are at the back of the bike just below the actual gears, they are quiet small)

it pretty easy to fix your bike if you fiddle around with it for a while

have some confidence and you'll get it done in no time

also you can look here

http://www.bikewebsite.com/



also check your chain, it might be too long, too short, (Not the right length)How best to stop ';crunching of gears'; on bike?
I have the same problem. Buy a new bike.
there should be an adjustment on the derailers......and maybe the chain is too long