confusing gear shifting problem

johnson1985 Posts: 38
edited November 2011 in The workshop
Hello, I've got a problem with my gears - I don't want to take it to a bike shop, I'd rather learn how to fix it myself, hopefully with some guidance from all of you experts!

I don't know how it happened but all of a sudden my bike is unable to switch to the lowest gear, even though it says it is in the lowest gear on the shifters. Furthermore, when it is in the highest gear, it says it is in the second highest on the shifters (so when I try and push it into the highest, the chain slips off the cog). Does this make sense??

(My bike is 21 speed)

For example, when I am in 3 (on the left handlebar) and 1 (on the right - I'm sorry I don't know the technical terms), the bike is actually in 3 and 2.

When it says 3 and 6 on the handlebars, the bike is actually in 3 and 7.

And so, if it says 1 and 4, the bike is actually in 1 and 5.

This means that I can reach the highest gear (although the shifters think there is one higher remaining), but I can't reach the lowest (because the shifters cannot move any lower).

Sorry that's so confusing, but it's a tricky one to describe.

Does anyone know how to resolve it? I'm guessing (hoping) it's just a bit of screw tightening. But then again, I don't really know what screws mean what on the derrailleur.

Any help would be much appreciated!