its not the cogs at the back is the problem ,if you look at the chainrings the ones at the front" were the peddels are" take the chain off and look at them ,if the teeth are worn out , they will look like sharksfins" point backwards" so the chain dont have anything to grip on , and when you go up a hill or put pressure on them ,it wil slip, its known as (chain jumps) you can just replace the worn out ring , the ring comes apart allen keys" ill check back here if you need to know more, mick
mick, August 2008