BTW your bike looks to be set up with a half step plus granny. I believe these normally had a slightly wider gear ratio in the back, and you shifted between the chain wheels more often to sort of 'split' the rear gear. Look up threads with information on half steps on here or check Sheldon Brown's site.
Depending on how your other bikes are set up, you find swapping out the middle chain wheel for a more common (these days anyway) alpine set up.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk