nothing but your imagination. provided you can reach the brakelevers and shifters while riding safely. I changed lots of bikes from drop bar tenspeeds to flat bar or 2 or 4" rise bars. you can change just the bar and brakelevers or the bars, levers, stem and shifters. lots of hipsters do it!
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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