Because they may look like the same type ie a "10 Speed" but they are different. My 87 Bianch Sport SX rides differently from my '90 Bianchi Proto which rides different from my '87(ish) Cornelo, which has a different ride than my '80(ish) Batavus Professional which rides... well you get the idea
variety is the spice of life. I mean you don't settle for just one lover do you? Why settle for just one bike.
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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