suprisingly we are talking ounces and grams indifferences between 'quality' frames. I used to get teased about how heavy my Max was. in reality the difference was simply my 36hole MA40s. with nice tubulars it was comperable to the smaller geurciottis and Rossins in the crowd.
I don't think going for a super light sub 20lb steel bike wil give you what your looking for. if you want a bike to ride smooth and turn heads build a nice SL ot full 531 frame with some old 36hole tubulars.
if you want something under 20lbs stick with your carbon frame, wheels, crank, seatpin and bars
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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