Except for the smallest frames sloping top tubes aren't really needed. They like brifters and carbon frames for all are just products the bicycle companies decided we needed. If your bike is properly sized you don't need a sloping anything. As I see it telling people they need a sloping TT gives the seller the advantage of only making 4 instead of 6 or 7 frame sizes.
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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