Richard Pinder very kindly pointed us towards the curved seat-tube Schwinn frame of the early 70s.. and it appears to be an excellent example of the short-wheelbase time-trialling type of frame. However if you enlarge the photo and look very closely at the juxtaposition of the leading edge of the rear tyre to the back edge of the seat-tube you will notice quite a large gap- you could easily slot your hand down it. In effect this gap defeats the object of the curved seat-tube.
With the photo enlarged, take a straight edge of some kind, a small rule etc, and place it along the back edge of the seat tube, so that the rule passes into the seat-tube's bottom bracket lug ie thereby "drawing" the line that a standard straight seat-tube would take. Look closely and you will note that the leading edge of the tubular does not cross that imaginary straight line. So what is the point of having a curved seat-tube if you are not going to tuck the rear wheel as close into the curve of the tube as possible? The builder might have "lost" as much as 1.5" of shorter chainstay.. and at best all he has achieved is a "go faster" look. In the UK the specialists of the ultra short-wheelbase time-trialling "out and back" frames, prided themselves on achieving what we referred to as "fag-paper clearances" meaning that you could just about thread a Rizla fag paper between the rear tyre(usually a 17/18mm tubular)and the wall of the seat-tube. Beware the rider who wanted to use 20s or 22mm., or didn't stick the valve of his tubular well onto the rim.. Such machines really are "mean" and perform as well as they look
If all you want is a slightly shorter than normal chainstay length then the best way is to press (or cut out on an oval tube) a "flute" into the rear of the seat-tube as Hetchins did on his 7-Day machine and Bruno Tardivo of CBT, Cuneo, has done on his road frames for the past twenty years. In this way you can just "hide" the tread of the tubular inside the flute. I think that the CBT in the ClassicRendez Vous archive will probably illustrate this type of treatment. Very neat..but make sure you anneal the steel tube at that point if you want to get a precise flute, and not just an ugly and deformed bulge.
Norris Lockley.. fighting my own battle of my bulge through not" getting enough miles in" recently.. Settle UK