Originally Posted by
Trakhak
Level top tube as the standard is a modern idea! Up to the early teens of the 20th century, "safety" bikes were routinely built with top tubes that sloped upward from the seat tube to the head tube.
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Edit: To clarify, using a level top tube design for lugged construction meant that manufacturers were able to keep their stock of head and seat tube lugs to a minimum. Fewer tube-to-tube angles, fewer lugs.
In fact, many bikes were designed with standardized parallel head and seat tube angles and standardized top and down tube lengths over the range of frame sizes, the only variables being the seat and head tube lengths, thus reducing stock requirements to the bare minimum.
History repeats itself. I haven't looked too hard, but it seems like most have gone to sloping and welding, so perhaps to your point about lugs. It certainly make sense for custom configurations in that it gives you more freedom. Looking the Disc Trucker - they seem to be riding the middle road - mildly sloping TT + tall head tube, evolving away from the level LHT.
Here is my early 20th century revision:
Not terrible, but I do prefer the flatty.
I must admit that it isn't immediately obvious to me that flat mass-production would result having to have fewer lugs in stock, but I can see about cuts. Maybe I need my thinking cap.
Don