Originally Posted by
d_dutchison
The Open University posted a series of interesting bike related videos. This one on frame design (
https://www.youtube.com/watch?v=3uXtiK5nDvo) suggested that the main impetus behind the move to sloping top tubes is that one frame size can accommodate a much wider range of differently sized cyclists, so fewer sizes of any given model need to be produced. The ultimate purpose was to reduce production costs, which in the case of custom carbon-fiber would be several orders above astronomical - to the point of being impossible.
I've seen this repeated whenever compact frames are discussed and I can see some truth in it. At the same time, there are a lot of compact geometry frames with a lineup of 5-8 frame sizes. The Trek Domane has 8 sizes, and the Cannondale Synapse has 7 sizes, for example. Meanwhile, bikes from Trek, Miyata, Schwinn and more had 5 or 6 sizes in the 80s.
Perhaps it became popular due to reduced costs because at the start, fewer frame sizes were produced?
Or perhaps it became popular because a smaller triangle is stiffer?
Or perhaps it became popular because it's easier to mount/dismount like an MTB as the trend started once MTBs went away from lugs and embraced sloping top tubes?
If it was initially only due to cost reductions by way of fewer frames, that didnt last since many popular compact bikes have a ton of sizes.