Is there any particular reason why space-frame bike designs like the ones Moulton use haven't become more common? Is the actual design of the frame in some way inferior to a traditional diamond-frame? Moulton could use diamond frames like this https://www.flickr.com/photos/sashae/2832307116/ for some of their small-wheel bikes, but they don't, so the space-frame must have something going for it. Obviously, there's more welding/brazing involved in producing them, so I'd imagine they work out at more expensive than conventional frames, but couldn't a well-designed space frame be potentially much stiffer than a diamond frame, or the same stiffness for less weight?

Anyone have any thoughts on this?

It's a lot more work to braze/weld such a frame, so unless you make a really high quality, expensive product for conaisseurs (like moulton) it's hard to pull off. Also, it's hard to make a jig for these kind of frames requiring a high initial investment. Then there is the question of perceived strenght, in these days of thigh-thick tubing such a spindly frame wouldn't raise much consumer interest. The final problem is that little tubes bend easy!

The late Art Stump made a very small batch of Bugatti "replica" frames, with more modern geometry. He spent hundreds of hours on them each. A quad tubed main triangle and fork.

There have been a number of twin seat tube or downtube frames over time. the Rigi comes to mind and the Colnago Bi-turbo or something like that, a twin down tube ti frame.

I drew one up, problem is suitable tubing, fixturing.

holy mac, I believe I have a new grail bike. These bugatti bikes are AWESOME. This article however confirms my suspicions about the trouble with getting jiggy with this kind of frame.

https://www.bugattirevue.com/revue37/bike.htm

I recall seeing on a magazine cover years back a Moulton like frame
enlarged to be a mountain bike with 26" wheel size

They are interesting frames, but they beg the question: "Is it better?"

From what I've seen, the answer is "No". From what I've seen and experienced, they are not any lighter, or stiffer, or better handling, or anything else. But they are god-awful expensive, which isn't surprising considering how much work goes into one. Just another interesting dead-end, IMO.

Regarding the Moulton space-frames: they are light, agile fun & comfortable to ride and they fold for compact storage or transport on trains and buses. That can have appeal in many situations.

I think, in general, that the traditional bike frame is at least marginally better than a space frame. However, a space frame is way cooler when you are playing your first solo hit for the 10500th time in concert