I guess the "best" motorcycle frames are the modern aluminum perimeter frames used on street liter bikes. They are the accumulation of racing experience and their design, and other modern improvements, make for a bike that has handling abilities way beyond the capabilities of the average rider.
A lot of the European manufacturers favor the trellis frame, but it always looks very "fussy" and complicated to my eyes.
I've never liked the "Backbone" frame design, I guess they get a lot of strength from the large tube fastened to the steering head, but it makes for inconvenience regarding gas tank design/shape/fitment/placement/size. The same is true with any frame with a single, center spine, whether they be of the cradle or engine suspension type.
Using the engine as a structural part of the frame (as in the CB77 frame below) always seemed "wrong" to me, but it has been used successfully on street bikes as well as racing machines......not least by Jim Redman, Ralph Bryans and Mike Hailwood on the factory Honda 4s and 6s!
They have made monocoque motorcycle frames through the years from Honda's 50cc step-thru to racing motorcycles but I don't think they make much practical or aesthetic sense for most motorcycles.
It has to be said that the immortal Norton Featherbed checks all the boxes for simple design, strong, rigid construction, ease of gas tank placement/size/fitment, versatility for adaptation to different engines and access to the engine for maintenance.
I guess my bottom line is I prefer the classic, tubed-steel, double cradle frame design, but also have to admit that, while not pretty, the modern, aluminum, perimeter frame design wins for rigidity and high speed stability.
Looks like the ZRX frame follows the double-cradle pretty closely,
Yes, double front down tubes, and double (braced) top tubes. The front down tubes are exceptionally wide, the engine providing the lateral brace. The top tubes' angle is fairly steep but least they allow for some leniency with gas tank shape/design, and it also allows for a lot of very strong bracing at the steering head.
That frame looks strong as hell. There's a lot of empty space there above and behind the engine - how much of that gets filled up when the bike is ready to ride?
It makes me wonder how important airbox considerations are to frame design. It also has me thinking about the packaging benefits of a low-slung engine like a BMW boxer or a Guzzi VTwin. Or those engines you see canted forward like the BMW 6. Or wide angle V's for that matter - what can you stuff in that gap.
Both pics are from the same machine family. The crabs 'inhale' the space.