This is the 'look' Arlen Ness popularized (above). Low, with the centralized mass of the engine and frame, and lighter-looking, less mass raked front end. Of course, his bikes used Harley-Davidsons. A generic name for the style is
'Frisco flyer. I like this, I'm guessing the rear wheel is a 18" and the front is a 21" with about a 44 degree rake.
I saw a few of this style when visiting Ness's shops, the old one, and the new one, in San Leandro CA. Here's a venerable Knucklehead in the
'Frisco flyer style (below). Who's that guy?
Here's a Ness-style
'Frisco flyer (above). It has a 32 degree rake.
Here's The Man and a creation (first pic, below). More extreme than the above. Feet-forward. There was a kit in the 1970's to make something like this, the Magna-Cycle. Built by Jerry Magnuson. 'Just add Sportster parts!"
Adhering more to what you'd expect to see in a
'Frisco flyer, (above) sporting a top-tube frame, I suspect it's a fuel tank of about 2 gallons, which won't get you very far when it's on-top of a Roots-type supercharged Sportster. The top-tube of larger size has several adopters. In drag racing, it's to minimize mass and has only enough capacity to get down the quarter-mile. It needs replenishing every run. No sense carrying more-than you need! The center/rear mass of the engine, the frame, and the rear wheel is opposite the much-less mass of the raked front end. The idea of a
'Frisco flyer.
The top-tube of larger size on a motorcycle has been used in several iterations. Fritz Egli has for many years sold kits to fit a variety of engines. The Egli-Vincent was one. This bare frame gives you an idea of Egli's basic design (below).
A famous Egli-Vincent (above). The nickname for the gas tank design (above) was 'banana tank'.
Triumph also used a larger top tube starting in ~1971 which was an oil tank, not used for gasoline. The design made the bike taller, and many riders prefer the older style, which had a separate oil tank. In a departure from the 'big top-tube' design, the Triumph 500 vertical parallel-twin (Tiger and Daytona) continued with the older frame design.
Dan Gurney was building something like this, he called it the 'Alligator.' No two were exactly alike, and he modified the design over time. He preferred a single-cyl engine, to make it as light as he could, w/a low center of gravity. Check-out the Gurney Eagle Indycar and the Toyota IMSA GTP car he built and successfully campaigned.
A souvenir I have from attending Daytona.
Signed by Dan Gurney.