The 1981 Elgi Grey Flash concept. Via: DOCSTYLE Blog

Vincent HRD's visionary proposal for a new motorcycle design to be constructed after the war in 1943, but nothing more transpired other than this drawing...a pity. Via: Motos Anglaises

The experimental scooters of Russia's TMZ. Most of these examples are believed to have survived and are quietly waiting restoration and/or re-creation. Via:

The 1928 AJW Super Four had an Anzani water-cooled engine but was never produced. Via:

"Though it never made it past the prototype stage, the Tuscan, capable of climbing a 45-degree hill, featured a 20bhp single-cylinder engine with a four-speed gearbox; a front ski attachment for snow trails; and a detachable headlamp to double as a flashlight. The gyroscope never made it to the Tuscan model pictured here."

Via: Hell for Leather

Via: stabilcar

The Gato Montés motorcycle by Spanish inventor Jose Artes de Arcos never made it past the prototype stage owing to the difficulties in its being stable due the the width of the wide wheels, a three wheel version was considered but other projects forced the little motorcycle to be abandoned.

Arcos fertile mind also created a second prototype fitted with a Sachs 250cc Rotary engine and even a 12 wheeled rover bike the he called "El Gato spider" (The Cat Spider )

What if ?...

The Laverda V6 racebike. Via: Breganzane

The obscure scooter club: The 1951 Bilka prototype.

Via: Stabilcar

What if ?...

The 1975 MV Agusta 750 S Prototype Turbo with fuel Injection. Via: Motorbike Search Engine