This started with a discussion on the MC-chassis-design list ( ) of various motors for projects.
I spotted the BMW 800 (Rotax) parallel twin.  It has a balance rod similar to the Ducati Supermono but underslung beneath the crank.

What attracted me was the arrangement of engine mounting lugs on the head as well as the mounting of the swingarm pivot to the rear of the engine cases.  I have always liked the idea of the motor as part of the frame, as with Ducati, Vincent, Britten, and the head-mounts make sense for a linkage front end.

Inspired by the Hossack design ( ) and helpful people on the list including (but not limited to) Michael Moore, ( ) Tony Foale, ( ) and Chris Cosentino ( ) et al, I decided on this for a 'frame-up' design with a 'non-conventional' front end.
Others such as Tony and Chris have built their own front end from scratch, using welded tube and joints, but in this case I decided to try to keep things as simple as possible, so aside from the literal 'frame', I will be using currently available components.  These can already be assumed to have sufficient design strengths and will allow me to get something rolling as soon as possible.  At a later date I can look at components or some of the design, but I will first try things as they are.
The front end is one example; BMW has adapted the Hossack design to their 1200 and 1300 4 cyl K-bikes.
I found a 1200 front end as well as a swingarm assembly from the 800.  It is a single-sided belt drive unit and I am sure I could make a lighter assembly, but that can wait.  This will bolt onto the motor with everything lining up.
You can see how the original frame mounts to the top of the swingarm mounting clamps.  I imagine I might be able to leave the clamps on their own, but as a bit of a backstop, and because I have to support the seat, fuel tank and battery etc. anyways, I will have a frame from the swingarm pivot up to the head-mounts and on to the front suspension mounts.  I had thought of tobes and plate but lately I have been leaning towards just alloy plate, boxed from side to side where required.  This is one of the original frames.
I took some weights of the components.
The front fork, no axle, but with the two ball joints and the steering linkage; 13.4 lbs.
The upper a-arm, no bolts; 4.5 lbs.
Lower a-arm, no bolts; 5.1 lbs.
Those are both forged steel I believe; heavy, but for now, a known quantity..
The swingarm, with sprocket, hub and disc, no caliper; 28.4 lbs.
The wheel with a 180 michelin tire; 24.4 lbs.
I have just received the bits as well as the stock injection setup; again simpler than figuring out a Megasquirt or equivalent setup.
One of the steering links was tweaked but the vendor is sending a good one and I am hoping that is the 'weak link' in the case of any damage. 
Next step; a plywood (cellulose / lignin fibre composite) 'plate' frame to see how things will lay out.  The original used an under-seat gas tank and the motor really wants an airbox right up top, so we shall see what fits where.
I will be looking at various placements for a front shock as well.
More pics as it takes shape.  Maybe I'll just leave it in plywood as a 'woody' ;-)

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