We're in agreement the early Motor-Bike or Motor-Cycle, was nothing less then a typical straight bar diamond frame with a motor attached.
Between 1902- 1914, a new two-wheel paradigm evolved. Bikes had fizzled and lost interest amongst the consumer...almost every bike maker recognized this diminishing trend and had a motorbike, either in production or on the drawing board to recapture brand loyalty.
Machismo, speed and convenience suddenly fell in alignment for the average man with the motorcycle. The excitement and thrill of witnessing a motorbike roaring down a street, or competing for a victory lap around a board track was exhilarating. Many motorcycle competitions and challenges sprang up across the countryl held on tracks, country roads, mud, gravel, rocks, and dirt! For a manufacturer to be a winner, their products had to endure and prevail under these harsh conditions. Hence the need for truss rods, then later the introduction of rear and front suspensions for comfort and stability.
One of the earliest engineer of the front and rear suspension on motorcycles was Joseph Merkel of the Merkel brand. It stands to reason that Merkel (an ex-bike part mfg) would introduced truss supports on his Flying Merkel to replicate its big brother, pre-dating other bicycle makers by a few years? Check out the ad copy featured in 1913.