Actually, the predominant vibration in the parallel twin configuration is that caused by both pistons moving together, one on the compression stroke and one on the exhaust stroke. This results in a vibration in the axis of the cylinders. There is a secondary vibration at 90 degrees to the cylinder axis.
In a vertical twin-cylinder engine with pistons on opposite strokes, the primary vibration is a rocking couple. The Honda 305 Dream is a classic example. Unfortunately, this configuration results in uneven firing impulses. This can be worse that the out-of balance vibration of a parallel twin, depending on overall capacity.
The only "vibrationless" configuration is an in-line 6-cyliner engine, though a 90-degree v-8 comes close. A 60-degree V6 is also not bad.
While I was at N-V, I tried to raise interest in a Commando with a horizontal 1200 cc in-line 6. With DOHC and a high rpm limit we could've got 100 horsepower or more. With shaft drive, it would have been smooth enough not to disturtb the skin on a pot of cream. Think Honda Gold Wing 10 years earlier.
Ariel had similar ideas and tried to develop its "Leader" 250 into a touring bike with a horizontal 600cc 4-cylinder engine, but the prototype engine only developed 30 horsepower and the project was killed shortly before Ariel went out of business. If only Val Paige and his development folks had done a better job of developing the engine, who knows where Ariel might have gone.
As a Leader owner, I was very pleased with the performance a 250 gave, and the handling was very impressive (brakes less so) . A scaled up 4-cylinder, 4-stroke 600, with DOHC would've been a world-beater. Why it had such abysmal power output, I don't know.
I remeber getting in touch with Ariel about "decoking" the exhaust system with caustic soda. It had aluminum inserts in the tail-pipes and I didn't want to dissolve anything inside the silencers. They replied that there were no aluminum components inside the silencers, so using caustic soda would not be a problem.
They then added that development testing had shown that the 16:1 ratio for petrol to oil was very conservative, and that 32:1 with regular mix 2-stroke oil or even 48:1 with Bardahl would be fine and would reduce the oil build-up in the exhaust system and the distinctive plume of blue smoke at full throttle..
Switching to Bardahl eliminated the "Old Smokey" moniker.
I finally traded the Leader for a 1955 BSA A7, a "real" motorcycle rather than an overgrown scooter. In retrospect, I wish I still had the Leader as a commuter bike. Maybe I'd have replaced that 2-stroke twin with something better and put better brakes on it, but the enclosed engine compartment and integrated winsdscreen, fairing, luggage grid and panniers were pretty impressive.
Lost chances foir the UK industry?
Apologies for rabbiting on out of context. Maybe it will be of interest.