Morris 1100 wrote:
If you want smooth you need a V8 or an inline 6.
It depends on the engine. A V4 with a balance shaft can be nicer than an inline 4, likewise a V6. Inline fours can have higher frequency vibration which is more annoying than a lower rev 'rumble'. Honda Goldwings have opposed sixes and nearly no vibration.
Anyway, the original point was to have some Harley rocker covers rumbling nicely in a cutaway mini bonnet. Harley couldn't make their motors smooth so they rely on a very clever 'solid' and rubber mounting system which transmits enough rumble to give it 'character' but not enough to vibrate the bike to bits.
Ultimately, the smoothest motors would be opposed or V12s, but it's chasing your tail to some extent. Other factors such as size come into play. The reason I originally mentioned HD motors was they're the last of the pre-unit construction, i.e. separate gearbox and motor, assemblies which would work with a transaxle. The more I think about this, the more I like it. There's a Volvo 440 transaxle for sale locally and my Mk II doesn't have an engine yet...
I'd assume when their designed (like any other V engine) they would be built to be smooth as possible...why is a V6 smooth and not a V4 
