Originally Posted by gearheadE30
It isn't based on engine layout, fwiw. 3000 RPM is about the minimum that my V twin really likes to see. The engines end up showing that trend because of the type of vehicle that each typically ends up in, though. The I4 in my GS750 is fine running from 2-4k rpm all the time if I'm riding sedately, but that would be miserable on a sportbike.
This is actually true. Even if you aren't specifically listening for it, your brain processes all the sounds that your ears hear. More RPM typically means more noise, and at a higher frequency. Smaller engines have to rev higher to make power, typically, and usually make a bit of noise doing so. Those things do contribute to fatigue, even if it isn't inherently bad for the bike. That said, if you can't hear the engine over the wind or it's just that quiet, then the RPM won't make any difference to fatigue.
Vibes tend to be worse at high RPM, too - BMW f800 motors come to mind.
OTOH my old wee was fine at 6k for hours at a time.