Originally Posted by gearheadE30
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.
Spot on. Had a '02 Softail with a 109 hp gear drive cam setup. Driving around town it was louder than other similar V-Twins and I didn't think much about it. On a trip wore tight fitting ear plugs and could't hear the engine. At a gas stop took the plugs out and swore the motor was coming apart and on pins and needles for the next 200 miles until I got used to the "normal" engine sounds at 3.5k rpm.
The XR1200 is geared so running 75-80 in 5th (top gear) is just over 4k rpm (1k less than redline on other Sportsters) and without ear plugs I constantly feel the need to shift up to the non existent 6th gear. With ear plugs in I don't have that feeling of needing to up shift.