I'm not a physicist but considering the high resistance your body creates, I can't see bars making much difference.
They don't notice because they had a lobotomy before buying a Harley when they could have bought a nice Vulcan and customised it for half the cost...Incidentally, how do those mean looking HD riders cope on the HWY with their open rigs?
I'd say it has to do with where the offending breeze is coming from. Helmets are designed for primarily straight on head wind. If that's what you're dealing with, the helmet you have should be pretty good with the visor down. It offers a smooth, round face in the wind that should break pretty evenly. If, on the other hand, you have a Big Shot windshield on your Custom (like mine), the headwind is deflected up and over your head. However, your knees may be kicking a large amount of wind up between the turn signals and windshield. Because this is basically an updraft it's going to catch the bottom of the helmet where it isn't quite so areodynamic. When I wore a full face I found that the updraft caught the chin guard and pushed my head side to side. My half helmet catches the air, but differently. I don't get the side to side action, but I get an annoying breeze swirling around my glasses that causes my eyes to water in temps under 60F and at higher speeds (which I enjoy) it tends to pull the helmet to the back of my head. I'm hoping my new Batwing takes care of some of that.That's good to know.
What characteristics of helmets make them more prone to buffeting?