Buffetting and physics, random musings
I noticed the other day on the highway above about 75mph a significant subsonic buffet hitting my helmet. Something like 4 cycles per second. The faster I went the stronger it got. It was minimal at 60 but was there. Once I had felt it at the higher speeds I could detect it at lower speeds, but mixed in with the other random turbulence.
Putting my hand in different places I was able to determine it was riding up the back side (rider's side) of the windshield, then separating close to the top and moving towards my helmet.
I had adjusted the windshield on an earlier ride to maximum tilt towards me. This is a Madstad windshield, but the physics would apply to all brands. Previously I kept it adjusted at mid tilt. It seemed to work well mid tilt and I liked the visual line the best.
We've all made noise blowing over the top of a soda bottle (at least those of us over a certain age) and know the frequency doesn't change with how hard we blow, just the magnitude changes. But changing the dimensions (fluid level in the bottle) changes the frequency.
Eureka! There must be a dimension somewhere involving the windshield with a natural frequency of about 4 Hz. At this maximum tilt angle the gap between the instrument cowling and the windshield is large and fairly constant.
Adjusting the windshield to more upright killed the 4 hz buffet without creating a different one. At the more vertical angle the gap is much narrower at the bottom and opens up. The gap is not at all constant either laterally or vertically, so there is no natural frequency.
I guess the moral of the story is that buffeting isn't merely whether the windshield deflects the air away from the rider. The aerodynamics are important, and the gap is actually a pretty sophisticated application of the slotted flap/slat found on aircraft. But there is the simple Coke Bottle effect happening too.
If you're getting buffeted, moving the windshield can make a bigger difference than might be expected.