Here what I posted on USA7s a while back.
Reducing Buffeting in a Seven
I have experimented with reducing wind buffeting on my Locost for several years now. The high vacuum “ low-pressure area” behind the front windshield is worst then if you are on a bike at the same relative road speed. I have tried several approaches to reduce the buffeting. Some worked, others did not, I have no way of measuring buffeting but these are my general findings for the various modifications.
Winglets will reduce buffeting around 10 to 15% depending on size. The attachment should allow some air to pass between the screen and the winglet.
Laying back the front windshield an addition 5* reduce buffeting by about 10%. It helped mostly at lower speeds and no change at higher speeds.
A solid Plexiglas wind screen behind the seats, attached across the full width of the roll bar and at head height did not help. I also experimented with cutting down the screen with a gap on the top, gap at the bottom, and both top and bottom gaps to allow some air to fill the low-pressure air behind the windshield. Using solid Plexiglas did not help, It actually made it worst.
Side doors, without the top. My doors are a lot longer then most. They run from the front of the windshield all the way to the roll bar. They reduce buffeting a good 40 to 50% when compared to an open cockpit.
A Bikini top, you still have high vacuum area, but a high percentage of the air flow is behind your head. It reduced buffeting in the 30 to 40% range.
Fixed sun visors that help defect the wind up over the top of the cockpit did help about 10%, but the visors probably need to 6 to 8” wide. My visors are only 4 ¼” wide, and worked the best when set about 20* from horizontal.
Make up air, I have a defroster, and a 5 speed heater fan, but I can not feel any difference in buffeting with the system running. I would venture to say that it would take a lot more air volume then a typical heater fan could put out. For the 5% of the Sevens that use bikini tops, I just installed a rear window that attached across the full width of the roll bar just a couple inches behind your head. The car was tested with Winglets and a bikini top installed. With a full width windscreen i.e. rear window, installed it increased the wind buffeting.” This is without doors” and the results where basically the same that I had with a solid wind screen tested without the top installed. But with the bikini top installed the air flow is back and forth alternating from the left side to the right side. Air flow was verified by using a single turf hanging from the roof bow. I design the rear window with center sliding section this allowed me to adjust the center opening. Adjusting the rear opening 1 or 2 inches wide in the center section had almost no effect on the buffeting. I next tested the windscreen with about 20 to 25% of the total rear window area opened in the center section to allow air to suck out into the low-pressure area behind the windscreen. A moderate 20-25% area opening actually reduces the total wind buffeting probably in the 10 to 15% range Vs a completely open rear area. It was more effective the higher the speeds. It was tested to 100+ mph and it prove to be even more effective at reducing the buffeting at those speeds when compared to a completely open rear area. Using an opening in the center area of the rear wind screen appears to work much better then trimming off the top and bottom of the full width Plexiglas windscreen and warrants further testing without a top installed or the used of an open mesh type wind screen which allows air flow. Your results may vary; Dave W