Some bike helmets have a smooth curve in the back and many others have an extra bump. The one I wear, Specialized Align II MIPS does. Why the bustle? A vented helmet creates turbulence so a rear bump will have little effect. I think shaving my goatee would have a greater aero effect than a helmet bustle. I think the main purpose is styling or “aero look”.
Those with a bustle
Smooth Rear Curve
A week ago there was a record-high temperature. Lots of news people gave all sorts of warnings about staying inside. Obviously, I went for a bike ride. I went south on the LFT , Lake Front Trail. When I got to the South Side Cultural Center I rode to the beach in the back. I decided to take a swim, even though it was before the lifeguards. I removed my shoes and waded out. Then I discovered a great advantage of the helmet bustle. When you float on your back the extra styrofoam is buoyant enough to keep your head at a comfortable angle in the water. Very relaxing, you could almost go to sleep in that position; until I noticed the west wind was blowing me farther out into the lake.