Originally Posted by Iride01

I bought a sweatvac for the very reason you state.,,,,, tan marks in the pattern of my helmet holes on my bare dome. It fit well under my existing helmet. Maybe mine wasn't as snug.

Added benifits I found was that it channeled the sweat away from dripping in my face and eyes and the cooling effect from evaporation while moving at speed seemed to be better than with my bare head in helmet.