Helmetss vary a lot from one style to the next. I had one patterened after a WWII wermacht one and it would start flying at about 40 MPH and threaten to lift my head off at 65+. I got a Z1R to replace it and that was much better. When I lived in the peoples demokratik republik of Kaliforneeya, I'd been up to 115 MPH with that lid and had little problem with it trying to fly away with my body bits.
