[jyl]

Auto racing helmets have the "SA" certification, motorcycle helmets the "M" certification. The difference, as I understand it, is that SA testing includes fire-resistance of the helmet materials / lining and also repeated impacts (like slamming your head repeatedly into the rollcage). Auto helmets often have smaller eyeports than motorcycle helmets, although not always. Also there are some auto helmets with features like aerodynamic design (for open-wheel racing), cooling tubes, etc that motorcycle helmets don't have. For casual AX and DE, you can usually use either type - check with the organizer.