Hi, I'm trying to understand the main difference between choosing helmets for Road Racing/Autox/and Motorcycle. Aside from the ratings, M and SA. What is the difference between the helments. I know you can't assume that a Helmet is a helmet but I don't understand the differences between an M and an SA. Plus also can you use SA for motorcycle or do they cover both? I'm very confused when it comes to these ratings. Thanks in advance for the info.