Excuse my ignorance, but what differentiates a motorcycle helmet from a racecar helmet?

 

Minor differences... for one the shield.. the way air goes into them... in a car u don't have the ram air force that u'd need for a bike helmet. I'm sure there's more, this is all i can thnk of right now

 

Motorcycle helmets generally have stricter demands to achieve certain certifications. There's more visibility through motorcycle visors. Uhhh, that's all I can think of off the top of my head (it's been a long day...).