Sports Photography Question

[Zulu109]

I know there's a few of you that shoot organized sports, specifically Mr. Bryant. By the way, following OPG on Facebook and love the stuff you guys post.

Anyway, I'm thinking about shooting some motocross and cross country racing this year when the season starts up in a couple of months. Basically just to expand my world as I just want to shoot artsy, fartsy landscape stuff. I'll be putting the pictures up for the world to see and was wondering if I offered any of the pics for sale, would I need releases from the riders? I would be showing up as a spectator if that matters.

Thank you.