Which zoom lens is best for shooting motocross outdoors?

I was talking with a fellow T2i owner over on my Facebook page and he was wondering which zoom lens is best for shooting motocross outdoors. I thought it would be beneficial to share our conversation with all of you...

Jessi's Question: I am in need of suggestions for a zoom lens that can compare to the crispness of my 50mm & 28mm prime lenses. I am in need of a telephoto for certain situations. I shoot primarily portrait/family photography, but I am branching out into the world of "action" shots and would love your suggestions.

Moose's Answer: I'd be glad to point you in the right direction. What does your budget look like and will most of your action shots be outdoors in bright daylight or indoors (gym or arena)?

Jessi's Reply: I'm not looking to spend more than $500 and it would be for outdoor (dirtbike races) shots.

Moose's Reply: If you're looking to spend around $500, the best outdoor telephoto lens would be the Canon 70-300mm IS USM lens. This lens has a wonderful reach, great for getting in tight to distant subjects (like motocross riders). It also comes equipped with image stabilization and USM (ultra sonic motor). You'll definitely appreciate the USM when trying to focus on riders as they zip around the track. Don't expect "L" lens or prime lens sharpness, but it's more than capable of a sharp shot with the right settings. It is a tad bit heavy, especially when compared to your 28mm and 50mm, but any telephoto lens will have some weight to it.