Looks good. If I were you I'd try and keep it tight (like you have a few shots down), thats a big trick to sports photography. Also I'm not sure if it was possible where you we're riding but I would try to get on an equal level with the bikes. It will help produce a more interesting photo that isn't a normal perspective.
Have you tried panning shots? Gordon has a tutorial here if you need it:
http://www.dslrtips.com/workshops/How_t ... hoto.shtml
One more thing, I would try to either shoot telephoto or wide angle. With extreme sports this helps give the image a bit more extremeness to it. (You did a good job of this in most of you images) just remember to avoid the 24-135mm range.