There are some very good, lightweight cameras now available on the market that could be well suited to the application. Go look at the Aerial Photography forum for much more informative advice. I'm waiting for my sample video to finish processing on You tube, and I'll come back and provide a link.

These small lightweight cameras are ideally suited to being flown on a 40 or 60 size trainer. Using a trainer for video is an excellent match due to the slow flight and stable platform.

Now couple the plane with an experienced pilot on a buddy box with a set of video goggles. The buddy box allows the pilot to fly FPV while the instructor box maintains the safety of the plane.

Add a second pair of video goggles and you (OP) can watch and direct the pilot, just like you were both sitting in the plane.

It didn't sound to me like the OP was looking for action tracking shots pursuing the mountain bikers, but more aerial shots (ala the final flyaway scene in Easy Rider). A few months ago, my son (who was taking a film class) wanted an action shot of a man running across a field, and he asked if I could use the plane to do this. I told him no, as it wasn't safe to chace a person with a plane, but if he wanted more aerial shots of the field, it would be easy to do.

As soon as I get the video processed, I'll post a link.

Brad