As above the Rider is the main item for the shot. You usually find that you get both in the first few shots then they part ways so you can get a good selection. The bike may bounce and smash but if you listen it is the Rider who gets a cheer when he stands up and walks away.
Also, after the actual crash follow the rider as he may get back to/on his bike which can provide you with a horrific looking crash from which you can show the rider actually carried on with the race.