I used to be a camera rig a while back.
Assuming you are shooting at the track, I would strongly recommend you get a GoPro or Contour HD. They are much lighter cameras and require a lot less rigging to attach to your car. They also come with very wide lenses which makes the vibrations from your car significantly more forgiving. With all things, there is always the chance of the unexpected. GoPro's and Contours can take an amazing amount of beating. I have had GoPro's fall of cars a few times and only once had to replace the clear plastic covering because it scratched around the lens.
This happened to one of my Contours...
You can rig your 7D to your car, but I strongly suggest against it unless if you are manning the mounted camera. For example if it were mounted to a door and you could man it out the passenger window or if it were mounted on the back of a car and you filmed through the trunk. An unmanned 7D on auto will not look much better than one of the new GoPros.
Also, make sure you have a good UV filter on your 7D. I once had a rock shoot up and put a nice chip on one of my filters.