justsomedude I have a question. How much does the 7D AF help you in capturing these shots(supposed you have a decent panning technique)?
I'm asking because with my 400d it is quite hard to get lots of in focus shots.. I usually put focus to manual and wait for the bike to come, else I may lose the shot..
Although the AF helps its more technique and knowing what shutter speed to use in comparison to the speed of the vehicle. Also alot is down to panning technique. My kit is fairly old-school in terms of technology, a 40D and a non IS 70-200mm F2.8 and a 2x extender. In my opinion, yes its nice to have the better kit but nailing the technique is more important, doesn't matter what kit you have your pics will still come out badly if the technique isnt great. The best way is trial and error there is no easy way of calculating because of available light. I generally use the AV mode, use a higher f stop to increase depth of field and decrease the shutter speed, anywhere between F5.6 and F11 works well for me, anything less than F5.6 and you will find that the vehicle will have a small plane of focus, which can be nice for some shots but more depth of field is better for the overall effect. But if the light is pretty consistent check your exposure and get it into manual and set the values to get a perfect exposure then you can worry about the technique.
In terms of your focus point, you can pre focus on a part of the road, then set the camera to burst, follow the vehicle from an apex or where ever you are, and when you get to your pre focused point shoot a few frames, generaly shoot the first one about a metre before the point. It is a tried and tested technique but gets a bit of getting used to. Especially shooting exactly where you pre focused, best thing to do is find a mark on the road like a tyre mark so you know exactly where to shoot the frames. It is paramount to follow the car and pan or you will find the pic is very flat without any movement or with your pre-defined settings the car will be blurred but not in an aesthetic panned way, more just a blurred image. The problem is any camera will struggle to continually focus but AI servo usually does a good job, if that doesn't work for you then try the pre focusing, the vehicles move so fast that manual focus while panning can be very difficult if not impossible.
The 400d isnt really geared up to this type of photography, but instead of fighting against it use its flaws to your advantage, work around them. With the burst mode shoot the 4fps at your pre defined point, the middle image will be pin sharp. Usually 3-5 frames is a good amount to work with, even pros use this technique to ensure they have a perfect image. Another piece of advice is buy a battery grip, it will make the camera a lot more balanced hand held with a big lens. Otherwise buy yourself a monopod, although I find that they are a bit cumbersome... and annoying because it is another factor to mess around with.
The more planted you are the more likely you are to keep a steady pan. Also how is your stance and how you hold the camera? a steady strong stance with a strong grip also helps, you want to use your left arm like a triangle pressed into your chest then by holding the lens barrel and pressing the camera up toward your face you create a steady posture, then use your upper body to pan rather than your hands, almost like using a monopod but more flexible. This dramatically aids your images. Otherwise you will find that your images look sharp on the camera then get them home and at 100% they are slightly out this technique helps to reduce this, although to give yourself a little bit of leeway bring your aperture up one step will still create a pleasing blur and a sharp shot, if not exactly what you want you can also add a little more in PS. Then once your technique starts to improve you can be more experimental with your panning technique and maybe think about upgrading your camera. Its a good tool to learn with because of the flaws then once you get hold of a quicker camera you will find the technique you have learned pay massive dividend. I did exactly the same started with a 350d and a crappy 24-300mm sigma lens and eventually decided it was the right time to get some better kit.
Hope this helps