Hey folks! Thanks for the replies, it's nice to know that y'all enjoy the pics..
I'll definitely drop by your pits MZAVARIN, thank you! I got to meet a few teams this year at Thompson but I never feel terribly comfortable dropping in to say hi 'cause it's not really meant to be a spectator sport. Plus I'm English so of course it's in my DNA to 'not be a bother'......
I'm not sure I have a lot of photography advice, I'm as much of a hack as the next guy! This is what I do though:
I have a pretty good camera body. It's a Sony A99 and about six years old. It's not very good in low light and not quick to focus but it has a few tricks a regular SLR doesn't have which are immensely helpful when using manual focus. Because the viewfinder is digital it can highlight any in-focus areas for you to help in knowing when something's in or not. It needs a lot of light to work though and there wasn't enough light at Thompson this year for it to work well. In this case I relied about 50/50 on auto focus or manually focusing on a part of the track the cars would be passing through and then tracking the cars as they came by. Then just checking the images and adjusting as I went.
The hardest trick is to track the cars smoothly as they pass. That just takes practise and a lot of patience.
I'm using a cheapy 75-300 f-5.6 kit lens which just happens to be extremely sharp. The problem is that I tend to be shooting on the long end, regularly at 300mm zoom. The general rule is that you should have a shutter speed equivalent to the zoom factor to get a sharp photo (300mm = 1/300s, 75mm = 1/75s). If you want blurred wheels and the illusion of movement that's not practical, you can't have a shutter speed faster than 1/160 and even that is really too fast.
As a result when you're tracking a car at 300mm zoom and a shutter speed of 1/100 you have to be extremely smooth but also just plain lucky. Higher f-stop values help as it deepens the depth of field but also reduces the available light meaning slower shutter speeds.
To give you an idea on a sunny day about 50-75% of my photos will be somewhat usable. This year I shot 2300 pics and could only publish 389. There were others that were ok, just not perfectly in focus and I was being picky.
Long and short is practise. Good equipment helps hugely, any mid/high level DSLR should be able to produce the results I get, high quality lenses aren't necessary as long as the one you have is sharp (mine cost me $150) but they can make your life a lot easier. Buy the best gear you can but focus on lenses.
Then just take 2300 pics and hope for the best!