^
I'd advise you to get a Sigma 300mm f/2.8 over a 100-400mm for field sports!!! The fast aperture and AF tracking is much more important, if I was you I'd wamt to have the Siggy on the 7D, 1D with the 70-200. It would be a great set-up. You can really crop heavy a 300 2.8 - especially with a hi res body like the 7D. I think that would be the best quality/price comprimise.
As for getting close to the cars stand right next to the fencing at the closest locations!!! #
At many of our circuits in the UK 200mm is enough for most locations. Other than our international tracks. (and when I go trackside I can even get to 35mm fills the frame!)
Full-frame at 17mm!!! Personally I like wide angles.
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Full frame 50mm, sometimes making the car less dominant works. (f/32, excuse dust, I haven't spent much time cleaning it it since it's not for sale!).
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This time a full frame at 200mm.
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But at the same track as I was using the 17mm, I can also use the 500mm and use that too. [500mm at a/13th

:p:p:p]
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