Frozen in motion.

[Derrel]

Zoom, for the right framing. You'll be fine with a slower aperture lens. You likely should pre-focus on the spot where the action will occur, and you'll probably get as little as 1 second, or less, where the car is in the exact, right spot. You want to pick the target up, get the framing on it, and start to track it at the exact right speed, and shoot when the subject is in front of a decent background, and when you're "framed up properly". The 50mm lens will probably not give the needed focal length flexibility. There's nothing wrong with panning at small f/stops like f/5.6 or f/11 or even f/22...the key is to get the right TIME interval based on how FAST the subject is moving in each situation. Focal length flexibility will be very useful. Sure, take the 50mm if you want to; if the races are under the lights, and you are seated close-up, it might be the exact right focal length. But again, you want a GOOD background, so the place where you shoot the image needs to have that first off. Then, you sit there, acquire the target, swing along, and frame up, and fire at the split-second when the timing and framing look "right". The faster the target, the less time there is to shoot, so again, pre-focusing can help. A racetrack is a very defined area, and the targets move in a regular, repeatable path, and will all be going at more or less the same speed from one car to another at the same places on the track.

 

[Johnways]

I like this frozen motion I took of a pelican. I watched him from about 40 feet away and when he took off I anticipated his route and got this: