You will be standing in a dust cloud for much of the time unless it is a National event. But even those can get dusty. You can also have to deal with flying mud and rocks. 24-70 and 70-200 would be my choices. Also a camera top mounted Speedlite for fill flash. Otherwise you will often not get the riders face showing up. Inside corners can be very good for closeup shots if you can be on the track. Often the deep ruts and sharp turns will slow the bikes a lot there. First curve or couple curves after the start are also good as you will have a lot of bikes coming down to a small choke point on the start. Try some panned shots too... Enjoy and good luck... A couple examples shot with a Digital Rebel back in the day...

Best,
Todd Ferguson

Much of the decision on which lens you use is base on a couple of things. I do racing photography at Daytona (Rolex 24) and the 12-hour race at Sebring. For shots on the banking at Daytona, I am shooting between 300-400mm on a crop sensor Canon 7D II. But on infield shots I use a 70-200 f2.8. A warning. The longer the lens, the shallower the depth of field, so you may need to stop down a little on your f-stop to get all of what you want in focus. Be prepared to crank up the ISO a bit. I usually pan my infield shots at at about 1/250th second. With the IS set on "mode 2" (panning mode), I can get clean shots, but the rotation of the wheels and the background are blurred, showing the "speed". Individual lenses will require different settings. I would take both lenses and be prepared to use each one, depending on how far you are from the action. Also, you may want to get a large part of the field on the first lap and then shift to shooting individual riders, even face and body close-ups which would require the longer lens. "Jump" shots can be a lot of fun, but get the hill in the shot, too, to help "tell the story" of what is actually happening. A motorcycle against a blue sky is kind of boring. Make sure you have a camera bag to keep your second lens in and some microfiber cloths with you (I buy mine in the automotive department at Walmart or Target. I am cheap) I carry about half a dozen with me to a race. Many people say they use a monopod, but I cannot move the camera fast enough with one attached to the camera or lens. I do everything free-hand. Try lots of different angles, coming right at you, panning as the go by you, although I am almost always getting a sort of front view as they get close. Pure side shots are harder to capture and keep sharp. remember, motocross is done on dirt and so their will be a lot of dirt flying around. Be prepared.