With motorcross, you are shooting dynamic shots. You will need to use a few tricks to maximise your rate of 'keepers' (good shots).
*Use High shutter speed : 1/1,000s or faster.
*Use Hi continuous shooting.
* AF.C is likely to be needed.
*You may consider also to use MF; first by focusing manually at a particular distance , and then shart shooting a burst before the motorbyke arrive and have a couple of shots in focus.
* If the motorbyke runs parallel to you, consider panning: that is, center the viewfinder on the byke, and follow the byke while shooting. You will need a bit of space around you to turn along and follow the byke.
The above 'tricks' will affect your shooting:
- To control the shuutter speed, you will be best in Tv mode (shutter priority), or full manual, and set up the shutter speed with the dial.
- high shutter speed means that you will need a good light (daylight) or a fast lens (f2.8, f1.8 or even f1.4) if the light is not good (dawn, dusk, storm).
- Hi continuous shooting means that you must setup your camera for JPEG (no RAW) and kill off all the in-camera processing (incl. lens distortion correction, high ISO NR, ...)
- Unless you are very close to the track, you will need a lens longer than 55mm. Possibly 90mm is you are close, or even longer if you are far away.
Hope that the above comments will assist.