I have shot a lot of bikes this summer. I am by no means an expert, but I normally shoot in AV, AI Servo, and use the AF-ON to track and the shutter button to take the picture. I use Shutter priority when I am attempting to get real panning blur, but this is hard as they are passing me in more than 200 km/h. This picture was taken on Thursday using a borrowed 1DX and the Canon 24-70 F2.8L II.
If you have the chance (and get permission from the ones running the track, try to get a good spot on the track, in a curve where you both have them leaning over with the knee down, also where you can catch them speeding away.