Hey mate,
Photos look good, couple of suggestions though..
- If your camera has a manual setting, try playing around with the shutter and aperture settings a bit. A faster shutter speed and lower aperture setting will help bring out the focus on the rider when you're moving at speed. You'd need to use manual focus for this to work properly. This should avoid the situation you've got in a few shots where the rider is fairly blurry, and the background is (fairly) in focus.
Of course if that's what you were going for (or if your camera doesn't have those settings), feel free to disregard everything I just said.