Really like these shots, especially the first one. Here's a couple tips to improve:
1. As hard as it may be (especially in the woods) try to get your subject placed infront of a solid colored background. If your subject is placed infront of trees, it's hard to differ subject from background, try placing the biker agains the sky, or ground. I know this may be very hard, since your in the woods after all, which leads me to tip numer 2....
2. A way to isolate your subject without worrying about solid color backgrounds is to use a slow shutter speed. With the UWA lense that you used in the first shot, set the shutter speed down low (enough to get motion blur if you pan while shooting) I would go with 1/50th of a second or even slower. Then pan quickly while shooting, if you pan at the same speed as the rider, the rider will be very sharp while the background is blurred. This not only gives the feeling of speed, but it isolates the rider (your subject)
The photos looked very nice, and I'm not saying they're bad, just giving a couple useful tips (you may have already known them). But keep up the good work!