One way to get rid of the back ground is to pan with the rider but keep your shutter speed around 30 to 45. This should keep the background very blurry and as long as you pan with the rider, he/she should stay in focus.
This will also give the feeling of movement and add a lot to the picture.
Great start as they look good now. Once you get the background out of focus, you'll be selling them to the riders for wall art....LOL