What ISO and shooting mode setting were you using? Were you using center weighted metering?
You may want to pick the turns you're shooting based on the angle of the sun. Based on the shadows it appears you had the sun in front of you for several of the shots. Combine this with the fact that you were shooting with the sun high in the sky and you get problems with contrast. This is especially true of dark bike/dart suit combinations.
I picked one under exposed picture to show you the histograph. If you adjust the camera to include this in the preview window, it can help you adjust your exposures for better overall contrast.
On the histograph, darker details are left and lighter right and the verticle is the level. Notice two things, the detail on the dark side is off the scale and on the right side isn't even registering. This indicates that this particular shot was underexposed. You could have probably slowed down the shutter two stops for this shot.