I find myself grabbing the 85 most of the time for the skating-rink shots. With the skate-boarding shots, you should be able to prefocus and wait for the person to come into position. I have some good luck with a "continuous-focus": focus at the farthest you want to shoot, move focus ring to the closest spot, then "interpolate" between them as you follow the moving subject. With digital- you can see if it worked.