Jedi Jump for Speed
Just an idea for recreating the same shot you did with the control you want. On the next try, shoot from a similar angle without the bikes going by. Get a couple of pics from google of bikers riding the type bikes you want in the riding position you want (close as you can get as this is just a test.)
Composite in the bikes (in most programs you can roto out the bikes and isolate them as a layer). From there you can animate them from left to right on the screen, build in your motion blur and have more control over there movements. *Do a youtube search on how the jedi jump is done, it's basicaly the same technique...just horizontal*
Do the same thing for the shadow on the ground, draw and cut out a shadow from a still picture and animate to match the bikes.
This will allow you to move the bikes at the desired speed and then keep the clip length as you shot it, or as you cut it afterward. Private message me if you have any questions or want some help.