The Hardware: Page 2
Figure 5. Measurments of the rear wheel cutout. The shapes in dashed lines represent where the drive motor, wheel and suspension rod are placed. The curved cut conveniently fits the size of your average compact disk (no problems tracing this one out).
Figure 6. Bottom assembly view of the rear drive design.
Figure 7. Front assembly view of the rear drive design.
Figure 8. Wheel hub design. The aluminum motor hub attaches to this, which attaches to the wheel. And since the robot is so light weight, it's reasonable that the three outer screws can simply be driven into the rubber of the wheels.