Take a look at many of the Pro Tour team cars and you'll see this type of racking system used a lot. You get two parts in the kit and they install in about five minutes using the supplied 5mm Allen key. Simply open up the clamp, fit the two parts to the front and rear cross bars of your roof rack and tighten the bolts down.
The front comes with a dedicated quick release axle, which mimics the diameter of your wheel hub to hold the forks. The rear wheel then sits in the cup, and is strapped down with the included strap. It's quick, simple to fit and simple to use. You need to put your front wheel/s in the car of course, but the stability offered by this system is fantastic. You'll also see slightly reduced fuel consumption figures thanks to the removal of your front wheel/s.
Bikes: 1. Weight limit: N/A.