When the kids were younger we used to have four bikes on the roof of our BMW three series touring using four Atera Giro AF carriers mounted to BMW roof bars, the problem is that the weight with four bikes makes the car corner like the QE2 as the centre of gravity is so much higher. IMO the best solution is a tow bar mounted system, however these days I use a couple of BMW branded Thule 561 carriers which hold the bike in the front drop outs which means the carrier doesn't touch the frame of the bike, making it ideal for carbon framed bikes.
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