For a hitch-mounted rack, look at the Draftmaster, Thule 916, Yakima Hookup, or Saris Cycle-On. All of these mount the bikes rigidly, so that they don't move around and bang against each other. The Draftmaster is a vertical mount, the others use a tray across the back of the vehicle.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue