Having raced and sold Cannondale mountain bikes, be very careful with any type of rack. I have seen every single type break or cause damage to the cars. (roofrack, towbar and clamp on)
I can fit two adult mountain bikes (one full suspension) in the back with the seat down without damaging the interior. Frames in first, wheels on top. Pad as required. I personally wouldn't use an external rack again, here being the reason:
Fiat x-19 Dented boot lid, bent engine cover up.
Golf - Dented rear body panel, damaged bumper
Audi- badly dented rear number plate panel.
Ford Escort- front carrier snapped, bike hit boot lid.
Merc- no damage. used a trailer with five bikes in it, two bikes in the boot.
Other twits
BMW M3- carrier snapped bike dragged along road at 90 mph. Not me, some idiot cutomer. Warranty claim? told him to get comprehensively lost.
Jetta- Border control boom not raised enough.... one snapped aluminium boom. Tirade in the vernacular ensuded. Glad it wasn't me as a strip search of the car ensuded.
Most damage occurs due to the wind load on the bikes denting panels not designed for such loads placed on them by the racks. Another massive problem was obscured taillights and number plates giving plod some pencil licking moments.