I know its easy to say but... plunk down the cash for a roof system. Its safer and more secure. If your transporting thousands of dollars worth of bicycles then the $500 for a yakima is money well spent.
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2010 Kestrel RT900SL, 800k carbon, chorus/record, speedplay, zonda
2000 litespeed Unicoi Ti, XTR,XT, Campy crank, time atac, carbon forks