I have traveled on multi-thousand-mile trips with a kayak or canoe on a roof rack, and the short answer is that the elements do take their toll on anything out in the sun and wind. I try to carry bikes inside the vehicle, if possible. Stay AWAY from large trucks, or any vehicles without good fender coverage over their wheels!
I definitely agree with removing the Brooks saddle for the trip, and letting a cheaper saddle take its place. Same for any nice saddle, unless I would be planning to stop the car and go for rides along the way.
I have noticed a difference of about 2 MPG when driving with a roof rack system in place, with no boats on the rack. I have not yet carried bikes on the roof.
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Have Colt, will travel...