Protecting cables while on bike rack...
 

The cables on both of my bikes run underneath the top tube. When setting the bikes in my bike rack on my car the bikes rest on the cables.

Needless to say, I'm not crazy about the bike grinding on top of the cables while I drive.

Is this something I need to be concerned with? And if so, what's the best way to protect the cables? I was thinking of slipping a piece of foam between the cables and the frame before putting the bikes in the rack.

Thanks!

Pipe insulation. Or slice a swimming pool 'noodle' about 2/3 through and put on the underside of the toptube before putting bike on the carrier. The cable will not be pushing against the frame. The cable will be in the deepest part of the V and the toptube will be cushioned from both sides.

Yup, pad the bike before you put it up there; also, properly secured, it shouldn't move around enough to 'rub'.