According to what I have read on this and other sources, attaching anything to your bumper is probably a bad move. No matter how light your bikes are their weight plus the weight of the rack will exert a twisting force on the bumper mounting, which it was not designed for. This is basic physics of the lever arm, which will eventually fail.
My wife and I travel with two road bikes which weigh 19lbs each. Our rack is about 50lbs. We had a regular hitch receiver welded to our 5'er's frame. That is really the best way to go. Otherwise somewhere down the road you will be replacing the rack, the bikes, and the bumper.
Bob
2007 Keystone Everest 293P, 32'
2006 Silvarado 2500HD Duramax, SRW, CC, Short Bed, 4x4
Sidewinder,16K Draw-Tite Slider
Chloe, Standard Poodle
Charlie, Tibetan Terrier