Been there, done that and never again. Had just one bike securely mounted and the very top ladder screws did start to pull out. Now when I say "securely" I mean just that. The bike had absolutely NO movement as it was bungeed down tight.
I have since come up with a much better idea that costs under 100 $$$$ and requires only 2 holes drilled into the frame.
My Bike Rack
Our 2 bikes have been riding back there from West central Florida to Branson, MO to Nashville, TN to East central Pennsylvania and the rack is just as solid as when I first installed it.
Hybrid Mode
