Make sure that all straps are super tight at all times. They loosen up while driving. Also put the bikes as close to the car as you can. And if it does bend your trunk or hatch, make sure to tell the insurance company that your bike weighs less then 35 lbs (or 30, check the manual for weight restrictions)
also stay off any roads that could be considered "off roads". Not only do bumps put you more at risk for stuff, but you won't be covered by any insurance, or liability's if you're on the "off road".
happy trials man.