It's a matter of personal preference . While the fuel consumption comment is valid , it made little difference if also towing a caravan at the same time . Some towbar carriers use the ball , so you can't tow at the same time . Some rear mounting carriers are pretty naff and insecure , with reported instances of the whole things or just bikes falling off then being hit by following traffic . Ones that hang off the bodywork can do damage too . My Thule roof mounted carriers lock onto the roof rails , and for added security with expensive bikes , I can run security cables through the frames and wheels and under the roof bars fixed into the car roof . I fabricated a carrier out of steel tube so my bikes hang across the front of my trailer when going camping ; this is very rigid and , again , the bikes can be secured to the trailer chassis , the trailer also being secured to the car . Most of the 'hang on the rear' types don't offer a lot of security for expensive bikes .