We have a tow ball and one of these

http://www.roofrackshop.co.uk/bike/t...ke-carrier.htm

so so good. I was so sick of having to lift a filthy wet bike too high to get it up onto our window mounted angled rack and struggling like no body's business. I'd never go back now, only problem is that it's only a 3 carrier. But it does leave the boot empty for all your gear which is nice.