And while we're on the topic of unknown risks from bike racks, don't hang bikes on a rear rack in such a way that the tyres are close to the exhaust pipe; you will get halfway down the motorway and the bike tyre will explode because of the heat from the exhaust. And don't forget to take the bikes off the roofrack before you drive into the garage...that one seems obvious doesn't it but the number of sorry looking people who bring mangled bikes into my shop to get insurance estimates after doing just that... :-)