technically you are actually breaking the law as you are putting more weight than is allowed on the towbar by the manufacturer and you could invalidate any warranty on the vehicle, If a memeber of our finest (plod) new the law and pulled you, you could be prosecuted but this is highly unlikely.
as stated by others, lots of riders carry there bikes on racks with no problems, and generally the highest nose weight on normal cars such as mondeos and the like is 75 kg or on smaller cars around 50kg.