Does the carrying of bikes on this kind of rack have to be taken into account when working out noseweight? If so is it because the extra weight is on the tow ball or the tow bar or both? With a combined weight of say 50kgs of rack and bikes this means that I will have only 25kgs left in which to use as noseweight for the caravan! With most cars having only 75kgs permissable on the tow bar are these type of bike racks only suitable for 4x4s?