I am keen to buy a sit on tow bar back rack - made up my mind on this type, so this question is about weights: Most suggest a max weight of 60kg, with max weight for each bike being 15kg each. 3 of the bikes are lighter (2 kids bikes at 10kg each, one 14kg adult hybrid). My bike is likely to be heavier - its an old Claud Butler MTB, I need to weigh it but haven't got scales. Is the 15kg/bike a guide, or an absolute...ie if the combined wt of the 4 bikes comes in under 60kg (heaviest near the car of course), is that likely to be a problem? Most of the time it would be for short trips, but there are likely to be one or 2 trips to the alps. Thank you in advance....(not willing to buy a new bike, but also don't want to buy a rack i can't use. Car is a Ford SMax. Roof rack not an option)