The formula
maximum trailer = gross train weight − gross vehicle weight
Where it comes from
Everything comes from three figures on the vehicle plate: the kerb weight, which is what the car weighs empty; the gross vehicle weight, the most it may weigh loaded; and the gross train weight, which includes the trailer. The difference between the last two is what you may pull. What almost nobody accounts for is the nose weight, the load the trailer rests on the ball: it does not travel in the trailer but in the car, so it eats into the payload exactly like one more passenger.
How to work it out by hand
- Subtract the kerb weight from the gross vehicle weight: that is the payload
- Take off the occupants at seventy-five kilos each, plus luggage
- Subtract the gross vehicle weight from the gross train weight: that is the maximum trailer
- Work out the nose weight, five to ten per cent of the trailer, and take that off too
What is worth knowing
A properly set nose weight is between five and ten per cent of the loaded trailer: too little and the trailer snakes at motorway speed, too much and it lightens the front axle and takes away steering. You adjust it by moving the load inside the trailer, not by changing the hitch. Occupants are counted at seventy-five kilos because that is the homologation convention; if your family weighs more, count what it actually weighs, because the weighbridge gives no discounts.