MIC’S MEMOS

Any serious caravan or boating enthusiast should check the weight of their towing rig to ensure that they are safe and legal, says

Irecently had a spirited discussion with a customer about tow ball weight, which is defined as the weight that a trailer or caravan puts on a tow bar tow ball. He had a suspension kit fitted to his bakkie to compensate for severe sag in the rear suspension when he hitched a boat and trailer. The upgrade was rated to carry 200 kg at a 40 mm increase in ride height over standard. When he hitched his boat, though, the rear suspension dropped substantially and he was obviously not happy. There are a number of things one has to consider regarding tow ball weight, tow bar capacity, vehicle towing capacity, and fitting uprated suspension to compensate. Any serious caravan or boating enthusiast should at least once check the weight of their entire towing rig to ensure that they are safe and legal. This should be done with the caravan or boat loaded with all the gear you would normally take on a typical holiday trip. The best option here is to visit an official weigh bridge. To get an accurate measure of your tow ball weight, weigh the caravan or trailer by itself standing on its wheels and jockey wheel. Next, hitch the caravan or trailer to your towing vehicle and weigh the caravan or trailer without your car wheels on the weigh bridge. Subtract the two weights from each other. This calculation will give you your vehicle’s tow ball weight. TOW BALL WEIGHT is very important for several reasons. Incorrect tow ball weight can adversely affect the behaviour of the caravan or trailer behind the tow vehicle. This weight should never exceed the specifications of the tow bar capacity. During my time in Australia, I picked up on the norm over there that tow ball weight should be between 7 and 10% of your caravan/trailer weight. The experts may be able to correct me here if that is not quite right. My Ford Ranger is rated to tow 3 500 kg and our Australian-designed bumper tow bar has a tow ball that is rated to carry 350 kg. My FJ Cruiser had a tow ball weight rated at 120 kg and the tow bar had a capacity of 1 500 kg, which is in line with the stated towing capacity for the vehicle that is 1 500 kg. On your average double cab bakkie putting 350 kg in the load bin on top of the rear axle is going to compress the rear suspension somewhat and decrease the ride height. Putting 350 kg on the tow ball on the very same bakkie will compress the suspension and decrease the ride height even more. The further away the tow ball is from the rear axle, the more that same weight will drop the rear of the vehicle down. This is governed by the physics pertaining to levers. THIS IS THE ISSUE that has befallen the customer I mentioned at the outset of this story. He tows a very large boat on a high-speed boat trailer with a new Nissan Navara. A high-speed boat trailer has the axle mounted further backwards on the trailer, which increases the weight that will have to be carried by the tow ball. The Nissan has rear coil springs as opposed to traditional leaf springs. I have previously sung the praises of this rear suspension setup from a comfort point of view. Unlike leaf springs, however, there is no secondstage overload part to the coil spring that can assist with the heavy tow ball weight. We have no idea what the tow ball weight is of this rig, but it has to be really heavy, and I am confident it would be on the wrong side of safe and legal. It’s near impossible to solve this problem. Merely fitting heavier steel coil springs on the rear may help, but when you unhook the boat you’ll be faced with the heavy rear springs on an empty vehicle. Fitting air helper springs inside the coils is a better option as you can vary the rate or stiffness of the rear springs to compensate. I can’t help but feel that this guy needs a small truck for his boat because the local double cab offering would all suffer under this kind of setup

Putting 350 kg on the tow bar of the same bakkie will compress the suspension and decrease the ride height even more than just placing the load in the bay above the rear axle.