Knowing your tow terminology can help when researching and buying a caravan. It can also help you to safely choose the right tow vehicle and caravan combination (and ultimately lead to you being safer on the road). Know your towing terms? Why not visit our ultimate guide to towing to learn more.
Towing terms defined
1. TARE WEIGHT
Tare weight is the empty weight of the caravan, as stated by the manufacturer; it is written on the van’s trailer plate. This means the weight of the caravan, fully built and fitted out by the manufacturer as it leaves the factory but before any dealer-fitted options are fitted and before any luggage or other items are loaded.
Aggregate Trailer Mass (ATM) refers to the total permitted weight of the caravan, measured on its wheels and on the coupling, with everything in it, ready to tour. Caravan manufacturers quote a maximum ATM (also found on the trailer plate, usually on the A-frame or in the luggage boot) devised as a safe maximum that by law must not be exceeded. This weight includes any options or accessories fitted to the caravan (such as an awning, for example) at the dealer prior to delivery, and anything (water, food, clothes, golf clubs, cooking gear) you put in the van before you tow it.
Gross Trailer Mass (GTM) refers to the maximum permitted weight of the caravan on its wheels, not including the weight imposed on the coupling. Again, the caravan manufacturer will quote a safe maximum GTM, which by law must not be exceeded.
3. TOW BALL MASS
Tow Ball Mass (TBM) refers to the weight imposed on the tow vehicle’s towball by a trailer. Vehicle manufacturers quote a maximum TBM that can be imposed on the vehicle. TBM varies according to how the caravan load is balanced but the goal is for 10 per cent of ATM as TBM for best balance. For example, if a loaded caravan weighs 2500kg, you should aim to have 250kg on the towball. Towbars are also given a rating for maximum towing weight and down-load. If the car and towbar have a different rating, the lower one applies.
4. GROSS COMBINED MASS
Gross Combined Mass (GCM) represents the total mass of the tow vehicle and caravan, with everything loaded in the vehicle and van. The vehicle manufacturer will quote a maximum permitted weight for GCM, so that the vehicle is not overloaded, or its engine, transmission and brakes put under too much strain. Knowing your rig's GCM is also important when towing across weight-limited roads or bridges.
5. JOCKEY WHEEL
The jockey wheel holds up the front of the caravan when uncoupled from the towing vehicle. It is either the removable type that is clamped into position, or is fitted on a pivot and locked in a horizontal position when the caravan is coupled to the vehicle. The wheel itself works on the same principle as a castor wheel, so the wheel can move 360 degrees for ease of manoeuvrability when the van is unhitched. The Trail-A-Mate combined jack and jockey wheel meets all relevant Australian Standards and is very useful accessory for safe wheel changing.
6. WEIGHT DISTRIBUTION HITCH
Also called load levellers, stabilisers, torsion bars, or level-rides, these accessories can reduce trailer sway. They can also help solve the problem of the caravan’s weight pushing the back of the car down, which makes the front of the car lift, thereby reducing steering control.
Weight distribution hitches consist of two or four torsion bars that are tensioned by chains on the A-frame and act on the towbar hitch to transfer weight from the rear wheels to the front of the tow vehicle. This ensures efficient steering and braking. The bars are fitted with the chains to the A-frame at one end and fit on to a special attachment on the towbar hitch.
Also known as a hitch, a coupling links your van to the towball on your vehicle. Several different types are used in general caravan towing. The ball coupling type is most common, and attaches directly on to the vehicle’s towball with a wedge within the hitch housing that keeps the coupling secured to the towball.
One version of this is a friction coupling, which clamps the towball to reduce lateral movement. A polyblock coupling (such as the Treg) is a composition block with swivelled sections on each end that is preferred for offroad use, as it allows for greater articulation. The AT35 from Vehicle Components features a double locking mechanism and offers a greater range of movement for offroad travelling. Vans that exceed 3500kg generally employ a light truck coupling such as Bartlett cone and ring style.
8. BRAKE CONTROLLER
Trailer brakes are a legal requirement for all trailers with a GTM of 750kg or more, and the typical brake system is electric brakes activated by an in-car brake controller.
The brake controller is fitted inside the car and power is fed from the vehicle’s brake light circuit into the controller, then back to the caravan through the connecting plug and socket to the (usually drum) brakes which are activated by electro magnets. This allows the caravan brakes to activate automatically when you apply the vehicle’s foot brakes.
The proportional braking capacity (pendulum) limits its ability to deliver smooth and effective braking. Two types are available: a totally electronic integrated circuit unit or a ‘motion sensing’ pendulum type.
9. TRAILER SWAY
This occurs when the caravan wheels lock when braking or when the van slides or swings from side to side. This ultimately compromise the stability of the towing vehicle.
10. COMPOSITE CONSTRUCTION
An increasingly common construction method for caravans is the composite type, which means the walls of the caravan are a bonded aluminium or fibreglass foam sandwich wall. The benefits are ease of construction and repair, and the smooth appearance, which some people like. Others still prefer the traditional method of building a caravan: a steel chassis topped by a timber or aluminium frame and clad in aluminium sheets.