03 Dec 2013
Custom Trailer - Customised Trailers

Trailer towing and weights

Weights, capacities & ratings: is your towing combination legal

Trailer towing and weights.

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By Johan Schronen and Mark Samuel

Have tough turbo-diesel – will tow! Not so fast, it’s not that straightforward. There’s a host of variables and restrictions that must be factored into your towing combination equation. But never fear, all will become clear …


In order to tow legally and safely, there are several factors you need to consider. Start by learning what the towing and load specifications of your tow vehicle are, and its towbar rating, as well as the specifications of your  trailer. These figures will help you determine whether your towing combination is legal – or a potential danger to yourself and other road users. If it’s the latter, you need to make some changes.

So let’s first get our heads round the terminology of the specifications used in the towing equation.


Tare: This is also known as ‘dry weight’. The tare of your car or trailer is the mass of the vehicle without passengers, fuel or luggage, but including all standard fittings.

Payload: This is the maximum mass the vehicle or trailer may carry, including passengers, fuel and luggage.

Gross vehicle mass (GVM): This is the sum of the tare and payload, and equals the maximum permissible, fully loaded mass of the vehicle.

Gross combination mass (GCM): As the term implies, it is the sum of the GVM of the towcar and the vehicle manufacturer’s maximum towing capacity for that vehicle – that is, the mass of the fully laden tow vehicle plus the maximum towing capacity as stated by the manufacturer.

Towbar static load: The maximum permissible downward force on a towbar is specified as its static load.

Towbar maximum drawing capacity: Towbar manufacturers specify what maximum mass of trailer  may be towed with a particular towbar.

Noseweight: The mass of  trailer measured at the tow hitch. By law in South Africa it has to be between 25 kg and 100 kg.

Manufacturer’s maximum towing capacity: Vehicle manufacturers specify the maximum towing capacity, in kilograms, that their vehicles can handle. (i.e. GCM – GVM = towing capacity).

Here are two brake-related concepts that also need to be understood:

Braked trailer: Certain  trailers come standard with what is known as an ‘overrun’ brake system, and are then classed as ‘braked’ trailers . (South African law stipulates that all  trailers with a GVM above 750 kg must be braked.) In simple terms, overrun brakes are activated when the tow vehicle’s brakes are applied: a shock absorber behind the trailers coupling compresses with the forward force of the trailer, which activates the overrun brakes

mechanically by means of cables. This causes the  trailer to brake simultaneously with, or fractionally after, the towcar.

Service (additional) brakes: These aren’t common in South Africa. Most often referred to as ‘service brakes’, they are hydraulic, vacuum or electrical braking systems trailers. Such a system needs to be fitted  trailer if it has a GVM in excess of the tare of the towcar. All three types are engaged by depressing the brake pedal on the towcar, and some systems can be independently controlled from inside the car with a switch or lever.

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