This arrangement is commonly used on light and medium sized trucks of category N2, but is not limited to this category.
Many of the vehicles of this category are equipped with power-assisted hydraulic brakes and have a towing capacity of around 3500 kg, making it ideal to tow trailers with overrun brakes.
Two compulsory specifications are relevant when a rear underrun protection with integrated tow ball coupling is fitted to a vehicle:
>SANS 1505-1 Ball-type couplings and towing brackets for towing caravans and light trailers - Part 1: Towing brackets and integrated towing devices.
The fitment of tow ball couplings to a vehicle with a mass of not more than 10 t must include the design of the brackets and members to lower the ball type coupling to the permissible height above the ground for certification.
>SANS 1055 Rear underrun protection devices.
Every vehicle above 3.5 t must have a rear underrun protection fitted to the rear of the vehicle, at maximum specified dimensions such as width and height from the ground..
In many cases this underrun protection may be found in the same position where a tow bar coupling should also be fitted to the chassis.
In EU countries, where the relevant ECE directives are applied (ECE R58, ECE R55), both directives are then applied to such an underrun protection with integrated towball coupling.
The same process should also be applied in South Africa, considering SANS 1055 and SANS 1505-1 for a test certificate of the rear underrun protection with integrated tow ball coupling and subsequent homologation of the vehicle to which this device is fitted.
If we look first at the specification for Rear underrun protection devices, SANS 1055, section 3.1.2.b) stipulates the fitment dimensions of this rear underrun protection as follows:
- the ground clearance of the rear part of the unladen vehicle does not exceed 550 mm over a width of not shorter than that of the rear axle by more than 100 mm on either side;
- the distance between the rear extremity of the vehicle and the rear surface of the structure at the rear or the tyres of the rearmost axle is not more than 450 mm, see and
- the structure regarded as replacing the underrun protection device conforms to the requirements in 3.3.
The specification therefore allows to have the rear underrun protection recessed from the rear extremity of the bodied vehicle by maximum 450 mm. Any body or chassis structure, and for that matter a tow coupling may therefore protrude beyond the rear underrun protection by this distance.
Figure 1 shows the positioning clearly.
It is needless to say that all the requirements of SANS 1055 must be met for vehicles with a Gross Vehicle Mass (GVM) of more than 3500 kg.
The scope of the specification for Ball-type couplings and towing brackets for towing caravans and light trailers - Part 1: Towing brackets and integrated towing devices, SANS 1505-1, reads:
This part of SANS 1505 specifies requirements for towing brackets and integrated towing devices intended to be fitted to towing vehicles of gross vehicle mass (GVM) not exceeding 22 000 kg, and intended for towing a caravan or a light trailer of GVM not exceeding 3 500 kg.
What this means is that it applies to
- vehicles of all categories, whose GVM is not exceeding 22000 kg,
- towing a trailer of category O1 or O2 behind such a vehicle and
- have a 50 mm towball fitted which complies to SANS 1505-2.
A towball coupling on its own will of course have to comply with all the requirements of SANS 1505-1.
This means that not only the 50 mm ball coupling must comply, but all the sub-frame components and attachment parts to the chassis or body must comply as well with the compulsory vehicle standards.
A typical example is shown on the image below.
Although both the rear underrun protection and the tow ball coupling can be installed independently to the rear of a chassis, is the more common and preferred installation to integrate both, the rear underrun protection and the towball unit, into one integrated unit.
Care must however be taken in this case to observe the limits for the minimum and maximum height of the towbar from the ground, which must be between 350 and 420 mm when the vehicle is in a fully laden condition to its GVM.
The rear underrun protection may not be higher than 550 mm from the ground, which may then require different adaptations during the installation to meet the limits of both specifications.
The subframe and attachment components for the rear underrun protection will then in this case also act as components for the ball type coupling and the complete assembly must be tested and verified for compliance to both specifications, SANS 1055 and SANS 1505-1.
The image below illustrates such an example.
In manyy cases will specialist companies have certified test certificates for universal rear underrun protection with integrated tow ball coupling which are readyly made for fitment to different chassis, together with comprehensive installation instructions.
The diagram below server as an example.