The primary responsibility for the safety of the trailer is borne by the axles and the suspension of the trailer acting in accordance with the tyres. The suspension is responsible for the road holding and the braking efficiency while the axles carry the load and the brakes. Both are safety critical items and need to be effectively maintained. Stephen du Toit, customer services manager, BPW Axles shares his expertise.

xles must not be forced to carry more than they are rated for, i.e. overloaded. Exceeding the mechanical strength limit of the axles can result in the axle failing under load, possibly resulting in an accident. Although all axles and suspensions have built in safety factors in terms of strength and carrying capacity, these are finite and can be exceeded. Consistent overloading can also lead to premature component wear, which can aggravate the situation.

Brake insights

In order for the brakes to work correctly, all the relevant components of the braking system need to be both correctly installed and correctly adjusted. The components of the braking system have to comply with the law regarding trailers – SABS ECE R13.

The brake boosters which the vehicle requires are indicated by a comprehensive brake calculation carried out either by the axle supplier or the brake component supplier. This calculation takes into account factors like vehicle weight, axle type, tyre size, centre of gravity etc, to work out the correct specification for the braking components of the vehicle. This specification then indicates the required booster size to be used as well as the correct slack adjuster hole height for the push rod to be attached to.

Only if the braking system is installed as per the specifications of the brake calculation will the braking system on the trailer provide the correct braking force – resulting in a safe trailer.

Once the brake  calculation has been performed you will know what size boosters to fit to the vehicle. The correct long stroke booster must then be fitted in accordance with the law ECE R13 as it applies to trailers. The booster push rod must be the correct length for the slack adjusters and it must be fitted to the correct height of hole in the slack adjuster. The desired angle between the slack adjuster and the push rod must be 90° with the brakes applied. If the angle is bigger or smaller than this the braking force will be reduced. If the incorrect height hole in the slack adjuster is used then the braking force will be too high or too low – both of which can lead to an unsafe vehicle. BPW branded brake boosters come with the push rod already at the required 227mm.

In order for the brakes to operate correctly, the material of the linings and of the drum must be to the correct specification. The use of linings that are either too hard or too soft will lead to premature wear in the braking system and the brakes will run far too hot. This increase in temperature in the braking system will percolate through to the hubs where it will negatively affect the working life of the grease. Heat is the main enemy of grease and prolonged exposure to higher than usual temperature will decrease the lubricating efficiency of the grease – leading to possible early bearing failure. The same applies if the drum material is not correct.

The material used in the manufacture of brake linings must be thermally stable in order to ensure that the correct slack set into the braking system is maintained. Linings which are not thermally stable – those that grow as they get hotter – will lead to dragging brakes with the attendant overheating and premature wear.

Brake drums that are consistently subjected to overheating – either by the wrong linings being fitted or by the system not being installed or set correctly can break. This will leave the vehicle without any brakes – an extremely unsafe condition in any one’s language.

If the vehicle is fitted with an ABS system this should be kept in working order. ABS systems ensure the safest possible braking performance on a trailer.

In order to achieve the safest and most efficient braking on a trailer all the above factors need to be addressed.
 

Neglected axles and wheels result in this kind of deathtrap.

Suspension savvy

Once again the safety of the vehicle lies in the adherence to the legal carrying capacity of the vehicle. If the vehicle is overloaded then the components are brought close to the limit of their mechanical strength. At this point it is possible to break things and an accident can occur.

Due to their inherently more efficient nature, air suspensions are a lot safer than mechanical ones on the road. Air suspensions do not allow the axles and tyres to skip around, meaning the tyres are always kept in contact with the road for better road holding and safer braking.

No matter what suspension is fitted to a vehicle, it must be installed and maintained in accordance with the manufacturer’s specifications. Incorrect installation can over-stress certain components, which can then break, leading to an accident. In the case of air suspensions this also includes the suspension ride height being correctly set.

Bottom line

The vehicle and its components need to be correctly specified for the task at hand. The components need to be correctly installed – this includes the braking system by way of the brake calculation. The vehicle needs to be correctly maintained in accordance with the manufacturer’s specifications. When doing maintenance, the correct specification and quality of replacement parts need to be fitted. The use of cheap or inferior replacement parts will definitely lead to an unsafe vehicle. Load the vehicle correctly. All of the above will come to naught if the vehicle is overloaded and the components are stressed to the point of breaking. Even the most carefully built and maintained vehicle will be dangerous if it is overloaded.