Four Western Star 4900SA custom built truck tractors were recently supplied to specialised haulage contractor Transsand, for pulling end tip trailers. The trucks are hauling brick clay from a clay mining operation outside Pretoria to a nearby brick works. Transsand purchased the Western Stars because most of the mileage covered by the trucks hauling clay is on corrugated dirt roads, which have proved to be the undoing of standard off the shelf on highway trucks.

The Western Star 4900SA 6x4 truck tractors were specified by Transsand owner Johan Nortier, with the assistance of Duncan Prince, product manager of Western Star trucks at DaimlerChrysler South Africa. The brief in building the trucks was to achieve a payload with a tandem axle Copelyn type end tip trailer of 26metric tons legally, and to survive continuous dirt road operation with a minimum of unscheduled maintenance, while still offering good fuel consumption. To make the task even more challenging, the short haul nature of the operation dictated that the truck be easy to drive for long periods in arduous stop start operation.

To meet the customer requirements, the trucks were specified with Cummins ISM 370 engines, developing 385 horse power or 287kw @1650 rpm. These Cummins engines are equipped with the famous Cummins C brake often referred to as a Jake Brake. This engine compression brake offers virtually the same retarding power as what the engine is capable of producing in terms of rated output.

Coupled to the Cummins engine is an Allison RDS 4500 six-speed fully automatic transmission. The transmission is programmed with two shift programmes, one that is used when the truck is under acceleration to optimise the shift point, the other that comes into operation when the engine brake is engaged, and the truck is slowing down. The secondary downshift programme aims to maximise engine revolutions while decelerating so as to keep the engine brake working at its maximum effectiveness, thereby maximising the life of the brake linings of the truck and trailer.

Thus far, the Western Stars are proving to be more fuel efficient than the trucks with conventional manual transmissions that they have replaced. Some people assume that a torque converter automatic transmission must be mechanically in-efficient. However, the Allison transmission uses a torque converter and the first two ratios in the transmission to cover the typical pull away speed range. Most trucks with a 12- or 16-speed manual transmission would require five or six gears shifts to get through. Once the Allison transmission gets to 2nd gear, the torque converter is locked and the rest of the shifts are from lock up - lock up, which offers similar mechanical efficiency to a conventional manual transmission. The added benefit is that no driver intervention is needed to effect gearshift.

A two pedal fully automatic transmission is far easier and less tiring for a driver to drive in short haul applications where numerous gearshifts are required. The soft shift between gears on fully automatic transmissions also means that the potential for damage to drivelines and drive axles is greatly reduced, and gear shifts can be safely made on steep inclines or declines, where a conventional transmission, whether it be automated or manual, could never make the shift.

Other features incorporated on the custom built Western Stars are double channel chassis, which is incredibly strong in harsh dirt road conditions. They are also very light due to the high tensile 768mPa steel that is employed in Western Star chassis rails.

The rear drive axles specified on these trucks are Meritor RT 46-164 EHP drive axles. These are a derivative of the ubiquitous RT 46-160P drive axle used in at least 1500 other new truck tractors in South Africa every year, but this derivative of the axle uses a 16mm thick banjo housing rather than the conventional 12.7mm housing for extra durability in arduous mining and dirt road operation.

The Meritor drive axles are attached to the chassis by means of a Chalmers rubber walking beam suspension. This suspension is particularly well adapted for tipper operation, as it has high articulation for good traction, excellent anti-hop properties to prevent wheel-spin, and doesn’t use any spring blades which can break when working in harsh dirt road environments. The Chalmers suspensions also stiffens rather than softens under load which means that the roll stability of the vehicle increases as the vehicle starts to lean with a high centre of gravity load.

Other small yet critical features worth mentioning on these vehicles that are incorporated are dual Sy-Klone self-cleaning active pre-cleaners, fitted to either side of the bonnet. These greatly extend the life of air cleaner elements. Also fitted are extra wide stainless steel fuel tank straps, which ensure that the 700 litre fuel tanks do not work themselves loose when subjected to the constant vibration of corrugated dirt road operation. Batteries are mounted under the passenger seat inside the air suspended cab where they are protected from extremes of heat and cold. Batteries will also last a lot longer than normal in this type of operating conditions because they are not subjected to the vibrations from the road that would normally cause the plates of a battery to break-up in arduous operating conditions.

The truck is also fitted with a heavy duty off-road cross member package that is built to cope with the rigours of continuous off road use. This complements the three-point mounting system employed to mount the radiator, cab and engine-transmission combination. Three-point mounting ensures that no twisting of the chassis is transmitted to any of the other components which are not intended as structural components.

Western Star trucks are distributed by both Freightliner Dealers, and Liebherr Africa who specialise in Western Stars employed in mining and pure off-highway applications.