Schweinfurt. ZF offers a high-performance clutch module for cross country rallying. Flywheel, clutch, bellhousing including all closure plates and a clutch release mechanism are supplied as a unit. In desert rallies, like the near 10,000 kilometre Rally Dakar, the transmission is subject to extreme loads. ZF also provides highly resilient telescopic shock absorbers for the suspension on rally prototypes.
The 300 hp plus prototypes entered by top teams plough through seas of sand, reach the crests of the highest dunes and cross river beds and scree deserts. Turbo diesel power plants have long since set themselves at the pinnacle of cross country rallying. As a result, the transmission systems must be designed to handle high torque loadings.
To transfer this power reliably, ZF Race Engineering has developed a complete clutch module. It includes the flywheel and a three-plate clutch as well as the clutch release system and also the bellhousing – a specially sealed housing with closure plate that protects the clutch from dust and water when driving off-road and when crossing rivers. The friction discs have an outside diameter of 184 millimetres and the clutch can transmit a static torque of 1,750 Newton metres.
The operating conditions are unusual in cross country rallying. For example, the module is subject to extreme loads when the vehicle is bogged down. Sensitive modulation of the clutch is required when trying to free the vehicle. If too much slip occurs when engaging or disengaging the clutch, excessive heat is generated. While a steel-sinter friction pair was initially used, the engineers selected a particularly robust and durable ceramic-sinter combination as friction lining. The advantage of ceramic discs: they are considered particular warp resistant even in high temperatures. The sinter linings are dimensioned with sufficient reserves in order to withstand the stresses and strains encountered on desert rallies. The clutch housing also withstands high mechanical and thermal stresses as it is manufactured from high-strength tempered steel.
The clutch and release mechanism are located in a bellhousing, which is produced from high-strength aluminium alloy and is equipped with a sophisticated sealing system. As a result, the moving parts are protected effectively from dirt, water and thermal shock loading during river crossings. However, if something unforeseen happens ZF has another solution in-hand: the clutch must not be integrated between engine and gearbox as is usually the case, but can in fact be installed behind the gearbox. As a result, the possibility to change the unit quickly arises.
Shock absorbers for extreme conditions
Jumps over dune crests, rough scree sections and high-speed stages over sand and gravel: probably no other motorsports discipline demands so much of the suspension as cross country rallying with the Rally Dakar as the most famous event.
Teams equipped with ZF technology have taken overall victories in the Rally Dakar in the past – such as the Volkswagen factory team between 2009 and 2011. The latest product on offer from ZF for its cross country rallying customers is a telescopic mono-tube damper with external reservoir.
To cope with the demands made of the suspension, the teams rely on long spring travel. The current top prototypes for instance have suspension that permits a vertical displacement range of 450 millimetres and more. The installation ratio of the damper allows a piston stroke of approximately 200 millimetres. A vehicle weight of around 1.3 tonnes, several hundred litres of fuel as well as driver and co-driver on board mean high dynamic loads – such as jumping over obstacles. Compression speeds of 4 metres per second are common-place.
The ZF shock absorbers are one-way adjustable in rebound. The bump damping is two-way adjustable – for low spring speeds of up to around 500 millimeter per second as well as for the range above.
In addition to the mechanical demands made of the components, intensive thermal loads also occur in cross country rallying. For many years now, it is normal for the leading teams to counter the permanently high loads in the cars by using two dampers per wheel. Rally stages often run for several hundred kilometres each day through rough terrain. The damper must therefore deliver top performance for hours on end. ZF designed its components to operate in temperatures of 150 degrees Celsius, but higher peak loads do not overtax the dampers. Seals and oil are developed for such temperatures, the reservoirs are ribbed to help heat dissipation, and active airflow in the vehicle often further optimises the thermal management.
ZF operates in this way as efficient and capable partner in one of the most demanding motorsports disciplines on earth.