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Power supply for the rear wheels: the cardan shaft
The cardan shaft is an important component of AWD and RWD vehicles. It transmits the torque from the engine to the rear wheels. Defects are relatively easy to repair. The cardan shaft is alternatively called joint shaft on account of the universal joint in its centre.
Transmitting power to the rear wheels
For practical as well as historical reasons, the engine is installed at the front on most cars. Nevertheless it can be driven via the rear wheels. This has certain advantages:
- sporty driving style through noticeable kickdown
- simpler construction: no need to integrate the drive with the steering mechanism
- advantages when driving straight ahead
- ideal weight distribution in case of transaxle construction
The transaxle construction has the manual or automatic transmission installed in the rear as well. This causes a better weight distribution of the car. Front and rear carry an equal load, which has a positive effect on the driving dynamics. The rear drive has disadvantages as well; especially on slippery roads it tends to swerve. Principally, the route of the torque leads from the engine to the rear axle via a cardan shaft.
Construction and function of the cardan shaft
The cardan shaft connects the engine with the rear axle. The torque is generated in the engine and transmitted to the differential on the rear axle. Because the rear axle has suspension, the cardan shaft must be able to absorb and compensate the up-and-down movements. Therefore it has a so-called universal joint in the middle. This joint is in fact a small cross with four arms of equal length. These arms are rounded and enclosed by a needle bearing, enabling the universal joint to cushion the rotating cardan shaft. On the connecting points of the drives the cardan shaft is equipped with toothing. This is connected to the engine drive and the differental drive with a flange plate.
Cardan shaft defects
The cardan shaft is a very robust component with a long life span. It has two minor points of wear, which can be easily repaired. At the engine side the cardan shaft has a so-called hardy disc. This is a thick rubber disc, damping the changing torque of the engine. If the disk becomes porous and hard over time, it can no longer adequately fulfil its damping function. The driver notices this when shifting gear. In the worst case, the hardy disc can break off from the cardan shaft. This will immobilise the car. The universal joint bearings have a limited life span. For reasons of space and weight they are built as needle bearings. Their replacement requires disassembly of the entire cardan shaft. This is less problematic than you might think. The driver notices the defective bearings through loud squeaking and knocking noises. Especially when the bearings have plenty of play, the knocking noises from the shaft are hard to ignore.
Cardan shaft repair
Cardan shaft repair requires its disassembly. To do this, the flange plates on the differential and the engine or gear are unscrewed and pulled backwards. Be careful: start by draining the oil on both points. Otherwise you end up with a spoiled garage. This is a good occasion for change of the gearbox oil for differential and drive. Once removed, the cardan shaft is easily disassembled. It should be checked for hairline fractures, particularly at the universal joint. This is a rare defect, but can occasionally occur. Usually, replacement of hardy disc and joint bearing is sufficient maintenance. Bearings are thoroughly lubricated prior to installation. When assembling and installing the shaft, the prescribed torques have to be observed. Otherwise the bolts might detach or an untimely failure of the shaft could occur. When installing the flange plate, all gaskets are replaced and fresh oil is added. Now you have a cleanly maintained cardan shaft, ready for many thousands of km of service.
Cardan shaft costs
The cardan shaft itself is hardly ever defective up to a point that it needs replacement. Its repair requires in addition to replacement of the joint bearings, renewal of both clamping bearings that fix the cardan shaft to the underbody. A repair kit is very cheap: with prices starting at 150 EUR (± £130) this will allow you to fully repair your cardan shaft.