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Proper rear wheel drive with the hardy disc
The hardy disc (joint disc) is a simple, though important component in all cars with rear wheel drive. It damps the torque generated by the engine and in doing so prevents untimely wear of the cardan shaft. Furthermore it makes acceleration and load change more comfortable.
Construction of the rear wheel drive
A real wheel drive is the combination of a front engine driving the wheels on the rear axle. The torque generated by the engine is transmitted by way of the cardan shaft from the front to the rear. Depending on car type, the cardan shaft connects the gearbox with the differential or connects the engine with the transmission, installed in the rear. This construction is called "transaxle" and causes an optimal centre of gravity in the car. In most rear-wheel drive cars the differential is used. This small transmission converts the longitudinal torque into a bilateral torque, driving the rear wheels.
Where to find the hardy disc
The hardy disc sits in the proximity of the engine as the connecting point for the cardan shaft. The cardan shaft is a rotating bar, interrupted in the middle by double joints to compensate the swinging movements of the car. The engine drives the rear wheels via the cardan shaft – and they offer permanent resistance. To guard the drive against untimely wear, the hardy disc is placed in between as a buffer. It is a simple component, which is easily accessible.
Construction of a hardy disc
The hardy disc is a massive, circular and disc-shaped rubber block. Within the disc is drilled a ring of holes. Every hole has a steel bushing. The disc is fixed onto the drive shaft of the transmission or the engine. The cardan shaft is connected on the other side of the hardy disc. The hardy disc is not encapsulated and needs no lubrication.
Hardy disc defects
Rubber, no matter how high-grade, loses elasticity over time. The more brittle the rubber becomes, the more its ability to absorb the forces of the load change is impaired. The steel bushings increasingly develop play and the entire construction loses stability. This can be perceived through knocking and beating sounds during driving. Replacement of the hardy disc should not be postponed for too long: the bolts can become loose or even break off. A broken cardan shaft constitutes serious damage. The loose, wayward drive shaft can cause a very dangerous driving situation.
Replacement of the hardy disc
Therefore it makes sense to replace the hardy disc either according to maintenance schedule, or in any case at the first signs of wear. For a successful replacement you need a hoist or a ramp. It is important that the right torque is applied to the hardy disc bolts. Although the bolts of the hardy disc in the literal sense of the word are no stretch bolts, replacement of the connecting bolts is recommended as well. You never know under what kind of strain they have been.
Costs for a hardy disc
The price for a hardy disc depends for a great deal on the transmitted torque. On motorcycles with cardan drive a hardy disc is available for less than 10 EUR (c. £ 9). On trucks with a high torque, the price of a hardy disc starts at 200 EUR (c. £180). For family cars the price of a high-grade hardy disc from a branded manufacturer like FEBI BILSTEIN lies at approximately 100 EUR (c. £90). This also depends for a great deal on the car model. Even for luxury cars such as Mercedes and BMW hardy discs are available starting at 60 EUR (c. £53). All in all, the hardy disc is a component which is easy to replace, considerably improving the driving quality and increasing the value of the car.