The truth about dual-mass flywheels

By: CAR magazine

THE constant drive towards greater refinement has given rise to a number of designs that are worth admiring. The dual-mass flywheel (DMF) is high on my list, because it successfully reduces the vibration that an engine will normally transmit into the gearbox, but there is a downside, as we shall see.

An engine that is properly balanced still vibrates in a number of ways. This is caused by the combustion forces acting on the pistons and con-rods at regular intervals. One of these vibrational modes is torsional and the effect is worse at low engine speeds and large throttle openings. Diesel engine vibration is up to four times more severe under these conditions than that of petrol engines.

A DMF is built-up from two flywheels that are about the same diameter as a single flywheel would be, so each one will have about one-half the mass of a single flywheel The first flywheel is attached to the crankshaft and spigotted into the second flywheel in such a way that the two ‘wheels are able to oscillate with respect to each other.

This movement is controlled by circumferential springs working against stops so that the first flywheel is able to vibrate with the crankshaft while the springs ensure that very little of this vibration gets transferred to the second flywheel. A normal clutch unit, but without springs in the hub of the driven plate, is bolted to the second flywheel and the gearbox input shaft is splined to the driven plate of this clutch. The result is that very little torsional vibration gets transmitted to the rest of the drivetrain, resulting in a smooth and silent driving experience.

The amount of oscillation that takes place is directly related to engine speed and the load. This means that when you combine a large throttle opening with a low engine speed the oscillation will be severe, but as the engine speed climbs, or the throttle opening is reduced, the oscillation will die down.

Unfortunately, the DMFs on some vehicles are not as robust as they should be. Recently, within one week we had three readers complaining about these units failing at mileages below 100 000 km. In the first case, one of the circumferential springs had broken, but in the other two cases the driven plates were worn out. All three vehicles were outside the warranty time period, so their owners had to pay in excess of R20 000 to have the complete DMF unit replaced. It seems that repairs to these units are not allowed, or are not feasible due to the parts not being available.

Such prices are unreasonable and bear no relationship to the cost price of the components to the OE manufacturers. This is one more example of the exploitation of motorists that is very common. Our contacts in the trade have provided an idea of the car models that are particularly prone to these failures, but it is unsubstantiated information. However, Googling the phrase dual-mass failure on the Internet provides food for thought.

What can a motorist do to give the DMF an easy time? Avoid the driving conditions that lead to a lot of torsional vibration. This means that lugging, combining a large throttle opening with a low engine speed, should be avoided. This is fairly easy advice to follow on a petrol-engined vehicle, but on a modern turbodiesel that develops maximum torque at about 1 500 r/min, lugging seems to be harmless, though sensitive drivers will notice the onset of vibration as the throttle is opened. This advice is especially relevant if you carry a heavy load or tow a trailer.

An alternative and much cheaper repair method is to replace the DMF unit with an old-fashioned flywheel and clutch assembly. This is being done in SA in a number of cases when the parts are available, and is already big business in the USA. The penalty is that increased vibration will be fed into the transmission, but if the driver avoids lugging this may not even be very noticeable.

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