Dual mass flywheels what they are for and how they work

One of the major issues all the manufacturers have is the dual mass flywheel (DMF) which seems to need to be replaced every time that you need a clutch and that puts the price of clutch replacement up to around £1000 which on a car that has probably done in excess of 100,000 miles will probably make it uneconomical to repair. So the first thing to understand is what a dual mass flywheel is designed to do. The dual mass flywheel is an aid to making the gear change on your car lighter and more positive while also making the engine and gearbox smoother and quieter. So how does it work, in order to make an engine smoother you need to increase the mass of the flywheel but as you do this the mass of the gear train effectively becomes less as a proportion of the total mass that is in motion and you will get noise, rattles and vibration from the gear train, in order to get around this you would traditionally have increased the mass of the gear train which in turn would mean that the synchro's (the brake that matches the speed of the gears that are about to be engaged) also had to become stronger and therefore the gear change becomes heavier and more agricultural. To get around this you have to increase the mass of the gear train without increasing the mass of the gears, this is done by splitting the flywheel into two separate masses (hence the name 'dual-mass flywheel'), one mass that is attached directly to the engine and one mass that is attached directly to the input shaft of the gearbox, these two halves are then separated from each other by a damper which stops the natural vibration of a large high capacity, high compression engine being transferred to the gearbox.

So how much trouble are dual-mass flywheels and when are they worn? Well if you own a Peugeot 407 2.0l diesel you may well find that you need a new one every 20000 miles or less while on Alfa Romeo's they seem to last well part 100,000 miles without trouble, I can almost feel the engineers amongst you bristle at this next comment but bare with me, and even with this sort on mileage on them they can often go again (not best practice) as long as there is only slight tangential rotation in the secondary mass relative to the primary mass, however, if there is any free play axially it should be replaced, however on later cars they have a fully floating dual-mass flywheel with internal clutch adjustment and that is almost impossible to assess the wear on it. These late flywheels also need special tools to stop them from adjusting up while you are doing gearbox repairs so beware!