So just what is it about the letters DMF that bring the very worst out of a normally sane and perfectly calm individual? To many the DMF (Dual Mass Flywheel) is nothing more than an additional charge or cost on motoring, always looming out there somewhere! Like a black cloud the merest hint of a rattle or vibration or even an odd feeling clutch pedal signals a load of horror that may just be coming our way.
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Precisely what is a DMF and what does it do? Why do we need them at all? In this respect we are in fact the authors of our own doom. The sole and single purpose of the DMF is to isolate engine vibration from the vehicle transmission and by extension the whole vehicle. There is in fact nothing new about this vibration damping as it is a standard component of automotive clutches going back to the middle of the last century. The only development to this technology is the level of comfort and refinement that we demand as a car buyer in the sales showroom. We have to be fair to the engineers who design our modern family hatch too as the internal combustion engine has become more harsh as higher levels of power and torque are demanded and delivered and yet with modern computer designed damping systems it can be difficult to know if the engine is even running without looking at a tachometer.
In terms of function, the DMF does little more than exploit the simplest physics to improve engine vibration damping but unfortunately at the cost of simplicity. Traditionally the clutch driven plate or “clutch disk” as it is often termed had a ring of damping springs between the splined hub and the friction linings on the driven plate. These springs would compress in the direction of both drive and over-run upon sharp engagement of the clutch, sudden application of torque, engine vibration or even during starting and idling. As these springs were placed between the hub of the clutch disk and the friction linings then there is a maximum size spring that will fit and maximum distance they could be placed “outwards” from the hub centre, the lever length. In an ideal world, these springs could be placed around the perimeter of the clutch disk where their size and performance would be optimised but that simply is not possible. The answer is to split the flywheel into two parts free to move relative to each other with springs between them but crucially placed out at the very perimeter of the clutch, where they are at their most effective and most efficient. Simply a longer lever against a more robust spring if you will.
So the damping springs are moved out further from the clutch hub:
Well that all sounds very simple and put like that it really is. The issue is that we now have two considerable masses supported on bearings and retained by springs and various other moving parts. To put a number on it, as many as fifty or more components in fact all rotating at thousands of revolutions per minute and soaking up the vibrations from harsh and knocky direct injected petrol or diesel engines, many with turbo charging, supercharging or both. Add to the mix the car owner who tows a caravan around Europe every summer, his neighbour who is a bit of a “racing driver” or the more mature driver who uses clutch modulation at full throttle to adjust their speed. Now we have a very robust and well engineered solution to the vibration and refinement conundrum but being operated at the very limit of its design envelope. Have a look at this video which explains the damping process quite well.
It is hardly a surprise then that such a complicated system might wear or suffer damage from abuse or surprising that the whole clutch system on your car is more expensive than a clutch of old. Dangerously the internet will often suggest a “solid flywheel” option to banish all of your DMF woes but most of the time this is nothing more than fools gold. Yes, the parts are considerably less expensive to replace than a full DMF clutch kit but however expensive the original parts are they will cost less than the repair or replacement crankshaft or transmission. It is vital to understand that the DMF acts like a pressure relief mechanism allowing all the vibration to leave your engine and transmission. Take it away and there is no way to dissipate the destructive vibration which will come back to bite you.
So is this the future for all of us? Indeed so, even with the widespread adoption of DKT or DSG systems using double wet clutches. These very high tech clutch and transmission systems have really revolutionised vehicle design but there is no escaping the harmful vibrations which have to be damped.
If you find this interesting do check back on our other blog posts describing both double clutch transmissions and engine vibration damping.