Can anyone explain the sudden growth in dual mass flywheels. My local independent tells me they have to change these on Transits and Sprinters quite frequently (£400 on the sprinter) as well as several cars. Not bad if under warranty, but horrendously expensive if you have to do it yourself
I appreciate that the theory is they help to cushion gearchanges and make this smoother, but I did not think this was really a problem, particularly on light vans
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My understanding of these devices is that they are in place to control torsional vibration in the crankshaft; the flywheel equivalent of the vibration reducing two part fan belt pulley typically found on the other end of the crankshaft.
Number_Cruncher
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Absolutely right, and with the explosion of modern refined diesel cars they are commonplace these days. It is true as you say that they can get knackered, in the same way as suspension bushes do, I understand thatr Fiat/Alfa JTD engines are particularly bad offenders in this area.
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Yes, supposed to make things smoother. I think its Audi who say that they should be changed at the same time as the clutch. £1000 clutch change basically....
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>>£1000 clutch change basically....
Ouch!!
I have had a quick search on the ZF/Sachs site, and the purpose of the dual mass flywheel isn't at all as I, incorrectly, implied in my post above.
The crankshaft pulley vibration absorber is there to protect the crankshaft from excessive stress due to torsional oscillation, and is specified as a reliability enhancement.
The dual mass flywheel is there to reduce torsional oscillation of the whole drivetrain, and is specified primarily as a noise, vibration, and harshness countermeasure.
Number_Cruncher
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OK
Same question
so why are they so popular all of a sudden
A solution without a problem, or is there a terrible vibration problem I have not noticed?
Probably my age
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My guess, more and more people want diesels that are as quiet and smooth as petrols so the switch to dual mass flywheels is an attempt to make them happy. I have read of some owners switching back to solid flywheels usually from a performance standpoint and the increased noise is very noticeable (online sound recording of a before and after VAG vehicle)
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These tend to be fitted to a lot of the modern, higher powered diesels.
They are there to prevent torque spikes through the crankshaft from damaging the transmission. Such spikes are more likely to occur in engines that put out a lot of torque and run with high compression ratios.
On the Ford 2.2 TDCi engine in addition to a dual mass flywheel they have had to electronically limit the torque output of the engine in the first two gears to stop it damaging the gearbox becuase it outputs so much torque.
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Lurching into the frequency domain...
The dual mass flywheel acts as a low pass filter in the drivetrain system.
The diesel engine produces torsional vibration in a broader frequency spectrum than an equivalent petrol engine owing to the sharp increase in cylinder pressure after the end of the delay period.
The incorporation of an intermediate mass-spring system attenuates the higher frequency components of this type of noise source, reducing the noise and vibration felt, and heard by the driver.
Number_Cruncher
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Quite so, Number Cruncher. So it's a transmission damper, like the heavy metal ring rubber-bonded onto a central flange that Land-Rovers used to have? Perhaps they still do.
It was an easy bolt-on accessory fitting on the gearbox output flange. I think it was intended really for diesels, for the reasons you say. I tried fitting one to my LWB petrol V8 conversion and it miraculously improved the quality of the gearchanges and transmission take-up, by smoothing the considerable slackness in the long drive-train.
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Very short tale of a failed dual mass flywheel from a A4.
www.carmechanicsmag.co.uk/cgi-bin/tftt.cgi?q=050303
Steve.
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I think Audi say to replace as a matter of course.
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That's got to be expensive?
Steve.
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It does seem an unnecessary complication, especially as they don't appear to be too reliable (and how many will need replacing after 100k miles or so?). I was amused to see this (US) site offering 'solid flywheel conversion kits', which look like they might just be made a bit chunkier to achieve a similar end...
www.standardtransmission.com/forddmf.html
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