DMF vs. Solid.

reeac

A neighbour with an Avensis had DMF problems and had it replaced by a solid clutch/flywheel assembly. He says that he can't feel any additional vibration as a result. Is this general experience? If so why do manufacturers instal DMF,s?

Retrogamer

DMF is to absorb the vibrations from the engine and gearbox.

Without the DMF the vibrations are transferred to other components such as the main shaft and bearings inside the gearbox or the crank shaft and supporting bearings inside the engine.

DMF was to bring some refinement to diesel engines. It keeps them nice and quiet and with less vibrations. Less vibrations was meant to allow them to cruise at lower RPM (longer gear ratios) and helping the environment by delivering more MPG

Using an SMF when sitting idle and the clutch isn't pressed then they normally sound like an early 90's Transit.
Using one long term is usually ok if it's a good kit but i've seen some cheaper kits fall apart from vibration funny enough (clutch broke up then made big gauges in the SMF)

Some cars don't like the added vibration and i've heard of gearboxes wearing prematurely and in extreme cases crankshafts snapping.

colino

Noise, Vibration, Harshness is a subset of vehicle engineering that has really split in to two camps. We have had very simple clutches since the invention of cars but of late, instead of building better balanced engines or adding balancer shafts or harmonic pulleys, DMFs have been introduced to hide the shortcomings of many diesel engines.
Some cars do just fine if converted to "SMF" clutches (some commercials can be specced that way from the factory), while others seem to add unacceptable vibrations and shorten the clutches lifespan. You really have to check with forums to find out which ones work and which ones don't like it.
Skoda Octavias for example are surrounded by myths of bone-jarring vibration and snapped crankshafts as a result of conversion. One taxi boy I know says his seems to "burr" a bit at idle, but has done this since he converted it 120,000 miles ago.
So, in short, lazy engineers masking a symptom instead of proper engine development, coupled to bad driving equals introduction of the unnecessary DMF.

forgotmyname

How good is your neighbour at detecting mechanical noises though?

Sometimes drivers dont spot obvious faults and thought that some noises were normal.

reeac

forgotmyname wrote: »

How good is your neighbour at detecting mechanical noises though?

Sometimes drivers dont spot obvious faults and thought that some noises were normal.

He's very experienced at car DIY. Re. other responses, yes I know the reasons for DMFs (as hinted at by the contents of my posting).


One other item: the Jag. XJ6 which came out in 1986 featured a manual gearbox option which used a conventional clutch initially for the 3.6 litre engine but a DMF when they upped it to 4.0 litres in 1989. I guess that even with petrol that was too big a swept volume per cylinder. I had a manual 4.0 which had no DMF problems at least to around 120k when I sold it.

colino

Too large a swept volume on a silky smooth Jaguar? My 944 at the time, with what was considered old-tech, competition clutch as standard, was equally smooth with its balancer shaft, and a greater swept volume in its 4-potter.

Gloomendoom

Jag were probably more interested in preserving the gearbox than refinement. Maybe the Porche gearbox didn't need kid gloves.

colino

While no Jaguar suits a Spanish gearbox, I've never seen a broken one. They are well designed, engineered and built pretty tough and even during the notorious Leyland era, didn't seem to need any special protection from the not very peaky, engines.

bigjl

DMFs have improved since they were introduced.

There was a decent article in Car Mechanics Mag a few years ago with input from the manufacturers.

If you ensure you start and stop the engine with the clutch depressed and don't labour the engine in too high a gear you will get much longer life from a DMF.

The wifes Clio is on the original DMF and it has been driven mostly round London. From memory it is on around 130k.

My V50 had over 150k in the original DMF.

My old mk3 Mondeo was on the original DMF when the bloke I sold it to traded it in, over 250k on it.

There have been issues with crank failure due to a solid DMF being fitted.

They certainly are not perfect, anybody that has driven a Transit will tell you that, to prevent DMF damage the vehicle was made ridiculously easy to stall, if the ECU thought you didn't have enough revs it would cut off! Likely improved in later ones.

atrixblue.-MFR-.

bigjl wrote: »

DMFs have improved since they were introduced.

There was a decent article in Car Mechanics Mag a few years ago with input from the manufacturers.

If you ensure you start and stop the engine with the clutch depressed and don't labour the engine in too high a gear you will get much longer life from a DMF.

The wifes Clio is on the original DMF and it has been driven mostly round London. From memory it is on around 130k.

My V50 had over 150k in the original DMF.

My old mk3 Mondeo was on the original DMF when the bloke I sold it to traded it in, over 250k on it.

There have been issues with crank failure due to a solid DMF being fitted.

They certainly are not perfect, anybody that has driven a Transit will tell you that, to prevent DMF damage the vehicle was made ridiculously easy to stall, if the ECU thought you didn't have enough revs it would cut off! Likely improved in later ones.

I have a doubts that a crank can fail due to vibration created from solid flywheel conversion after all they are made from a single billet of steel and balanced accordingly and before DMF's there were and still are cars a plenty running their original SMF's by you reckoning these are ticking time bombs. Also If this was the case, then SMF manufacturers like LUK and valeo would NOT make them for DMF applications to be converted to SMF as they would be liable for costs of the damage.

Retrogamer

atrixblue.-MFR-. wrote: »

I have a doubts that a crank can fail due to vibration created from solid flywheel conversion after all they are made from a single billet of steel and balanced accordingly and before DMF's there were and still are cars a plenty running their original SMF's by you reckoning these are ticking time bombs. Also If this was the case, then SMF manufacturers like LUK and valeo would NOT make them for DMF applications to be converted to SMF as they would be liable for costs of the damage.

Is this the case?
I thought the majority of crankshafts were made from molten metal (cast) rather than machined from a solid billet.
Hence all the roughness on parts not machined down to be smooth.

The only car i know off the top of my head that doesn't like SMF is the 4x4 Octavia TDi. As the engine is the same as other TDI models with no SMF / Crank issues it's likely the extra vibration from the 4x4 transmission is what causes the crank to fail