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Dual Mass Flywheel info
Posted by: UrS4boy (137) on 2009-12-27 16:22:57

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Our C4 UrS cars came equipped with a dual mass flywheel. Its there for a reason (take shock out of the drive train while shifting). Some people don't like them and switch to single mass (7A) flywheels (and clutches). This post focuses on the Dual Mass Flywheel


What is the DMF's function? It is designed to isolate torsional crankshaft spikes created by high power, high torque engines including diesel engines with high compression ratios. By separating the mass of the flywheel between the engines and the transmission, torsional spikes can be isolated, eliminating potential damage to the transmission gear teeth. Luk, the OE supplier of our DMF's says:

"dual-mass flywheels absorb engine vibrations before they are transmitted to the driveline where they can create gear rattle. These units feature a built-in damper designed to isolate engine torque spikes and prevent transmission damage. By design, dual-mass flywheels cannot be resurfaced." (We'll talk more about their last statement)

Here is a cut-away of a DMF (Courtesy of European Car):

There are several components:

Engine-side flywheel damper springs: The damper springs that are visible on the engine side of DMF are designed to dampen heavy torsional spikes that occur when the engine's torsional frequency matches the torsional frequency of the transmission. When torsional frequencies match (have the same amplitude), severe damage can occur to the transmission if not isolated. DMF's isolate the torsional frequency match between the engine and transmission to an r.p.m. range below the operating range of the engine (usually between 200-400 r.p.m.). These damper springs only work hard when the engine passes through 200-400 r.p.m. at vehicle start up and shut down.

DMF damper springs and/or damper springs nylon retainers usually fall because the engine is not running correctly. Bad fuel injectors, worn pistol rings, bad valves, etc, will change the resonant frequency of the engine. A change in the resonant frequency of the engine can force the torsional frequency match between the engine and DMF to fall within the operating range of the engine. This forces the damper springs to work continuously, resulting in failure.

Friction Ring: The friction ring located between the inner and outer flywheel is designed to allow the inner and outer flywheel to slip. This feature saves the transmission from damage when torque loads exceed the vehicle rating of the transmission. (Read: Burnouts/high RPM shifts) The friction ring will wear out if excessive torque loads are continuously applied. Loading the vehicle beyond the rated load capacity is often the root cause of friction ring failures in DMF's.

Center Bearing: A sealed double row center ball bearing carries the load between the inner and outer halves of the DMF. The leading cause of center bearing failure is often related to out of balance vibration caused by not aligning the pressure plate with the DMF dowel pins. The center bearing may also fall if the clutch pilot bearing is destroyed by a worn transmission input shaft (see Pilot Bearing) or if the rated load/towing capacity of the vehicle is exceeded.

Pilot Bearing: The pilot bearing supplied with most DMF's is a caged needle roller bearing. If it fails, the transmission input shaft must be repaired or replaced. If the input shaft is not repaired correctly or replaced, the lack of input shaft support will result in DMF center bearing failure.

Here is what LUK says about their DMF in the diagram above:

"A standard Dual Mass Flywheel consists of the primary flywheel (1) and the secondary flywheel (6). The two decoupled masses are connected via a spring/ damper system and supported by a deep groove ball bearing or plain bearing (2) so they can rotate against each other. The primary mass with starter ring gear is driven by the engine and tightly bolted to the crankshaft. It encloses, together with the primary cover (5) a cavity which forms the arc spring channel. At the heart of the spring/ damper system are the arc springs (3). They sit in guides in the arc spring channels and cost effectivley fulfil the requirements of an "ideal" torsion damper. The guides ensure correct guidance of the springs during operation and the grease around the springs reduces wear between the guides, channels and the springs. Torque is transferred via the flange (4). The flange is bolted to the secondary flywheel with its wings sitting between the arc springs. The secondary flywheel helps to increase the mass moment of inertia on the gearbox side. Vents ensure better heat dissipation. As the DMF has an integral spring/damper system, a rigid clutch disc without torsion damper is normally used."

Here is a video of how a DMF works to dampen out shocks

There is a wear limit for the dual mass flywheels. Bentley just doesn't tell us. Sean D. remembers a "15 mm" limit. Here is some info that might be meaningful:

Link to some DMF play info (20 mm max play, but it's a Sachs DMF)

or this one featuring our very own LUK DMF (but no answer as to play):

Link to some LUK DMF play info

(Note: LUK and FAG are in the same group of Schaeffler companies)

A little more digging finds the answer(sort of):

Link to more LUK DMF info

One problem, which DMF is ours? We know our flywheel has an Audi PN of 034105266C, as shown here along with the pertinent related parts, e.g. needle bearing (034105313A) and the single use flywheel bolts (90705901).

This is what our AAN DMF looks like (note the timing reference pin used by the G4 Crank Position Sensor as the square pin at 8 o'clock in the left hand photo):

Then there is this almost too much information LUK DMF Failure Diagnostic (all of which end in "Replace DMF" (Think Roman Catholic service with a chant back to the priest "Reeee..Place..theeeee..Deeee...Emmmm...Efffaa"):
Link to LUK DMF Failure Diagnostic

Still no real answer, so I downloaded the LUK catalogue (8.4 Mb) (276 pgs). All it says is the LUK DMF PN for the UrS4 and S6 is DMF 038 and the clutch kit is 02-025 (kinderutz, take note, I suspect your eBay seller had the wrong PN)

Link to the LUK catalogue
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