Dual-Mass Clutches Double-Edged Opportunity

Well, other than the obvious complement of wheels, tires, and standard running gear, they also have what is turning out to be a blessing in disguise for the aftermarket: a dual-mass flywheel.

The dual-mass flywheel is a development of the standard clutch mechanism on manual transmission-equipped vehicles. It is designed to reduce the degree of shock loading transmitted through the drivetrain when shifting gears.

On the Z4, a specially developed dual-mass flywheel is used with the H-gearbox. Because the 6-speed manual transmission has more gears, the flywheel is modified to compensate for vibration and noise.

In this flywheel, the two halves are connected via inner and outer dampers (rubber-like damping material), supported by a plain bearing. The plain bearing requires less space than a ball bearing and enables an additional inner damper to be used.

The transmission input shaft is supported through the flywheel in a needle bushing with a roller pilot bearing. The flywheel is connected to the crankshaft flange with Torx socket bolts.

The use of the technology on passenger vehicles points to another developing trend. The combination of the ability to drive at lower rpm, due to the expanding number of gears in manual transmissions, and increasing attention to reducing wind and engine noise means that other noises become more noticeable.

All the while the natural damping effect of a car’s mass is reduced due to its lighter weight and use of thinner materials. The engine still creates torque and noise, and lighter-weight gearboxes can be prone to gear rattle.

The sum of all these forces is an increase in noise and shock being transmitted to the driver, which is counter to the whole design trend. This is why Luk developed the dual-mass flywheel, a development that was released worldwide as far back as 1985.

But, as interesting as that small application is, the big mover–in many senses of the phrase–is the light truck application of the technology. It is here that the aftermarket is seeing more of its potential realized.

“The original dual-mass flywheel is subject to unexpected failure and is non-rebuildable,” says Joel Fenwick, vice-president purchasing, Fenwick Automotive Products. Fenwick says that the trend toward being able to offer a solid-flywheel option is one that his company has picked up on, as well as being able to still offer a dual-mass flywheel replacement clutch or a complete kit.

“By converting it to a solid flywheel, at the same time you have to use a revised design. The dampening of the flywheel has now been transferred to the disc,” he says, so it must also be more robust.

Robert Bean, director of product development, Mevotech Inc., says that the dual-mass flywheel in heavy-duty applications has a bad habit of failing when it’s most needed. Then the customer is faced with a significant decision to make.

“People want to replace [the dual-mass flywheel] with a solid one. People want to save money, but then it’s jerky and rough. That’s what it was put in to eliminate. The same guy eliminates the hydraulic transmission mounts and engine mounts, and that just magnifies the problem.” Still, says Bean, he understands the desire by a consumer to want the lower-cost option.

“The cost is double or triple sometimes. Really, can you get away without it? Yes you can, but I always caution people that they may experience a little more shock.”

There are a number of designs of dual-mass flywheels percolating out of suppliers–from the original Luk elastomer/spring design, to an innovation out of Sachs AG which uses a set of planetary gears that allows for the two flywheel halves to move against each other and damp the shock loadings.

In all cases, however, the goal is about both driver comfort and preserving the integrity and function of the drivetrain.

“On the diesels they tend to break pretty easily. They are put on to help get rid of the vibration and noise and they tend to break when you really need them,” says Bean.

Technicians and counterpeople faced with replacement opportunities must always accurately convey the tradeoff with making the solid flywheel retrofit. There is a price advantage to be sure, but the dual mass flywheel was created for a reason. The price advantage is significant, though, and may actually be sufficient to keep a vehicle in operation.

By using the full complement of parts required for the changeover, the customer, the repair shop, and their jobber can all be satisfied with the result.

DIAGNOSING CLUTCH PROBLEMS

The dual-mass flywheel, such as that used on Ford F-Series pickups with the 7.3 diesel engine and its full-sized cousin from General Motors, the C/K, has unique features as discussed in the article and these can make diagnosing problems more challenging.

The flywheel’s two sections allow for slippage to help reduce the amount of shock being transmitted to the drivetrain, but this can cause misdiagnosis for a technician not vigilant about the effects of a failing DMF.

Is the clutch slipping or is the flywheel failing?

If the driver is complaining of a slipping clutch but the clutch does not show the telltale evidence of overheating, such as disintegrating friction material, or discoloured clutch cover or disc material, then it is quite possible that the slipping sensation being experienced by the vehicle’s driver is due to the slippage at the flywheel instead.

Comprehensive testing of the dual-mass assembly is impossible without a dedicated test rig, but some information on dual- mass flywheels points to the fact that if they can be rotated more than 20 mm against each other by hand as measured on the outer circumference, then the wear limit has been reached.

In the more extreme cases, it may be possible to determine that lubricant has been shed by the DMF due to overheating and this is incontrovertible evidence that the unit has failed.

FLYWHEEL RESURFACING

While resurfacing of dual mass flywheels is not considered an option, there are sufficient clutches out there to warrant the inclusion of some words to that subject here.

Proper resurfacing of the flywheel is critical to clutch operation.

In clutch replacement, the flywheel must be resurfaced or replaced; failure to do so will void the warranty of the clutch set, according to major manufacturers.

Increasingly, though, original equipment clutch systems are designed without allowance for flywheel resurfacing. If manufacturers’ specifications indicate this, the flywheel cannot be resurfaced and must be replaced during clutch installation to avoid catastrophic failure.

It is particularly difficult to ensure proper disc to flywheel contact following the resurfacing of a stepped or cupped flywheel, as all dimensions must remain identical relative to each other.

These dimensions are generally available in a supplier’s catalogue, but it can be time-consuming for a machine shop to do correctly, which means that it might push the resurfacing cost above that of a replacement unit.

Machinists need to ensure that the specified dimensions are used, as simply measuring the various steps on the existing flywheel may only lead to the machinist replicating the problem that caused the original failure.