On the Fly
The function of a drivetrain is to serve as the conduit for power from the engine to the wheels. Depending on vehicle configuration, engine power is transferred to the driveshaft and rear wheels in rear-wheel-drive vehicles, to axle half-shafts and front wheels in front-wheel-drive vehicles, or to all four wheels in all-wheel-drive vehicles.

The drivetrain starts at the engine, where pistons spin the crankshaft, then the flywheel, clutch disc, clutch cover, transmission, differential, and ends at the drive axles. It is at the flywheel where we begin our drivetrain journey. The flywheel provides a smooth, flat surface for the clutch to operate properly and serves to take heat away from the clutch pressure plate and disc friction material.

Dual Mass Flywheel (DMF)
The OEM flywheel found in most modern manual transmissions (as well as semi-automatic transmissions like the Audi/VW DSG and BMW SMG) is classified as a dual-mass flywheel (DMF). The DMF is designed to eliminate engine vibrations before they are transferred to the drivetrain.

A DMF is tuned by the manufacturer to match engine torque and horsepower curves, engine harmonics, and vehicle load dynamics, based on transmission gear ratios as well as overall rolling diameter of the rims and tires. It is constructed by splitting the standard single mass flywheel in half. The primary section bolts to the engine crankshaft and the second section is where the clutch assembly is installed. Contained within the two sections is a complex system of varying rate springs, axle and radial bearings, and lubrication. The two halves are rubber-sealed. The aforementioned series of components is referred to as the friction pack. The main purpose of the friction pack is to absorb vibration, allowing for smooth, comfortable shifting, low speed driveability, and reduced drivetrain noise.

When increasing the horsepower of a vehicle, the DMF can be the weakest link. It is designed with an over-torque friction release. As engine torque is increased, the flywheel has the potential to overload. To protect itself, the flywheel will slip, causing a reduction in torque, protecting the internal springs.

Pros: Reduced noise, smoother shifting, increased fuel economy, longer engine life.
Cons: High replacement cost ($799-$1599), can prematurely fail if horsepower and torque are increased, can slip under increased load, 8-11 degrees of movement before clutch engagement.

Single Mass Flywheel (SMF)
This flywheel bolts directly to the crankshaft and the clutch assembly connects directly to the flywheel. The most common and reliable flywheel upgrade is to replace the standard dual mass flywheel with a single mass flywheel. The SMF is typically sourced from an earlier application within the model line. For example, when upgrading a clutch on a transverse-engined VW/Audi five-speed 1.8T, the stock dual mass flywheel is replaced with an SMF from a 1990-92 VW Corrado G60. In the case of the VW/Audi conversion, the matching clutch kit is also needed to make it work. Upgrades for BMW and Porsche involve scrapping the stock dual mass for the European-spec SMF from the same application. When Porsche released the specifications for the GT3, it was quick to point out the use of an SMF for increased performance.

Pros: Increased performance, increased throttle response, increased durability (vs. dual mass), increased service life.
Cons: Increased engine vibrations.

Aftermarket Lightweight Flywheel (ALF)
Aftermarket lightweight flywheels are designed to weigh less than the SMF and the DMF by at least 50 percent. The most common ALFs are constructed either of aluminum or steel. Most aluminum flywheels feature a replaceable friction surface, making them easy to rebuild. On the other hand, a steel flywheel can be resurfaced only once.

Pros: On a naturally aspirated vehicle, an ALF is a good way to reduce excess rotating mass, allowing for increased acceleration.
Cons: Excessive low end clutch chatter, premature synchromesh wear, and possible engine damage will occur if the ALF is too light or not properly balanced. If the ALF is too light, a lack of initial inertia can cause the vehicle to stall easily when moving off from a standstill.

Conclusions
A properly designed flywheel that is chosen to match the vehicle application is integral, since the flywheel is an energy storage device that determines the amount of power available to get the car moving. A heavier weight flywheel can store more energy as it rotates, which is perfect for cars with engine and gear combinations that need more power or inertia to make the car accelerate. A lighter flywheel creates less inertia at launch, but accelerates quicker through the gears. If the vehicle is turbocharged and German, sticking with an SMF steel flywheel around the same stock weight as the OEM works the best. This setup tends to give peak low end torque, smooth gear transitions and mid-range boost response. Consulting with a specialist who has experience with your vehicle type is vital to getting the best flywheel for your application.

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