A transmission flywheel is a device used to apply the torque and horsepower of the engine to the drive line of a stick shift-equipped vehicle. Made of heavy cast iron, steel or aluminum with a steel insert, the transmission flywheel provides half of the surface that the vehicle's clutch disk operates on. The weight of the stock transmission flywheel aids in building inertia, which is used to assist the vehicle in accelerating from a dead stop. In high-performance applications, the transmission flywheel is commonly a very lightweight component, which allows the engine to rev and decelerate faster. In any form, the transmission flywheel is attached to the engine's crankshaft using very strong bolts to ensure they do not shear off easily.
In addition to providing a surface for the clutch disk to ride on, the flywheel also contains the vehicle's starter ring gear. This steel gear surrounds the flywheel and is comprised of teeth that mesh with the starter's spur gear to crank the engine when starting it. This steel gear is commonly pressed onto the flywheel and held in place with several small welds. In the event of damage such as broken or ground teeth, the starter ring can be replaced by grinding the welds and pressing the ring off of the flywheel.
Many times, the starter ring or flex plate used in an automatic transmission application is mistakenly referred to as being a transmission flywheel. While the flex plate serves a similar function, a flywheel by definition is a heavy component intended to increase and retain inertia as the engine spins. An automatic transmission uses a torque converter to increase the required inertia to accelerate a stopped vehicle. The two functions of a flex plate are to provide a location for the torque converter to attach to the engine's crankshaft as well as to hold the starter ring gear so that it can contact the vehicle's starter mechanism.
In many racing-type drive lines, an automatic transmission's flex plate will be used to mount a special small diameter clutch assembly in place of a typical transmission flywheel. By eliminating the heavy weight of the flywheel as well as the rotating mass diameter of the clutch package, the engine is able to accelerate much faster, meaning the vehicle moves forward much quicker. Conversely, pulling applications such as farm tractors or semi-tractors use an extra-heavy transmission flywheel to aid in bringing a very heavy load into motion.