Flywheels are rotating mechanical devices more common as an inertia device. These are used as a storage of rotational energy which enables the whole vehicle to move as the clutch engages.
The maximum amount of stored energy could be applied by factory flywheels which improve vehicles’ drivability. Smaller vehicles are said to have a bit of a heavier flywheel because there would be extra inertia required for the vehicle to move smoothly upon transition to engagement. Experts have to clarify and keep in mind that lighter flywheels with increased performance will lead to reduced vehicle drivability.
Flywheels are specially altered during racing conditions to control the inertia exerted onto the drivetrain. However, too much flywheel weight may provide too much inertia on the train resulting for the vehicle tires to spin uncontrollably. But given a condition where the vehicle seemed to experience a ‘bog’ upon engagement will need increased flywheel weight so that there would be enough inertia applied for a smooth launch. Since too much flywheel weight is not good, there would be a need to reduce the weight for a much lower amount of inertia to be applied to the vehicle leading to smoother acceleration upon engagement.
Normal and everyday vehicles seen on the streets use steel flywheels because it applies enough inertia to the vehicle for safe but smooth driving. Drag racing vehicles use aluminum flywheels because it applies higher horsepower, but this requires clutch slipping when the vehicle leaves the starting point.
There could also be other factors in selecting the right flywheel for specific conditions like transmission gearing or rear gearing. There is a huge market of gearing choices in modern vehicles allowing almost any kind of flywheel for any car given the condition that the right gear is being used. This was not the case in the 1970s since there were very limited gearing choices available leaving racers with no choice but to manipulate their vehicles’ flywheel weight to achieve more effective launch and vehicle control.
Take note that it is much better to settle for proper gearing equipped with a light flywheel as compared to a tall gearing partnered with a heavier flywheel.