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The Critical Difference

New in Clutch University

What's the best way to drive my manual transmission car?

Chapter 4

Flywheels

(click on any image to enlarge)


Billet steel flywheel	Billet aluminum flywheel with steel insert

The flywheel is primarily an inertia device. As the flywheel spins it stores energy or inertia that helps move the mass of the vehicle as you engage the clutch.

Factory flywheels are designed to apply the optimum amount of stored energy to provide good drivability for the vehicle. Vehicles with smaller engines have relatively heavy flywheels due to the extra inertia needed for a smooth transition to engagement. Reducing the weight of the flywheel, while increasing performance, could reduce the drivability of the vehicle.

Under racing conditions, the flywheel weight can be used to control the inertia applied to the drivetrain. For instance, if a vehicle tends to ‘bog’ upon engagement of the clutch, increasing the flywheel weight will increase the inertia needed to launch the vehicle smoothly. Too much flywheel weight may cause excessive inertia to be applied, causing the tires to spin. Reducing flywheel weight under this condition will reduce the inertia applied to the vehicle and allow smoother acceleration.

Aluminum flywheels are used in drag racing high horsepower applications which require the clutch to slip as the vehicle leaves the line. Steel flywheels are used primarily in street driven vehicles.

Other load factors can effect flywheel selection, such as rear gearing or transmission gearing. With the abundance of gearing choices available today, it is possible to use almost any flywheel if the proper selection of gears is made. This was not always the case – in the 70’s when the gearing choices were not available, racers had no choice but to use the flywheel weight to control the vehicles on launch. It is more efficient to use a light flywheel and proper gearing than to use tall gearing and a heavier flywheel.

TECH TIP – FLYWHEELS AND DRIVABILITY

Selecting the proper flywheel helps you achieve the drivability you desire for your vehicle. Heavy street cars will benefit from a heavier flywheel to generate the inertia to get you moving. An aluminum flywheel will not generate as much inertia to move the vehicle, and thus it would be necessary to slip the clutch more on takeoff. Some street vehicles may benefit from a lighter flywheel, but only if there is enough rear gear to help you transition the clutch smoothly without excessive slippage.

TECH TIP – HOW DO I SELECT THE PROPER FLYWHEEL?

When we are asked for a flywheel recommendation, several factors are considered to make a proper recommendation.

What is the primary use of the vehicle? For street driven vehicles, a steel flywheel will provide easier engagement and longer clutch life.

What is the rear end gearing? Lower (higher numerically) gears will make engaging the clutch easier, while higher gearing requires the clutch to be slipped more on takeoff for a smooth transition.

What is the weight of the vehicle? If it is over 2800 pounds, we will generally recommend steel.

Remember what the flywheel’s job is – to help you get the car accelerating smoothly. If you have ever driven a vehicle and tried to pull off in second gear, you know you had to slip the clutch significantly more to get moving than if you started in low. This is the same effect you would notice if the flywheel is too light.

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