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Rotational Energy and Challenger Flywheels

Rotational Energy and Challenger Flywheels

Manual transmissions might be a dying breed, but nothing quite beats the feeling of blasting through your Challenger's gears on an open stretch of highway. To make all that happen, however, your clutch and flywheel need to up to snuff. With upgrades to horsepower, you'll want to consider lightweight flywheels to keep you in the power band. But how does that all work exactly?

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If you're planning on upgrading your Challenger's clutch, you should be considering a compatible flywheel as well. Much like brake pads and brake rotors, the two go hand in hand. Where a clutch transfers the power to the drivetrain, the flywheel stores the rotational energy, keeping your RPMs from plummeting every time you shift, among other benefits.

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What is the Role of the Flywheel in the Drivetrain?

The flywheel is connected to the crankshaft. As the pistons move up and down, the rotational inertia of the crankshaft is harnessed by the flywheel. In actuality, the flywheel is a solid and heavy wheel. Due to the mass of the flywheel, it takes a lot of force to spin the wheel around. If it wasn’t for the rotational energy of the flywheel, your Dodge Challenger will jerk or hesitate every time you change gears or brake to a full stop.

It is the job of the flywheel to store rotational energy to keep the engine in motion. It modulates the individual torque spikes (produced on the power stroke) of each cylinder and smoothes them into one constant rotational motion. With that being said, the flywheel performs four major functions:

  • The weight of the flywheel is perfect for balancing the crankshaft. This results in a smoother-feeling engine with lesser vibrations even at higher loads.
  • The flywheel is equipped with teeth on the outer edge (called a ring gear). This enables the starter to engage the flywheel and turn the engine over to start it. 
  • It also provides a connection between the engine and the transmission. The rotating flywheel transfers the power to the clutch and provides the acceleration and torque that you can feel when stepping on the throttle.
  • The mass of the flywheel stores the rotational inertia to keep the engine spinning no matter what gear you’re in, even in neutral. A heavy flywheel takes more energy to spin, but then becomes more difficult to stop (moment of inertia).

How Does the Rotational Energy of the Flywheel Affect the Engine RPM?

Kinetic energy is based on mass and velocity. In regards to a flywheel, one that is heavier or spinning faster will have more kinetic energy. Since the engine is the source of power, then flywheel velocity (and engine RPM) are dependent on the mass of the flywheel.

In the case of a larger and heavier flywheel, it does not need to spin as fast as a lighter flywheel to have the same amount of energy. Alternatively, a heavier flywheel would require more energy input to accelerate it. Due to the effect of inertia, flywheels are designed to modulate and maintain RPM and therefore are resistant to change. 

Applying this to a Challenger, a heavier flywheel will be more difficult to accelerate, but once at speed will be more resistant to slowing down. For example, the stock flywheel on a 2017 Challenger with a Tremec TR-6060 weighs 30 pounds. To accelerate, more throttle input will be required before a change is seen in engine RPM (remember, the flywheel is directly connected to the crankshaft, thus the flywheel is always turning at the same speed as the engine). When changing gears, however, note the absence of a big drop in RPM when the clutch is disengaged. This is due to the mass of the flywheel attempting to maintain RPM. 

If you were to equip your Challenger with a lighter flywheel, it would be easier to accelerate the flywheel for any given throttle input and therefore rev the engine quicker. Due to the lower mass, however, it will decelerate faster and be more difficult to get the vehicle moving.

Why Should I Upgrade to an Aftermarket Flywheel?

If you shoot for a lighter-than-stock flywheel, you will instantly feel an increase in engine acceleration and responsiveness. This is expected given the lighter mass of the flywheel.

However, we have a slight word of caution. While upgrading to a lighter flywheel will yield some performance gains, choosing an overly light flywheel can affect the drivability of your Challenger. The clutch engagement will be trickier since you will need to rev the engine higher before engaging gears. This could prove to be a chore in city driving. In fact, even any flywheel lighter than stock will affect the drivability on the street. 

Billet Steel Flywheels: Billet steel flywheels are marginally lighter than the cast iron variety. This type of flywheel is also engineered to be a bit stronger and can handle more horsepower at higher RPM. If strength is your primary concern but you want something that performs similar to stock, billet steel is a great choice as it will maintain close to OEM drivability. One downside to this type of flywheel is that it will always need to be resurfaced when changing clutches, which can only be done a few times before requiring complete replacement.

Aluminum Flywheels: Flywheels crafted from aluminum are engineered to be strong and much lighter than the OEM flywheel. Sharp throttle response and acceleration is to be expected when outfitting your Challenger with an aluminum flywheel. On a drivability front, day-to-day driving will take a hit as you will need to rev the engine higher (and slip the clutch more) in order to get the car moving. As aluminum is malleable, flywheels made of this material often have a replaceable wear plate (made from steel) that is in contact with the clutch disc. Heat dissipation is also improved when using an aluminum flywheel.

Chromoly Steel Flywheel: Chromoly 4130 steel contains more carbon molecules than ordinary steel. The word ‘Chromoly’ is short for chromium-molybdenum steel, which is added to high carbon steel. With a very hard surface area, this material is lightweight yet extremely strong - intended to be used in high horsepower racing applications. As with all lightened flywheels, driving with a chromoly flywheel will also require changing your driving style. 

Is There Such a Thing as a Flywheel That is Too Light?

Yes, but it all depends on the application. In the case of race-tuned Dodge Challengers, an extremely light flywheel will enable the motor to spin faster and deliver better acceleration. But since the flywheel is lighter, the engine revs will drop faster and will demand the reflexes of a proper racing driver to motivate the vehicle.

But for street applications, installing a flywheel that is too light will make your muscle car almost impossible to drive. The lower inertia of the flywheel will also affect the performance of the vehicle when climbing hills or when pulling away from a dead stop. Ultra-light flywheels will also emit more vibrations in the cabin, steering, and drivetrain as there is simply less mass to absorb the irregularities.

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