A rather simple device that has three uses; the flywheel helps start the car, smoothing out speed and store momentum.

The flywheel is located between the engine and the transmission. It is a large wheel of considerable weight by design. At the engine end it is attached with bolts to the crankshaft. On the other side lies the clutch. The outer rim of the flywheel acts as a gear with teeth cut into it. The teeth of the starter motor engage the flywheel teeth and spin it to crank the engine.

Once the engine has started, the flywheel has two more jobs to carry out. As you accelerate, or free rev the engine, the flywheel smooths out the rotational movement of the crankshaft and stores energy. When people say the flywheel stores energy they really mean that it absorbs and returns energy with each combustion cycle. During the power stroke, the crankshaft is being twisted (pushed by the piston) and during the compression stroke the crankshaft is doing the twisting (pushing the piston). The flywheel absorbs the energy when the crankshaft is being pushed and returns it when the crankshaft is pushing. The faster the engine is turning over (high rpm), the more energy is being stored by the flywheel.

When selecting a weight for the flywheel, car manufacturers need to consider (among other things); engine acceleration (a lighter flywheel will build speed quicker than a heavy one, but drop revs too quickly) and inertia (a heavier flywheel carries speed further, but is harder to slow down than a light one). It’s a balancing act and production cars strike this balance well for their intended use. Also, the larger the car, the heavier the flywheel required.

A race car on the other hand is usually very light and does not need to concern itself with going to the shops to pick up milk and cookies. A lighter flywheel, within reason, is used to reach the redline quicker and to control the revs with greater precision throughout the range. Many weekend warriors buy lightened flywheels for their cars and nearly all video game racers have at some stage purchased the ‘lightened flywheel stage 1 kit’ which adds at least 35kw to their car. In reality, the weight of the flywheel is more of a preference thing and does not add power to your car.

If you accept that you will accelerate faster with a lighter flywheel, you also have to accept that you will lose speed faster and have to work harder (more frequent input) to maintain your momentum. There isn’t any power increase in the scheme of things, just a change in the rate of rotational momentum and the storage efficiency of this momentum.

Let’s look at some examples.

Harry and Larry are twins and two months ago they turned 18. Their sugar daddy (ed: name removed under court order) bought them identical cars for their birthday and gave them some spending money. Harry spent the money on 3 midget hookers while Larry installed a lightweight flywheel in his car. Their cars are identical except Harry has a heavier flywheel and Larry has a lighter flywheel.

To show off their sugar daddy rides, the boys decided to cruise George Street on a Friday night. On their way they encountered lots of traffic. Larry discovered that his car now accelerates faster. Each time he steps on the gas the revs respond quicker but when he changes gears he noticed that his revs drop off faster as well. He had to adjust his driving style to shift quicker. He also noticed that he was using the accelerator more frequently to compensate for his fluctuating speed. This did not please him since he couldn’t cruise in a relaxed style like Harry.

Halfway through George Street, to his surprise, Larry stalled his car trying to take off from a set of lights… on several occasions. Quite an embarrassing situation and he attributed this to the flywheel since Harry had no such issues with his factory spec flywheel.

When the boys passed The Rocks, they came across some clear road. Larry looked over to his brother who nodded in agreement. They, like many of their peers, live their life a quarter mile at a time. A furious drag race commenced. Larry noticed that he could shift earlier through the gears. It was like his car felt lighter to his engine and his instincts proved right. His lighter flywheel effectively lightened his car.

Unfortunately for Larry, he missed a gear and his revs dropped quickly which allowed Harry to catch up. Side by side the boys raced while the world warped around them. A quarter of a mile seemed to last minutes. Larry was amazed to discover that his flywheel didn’t actually give him more horsepower like the guy that sold it to him promised.

All of a sudden, the boys saw a defect station up ahead. They both geared down to assist their brakes with engine braking. Larry’s revs shot up very quickly to match his wheel speed and didn’t slow him down as much as what Harry’s car did. The lighter flywheel just didn’t offer enough resistance to change in speed. Seeing that he could not slow down in time, he made a rash decision and started to drift his way into a side street before the defect station.

Very quickly, Larry discovered that the lighter weight of the flywheel hindered his clutch dumping. He had to use higher revs and it was harder for him to break traction which he suspected was related to a lack of weight behind his flywheel. Like a pro, Harry easily overtook his brother while drifting on the outside lane.

Dirty with his inability to drift donuts to get chicks numbers, Larry dropped the hammer and went all street racer style on his brother. With a few tight back street corners between the brothers, Larry made up the difference in no time and was grinning into the disappearing reflection of his brother’s car in the rear view mirror. He realised that the flywheel he had installed was well suited to high rev driving and accelerating out of corners as quickly as possible.

So in summary:

City driving

• acceleration has improved with a lighter flywheel
• the engine has to rotate less mass so it works more efficiently
• you will have to adapt a new driving style to avoid stalling
• gear changes are more frequent and need to be executed faster
• it requires more frequent adjustments to maintain cruising speed and hence burns more fuel

Drag

• acceleration, as above, is quick with a lighter flywheel
• you run through the gears too quickly, spending precious time off the accelerator
• when changing gears, you lose more revs
• a lighter flywheel does not add horsepower
• too light a flywheel in a heavy car will struggle to get it off the line quickly

Drift

• the lighter the flywheel the more revs you need to break traction
• the fluctuations of a light flywheel require more frequent adjustments upsetting the balance of the car

Race

• a lighter flywheel is an all round improvement
• quicker acceleration
• lower effective car weight
• precise rev control

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