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This previous question explains what a flywheel does and why it is needed. That explanation means that the flywheel needs a certain amount of mass to do its job. However, an "upgrade" for many cars (when looking at performance enhancements) is a "lightweight" flywheel.

Removing weight from a performance car is a normal thing to do, so a lightweight flywheel seems like a good choice, but doesn't it need that mass? What are the advantages other than sheer weight loss to the upgraded flywheel? If it was only a good thing than the OEM flywheel would be lightweight already, so what are the cons?

Is there such a thing as a heavyweight flywheel?

Ultralight flywheel

Ultralight Flywheel for Honda Civic

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The main advantage to a lower flywheel mass on race cars is that the reduced mass allows the engine to rev more freely. The overall weight loss to the car is really not the key thing, it's letting the engine vary RPM really quickly.

Being able to change RPM quickly can mean faster shifts, getting to a power band more quickly, etc, etc. It also makes the car much more twitchy to drive since it doesn't like low RPM very much. At low RPM the lower mass of the flywheel means you need to use the throttle to keep the RPM above where the flywheel isn't going to carry things around as well. For a race car that's not a big deal. For the family driver with the manual transmission, it's a big problem.

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    +1 for highlighting that a lightweight flywheel isn't really ideal in a daily driver. – Ellesedil Apr 19 '16 at 18:13
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    There is no such thing as flywheel in automatic transmission, just FYI. – Alexus Apr 25 '16 at 21:58
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cdunn's answer is spot on.

To add a bit (especially for cars), in a race car you often want to be dropping from something like 7000 rpm to maybe 5500 rpm (or an even higher band, depending on the engine) in a split second to make an upshift. Especially at higher engine speed, that's a lot of energy to bleed (and goes as a function of the weight of the flywheel).

The lighter the flywheel, as soon as you hit the clutch, the faster the engine speed will drop. This is a double-edged sword. Not only does it let you shift faster, you must shift faster - and more precisely. The time between clutch and shift (at ideal rev-match) is reduced, and the window of opportunity to make a perfect shift is also reduced. Your revs won't just hang around, slowly bobbling down (giving you lots of time to fuss with the shift lever) - they sink like a stone and you must make the shift before they drop too low (or else you get a rude shock when you let the clutch out again).

Same deal for downshifts - a heel-toe gives you a very fast RPM bump but, again, you have to make the shift in the brief window that presents itself.

Clutch springs are usually stiffer on racing clutches as well, so badly timed shifts are even harder on your vehicle - not only will a sloppy shift kill your race time, it will hurt your car as well.

The low weight of a racing flywheel also makes moving off (from a stop) harder. When you're racing, you only do this once per race - sometimes not at all if it's a rolling start. When you're commuting you do this all day long. A heavier flywheel carries much more momentum - when you feather out the clutch from a stop, this momentum helps to keep the engine running at a steady RPM as you add load and gives you more latitude in your throttle control to keep the car going and setting off smoothly.

With a racing flywheel, you get no help here - as you feather out the clutch from a stop the engine will bog down rapidly (and stall) if you don't add fuel to keep the revs up. But because the flywheel is so light, adding just a bit of fuel sends the engine racing off to high RPM if you jump the gun before the clutch starts to engage. This whole procedure is much more sensitive with a light flywheel (and a stiff, grabby clutch). Daily driving like this would be a nightmare - stop-and-go traffic would be a full body workout and mental exercise in concentration.

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    @nocomprende No, I think I said that daily driving a race tuned vehicle in stop-and-go traffic would be a more laborious and stressful exercise than a typical commuter would tolerate. It probably wouldn't be any fun for a race car driver, either. – J... Apr 19 '16 at 19:19
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    @nocomprende Yes. Because the post clearly states that the race car drivers organic diet and daily work regiment are a key to success. Especially with the Yoga and and Spinning detailed in the answer. – DucatiKiller Apr 19 '16 at 19:32
  • +1 for major enlightenment on what "performance" parts actually means. Basically if I drove a real race car to work every day, it will be very irritating at best, I will probably stall and have all sorts of very unpleasant experiences, and it will probably kill the car to boot. – Nelson Apr 20 '16 at 14:47
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    +1 for all of the tradeoffs. Also, I have driven a race tuned vehicle in stop-and-go traffic, due to some unfortunate maintenance issues in my commuter vehicle. It's a bear to start from a standstill repeatedly without the help of a heavier flywheel to smooth things out. Might qualify as good practice for learning exactly how your vehicle behaves, but it's mentally exhausting. – Cort Ammon Apr 21 '16 at 0:51
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    @dotancohen I think if you're pushing strip tires to their limits on the street then you're probably trying to go to jail or something. Suspect if he's driving in that it's to get used to the clutch feel and engine response - not high speed handling. – J... Apr 21 '16 at 13:52
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Physics dictates that rotational inertia impedes acceleration...

which is why a lighter flywheel is considered to be a performance mod

Less rotating mass = more acceleration, if all else is equal

But rotational inertia helps an engine idle stably

which explains why manufacturers don't go for lighter flywheels from the get-go

If they opt for a lighter flywheel, the idle RPM will have to be higher in order to maintain engine stability, which translates to more fuel consumption and lower fuel economy. For an average vehicle where performance isn't a priority, it makes little business sense to go down this route.


More often than not, folks who install a lighter flywheel in the name of performance have to raise their idle speed in order to keep the engine running. This is especially true for engines with fewer cylinders and smaller displacement.

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    So, improving fuel economy lowers fuel economy? – user15009 Apr 19 '16 at 19:13
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    @nocomprende where did I say that? – Zaid Apr 19 '16 at 19:28
  • @nocomprende - I'm not seeing Zaid say that either ... are we missing something obvious? – Pᴀᴜʟsᴛᴇʀ2 Apr 19 '16 at 20:59
  • @Pᴀᴜʟsᴛᴇʀ2 Yes, he said that lightening the flywheel (which improves vehicle weight and engine responsiveness) requires raising the idle rpm. Many physical systems can be modeled in terms of local peaks or valleys, depending on how you lay things out, and there is a sweet spot of two conflicting requirements, Taking it further one way or the other makes things worse. So the overall weight of the car is predicated on having all the things it needs, and no things that it does not need. If we could make cars weigh 10 pounds, wouldn't we have done it by now? A "lightweight flywheel" is just silly. – user15009 Apr 19 '16 at 23:46
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    @nocomprende - No, he stated, "... translates to more fuel consumption and lower fuel economy..." which is exactly right, if not redundant. He never stated anything about improving fuel economy. – Pᴀᴜʟsᴛᴇʀ2 Apr 20 '16 at 0:23
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The flywheel keeps your engine spinning

The inertia of a flywheel is in direct proportion to the mass of the flywheel.

Newton's first law of motion states, "An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force"

With that in mind, motorcycle engines typically have less resistance to inertia to overcome than most car engines because they have fewer cylinders compressing air due to fewer cylinders and (in many cases) lower compression. As well, there are fewer resistant forces related to air conditioners and the various accessories associated with a car engine.

Here is a great post related to flywheels.

Flywheels on motorcycles will frequently be used in a dual-homed role, taking on the role of an alternator as well.

enter image description here Image from KTM and MotoUSA

Attributes

A lower weight flywheel will allow for a faster response to fuel input from the rider due to it's lower weight and Newton's First Law of Motion.

Attributes include

  • Faster shifting

  • Longevity of transmission

  • Higher RPM's needed engaging first gear for more HP due to lower inertial mass

Caveats

Single and two cylinder motorcycles will frequently have a counterbalancer in order to reduce the vibration and imbalances inherent to fewer cylinders in an ICE. These additional components help to bring up the inertial mass and thus reducing the need for a heavier flywheel as the sum of the additional components will total the overall inertial mass of the ICE.

  • +1 for great info and of course for an illustrative picture. I never knew that motocycle flywheels were also the alternator. Very clever design. – cdunn Apr 19 '16 at 18:16
  • Don't motorcycles typically have a higher compression ratio than the average car? – Scott Hillson Apr 19 '16 at 18:47
  • Yes, but cars typically have more cylinders and a ton of accessories to spin. AC, power steering, etc. – DucatiKiller Apr 19 '16 at 18:52
  • "The inertia of a flywheel is in direct proportion to the mass of the flywheel." - not exactly. This is true for two flywheels of the same overall shape and size, but a flywheel with a larger radius (or most of the weight near the edge vs the center) has more inertia, since we're talking about rotational movement rather than linear. – Random832 Apr 21 '16 at 14:42
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One thing which hasn't been mentioned here about why a lightweight flywheel may not necessarily be a good thing for the street is, just like brakes, the rotational mass of a regular weight flywheel will allow for better heat control which is caused by engaging/disengaging the clutch during normal operation. The mass of a regular flywheel will be more thermally stable than would a lightweight one. This provides for more longevity of the flywheel. The lightweight one will have more problems with overheating and losing grip after such events. With race teams, this is usually not an issue as this would be a planned replacement item and will get replaced much sooner than would a street flywheel.

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    To paraphrase Einstein: everything should be made as light as possible, but no lighter. – user15009 Apr 19 '16 at 23:55

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