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Old 09-09-2006, 01:02 PM   #1
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Larger/Heavier Flywheel

I've been doing a lot of inadvertent brainstorming lately.

Anyone familiar with the PlayStation-2/Gran Turismo game series knows that a lighter flywheel allows faster acceleration, but loss of power (torque?) on hills.

For a lot of us, we do a lot of hilly, highway driving.

Would the installation of a larger, heavier flywheel help with Pulse 'n Glide? I know it would take more energy to get it rotating from a starting stop; but conversely, once it gets going, the rotational energy could really help keep the momentum -- especially for automatic folks whom do not engine-off coast regularly

Any thoughts???

RH77
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Old 09-09-2006, 03:39 PM   #2
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I know drag racers like keeping the older, heavier flywheels because it supposedly allows for more torque/power on take off, but I'm not sure how it would influence FE. On one hand a lighter flywheel may allow a driver to get into the better BSFC regions faster, but otoh, with more mass/energy at the same rpm an auto will coast longer? Then again, I'm not even sure engine-on coasting helps with P&G...
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I think if i could get that type of FE i would have no problem driving a dildo shaped car.
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Old 09-09-2006, 03:49 PM   #3
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Flyhweels are used to maintain rotational momentum. Heavy is preferred in an internal combustion engine automobile due to high rotational inertia. That inertia is needed to keep the engine idling at low RPM without losing too much speed between each cylinder firing. If your engine speed gets too low, it doesn't make enough torque to increase speed, and it stalls.

Given the laws of physics we know, a heavier flywheel really wouldn't help with FE. Whatever energy you spend to rotate more weight, at best you'll merely get it back as increased distance rolled.


Lighter flywheels are often used in racecars to increase acceleration, but again that's at the expense of practicality. What I like about electric conversions: peak torque at 0 rpm from a series DC motor. My flywheel can be as light as I want it, and accordingly, I can reduce the rotational mass and the negative effects on acceleration that increased rotational mass entails. Shaving 10 pounds or so off my flywheel would reduce my 0-60 time by ~0.2 seconds and around ~0.1 off the the 1/4 mile. It would very marginally reduce the energy used to accelerate as well.
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Old 09-09-2006, 04:10 PM   #4
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better yet, why not have the flywheel have a series of small weights in the center. When the flywheel is moving, the weights move out to the edge. This will make it so that it is easy to spin during acceleration.

THen again this idea could be silly, but I just thought I'd mention it.
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Old 09-09-2006, 04:15 PM   #5
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Then it's a matter of making sure the centripetal forces generated on those rotating weights as they move outward don't make the weights come off and fly all over the place.
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Old 09-09-2006, 05:07 PM   #6
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In GT4

I know that a lighter flywheel gives you greater acceleration, and the cost of loss of speed up hills, not power. Lighter is better.
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Old 09-09-2006, 06:57 PM   #7
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for whatever it's worth, all of honda's fuel economy vehicles (CRX HF, Civic VX, Civic CX) all had lighter flywheels.
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Old 09-10-2006, 01:21 AM   #8
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Hi All

Yup . lighter flywheels are prefered on sports cars , and race cars but not for the reason that looks at first ovbvious.
A flwheel is a torque storage device , what you put in you get out.- but you dont get out more.
An engine fitted with a light flwheel wont make more power.

In the old days we used to do a lot of flwheel machining.
They used to be heavier back then due to less use of hollow crankshafts etc.
Engines made more vibration at lower speeds and a heavier flywheel were used to smooth out the engine especially at and near idle. (harmonic ballancers work differently than flywheels but they too smooth up an engine)

Flywheels were made so light that they could explode , so we used to make scatter shileds - plates of thick steel that fitted closely around the transmisions bell housing that would take the force of a disintergrating flwheel (we like keeping our legs - ON)

Drag racers would like a heavier flywheel so that there dont bog down on the line when the clutch is dropped.
Track racers would prefer a light flywheel because of quicker rpm response change during gearshifts.
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Old 09-10-2006, 03:27 AM   #9
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in an automatic you want a light flywheel so the load on the clutches is reduced and the engine rpm will drop more quickly when the tranny shifts to the next gear. Heavy flywheel also helps reduce the power pulses in large displacement low piston count engines i.e. my 1000cc boxer BMW but unfortunately it also makes it wobble because of the gyroscopic effect the heavy flywheel makes when the bike tips forward and aft it twists sideways.
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Old 09-10-2006, 06:42 AM   #10
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Hi JanGeo

The torque convertor found on car auto trany's weighs quite a bit and then its filled with a couple of kilograms worth of hydraulic fluid ., this gives the flywhell effect.
An auto car doesnt actualy have a flywheel as such they call them ?flex plates? instead.
A flex plates is only 2 or 3mm thick compared to 20 to 30mm thick for a flywheel.
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