# Flywheel Weight Reduction Matter?



## PeterH (Mar 20, 2009)

Hi there,

Yesterday I dropped off a flywheel to be machined for my EV conversion. The machinist is going to remove the ring gear for me because I obviously don't need it. I'm wondering if it would do any good to ask him to remove any of the mass by machining the side opposite the pressure plate while it is in the shop. 

I've read other posts suggesting it made a big difference. Anyone have an opinion on this they'd like to share?

I did a quick search before posting and didn't turn up any useful information.

Thanks
Pete


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## PZigouras (Jun 5, 2010)

The ideal weight of the flywheel will be determined by your application, and has nothing to do with if your car is electric or not. 

The weight of the flywheel determines the RPM drop when you shift. It simply stores energy. It also controls how fast your motor spools up. If you tell your machine shop what the application is (drag racing, closed track, etc.) they may be able to give you a better idea of what weight you would need.


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## DJBecker (Nov 3, 2010)

PZigouras said:


> The ideal weight of the flywheel will be determined by your application, and has nothing to do with if your car is electric or not.
> 
> The weight of the flywheel determines the RPM drop when you shift. It simply stores energy. It also controls how fast your motor spools up. If you tell your machine shop what the application is (drag racing, closed track, etc.) they may be able to give you a better idea of what weight you would need.


A flywheel is needed on an ICE to store the energy needed for the next compression stroke. To reduce vibration it needs to be rigidly coupled to the crankshaft, fairly close to the rods. The energy storage is often combined with crank counterweights that balance the first order harmonics of the rod and piston mass, and sometimes countershafts to balance higher order imbalances of rods and asymmetrical parts.

The flywheel also smooths out the power delivery of the combustion stroke. These vibrations are better handled by dampers, clutch springs and dual-mass flywheels.

An electric motor typically is fully balanced and has negligible harmonics. Even the slight pulsed nature of power delivery can be all but eliminated by skewing the poles/windings.

Bottom line: if you can find a zero-mass flywheel, use it.


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## PeterH (Mar 20, 2009)

I guess now my question is has anyone done this before and have experience with the subject to share? 

As it is, I wouldn't know how much to ask the shop to remove from the back of the flywheel. I doubt he will have much of an idea. When I told him why I wanted the ring gear removed, he commented that he'd never heard of such a thing... this is a small town. 

Pete


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## dougingraham (Jul 26, 2011)

PeterH said:


> I guess now my question is has anyone done this before and have experience with the subject to share?
> 
> As it is, I wouldn't know how much to ask the shop to remove from the back of the flywheel. I doubt he will have much of an idea. When I told him why I wanted the ring gear removed, he commented that he'd never heard of such a thing... this is a small town.
> 
> Pete


You need to be careful in what you remove because you could compromise the integrity of the flywheel. However you could probably remove everything outside of where the pressure plate bolts to the flywheel safely. You will want to have it re-balanced as a unit once you do this.

It will help your acceleration, I just don't have any idea how noticeable it will be.


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## mizlplix (May 1, 2011)

To add a little to the previous comments.

1. An ICE flywheel provides mass to store energy to overcome the dead weight of the vehicle and start it in motion. <Heavy car/heavy flywheel. light car/light flywheel> An EV does not need this assist as it has abundant torque at this speed. A heavy flywheel is actually a hindrance to an EV.

2. A clutch in an ICE needs a greater thermal mass to dissapate the heat generated by the slipping clutch during the initial acceleration. An EV does not use a clutch that way. It merely is a decoupling device to permit easier shifts. So an EV flywheel can be of MUCH smaller thermal mass than the ICE flywheel.

What I have done....Removed the ring gear. Removed all metal outside the clutch cover mounting bolt circle, (leave about .250 outside the bolt holes), Removed any weights cast into the back of the flywheel with a mill. Welded in any drilled balancing holes, THEN cut the portion under the clutch friction area. It can be thinned to about 2/3 rds of it's original thickness because the clutch will not slip and generate heat like in the ICE application....

(after all this I didn't even re-balance mine, that is your call, the guy running the lathe can tell you how smooth it ran......)

ANSWER: Cut down as much weight as you possibly can OR buy an aluminum flywheel. You will be happier and your car will be more responsive.

Miz


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## tomofreno (Mar 3, 2009)

The moment of inertia is 1/2M*R*R and the kinetic energy is 1/2*I*w*w = 1/4*M*R*R*w*w, where M is the mass, R the radius, and w the angular velocity of the flywheel, assumed a simple disk. If a 10 kg, 1/4 meter radius flywheel is accelerated to an angular velocity of 523 radians/second (5000 rpm) the work done (equal the kinetic energy, neglecting windage) by the motor is 1/4*10*1/16*523*523 = 42.7kJ = 12 Wh. 

Let's say the overall gear reduction in first gear is 12:1, so the wheels are turning at 417 rpm when the motor is spinning at 5000 rpm. Let's also assume 25", or 0.64 meter diameter tires, and a 2500 lb, or 1136 kg vehicle. Then at 5000 motor rpm the vehicle is moving at 14 m/sec or 31 mph. It's kinetic energy, and roughly the work done to accelerate it to this velocity, is 1/2*m*v*v = 1/2*1136*14*14 = 111 kJ = 31 Wh.

So seems it is well worth reducing the flywheel mass. I didn't expect it to be so significant.


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## TomA (Mar 26, 2009)

Interesting numbers, Tom, but is it really such a stark contrast?

Isn't a fair amount of the energy put into spinning the flywheel up to 5000 rpm available for moving the car? Sure, there are bearing losses, and if you're on the brakes its probably wasted, but I always thought a flywheel was a pretty efficient energy storage device.

Just wondering...

TomA


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## tomofreno (Mar 3, 2009)

> Isn't a fair amount of the energy put into spinning the flywheel up to 5000 rpm available for moving the car? Sure, there are bearing losses, and if you're on the brakes its probably wasted, but I always thought a flywheel was a pretty efficient energy storage device.


 If you just accelerate up to 5000 rpm and then stop, obviously not. If you continue driving at constant speed, yes, you could use a bit of the flywheel energy to move the car, but only by easing off the accelerator allowing the vehicle speed to lower a bit while coasting to transfer the energy. If you want to continue at constant speed you have to continually supply the energy per unit time necessary to move the vehicle at that speed plus compensate for the small bearing and windage losses of the flywheel. Eventually you brake to stop for a light or stop sign, or slow for a vehicle in front of you, and dissipate some or all of that flywheel energy in frictional losses. Then you accelerate again and do more work to increase the kinetic energy of the flywheel.


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## madderscience (Jun 28, 2008)

I used an aluminum flywheel and removed the ring gear from it in my MR2. It removed about 9 pounds of rotating weight in my case. I could have gone even further since I still used the stock pressure plate. My math said that was an equivalent effect to reducing the weight of the wheels and tires by about 1/4 if the car was in 2nd gear.

The only reason for having a flywheel in an EV is if it still has the clutch. Therefore, it can be as light as possible as long as it is strong enough to safely engage the clutch. 

There are plenty of clutchless and direct drive conversions out there with no flywheel at all. The rotating mass of the motor is usually in the same ballpark as the rotating mass of the flywheel anyway and with so little motor friction there is no need for additional kinetic energy storage.


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## Guest (Oct 21, 2011)

A stock 22 lb flywheel lightened to 11 lb. Works fine. The stock one worked fine too. You don't need a heavy flywheel with an electric motor. For the VW you must have them cut down to lighten them. I have two different VW flywheels that have been lightened. I am not sure how light the small one is but its light like in the 9 lb range and maybe even 7 or 8 lb range. Not sure. It was cut very very tight. Most of the rear mass remains. I'd have to remove it to show you. The 11 lb one is lightened and had the ring removed from the aftermarket flywheel. It was a flywheel built then a ring would have been press fit in place. The rings were removed with out cutting on these. I sold one and should have kept it.

Anyway you should be able to get away with a much lighter one but don't make them too thin.


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