# Power comparison of a clutch less design verse clutch.



## major (Apr 4, 2008)

Rob_of_Waterloo said:


> I am looking for comments on my logic.
> .......If you start from a standstill the motor puts the energy (power) into the flywheel before you pop the clutch.


Hi Rob,

I'd say it is illogical to pop the clutch in an EV 

major


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## rwaudio (May 22, 2008)

When testing the Cobra, the folks at EVTV tried popping the clutch vs just flooring it, using the clutch was slower in 0-60 tests. The only plus for the clutch would be if you need to quickly shift during your acceleration. If you can accelerate to the desired speed in one gear then I would say ditch the clutch. If you want some 1/4 mile runs and need to shift, then keep the clutch.

My car should do 0-60mph in 2nd gear so no need to shift, so I went clutchless.


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## Rob_of_Waterloo (Nov 2, 2011)

Why would it be illogical to pop the clutch of a EV?


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## cruisin (Jun 3, 2009)

Rob_of_Waterloo said:


> Why would it be illogical to pop the clutch of a EV?


 
With a AC-50 using the motor from 0-8000rpm in 2nd or 3rd gear, what is the purpose of a cltch?


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## Ziggythewiz (May 16, 2010)

A clutch is designed to allow slippage, aka wasted energy. There's no need to waste the energy in the EV as you can shift at 0 rpm. Clutch is only good for faster shifting. Starting a race with the clutch engaged would be like starting a duel with the safety on. Just one more step.


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## Duncan (Dec 8, 2008)

Hi Rob

Its not the clutch you need to think about its the flywheel, 
an IC car has a massive flywheel because of its pulsating power delivery,
With an electric car that mass is not needed to smooth out the power (and idle) so you should get rid of that wasted mass - it's the equivalent of adding a couple of hundred Kg of lead to your car

You don't need 14Kg of flywheel you just need a coupling between the motor and the gearbox

Other reasons an IC car needs a flywheel;
to get over the lack of torque at idle, 
to absorb the energy wasted when the clutch is slipped - in an IC car a very light flywheel would get hot very fast when you slipped the clutch to start off

The clutch/no clutch argument is more about gear-changing and the speed you can shift - if you need to shift


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## bluefxstc (Dec 29, 2007)

I don’t have a clutch and really don’t think I need one but if you are looking for the quickest 0 to 60 times you will. My car is pretty quick but it take about 2 sec to shift without a clutch so 0 to 60 times suffer.

http://www.diyelectriccar.com/garage/cars/242


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## mizlplix (May 1, 2011)

Speaking as an old drag racer, the flywheel was a tuning tool for our launchs.

Heavier wheel to band-aid for weak low-RPM torque.

An EV does not have that problem.

Drop the flywheel.

If you want lightning fast shifts, get a Dog Ring transmission.
{Jericho, Lenco, Wiseman, Hewland}

Here is an explanation of their operation. http://www.circletrack.com/techarticles/road_racing_transmission_tech/index.html


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## DavidDymaxion (Dec 1, 2008)

Some EVers use the clutch to capture energy from the still spinning motor for the next take-off.

Not slipping the clutch also has a cost in the electric. Electric motors tend to have poor efficiency near 0 rpm. Does that moment of slipping the clutch waste more power than running the motor near 0 rpm?

Another thought: If your controller has a slow ramp rate, and it isn't adjustable, it might be faster to rev the motor and slip the clutch to get going.


Ziggythewiz said:


> A clutch is designed to allow slippage, aka wasted energy. There's no need to waste the energy in the EV as you can shift at 0 rpm. Clutch is only good for faster shifting. Starting a race with the clutch engaged would be like starting a duel with the safety on. Just one more step.


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## major (Apr 4, 2008)

Hi David,



DavidDymaxion said:


> Does that moment of slipping the clutch waste more power than running the motor near 0 rpm?


Sure it does. It wastes half of the energy. And that is from the motor shaft, so you have motor (and controller) losses also.

And yes, the electric motor has poor efficiency near zero RPM (0% at 0 RPM). But two things:

1) You have low power at very low RPM (zero power at 0 RPM), so actual loss is low (% of a small figure is a small number).

2) As RPM increases from zero, efficiency rises very quickly. You may only need to come up to a couple 100 RPM to start seeing decent efficiency.

Regards,

major


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## JRP3 (Mar 7, 2008)

I'll toss in one other point about shifting clutchless. If your motor has regen you can actually shift faster without than with a clutch because as you lift off the accelerator the regen quickly slows the motor. In fact you have to shift fast otherwise the motor will slow too much and you'll miss the shift.


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## Rob_of_Waterloo (Nov 2, 2011)

Thanks for all the good input everyone. I like the idea of a clutch-less design, but components for a clutch design are more readily available. I am taking the simpler path on my first project. As it turns out a friend of my sold me a light weight flywheel. Reducing the weight from 19 lbs to 9lbs.


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## DavidDymaxion (Dec 1, 2008)

I thought it would be fun to put some back-of-the-envelope calculations to it.

Let's do a 0 to 200 rpm comparison. You should be able to idle an electric motor at 200 rpm, and get the clutch fully engaged by that rpm. In a typical car, in 1st gear, you are going about 1 mph or 0.44 m/s at 200 rpm.

The clutch is alot like the brakes. The brakes soak up most of the energy of a stop, so lets assume the same for the clutch to get going. Assume a 1500 kg car.

Eclutchloss = 1/2mv^2 = 0.5 * 1500 kg * (0.44 m/s)^2 = 145 J of energy lost in the clutch.

What about the controller? I'll use Major's numbers of 0 efficiency at 0 rpm, and 200 rpm for "decent efficiency." Let's call that linear 0% at 0 rpm to 75% at 200 rpm.

Energy lost in the controller at a given rpm:
Econtrollerloss = 1500.0 / 2 * 0.44 * 0.44 * (1.0/0.75 - 1) = 48 J

Note the clutch case wastes a mere 97 J over the controller. I calculated this is a loss of about 16 centimeters of range per clutch actuation.


major said:


> Hi David,
> 
> Sure it does. It wastes half of the energy. And that is from the motor shaft, so you have motor (and controller) losses also.
> 
> ...


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## DJBecker (Nov 3, 2010)

JRP3 said:


> I'll toss in one other point about shifting clutchless. If your motor has regen you can actually shift faster without than with a clutch because as you lift off the accelerator the regen quickly slows the motor. In fact you have to shift fast otherwise the motor will slow too much and you'll miss the shift.


Does any commercial controller do this? Regen powerful enough to slow the vehicle will very quickly slow the motor. The controller would need to actively speed match, which means knowing the road speed and predicting the next gear.

Not impossible, but certainly not trivial.


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## Ziggythewiz (May 16, 2010)

I've often thought it would be cool to have a couple momentary push buttons that would set the controller to the speed necessary to smoothly shift to the desired gear.


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## JRP3 (Mar 7, 2008)

I've thought of something similar, computer control that matches motor speed to gear speed when shifting. I know Tesla broke a bunch of transmissions trying to get full power shifts on their Roadster before giving up and going with a single speed.


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## fb_bf (Jul 6, 2011)

I just rebuilt my transmission on a the MG Midget I got from Pete McWade. I looked into what effect reducing the flywheel inertia would have on my car. I found someone on the internet who had reduced the math down to lbs of flywheel removed at a certain radius VS effective mass reduction of the car. He used the kinetic energy stored approach to get to the final answer. It varies with each gear so you plug in the final drive ratios for each gear and the tire diameter, etc to get the final estimates. For The midget, in first gear, the amount of effective weight reduction was over 200 lbs (for my first estimates) by reducing the outer diameter of the flywheel. I found a machine shop that could do this and I removed everything down to the diameter of the pressure plate. It took the weight down from 16.5 lbs to 9 lbs. The effective weight savings is now over 300 lbs in first gear, and over a 100 lbs in second. My car currently weighs 2020 lbs, so this is a big savings. I can try to post some pictures of before and after when I get home tonight if people are interested. I'm new to forums and such, so be patient if I do things wrong for awhile.


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## RET (Jan 3, 2012)

fb_bf said:


> I just rebuilt my transmission on a the MG Midget I got from Pete McWade. I looked into what effect reducing the flywheel inertia would have on my car. I found someone on the internet who had reduced the math down to lbs of flywheel removed at a certain radius VS effective mass reduction of the car. He used the kinetic energy stored approach to get to the final answer. It varies with each gear so you plug in the final drive ratios for each gear and the tire diameter, etc to get the final estimates. For The midget, in first gear, the amount of effective weight reduction was over 200 lbs (for my first estimates) by reducing the outer diameter of the flywheel. I found a machine shop that could do this and I removed everything down to the diameter of the pressure plate. It took the weight down from 16.5 lbs to 9 lbs. The effective weight savings is now over 300 lbs in first gear, and over a 100 lbs in second. My car currently weighs 2020 lbs, so this is a big savings. I can try to post some pictures of before and after when I get home tonight if people are interested. I'm new to forums and such, so be patient if I do things wrong for awhile.


I would be interested in pictures of your clutch setup , I am planning a Triumph conversion and plan on doing the same to my fly wheel , what kind of 
conversion are you doing ( motor , controller , batts ? ). I would like to see the coupler you are using also . Thanks for the technical info also very interesting.


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## shoup (Feb 10, 2009)

I also went the lightened flywheel route. You can see pictures here along with a CAD drawing of my lightened flywheel:

http://www.diyelectriccar.com/forums/showthread.php/e-e30-bmw-build-38762.html

By removing the weight the flywheel essentially becomes a coupling mechanism. The flywheel does not need to keep momentum in an EV. Start by removing the ring gear for the starter, then have the thickness machined down to be the same as the attachment surface. You don't want to go too thin because it still needs to transfer torque and you don't want it to explode.


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## RET (Jan 3, 2012)

shoup said:


> I also went the lightened flywheel route. You can see pictures here along with a CAD drawing of my lightened flywheel:
> 
> http://www.diyelectriccar.com/forums/showthread.php/e-e30-bmw-build-38762.html
> 
> By removing the weight the flywheel essentially becomes a coupling mechanism. The flywheel does not need to keep momentum in an EV. Start by removing the ring gear for the starter, then have the thickness machined down to be the same as the attachment surface. You don't want to go too thin because it still needs to transfer torque and you don't want it to explode.


Nice write up . I also have an e30 325i Vert 1991 . I am doing my Triumph first then maybe my BMW if the tests go well with the Triumph . Thanks for the good info . I will be doing the same to my flywheel and retaining the clutch .


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## fb_bf (Jul 6, 2011)

fb_bf said:


> I just rebuilt my transmission on a the MG Midget I got from Pete McWade. I looked into what effect reducing the flywheel inertia would have on my car. I found someone on the internet who had reduced the math down to lbs of flywheel removed at a certain radius VS effective mass reduction of the car. He used the kinetic energy stored approach to get to the final answer. It varies with each gear so you plug in the final drive ratios for each gear and the tire diameter, etc to get the final estimates. For The midget, in first gear, the amount of effective weight reduction was over 200 lbs (for my first estimates) by reducing the outer diameter of the flywheel. I found a machine shop that could do this and I removed everything down to the diameter of the pressure plate. It took the weight down from 16.5 lbs to 9 lbs. The effective weight savings is now over 300 lbs in first gear, and over a 100 lbs in second. My car currently weighs 2020 lbs, so this is a big savings. I can try to post some pictures of before and after when I get home tonight if people are interested. I'm new to forums and such, so be patient if I do things wrong for awhile.


I'm posting my pictures of my flywheel, and I'm quoting myself because in an effort to find the site where I found the equations for estimating the weight reduction of the car when you reduce the flywheel inertia I found that my original equation was wrong. So it turns out that I have in fact effectively reduced the weight of my car (in first gear acceleration) by 123 lbs, not the 300lbs I had claimed before. I'm sorry about the mistake. This link shows the final equation I believe is now correct. The basic concept is to compare the kinetic energy stored in flywheel with that of the car. If the flywheel is turning at 3000 RPM in first gear, then your car is moving at X miles per hour as well. You calculate the stored energy in both, and that gives you the ratio of the gain possible. In my case I took all of the weight off the outer diameter, which gives me the biggest gain possible for the 7.5 lbs removed. Here is a link to the site that has the calculations. http://www.uucmotorwerks.com/flywheel/how_a_lightweight_flywheel_works.htm

Here are some pictures that show how much material I was able to remove.


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