# center of gravity



## sunworksco (Sep 8, 2008)

Battery placement depends if the vehicle is front wheel drive or rear wheel drive.
Or reverse trike or 4-wheel. If you can keep the batteries in the middle of the floor pan, the better. Weigh each corner of the car with the batteries uninstalled and find out where the percentage of weight balance is. Then try and install the batteries at the midpoint of the sweet spot of the chassis.
Look at the Tesla. It has the batteries in the belly pan. Of course this is a perfect world car.
Good luck with your build.


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## dougingraham (Jul 26, 2011)

More correctly this should be the Center of Mass. Also called the CG (Center of Gravity) is is the point at which if you were to attach a cable the object would just hang and not try to rotate. In a zero gravity environment it is the point around which the object would spin if given an off center push. For solid regular object it is obvious where the center of mass is. For a complex object like a car with driver it is not so obvious.

In a vehicle that is expected to turn corners you want the mass to be concentrated in the middle of the vehicle as low to the ground as possible. Being as low as possible will minimize the load shift towards the wheels on the outside of a corner. You want the mass to be concentrated in the center to minimize the moment of inertia. This is one effect that slows a car from initiating a turn and once turning tends to want the vehicle to continue to rotate in the direction of the turn. With the mass in the center the tires have to do less work. It is true that if you placed the batteries at the far front and far rear of the vehicle it would be more difficult to get the vehicle to spin if struck but it will also tend to continue to spin once it happens. A floor pan filled with batteries like Tesla used in the Model S is about the best that you can do. The mass is centered and low. This also lessens the tendency to nose dive when braking hard or nose lift during acceleration.


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## sunworksco (Sep 8, 2008)

Thanks Doug.
I knew someone brilliant would chime in with the technical know-how.


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## Duncan (Dec 8, 2008)

Hi BW

You are asking about the "moment of inertia about an axis"

This is how far out from the axis that the mass is concentrated,

Think of a javelin - it would be easy to spin it about the middle of the length axis
Much more difficult to spin it about the middle with the length at right angles,

This is the reason a flywheel will have its weight concentrated as far out as possible

In a car you will be interested in the moment of inertia around an axis vertically stabbed through the centre of mass

If your car has weight far out (as in your example) it will be more stable 
It will react slower to an input
But when it goes it will take a lot more stopping

If the mass is all to the center it will react faster - possibly too fast!
When I was younger all of my pals with mid engined sports cars spun them!!


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## bwjunkie (Jul 31, 2013)

Duncan said:


> Hi BW
> You are asking about the "moment of inertia about an axis"
> ...
> This is the reason a flywheel will have its weight concentrated as far out as possible


excellent, thank you! 
I also have another question, but since you mentioned flywheels I'll ask it here!

My Ford 157 tooth flywheel is 50 oz *imbalanced *purposefully to work with the motor internals to create a system which is more resistant to vibration from imperfections in other parts (lol that is how I understand it anyway)

So I can't use this flywheel right? I mean it has to be balanced first?

I might end up buying an aluminum flywheel anyway, but still... 

Josh


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## Duncan (Dec 8, 2008)

bwjunkie said:


> excellent, thank you!
> I also have another question, but since you mentioned flywheels I'll ask it here!
> 
> My Ford 157 tooth flywheel is 50 oz *imbalanced *purposefully to work with the motor internals to create a system which is more resistant to vibration from imperfections in other parts (lol that is how I understand it anyway)
> ...


The question is why would you want to?

Yes it is not a good idea to use that unbalanced flywheel

Are you using the clutch? 
If you are you need to keep the flywheel that the clutch bolts to 
Get rid of the rest
Machine everything outboard of the clutch mounting bolts
And you can thin the flywheel down a bit as well 
An IC clutch is slipped on takeoff - this generated heat and needs some metal
An EV clutch is never slipped

If you are not using the clutch - get rid of the whole thing


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## bwjunkie (Jul 31, 2013)

Yes i want to try a clutch. Make the long term choice by using one myself for a while or maybe forever. 

Thank you for confirming I need to balance. I will probably just buy an aluminum flywheel and then cut down the teeth. 

The steel one would have to be balanced removing the 50 oz imbalance.

But as to your comment about "slipped" , why is an EV not allowed to slip a clutch to get some minor torque effect?

josh


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## Duncan (Dec 8, 2008)

bwjunkie said:


> Yes i want to try a clutch. Make the long term choice by using one myself for a while or maybe forever.
> 
> Thank you for confirming I need to balance. I will probably just buy an aluminum flywheel and then cut down the teeth.
> 
> ...


You can "slip" your clutch - but to no real benefit plus a significant risk of overspeeding your motor


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## dougingraham (Jul 26, 2011)

bwjunkie said:


> Thank you for confirming I need to balance. I will probably just buy an aluminum flywheel and then cut down the teeth.


The ring gear can usually just be removed. Often just heating and it will fall off.



bwjunkie said:


> But as to your comment about "slipped" , why is an EV not allowed to slip a clutch to get some minor torque effect?


An electric motor develops almost full torque at zero rpm so there is no advantage to doing this with an EV. I don't even use first gear because if you step on the pedal a little too far the wheels just break loose. No need to slip the clutch to misbehave. In second I can still do this but it is easy to moderate. If I am in misbehave mode I will push the throttle until I hear the tires just start to complain and then wait for the shift point.

If you run the motor up to redline and drop the clutch in an EV which already has significantly more torque than the OEM planned for you tend to break stuff. An ICE car must slip the clutch in order to start moving or the engine will stall.


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## piotrsko (Dec 9, 2007)

Depending on the donor's motor, you may be stuck machining to remove the imbalance on the new flywheel, too. I don't recall when they started that, I remember when 302's were internally balanced. I know the cologne series are internally balanced.

Rule of thumb: Dowel, offset bolt holes, spotwelded tabs, or drill marks are signs of external balance

I use the clutch a: rotating mass is 3 lbs vs 35. Shifts way faster, doesn't burn synchros.

B: my power steering don't work below 200 rpm in the Wal-Mart parking lot.


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