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EV Conversion: Why Keep The Transmission?

by Steve Smith
(Rio Rancho, NM)

Electric Car Motor Installed

Electric Car Motor Installed

Everyone seems to be keeping the transmission in their electric car conversions. I don't get it. If I upscale the motor why can't I lose the transmission and clutch etc? Of course I would need to mount the motor where the tranny is now and get a part milled to connect the drive shaft to the motor.

I'd be inclined to lose the drive shaft too, if I could figure out how to secure the motor to the differential and still let the whole unit rock with the road.

What do you think?

Hi, Steve -

Allen Antonucci here from Duke's Garage.

All great ideas!

For every drive train component to work correctly with your design, you'll want to maintain the stock suspension geometry.

If you attach the motor directly to the stock rear differential it still needs to pivot up and down while being secured to the body somehow like on a hing mount. You may put too much stress on your motor case or over flex your wires because the will need to move up & down constantly with the suspension travel. Best thing for you to do is weld in a VW Beetle rear end, easy to get parts and motor bolts right to the transaxle with a bell housing adapter.

We built a 1965 Mustang with 2 DC motors coupled together "No tranny". Attached by a machined u joint coupler & bearing to the driveshaft. It works great, but the battery packs state of charge is decreased faster and the motor will overheat quicker running at higher RPM's.

If you use a 4 or 5 speed transmission you can reach a higher rate of speed and maintain lower RPM's. That will increase your distance and keep your electric components running cooler.

We have tried lots of ways to accomplish integration of EV parts to a variety of vehicles: 4x4 jeep with a push button shift LEMCO 2spd transmission which was a really tough build...VW Beetles, 356 Beck Speedsters, Ford rangers, GMC Sonoma.

- Allen

Duke's Garage is an electric car conversion company in Westminster, CO specializing in classic car conversions.

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change differential ratio? Motor heat sink...
by: Steve Smith

Thanks for the feedback guys.
Yeah, I guess coupling the motor directly to the differential would be an engineering disaster. Maybe just a shortend drive line if space allows. Dumping the weight and problems of clutches and transmissions still appeals to me at a gut level.
It is my understanding that for the same load a higher RPM in a DC motor is more efficient. Most ICE vehicules are in direct drive at 4th gear and 2-3000 rpm somewhere around 50-70 mph. If the electric motor is rated at 4-5k rpm then it would seem to me that putting in drag racing differential gears would improve acceleration (resulting in higher rpm and less battery draw). It would also bring the rated speed of the motor closer in line with typical highway cruising speeds? DISCLAIMER: I have never built an EV, so yeah I'm just pulling this stuff out of basic DC theory and 1975 high school auto shop class. Would an AC/DC motor be subject to the same issues?
One more thought I had is why is there such delicate mounting brackets on the electric motors? Since heat is such a major problem I would think that making a heavy steel bracket that engulfs much of the motor would allow that 1000lbs of metal around the motor (the car itself) to act as a heat sink. I'm pulling that from decades of work in computers.
Thanks again for the conversation.

How many electric cars have you built?....
by: Allen,

For every question I answer there's always some smarty pants who wants to throw in their two cents which is fine. The proof is in the range so thats why I say use a transmission. You can keep it simple stupid or you can over evaluate everything. The more electricity you push into the motor to make it go faster the hotter the battery's, controller and motor will get. I was refering to a lithium ion type pack because thats what we use. Lets say you built two identical cost efficient EV's, one with a transmission and one without. Lets say they are doing a constant speed of 70 mph. The one car is in fourth gear at half throttle and the dirrect drive at full throttle. It does not take an rocket scientist to know that the car with the transmission is using less juice from the pack. The torque loss caused by the transmissions drag is barely noticeable depending on gearing. I am not the all knowing master of building EV's by any means. I am just trying to help those who are trying to build a car of there own.

how wrong can you be......
by: Antiscab

"the battery packs state of charge is decreased faster and the motor will overheat quicker running at higher RPM's.

If you use a 4 or 5 speed transmission you can reach a higher rate of speed and maintain lower RPM's. That will increase your distance and keep your electric components running cooler"

Completely wrong.

The resistance (copper) losses are far greater than the magnetic (core) losses.

copper losses = resistance * current^2
core losses = constant * current * rpm

output torque = constant * current ^ another constant
output power = torque * rpm * constant

don't be tricked into thinking that because peak power is around 2-3000rpm, that thats where best efficiency is.
Thats true for an Petrol engine, but not for an electric motor.

Electric motors are most efficient at high rpm and low torque, the complete opposite of an ICE.
generally for a highway capable car using an ex-forklift motor, 4-5000rpm is best.

max power at that rpm is quite low, so if you want to accelerate, you will need to change up a gear.

If you have a transmission, use measure the battery amps and try every gear, you will sound find the most efficient operating point for your speed and load.

Your average series DC motor will run cooler at higher rpm, while the output torque is the same.

This is because the shaft mounted cooling fan is able to move more air and dissipate more heat.

Your average series DC motor will run *much* cooler at higher rpm when putting out the same power, as the resistance losses are far lower, due to the reduced current.

The same goes for your controller as it is operating at a higher duty cycle.

This is actually very important for alotta controllers (Curtis, logisystems, etc) as the freewheel diodes aren't nearly as strong as the main gates.
Meaning they were designed to be operated with output voltage being as close to battery voltage as possible for the majority of the time.

Better controllers (zilla, soliton1, etc) don't have this limitation, but 100V 200A out still has roughly half the losses of 50V 400A out.

The only downside to running higher rpm is the shaft mounted fan may be using more power than it needs to (moving more air than necessary).

If that were the case (and we are talking ~200W here) than you could change to an independently powered fan, so it runs faster at high torque load, and slower at low torque load.

how fast your battery depletes is entirely dependent upon how much power you are drawing.
Your speed will matter much more than the ~15% variation in efficiency because you got the gearing wrong.

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