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Can I Hook the Electric Motor Straight to the Rear Differential?

by Mike Avila
(Turlock, California)

I am considering converting a 4 door (Crew Cab) pick-up like 2000-2004 Nissan Frontier/Toyota Tacoma/Ford Ranger/Chevy S-10 etc.. & have a question that I havn't been able to get a straight answer on.

I'd like to hook an AC motor straight to the rear differential.

Reasons:

(1) It would save a few hundred pounds of transmission;
(2) Make a small pick-up search easier with not having to just look at manual transmissions;
(3) allow me to make more money on selling the ICE parts;
(4) allow more overall power to make it to the rear wheels without all the wasted energy of going through the tranny;
(5) Allow more room in the engine compartment for batteries therefore making weight distribution easier;
(6) Allow my wife & other non-stick driving people to drive it;
(7) Not having a transmission means one less ICE component to have to work on & find parts for later on.

The answers I've found said that it would be like taking off in 4th gear all the time, this is true for an average vehicle with about 3.73/1 rear differential ratio but there are many rear axles out there with ratios of 5.71/1 & greater therefore making it closer to taking off in 2nd or 3rd gear right?

So my questions are...

(1) Is my theory of using about a 5.71/1 or more gear ratio on the rear axle a good one?
(2) I'd prefer an AC motor but should I be considering a DC motor for reasons of low rpm torque? From what I heard AC motors have less low rpm torque but greater range of rpm's up to 10,000 so it might help my not being able to shift situation.
(3) Would road vibration straight on to the motor be bad for it? Would it be better to have the motor a little forward more with a drive line to the differential?
(4) Would adding rear independent suspension from another vehicle help out the idea about motor vibration & lighten it up but allow less rear weight carrying capacity?
(5) If I had to would changing the rear tire size to a smaller one help the lack if any of low rpm torque?

Hi, Mike -

You must be lost, darlin'; )
Allow me to point you in the right direction: You're looking for the EV Discussion List, where you can chat with extremely smart, gifted - not to mention quite handsome - experienced electric car converters about torque, gear ratio, drag coefficient, and cathode chemistry to your heart's content. Your question(s) are bound to put the cayenne in somebody's cocoa over there.

If you can't find a satisfactory map, however, it may be because you're in uncharted territory. Sometimes you gotta strap on them Nikes and just DO it. What's the worst that could happen?

Remember, the lousiest running EV will carry your butt further without petrol than the finest theoretical construct; )

Regards,
Lynne



Comments for Can I Hook the Electric Motor Straight to the Rear Differential?

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a minor mod
by: George U

Im thinking about this approach too, only i would mount the motor "midships" to the car frame/body with vibration mounts and add a short telescoping drive-shaft to the rear end. This allows a cover to keep the motor out of the weather (provided i figure out how to pass the drive shaft thru it that keeps most water out. Maybe just mount an automotive transmission male spline part on the motor shaft and use the matching tail seal and mating yoke. The drive shaft could be a standard configuration, only much shorter. The drive shaft could also incorporate some weight if needed for rotational inertia (a flywheel). But this is just added weight.

The motor can be mounted high for road clearance but at an angle and the rear end (toward front of car) rotated up to set 0-degree pinion angle with normal car stance. This might require the motor to stick through a hole in the center of the floor though. This would require a soundproof cover.

I have calculated speed vs RPM for various rear end ratios but don't have any torque or efficiency data for the AC motor im considering. So the low end starting torque and top end speed is definitely an issue.

Note in the JEGS catalog, GM pinion sets are available for car GM 12 bolt 'C' car axle ratio up to 6.14, and for the popular Ford 9" rear end up to 6.5. You can use the ratio with tire diameter to figure speed vs RPM.

Good luck on your conversion.

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direct drive conversion
by: KGround

I have done this (direct to rear end) with a small car - Fiat 1500 and DC components.
http://falconlabs.com/ELECCAR/eleccar.htm
The car was very usable in the flat territory here in FL, but it WAS sluggish on take off. That problem could be solved with a higher amperage controller, but at the expense of decreased range in proportion to the extra amps you use each time you take off from a stop.

As far as schemes like higher rear end ratio, smaller tires, etc.: these will all address the starting torque problem, but what you gain in low end torque you lose in high speed capability. For good starting AND freeway speed you really do need at least two gears. I have yet to see any of these 'industrial gear reducers' that are lighter than automotive transmissions. If it is industrial and can take the load (not just the DC motor's nameplate Hp, but the actual torque you are going to apply at starting, then it is going to be WAY heaver than a car transmission.

DC is the only technology which is going to be practical for this. With a DC motor the torque is limited by how many amps you can pass from the batteries through the controller to the motor and then ultimately by motor heating. You can easily and safely exceed the DC motor's continuous rating by several orders of magnitude for a minute on starting, but the AC motor just won't give you that zero speed torque unless it is also designed to provide a lot more torque than is needed for cruising. With an AC motor oversizing is not a good idea since your fixed losses (which are higher anyway with the AC motor) will increase as you increase motor size.

Every part of the design of an electric vehicle is about compromise. If you balance your factors well you will get a useful vehicle, if you balance your factors poorly or start off with the idea that no compromise is needed, then you will be dissatisfied with the result.

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direct drive
by: bill k

i forgot to mention in my previos post. there are many trucks on the road with two speed differentials,most are too large for our application. the smallest i've ever seen was a IHC 1500, a very small dual wheel pick-up,perfect for what we are doing--problem, hard to find. happy trails ---bk---

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direct drive
by: bill k

hey i had this same idea with many of the same questions. dont forget road grime and sludge,water and salt. heres a couple answers i came up with--use a gear reduction such as you would buy from a industrial supply. you can get 1 speed,2 speed, reversing,--all at a fraction of the weight of a auto trans. you could also mount your elec motor off to the side or above your diff and chain drive (noisy) to the diff thus allowing you to install any ratio you want. with chain drive you can select low or high speed driving final ratios with the ability to change. also you could mount two motors in tandem, for low speed or hills,and economy at high speed

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Drive setup
by: Dan Bentler

(1) Is my theory of using about a 5.71/1 or more gear ratio on the rear axle a good one?
I think it may work. I would tend to think more about driveshaft speed. IF driveline speed is 3600 RPM at 60 mph then you are going to do real well with an AC motor IF it is rated for 3600 RPM.

(2) I'd prefer an AC motor but should I be considering a DC motor for reasons of low rpm torque? From what I heard AC motors have less low rpm torque but greater range of rpm's up to 10,000 so it might help my not being able to shift situation.
HP for HP an AC motor will have less starting torque. In industry the common practice is to get the next size AC motor when switching from DC motor.

(3) Would road vibration straight on to the motor be bad for it? Would it be better to have the motor a little forward more with a drive line to the differential?
AC motors are often used where there is vibration and it does not seem to harm them electrically. It may decrease bearing life however so get a motor with heavy duty bearings.
Mounting direct to rear axle may result in a problem where the axle tube is not designed for the added weight of a motor so you may have to design additional bracket to springs to carry motor load.

(4) Would adding rear independent suspension from another vehicle help out the idea about motor vibration & lighten it up but allow less rear weight carrying capacity?
YES but I think you have enough to do without this.

(5) If I had to would changing the rear tire size to a smaller one help the lack if any of low rpm torque?
YES. But you may affect vehicle stability. I ran oversize tires on my small pickup and it turned out to be quite unstable and downright dangerous.

I am playing around with these ideas also.

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