Multiple gears/transmission in EVs

This thread can be used for discussion related to the gearing or lack thereof in EVs.
Basically there are very few EVs (if any) that make use of a transmission or gearbox.
Mainly the electric motor is connected directly to the tires through a single gear ratio.

Some comments so far have been:
Nozferatu:
I would love to see and expect to see future EVs with gearboxes...that is to say at least perhaps 2 gears? These cars could really do with taller gearing at high speeds. The Spark EV pulls like a freight train up until about 60. And it still does well after that given how quickly the torque curve drops off after that. Imagine a flatter torque curve after 60 in this thing. I've embarrassed some seriously quick cars on the roll and held my own up to top speed. But clearly their gearing and extra power dominates at freeway speeds.

DarrenDoonovan: Why [does] the torque drops off significantly after the initial jump off the line.


Here's some info I found off reddit: "http://www.reddit.com/r/AskEngineers/comments/1wymlw/why_dont_electric_cars_use_transmissions/

Basically not worth the complexity... The electric motor has a wide enough powerband (ie- range of RPMs that it can deliver torque effectively) that it isn't needed. Also worth keeping in mind-companies design the motor with the weight of the car in mind. You can design a motor with a certain number of windings, magnet shape and location and geometry to best deliver torque and power at certain speeds.

Also: Motor torque is a function of the current and the torque constant.
T = Kt x I
As the motor speed increases the back EMF also increases. More back EMF means less applied voltage and therefore less current. Therefore you get less torque. To overcome this, you can increase the applied voltage.
With an electric motor, you can control the available torque across the whole range of speeds but you will always get zero torque at maximum speed and only get maximum torque at zero speed. What you do in between depends on the motor controller.

Chaconzies said:

This thread can be used for discussion related to the gearing or lack thereof in EVs.
Basically there are very few EVs (if any) that make use of a transmission or gearbox.
Mainly the electric motor is connected directly to the tires through a single gear ratio.

Some comments so far have been:
Nozferatu:
I would love to see and expect to see future EVs with gearboxes...that is to say at least perhaps 2 gears? These cars could really do with taller gearing at high speeds. The Spark EV pulls like a freight train up until about 60. And it still does well after that given how quickly the torque curve drops off after that. Imagine a flatter torque curve after 60 in this thing. I've embarrassed some seriously quick cars on the roll and held my own up to top speed. But clearly their gearing and extra power dominates at freeway speeds.

DarrenDoonovan: Why [does] the torque drops off significantly after the initial jump off the line.


Here's some info I found off reddit: "http://www.reddit.com/r/AskEngineers/comments/1wymlw/why_dont_electric_cars_use_transmissions/

Basically not worth the complexity... The electric motor has a wide enough powerband (ie- range of RPMs that it can deliver torque effectively) that it isn't needed. Also worth keeping in mind-companies design the motor with the weight of the car in mind. You can design a motor with a certain number of windings, magnet shape and location and geometry to best deliver torque and power at certain speeds.

Also: Motor torque is a function of the current and the torque constant.
T = Kt x I
As the motor speed increases the back EMF also increases. More back EMF means less applied voltage and therefore less current. Therefore you get less torque. To overcome this, you can increase the applied voltage.
With an electric motor, you can control the available torque across the whole range of speeds but you will always get zero torque at maximum speed and only get maximum torque at zero speed. What you do in between depends on the motor controller.

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Interesting and thanks for creating a new thread!

I personally believe EV's need two gears...one for up to 70 mph and from 70 and above. Our cars are more capable than they even are now. The Spark EV, as an example for us, can pull even harder beyond 60 mph. The torque curve is flat up to about 50 and then drops off. It goes from pulling like a muscle car to pulling like a mid-performance 4 cylinder hatchback.

The extra high speed gearing could put bring the power/torque-band back into use at higher speeds. I personally would like to be able accelerate even harder at 70 and on.

Obviously from a cost point of view it's not worth offering so much performance in a small limited production car. The operating regime of this car IS NOT at 70 and above most of the time simply because A) it's too small and B) battery capacity would not allow it for long periods of time.

Perhaps a simple torque converter could suffice.

But a few things this car could do with that I think would work wonders for the overall performance are:

A) slip diff
B) better tires/bigger rims for higher torque output at the tarmac
C) better suspension setup

I don't think any of these items would cause a drastic increase in price.

Or the EV needs two motor AWD like the Tesla Model S 70D, 85D and P85D.
The front motor with different gear ratio from rear motor.
When cruising on highway, use the front motor for most of the work, while rear is idle.
For in town, use rear motor mostly, with front helping when traction is needed.

I like the 2 motor all wheel drive idea. It would help extend the life of the rear brakes by adding regenerative braking to all wheels.

I wonder what effect the different gear ratios would have on regenerative braking?

Probably some years away yet, but when costs come down and the technology is perfected, we will see AWD 4-motor electrics. Having a motor on each wheel would improve control and handling sooo much. I've seen raves of the Mercedes-Benz SLS AMG Electric Drive with its torque vectoring, and it sounds like such a phenomenal improvement in handling and control. Few things get me excited in automotive engineering, but this possibility makes me drool.

Here's a quote, from ExtremeTech

When every wheel is powered by its own electric motor, that’s attention-getting, and it makes the $450,000 gull-wing Mercedes-Benz SLS AMG Electric Drive a harbinger of the future. Imagine if you could do away with the traditional driveshaft that dictates where the powerplant goes and imagine if you could speed up or slow down each wheel. Safety, performance, and space efficiency would all improve. So would bragging rights at the gym, so long as you can explain it to non-techie colleagues.

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