4WD can simulate 2-speed transmission, maybe?

[Martini]

At the risk of sounding like just another loon with an idea on the internet, I was thinking that if you implemented a 4WD system with different final gear ratios in the front and back, it could effectively operate like a 2-speed transmission. At low speeds, all four wheels will drive the vehicle, but with more acceleration coming from the wheels with the lower drive ratio. At higher speeds those wheels could free-wheel using a clutch or whatnot, leaving the high drive-ratio wheels to continue accelerating the car in 2WD mode.

In this manner, you can preserve low-end acceleration while adding a higher top speed.

 

[arg]

At the risk of sounding like just another loon with an idea on the internet, I was thinking that if you implemented a 4WD system with different final gear ratios in the front and back, it could effectively operate like a 2-speed transmission. At low speeds, all four wheels will drive the vehicle, but with more acceleration coming from the wheels with the lower drive ratio. At higher speeds those wheels could free-wheel using a clutch or whatnot, leaving the high drive-ratio wheels to continue accelerating the car in 2WD mode.

In this manner, you can preserve low-end acceleration while adding a higher top speed.

There is speculation, backed by comments from a Tesla rep in a Model X test drive video, that they are already doing exactly this in the 4WD Model X.

See http://www.teslamotorsclub.com/showthread.php/13332-Comparison-between-2-Model-X-85kWh-cars-with-a-single-and-a-dual-motor-5-Questions/page2?p=267358&viewfull=1#post267358

 

[Martini]

As usual, others are way ahead of me.

 

[Discoducky]

Why I have an X reservation :wink:

 

[J in MN]

At higher speeds those wheels could free-wheel using a clutch or whatnot...

I think this is the fundamental problem with your idea. Increased mass, cost, complexity and durability concerns.

 

[stevezzzz]

I think this is the fundamental problem with your idea. Increased mass, cost, complexity and durability concerns.

Compared with a 2WD/one motor system, yes. But once you've committed to 4WD and decided to implement it with different front and rear motors, a different gearbox and different power electronics are required up front, too. Beyond that, everything the OP suggests is just software.

 

[AC1K]

do you want to go faster than 200kph? the only reason why any car has gears is because of the terrible powerband of an ICE, electric motors have 100% power until a very high RPM, in some kick ass electric motors they can rev as high as 18,000RPM. in smaller applications i've seen electric motors exceeding 50,000 RPM.

gearing/transmissions just adds weight and weight imbalance (changing center of gravity or raising it) complexity, more points of failure, delays in power delivery and robs total power to the wheels,

 

[RDoc]

The issue might not be weight and complexity due to a different gear set since there is a transmission no matter what. Changing the gear ratio isn't going to affect either weight or complexity.
There might be a problem though with not over-reving the lower geared motor at higher speeds which would require some sort of clutch. That would be an issue, although with electronic control of the motors, it would likely not be a very big or complex device.