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I was wondering what the max power draw from the volt battery pack is likely to be without damaging it? Would it be possible for GM to add a second electric motor of the same or similar size to make the car AWD and give it more umph. Don't get me wrong what they're planning with the volt is great but I've really become attached to AWD cars with a descent amount of acceleration that I can drive year round without any worries about not being able to make it up a hill; because of limited traction. I don't live in the mountains or anything, but it is hilly and I've seem plenty of fwd cars with snow tires loose traction halfway up a hill and slide down into the ditch or simply get stuck. I realize that this would decrease range a bit, but I wouldn't think it would be a huge amount. So long as the motors wouldn't pull too much wattage during heavy acceleration and cook the batteries.
 

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The Cadillac Provoq concept car was a 4WD E-flex vehicle. It was shown as a fuel-cell vehicle, but it used the same E-flex architecture, with the addition of two hub motors to drive the rear wheels. This gave the car both four wheel drive, and four wheel regenerative braking.

Putting hub motors on the rear wheels sounds like a really good idea. They provide both drive and braking electronically, and have a built-in locking gear to support the parking brake function. They work work well on the rear because the hub motor mechanism tends to act as a gyroscope, and this would cause issues if they were put on the steering wheels. However, most cars don't steer from the rear, so this isn't an issue when driving the rear.

The one outstanding issue on hub motors is that they can't include a friction brake in the same unit. The motor manufacturers don't think this is an issue, as they provide more than enough braking power through magnetic braking. However, federal law currently requires friction brakes on all four wheels, so this would have to be changed before this becomes reality.
 

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The MiEV uses in-hub motors with (very small) disk brakes (with the rotors outside the stator).



The Volt really should have 4 wheel in-hub motors. A single motor providing front wheel drive isn't very innovative. Just think of the traction control benefit alone (using the motors to provide independent traction control instead of just wasting energy with brakes like we do it now). You can also skip gears/transmissions which reduces wasted energy in friction and unnecessary parts and maintenance.

But I know why they chose a single motor. It's cheap. It's simple. It's faster to bring to the market. E-Flex really should offer 4x4 as standard though.
 

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Actually, they used the single motor because they learned a few things from the EV1. Some of the early EV1 prototypes had independent fron motors, in an attempt to eliminate the friction and weight of the differential. The result was a problem, as the car had a tendecy to 'wander' at high speeds. Because of internal friction, a differential causes the car to drive in a straight line, while the two independent motors (even with the most sophisticated computer controls available) resulted in the car wandering back and forth within the lane.

Hub motors have always had issues on front-drive cards running at highway speeds because of gyroscopic effects. The state-of-the art hub motors are on the Mini-QED prototype, and it has problems with steering because the front wheels act as gyroscopes. In addition, there's the issue of the sprung-to-unsprung ratio. Hub motors add a lot of unsprung weight, which results in poor ride quality. The mini-QED even eliminated the brakes at each wheel to minimize the unsprung weight.

Finally, remember that GM had to be conservative with their technology because the first Volt has to work. They learned from the EV1 that if they have issues with the car the'll have ro recall all of them and cancel the project. Unlike the EV1, they don't have the opportunity to make a mistake with the Volt. It has to work first time, or it's a flop and GM goes under.
 

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There is another issue with hub motors if you plan on off road activities - you can't wade a stream or deal with deep puddles. I do have a hard time believing that you can't sync the motors however. I can easily believe that somebody had problems, but I refuse to believe that it can't be resolved by somebody else. I've heard the 'if we can't do it it can't be done' story many times. Always been BS. This could be different I guess, but I doubt it.
 
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