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Old 04-23-2010, 10:29 AM   #1 (permalink)
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Why hub motors in an EV?

Why do people keep pumping money in to the hub motor idea for EV's? the only time I can see a hub motor working well is on a bicycle because it's low speed, the rest of the bicycle is unsprung and you have limited space.
Am I missing something? Reducing unsprung weight is a good idea, so why stick a 50 pound motor in the wheel? the rest of the suspension is going to need to be beefed up, the motor is going to see every bump the tire hits, it's going to be adding to the gyroscopic action of the wheel when you turn to turn the steering wheel, why not put a drive shaft on it with a pair of CV joints if you really want to do a direct drive.
My electric car has the motor mounted on the rear axle, it makes the rear suspension very slow to respond because of that extra weight, I could only imagine having two motors back there.

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Old 04-23-2010, 10:58 AM   #2 (permalink)
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Yes you've pointed out all the drawbacks for hub motors. Some of the "pro", drive shafts, cv joints, etc are all moving parts that take energy to move and introduce friction and thus lower effeciency. Also with the motors out at the wheels it gives the designers more oportunity to put the people / luggage and other stuff in different places. You are not constrained by having to put the engine in a certain location and then building everything else around it.
But I agree, you have more disadvantages then I can think of advantages.
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Old 04-23-2010, 11:41 AM   #3 (permalink)
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I'm thinking a hub motor would be advantageous for a hybrid.
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Old 04-23-2010, 01:42 PM   #4 (permalink)
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Quote:
Originally Posted by vpoppv View Post
I'm thinking a hub motor would be advantageous for a hybrid.
but why? you could shorten the drive shaft and have the motor right off the tranny.
As for CV shafts being another moving part, that is true, but as an example Honda civic drive shafts tend to last 200,000 miles and I would think that the reduction in unsprung weight would increase the efficentecy enough to make it worth while, the slightly larger heavier motor should be cheaper to build as well and that should offset the cost of a cv joint or two.
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Old 04-23-2010, 02:05 PM   #5 (permalink)
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Quote:
Originally Posted by Ryland View Post
but why? you could shorten the drive shaft and have the motor right off the tranny.
As for CV shafts being another moving part, that is true, but as an example Honda civic drive shafts tend to last 200,000 miles and I would think that the reduction in unsprung weight would increase the efficentecy enough to make it worth while, the slightly larger heavier motor should be cheaper to build as well and that should offset the cost of a cv joint or two.
Well honestly I don't think trying to pack two technologies in one car is a very good idea: it takes away from the efficiency of BOTH. But a hub motor would seem to provide more room in the engine compartment for an engine. Until the masses get over their distance anxieties, hybrids will be the "in" thing.
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Old 04-23-2010, 05:27 PM   #6 (permalink)
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Most hub motors I've seen tend to be outrunning hub motor (think of the shaft as staying still, the housing turns, the wheel bolts to the housing) making it hard to put a drive shaft in the center of it, thus it's hard to connect a traditional drive shaft to it, but I do agree on your point of making it so the components can work separately, no point in pulling the engine or tranny and having to replace the motor at the same time, make it so it can be taken appart, but it would seem to be true the other way, you don't want a $1,000 repair bill to replace a wheel bearing, but if the wheel bearing and the motor bearing are one in the same that just might be the case, also how long would a hub motor last with a bad wheel bearing?
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Old 04-23-2010, 07:59 PM   #7 (permalink)
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Why would you need a transmission with an electric motor?

Wouldn't 60 MPH be approximately 8-900 RPM, depending on tire size, and well within the speed range of most motors?

I just looked at a 50KW electric motor that is rated at 180NM (133 FT*LB) torque continuous, more than enough for most applications I would think. Especially if the inefficiency of the transmission and differential are removed.

Sure, unsprung weight is bad for the suspension, and it is going to be sever on the motor, but this effects ride quality more than drive train efficiency.

I would think that the disadvantages can be overcome and some real efficiency gains made, with some appropriate resizing of spindle bearings.
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Old 04-23-2010, 08:15 PM   #8 (permalink)
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Basically you have to oversize your motors/battery/controller if you don't have a transmission, which could conceivably cancel out the space savings of hub motors (and cost).

And oversized is not optimized. It is going to be heavier than it could be with a couple gears. Electric motors also have peak efficiency under the right conditions, you can approximate those conditions more closely/frequently with gears. Gears do not have to be horribly inefficient.

Also this mass is not just unsprung, but it is spinning. You spend more energy accelerating and decelerating (and trying to change direction).
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Old 04-23-2010, 08:32 PM   #9 (permalink)
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Yet another classic engineering conundrum.

The efficiency of eliminating shafts and whatnot is highly appealing, but then there are the downsides noted above, as well as the motors' greater susceptibility to damage.

Then all the opinions, educated opinions, and trading-off begins.

I think there isn't a single right answer. I agree that bicycle hub motors are great. I think that if the vehicle sees very smooth and clean roads, hub motors could be the best choice. In filthy, bumpy conditions- which are also more prone to damage- I'd want an inboard motor with shaft drives. And/or: is there a lot of accel/decel or are the trips predominantly cruising? And then there is, waddya gots to work with and how much do ya wanna spend? Do you want some sort of transmission tween the motor and where the rubber meets the road?

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