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Old 07-19-2010, 01:58 AM   #1 (permalink)
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Question --- The implications of doing 4 wheel motors ---

So using 4 motors, each on one each wheel, is a dream for many here... But what is the problem and why hasn't anyone done it yet?

What I'm thinking;
- Would you need 4 controllers, and 1 potentiometer hooked up to all 4 so they all "react" at the same exact time?

- One big expensive controller needed?

- Weight?

Anyways... Again... Why haven't more people don't this? Or a little simpler using 2 motors?

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Old 07-19-2010, 04:21 AM   #2 (permalink)
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I belive that someone has added an electric motor to the rear differential of a 4 wheel drive car as a hybridization project. Can't remember what car it was and who dun it.

A potential problem with 4 independantly driven electric motors is that when steering each wheel spins at a slightly different speed. The controller would have to take into account vehicle speed and steering wheel angle to calculate how fast wheel wheel should be spinning. It's doable, but a differential does this much easier.
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Old 07-19-2010, 04:19 PM   #3 (permalink)
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I think it would be easier to use two motors and run one to the front and one to the back. Using the motor as the drive shaft for the rear end putting the power into a differential, that will deal with the side to side wheel speed differences for the back. The front would be the same but would need a FWD trans or something to deal with the wheels turning and the differential.

Then have the controller(s) keep the average speed of the rear and front with in say 5%. The gearing would need to be the same or close to make it easier to deal with if you want it 4WD and more programing would need to be done if you want it closer to a AWD system.

Then if you wanted with more work you could go with a FWD/RWD hybrid. What I mean by this is you could have a combination of driving forces depending on the conditions. Since the motors are independent of each other the rear could have the motor optimized for highway speeds and front for city. That would be accomplished by selectively choosing the right gearing for each condition and placing a logic circuit between the controllers saying if below xMPH FWD if between x and x MPH FWD/RWD and if over X RWD (TPS% might be good to include in there also).
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Old 07-19-2010, 06:46 PM   #4 (permalink)
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And so with all this... Would you say you would need two controllers instead of one (so you could operate the front only or the rear only... ?)
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Old 07-19-2010, 09:01 PM   #5 (permalink)
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I think you can just use 4 controllers and not worry about a differential, conceptually anyway. The dolphin uses two controllers instead of a differential.

But hub motors are heavy (unsprung weight=bad) and some gearing is usually called for, so sprockets and chain are a hard-to-beat-for-efficiency option, and is trivial and inexpensive to implement on the non-steering wheels, and provide super-easy ratio changes, plus redundant powertrains for uber reliability (especially if you have a pack for each controller like the dolphin).

So it is very cost effective and simple and flexable and potentially very reliable to just drive the rear two wheels with two motors. It isn't clear to me that the "dream" of 4wd is all that universal.
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Old 07-21-2010, 02:37 AM   #6 (permalink)
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IIRC, the vaporware ZAP-X intended to do something like this, and it opens up a number of interesting possibilities, like easily tweakable software differentials and having the entire bottom of the car free to use as a battery pack. I believe the extra weight of the motors would be offset by the axles being made essentially unnecessary. The issue is, though, that this is really something where the vehicle has to be designed with this in mind (e.g. the axles in most production cars serve structural purposes as well), plus it might require programming, etc....it's just a complex undertaking, likely beyond the scale of hobbyists.
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Old 07-21-2010, 08:28 AM   #7 (permalink)
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Its totally do able, the software is out there (as above^) but its the complexity of balancing the out put side to side and front to back in a straight line and then the even bigger headache of the 'Diff action' require during a turn. Although It would be cool from and ESP point of view shunting the power the correct wheels to increase rate of turn etc....
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Old 07-21-2010, 08:49 AM   #8 (permalink)
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What complexity? Outside of snow/ice ops, each wheel shouldn't need to put enough torque down to make individual regulation necessary.

If four-wheel hub motors can be 1/2 the size/weight of two-wheel hub motors perhaps the unsprung weight penalty/axle wouldn't be noticeable...?
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Old 07-21-2010, 08:55 PM   #9 (permalink)
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Well the way I always though of doing this was with super inExpensive golf cart motors. Sure one golf cart motor is only 48v and 10hp. But multiply that by 4 and you get 192v and 40hp which is quite a bit!

And what i was thinking you could run only two motors (96v) for under 35mph (city driving) and then the other two for highway. And if you can make the battery array output either voltage then during city driving you would get double the range
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Old 07-21-2010, 09:09 PM   #10 (permalink)
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Quote:
Originally Posted by basslover911 View Post
Well the way I always though of doing this was with super inExpensive golf cart motors. Sure one golf cart motor is only 48v and 10hp. But multiply that by 4 and you get 192v and 40hp which is quite a bit!

And what i was thinking you could run only two motors (96v) for under 35mph (city driving) and then the other two for highway. And if you can make the battery array output either voltage then during city driving you would get double the range
You can split the pack in half then use contactors to connect those two halves either in series or parallel to change the voltage. But changing the voltage won't get you double the range. The range is dependent on the power (kWh) available in the pack, which is constant regardless of how you set up the voltage.

If you have 4 motors you will get 40 hp at their rated voltage of 48, no need to run them up to 96V. And, I wouldn't run a 48V motor at 96V for very long because it may fry. If you do, at least add some more cooling for the motor.

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