Yes, I understand what you're saying, but guys I'm not saying putting the open differential on the rear will not work. It will, just not efficient because the outside rear wheel would want to go turn the rear truck back. You can fight it but its not efficient.
The best way is to put the power on the rear wheel with the most traction, in this case its the inside wheel. Its still a differential, but with LSD/Positraction.
Here is Josh, maker of Gnarboards, talking about his 4wd setup:
http://visforvoltage.org/forum/10352-4w ... -questions
"another interesting thing is how I am running 4 motors off of just one controller. Some people would advise against that since you have a chance of frying your motors. But here is the benefit to running 4 (Brushed DC) motors (In parallel) off of just one controller: You get the ultimate traction. It's like having a positraction system on your vehicle and here is why: When 1 of the 4 tires loses traction, it's (motor) Amperage demand drops and the controller stops feeding it lots of amps and instead feeds the other 3 motors however many amps they are demanding. The end result is "amps only when you need 'em". Now here is where it could be bad: If for some reason you lose traction on 3 of the 4 tires, and the 1 tire is stuck in a rut or something, then that 1 motor could "eat itself to death" by demanding too many amps, and the controller would have no problem supplying the extra amps since the other 3 motors aren't using them.
If you were to run 4 motors in series w/ the motor controller (and battery-pack) you would get the opposite effect. When 1 of the 4 tires would leave the ground it would become the only tire spinning and the other 3 tires would lose all power. (like an open differential).
So thats why I chose to just run a 24V system and 4 Brushed DC motors in parallel w/ a single Brushed DC motor controller.
I remember people telling me that this isn't an option for Brushless motors because they would eventually become out of sync with each other and the controller would have no way of syncing multiple motors. I do wonder though:
Can you run multiple sensorless brushless motors in parallel with a single brushless motor controller?"
Basically, he is saying you need to put an open differential in the front, because the outside wheel needs power; and an LSD type differential (Positraction) in the rear because the rear trucks turns oppositely compared to the front truck.
If you put an open differential in the rear, it will power the wheel with the least traction, the outside wheel because you are leaning on the inside wheel. Therefore it will not be efficient, and you will just get wheelspin, same as getting stuck in mud or snow.
I know he's talking about electronic differential and that he opted for a LSD-type differential on all wheels, but this is because I think he just wanted to use one controller.
I know its impossible to end up having a crab steering because you're leaning on the board, I'm just saying its the same principle.
Just remember, its not a car with front wheels steering and fixed rear wheels. Its a board with skate trucks that turn to opposite sides. Therefore you need a differential that works on opposite directions too.