# mounting motor to axle?



## RedBandit (May 27, 2009)

I've been looking around for a while, but haven't seen anything on this yet, so excuse me if I'm asking a redundant question. 

I'd like to do an AC system with direct drive on a small truck. If I can drive with a single gear ratio, the drivetrain will only add weight and take up space, so I'd like to mount the motor on top of the axle. Ideally, I'd have the motor driving the ring gear directly, but even if I could have a gearbox driving the axles from where the u-joint connects, I'd be pretty happy. (I'm not a gear head, so I'm not familiar with all the terms).

Has anybody mounted a motor similarly? If so, how did you do it?


----------



## major (Apr 4, 2008)

RedBandit said:


> I'd like to do an AC system with direct drive on a small truck. If I can drive with a single gear ratio,


Hi Red,

You are most likely going to need a gear reduction between the motor and axel. Without this, you would need a much larger motor. Even with a gear reduction, without a gear shifting transmission, it is called direct drive. To give you an example, the GM EV-1 electric car was direct drive and had about an 11 to 1 overall reduction from motor to wheel. And the AC motor would max out at 12,000 RPM for a top speed of like 75 mph, IIRC. And GM put this drive system into their S-10EV pick-up trucks which did very well. The Ford Ranger EVs were also direct drive with AC motor.

So, while direct drive and AC motors are possible, you still need more gear reduction than available in the rear end. So, finding a suitable gear reducer becomes a challenge. And most donner cars have a transmission. Most converters just use the free tranny, sometimes leaving it in a single gear.

And then you have to find an AC motor and inverter. If you can put it all together, you'll have a nice set-up. Do your homework first and make sure you'll have enough torque for decent acceleration or hill climbing.

Regards,

major


----------



## RedBandit (May 27, 2009)

That's exactly my point. I should be able to run direct drive on the truck without any probems. I'd like to mount the motor on top of the axle to save space and weight. To get the gear reduction I'll need, I would have a gearbox drive the axle through the rear end gears. 

I'd like to know if anybody here has tried something like this.
Thanks


----------



## danimal (Apr 24, 2009)

If you mounted the motor "on top" you would have to turn your diff 90 degrees and your pinion bearing would never get lubricated. The motor would have to be in front of the diff.


----------



## RedBandit (May 27, 2009)

If you look at the picture on the lower left, you can see my desire for a gearbox driving an axle. The shown application is for a bus.

http://www.automation.siemens.com/ld/bahnen/html_76/elfa/elfa-12.html

Driving the axle this way would do away with complicated drivetrain. I figured someone somewhere must have done this before, other than on a bus.


----------



## CNCRouterman (May 5, 2008)

Can it be done? Sure, with the already stated advantages. I do not know if it has been done. 

Do you really want to though?

+Upsides:

+ Lower overall weight.

+ Mechanicly simple, straight forward drivetrain.

+Down sides:

- If axle is rotated so pinion is pointed up, then you have lubrication problems, and the fabrication issues related to spring pad mounts or other suspension components needing fabrication.

- If you mount the motor on top of the axle, you will likely have a clearance issue with either the chassis, or the underbody, not an insurmountable problem, but you will have to plan for it.

- If you mount the motor face to face with the axle input, the motor will have a significant moment arm, the cantilevered arrangement will put a lot of stress on the mountings. Axles have rather harsh living conditions, even worse than the transmission, lots of shock, significant g loads, road debris, water, salt, mud, etc, without the option of shrouding that you might otherwise be able to take advantage of.

- The high voltage power cables will be constantly flexed, more like whipped with the normal suspension articulation of the axle.

- You will have to take into consideration almost as many constraints as a wheel motor, though without most of the most expensive considerations.

It would seem that you could get almost all of your design goals with a chassis mounted motor and reletively short drive line. That would allow you to protect the motor, and not have to worry about having high voltage wires so exposed to the elements, road debris and constant flexing.

Just my opinion, your milage may vary.


----------



## roadrailer (Jun 10, 2008)

CNCRouterman said:


> It would seem that you could get almost all of your design goals with a chassis mounted motor and reletively short drive line.


Hello RedBandit: I am doing exactly what Routerman suggests. He makes excellent points, and I would add that having the motor mounted to the axle in any position increases the mass of the axle. This is a bad thing because it changes the suspension characteristics and degrades road-handling.

My approach to the gearing issus is to use a lower-speed motor, such as a four-pole (1800 rpm) or six-pole (1200 rpm). You have to match the motor speed curves to your expected driving needs.


----------

