Re: (ET) I was wondering

["David Roden (Akron OH USA)" <etpost drmm net>]

On 8 Jun 2013 at 17:29, Geof Thompson wrote:

> If two motors were connected in series, but with opposite polarity.
> Then mounted end to end, what would happen if they were used as a rear
> axle? would  they act like a differential or would they work like a
> posi- trac. or would they just not work at all? 

In terms of the way they drive the wheels, DC motors in series will act 
like 
a conventional differential. DC motors in parallel will act more like a 
limited slip differential.  

The amount of "slip" with parallel motors will depend on the type of 
motors. 
I think that series motors might be more tolerant of differing rpm going 
round corners, but PM motors tend to be more rigid about voltage vs rpm 
and 
might fight you more.  Not having tried it, though, that's just a guess.

How you make this idea into a usable drive axle is another matter.  I 
don't 
see how you could just bolt two motors together and make a drive axle.  
Other folks have mentioned the issue of reduction gearing.  Even if the 
speed were correct, I'm not sure that conventional motor housings would 
have 
the mechanical strength necessary to handle supporting vehicle weight over 
rough ground.

Another concern in a road EV would be the large unsprung weight at each 
wheel.  That probably wouldn't matter much, if at all, with something like 
a 
tractor.  

What would be more likely to work would be to forge an axle and mount the 
motors outboard, driving the wheels through chains or belts.  Bob Beaumont 
did something like this in the Tropica roadster in the 1990s. The rear 
wheels were on trailing arms.  It used two motors, each with its own 
controller.  Each motor drove one rear wheel through cogged-belt 
reduction.  
It worked, but it was a fairly expensive setup compared to one motor 
driving 
a conventional differential.