[ausev] VW FOX CONVERSION, GEAR RATIO QUESTION

[Chris Robison]

loopcat wrote:
> Your right Chris, The motor is OVERSIZED for the car. The weight difference between this GE-11 and a ADC-9 is about 90 lbs. I can live with that. But I am very concerned about the torque effects on the fox tranny.

I think a 9" motor is a bit much for a Fox. I'd say an 8" is about 
right. Also be aware that a pound of rotating mass (larger, heavier 
armature) is worth potentially several pounds of mass on the body or 
chassis, in terms of the energy required to overcome inertia and the 
energy subsequently lost during braking.

Weight is also not the only consideration. The motor's additional volume 
will take up space that could otherwise be used for batteries or other 
components. This actually ended up being a challenge with Brian's 
Saturn; choosing a 9" instead of an 8" motor forced some change in plans 
for fitting things in the front. Just that extra 1/2" of height over the 
top of the motor affected the design significantly.


> If I were to keep the motor, I could make adjustments to the Zilla LV. The GE tag states 1822 rmps at 70volts, So at 156v it should bring it up to about 4000 rpms. Right? The fox engine produced 81 hp and 91 lbs of torque. So what is a safe current to run it at?  I'm guessing that my battery selection would also play a part.

Series-wound motors don't seek to a particular RPM for a given voltage, 
like shunt-wound and permanent magnet motors do. The RPM on the tag is 
just a point at which the motor's other performance values are measured 
and rated. For example if you powered the motor without a load at 70 
volts, it would NOT spin up to 1822rpm; it would quite likely spin up 
well past its redline and the commutator would explode.

I don't know how much torque per amp this motor will make. I also don't 
know how fast back-EMF will climb with RPM (which determines how 
controller output voltage must rise to maintain the same current flow). 
It's pretty hard to say what will be "safe" but bear in mind that at 
1000A, your motor will quite possibly produce 3+ times the amount of 
torque that the stock engine did. I've read that the Fox's 4-speed was 
actually a durable, well-made transmission, but this is so far beyond 
it's intended operating area that it's hard to predict how long it will 
stand up to the abuse. Turn the current down to 500 or 600A or so and 
you may solve this problem, but then you'll have a lot of unused 
capacity in both your motor and your controller.

Generally speaking, larger motors are capable of producing more 
continuous mechanical horsepower than small ones. However, this higher 
output power is in the form of higher torque, not higher RPM (a larger 
motor tends to have a lower redline). If your vehicle cannot take 
advantage of the higher torque, then the larger motor is dead weight.

   --chris