RE: High-Low gears [TANKS]

["jon s kowitz" <jonskowitz-at-hotmail.com>]

  Motor torque does not equal Sprocket torque.  Your transmission will act 
as a lever, so the EV Warriors in your T-34 (wonderful job on that tank btw) 
that you've got stalled and smoking are producing thier maximum motor 
torque.  Because of the 9:1 transmission ratio however, the force being 
applied ot the tracks by the sprocket is 9x the motor's force.  If your 
transmission had a 21:1 ratio then you'd be applying 21x that force (and 
this is the point things start exploding).  Or, that same motor/transmission 
combo could produce the same torque as the stalled motor at 9:1, only it'd 
still be turning, using less power and generating less heat.

  I think what we want to do is how to vary the transmission ratios between 
maximum speed and maximum power in a cost-effective manner, and solving all 
of the technical challenges that go with it.

>Any decent DC speed controller will use pulse width modulation (PWM) to 
>control a motors speed.  PWM controls a motors speed by pulsing the current 
>to the motor at a constant rate (modulation) and varies the pulse width 
>from 0% to 100%.  At full power the pulses meet and become one long stream 
>of constant current.  There is no reduction of voltage to the motor, only a 
>variable reduction of time when current is available to the motor.  This 
>all happens several thousand times a second so looks very smooth at the 
>motor end.  The big advantage PWM has over a resistance type of speed 
>control is that excess voltage is not shaved off and wasted as heat thus 
>conserving battery power.  As this relates to the question at hand 
>concerning a multiple ratio transmission.  You should consider that an 
>electric motor produces its maximum amount of torque at 0 RPM (i.e. when 
>stalled) and the least amount of torque at it's max RPM.  So in the example 
>sited (rough terrain, skid steering, etc.) a PWM controlled electric 
>drivetrain optimized to give you the top speed performance desired will 
>still have the most available torque at close to stall conditions (skid 
>steer in tall grass as an example).  At least that's how I understand it 
>and I'm no expert.
>
>Steve Tyng
>