Cheapest and easiest way to slow down a motor

[DonRotolo]

Wow, that's quite a thread.

1. To reduce the speed of the motor, cut the voltage. R=V/I, if you want 6 volts to the resistor and I = 4 Amps (You measured that?), 1.5 Ohms. A dozen feet of 18 gauge wire will cover that pretty well, just coil it loosely so it can dissipate the heat. BUT all this is OK only if you can accept significantly reduced torque and power.

2. To control the speed to any value, and not lose torque or power, use a PWM motor controller in "Locked Anti-phase" mode. Victors (at leats 2005 version) don't do this, but something like that should be easily findable, try a google search or visit IFI and other robotics sited. PM me if not.
(FYI: Locked Anti-Phase control switches voltage polarity quickly, so the motor actually has full current at all times (max torque) but at, say, 50% duty cycle the net current flow over time is zero, so the motor stands still (hums, actually). At 75% duty cycle, it moves forward around half speed, but again with full torque.)

3. To control the speed with a Victor, just build the circuit I posted near the end of this post and get a victor, or similar. You do lose some torque though, but it seems to work for most FIRST teams...

4. Modern cars use a special motor for the wipers, with two windings, one for low and the other for high speed. Both windings take 12 volts. Resistors haven't been used for decades.

5. The cordless drill idea from Gdeaver is pretty good, though.