There's a schematic for the C-Car contactor controller here:
http://autos.groups.yahoo.com/group/C-Car/files/
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In a nutshell, a contactor controller is a number of switches and (usually) a big fat resistor. In the C-Cars, there are 3 stages in the setup: 1st speed is 24v through a resistor for creeping; 2nd speed is straight 24v (no resistor); 3rd speed is straight 48v.
Check this vid for an example of one in action.
@ about 1:50 he engages the 1st stage
@ about 1:56 he engages the 2nd stage
@ about 1:57 he goes straight for the 3rd stage
And he has a bunch of fun doing it over and over!
@ about 4:40 he gets lined up to go direct-to-4th stage from stopped ... and lights up the tires! It's a goofy but fun vid to watch...