I just got a Rover 5 chassis and I was trying to figure out an easy way to control the motors. Would it work to take the electronics from a servo, disconnect the original motor and solder it to the Rover 5 motor?
If you scroll down to the bottom of the posts, you will see how to get a high powered "servo" to work.
This guy has walked down the road beforre...
The Rover 5 motors have a stall current of 2.5A each so unless you are very gentle or use the guts of super heavy duty servos then I see blue smoke in your future!
Assuming you have the 4 motor, 4 encoder version of the chassis then it will be cheaper to buy the proper 4ch motor driver. SparkFun have them for under $25!
This controller will not only drive up to 4 motors at 4A each but it will also alow you to monitor the current draw and it will take the two outputs of each quadrature encoder and mix them into a single output so you need less external interrupts to monitor your encoders.
the current draw of the Rover 5 motor vs the servo motor.