Overview

 Picking the motor that you use for your battlebot will make the biggest impact on your battlebot design. The motor will determine the hardware layout, gearing, battery and speed controller that you will need.

You want to find a motor that is efficient and powerful. You also have to consider the battery and speed controller that you want to use.
Starter motors are very powerful, however some are not very efficient. You will want to use a permanent magnet motor since they are more efficient.

I chose to use two briggs and stratton starter motors (499521) .
I also used one slightly shorter briggs and stratton starter motor (497595) for powering the weapon.
Pros
   1. They produce a lot of power (> 700 watts)
   2. They have a slipper clutch (friction clutch)
   3. They are readily available.
Cons
   1. They draw a lot of current.
   2. They are only available at 12 volts.
   3. They will over heat quite quickly.
   4. They will get hot if they are run at full speed without a load. (inductive loss)
   5. They are set so that the timing favors counter clockwise. (faster and less efficient in reverse)

Technical info
(499521)
Forward Speed: 6100 rpm
Reverse Speed: 6900 rpm
Armature ohms: ~.05 ohms
No load Current draw: 35 amps
Full current: > 60 amps
Stall current: 12V / .05 = >240 amps

Speed controller issues

The biggest problem you will have is supplying the motor with enough current while protecting both the speed controller and the motor.
The vantec controllers can handle these loads, however they have long lead times. I decided to use a inexpensive controller from innovation first. This controller is rated at 60 amps and does not have any current limiting. I used the friction clutch to protect this smaller and less powerful speed controller.

Modifications

I modified the starter motors quite a bit.
I did leave is the timing so that it had more power in forward for better pushing.

1. Sprocket chain drive. 
2. Dual wire connectors
3. RF suppression caps.
5. Bearings