Building Battle Robots on a Budget

Lets be honest, building fighting robots is a hobby. Its an awesome hobby but it can be expensive. 

Bot Brawl asked Jason Ribeiro of Team Terror to write an article regarding budget building.

Jason might be one of the worlds best budget builders. If you ask him about it on September 19th at Maker Expo he'll likely tell you hitting things with hammers and repurposing items is almost therapeutic. 

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The repurposed lawn mower and steel

 making up Gloomsday's (220b) frame and drivetrain

Classifieds, flea markets, eBay, garage sales.  These are the supermarkets for budget builders.

That noisy power wheelchair with the sluggish batteries and the ripped seat.  That power wheels jeep 

that someone’s child has long since out grown.  Those cordless drills whose batteries are too expensive to replace.  These are some of the potential ingredients perfect for the budget builder’s recipe.

The design for a new machine is usually centered around the drive motors.  This is because the frame 

must fit around them, the wheel size is chosen based on their output speed, and the batteries and 

electronics are sized to suit their power requirements.  So where does one get cheap motors that could 

power a large robot?  

Motors

Power wheelchairs would be a number one candidate.  During their service life they’ve hauled around their owner, which when combined with the weight of the chair itself, could be approaching 450 pounds,  at a speed that is very respectable for a sumo robot.  Once their gearmotors have become too noisy, or batteries become too weak they can be found on local classifieds such as kijiji or craigslist for a bargain.

Powerwheel jeeps (kiddie ride-on cars) and cordless drills have similar drivetrains as they are both 

powered by hobby-sized 550 or 775 motors with an attached gearbox. Although their output speeds and mounting configurations are vastly different, a machine that uses 4 of either of these motors will have enough grunt to push an opponent off of the sumo platform. Perfect for a 30lb robot.

Stripped down drill motor/gearbox attached to a 4" lawn mower wheel

If you are adventurous and want to do your own gearing via gears, sprockets, or belts or some 

combination of the aforementioned, then there are several options for cheaply found DC motors.  

Electric ride on scooters and cordless lawnmowers will yield powerful motors, and there are many

others to be found on ebay and surplus stores.  For a 150lb machine between 1/4hp – 1 hp 

would be desired.

Tutorial for stripping down cordless drill motors

Frame

The framework and body of a bargain build could be made of materials such as plywood, angle iron, and sheet metal.  Even high quality structural plywood such as birch isn’t that expensive and can be found at home depot or other building centres.  Old bed frames are an excellent source of angle iron and can be found locally at the side of the road on garbage days at scrap yards.  The chassis and framework has to be strong and ridged but exotic materials are not required.

Wheels

Wheels should be sized to the output speed of your gearmotors, or the gearing chosen for the 

drivetrain.  The mounting options for these wheels, whether for a live or dead axle are an important 

decision.  Cheap wheels such as lawnmower wheels can be coated with a grippy rubber such as bike 

tread or urethane.  Soft tread caster wheels are a good choice, and pneumatic wheels can be too.

Batteries

The weight available in a large sumo robot gives you some options for battery choices.  I would 

recommend being careful about scavenging batteries as they are typically the reason why the 

equipment they’re powering is discarded.

Control System

The ruleset for the sumo allows for tether control, and this can be a cost effective but sometimes 

awkward route to take.  A hand held controller consisting of two DPDT switches can control small 

motors directly, or switch a set of relays or winch solenoids to give you forward/off/reverse control of 

your machine without speed control.  Using the stock wheelchair controller and joystick is another 

tether control option although the control can be very sluggish as the wheelchair controllers are 

programmed to avoid quick jerky movements.  

If radio control is the method you’d like to pursue then you have a couple “lower” cost options for 

suitably sized motor controllers.  VEX Pro has some cost competitive offerings that could control robots this large, as well as Victor ESC’s (883, 884, or 885 series) if one could find used units.  Companies like RobotPower, and Dimension Engineering have products worth reviewing as well. On top of a motor controller you will also need a radio system, its best to stick to a 2.4ghz radio system. If you can afford the wait time www.hobbyking.com has a great 2.4ghz radio selection, make sure you order from the US warehouse or the will ship from China.

-Jason Riberio
Team Terror

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