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Sumo Bots is a building event which was run in the C division in 2011, and is being run as a trial event for the B division in 2014. The event consists of building a robot which is able to push another team's robot out of a designated area. Currently the robots must autonomously operate. Before 2013 the robots were remote controlled.
There are many ways to build your robot. You could use Vex Kits or a similar building kit, or custom-build a robot from parts you attain yourself and are not part of a kit. The method that has had by far the most success is the custom built robot, since you can modify the size of the motor and the wheels into the most beneficial configuration without being constrained by the parts in a kit.
Under the trial event rules, Sumo Bots is a battle of the wallets, in a sense, since most of the components needed to build a good robot can cost a lot of money. If it is possible to salvage parts such as wheels and motors from old robots or RC cars and the like, you may be able to reduce this cost a bit.
Hopefully in the future the national committee will make some changes to the maximum size and weight of the robots so as to make the event less dependent on money and more dependent on who has the best design.
There are lots of different design possibilities, especially if you're custom building your Sumobot, but by far the most popular design is the wedge.
The wedge design is basically a robot that is shaped like a wedge. This way, when the wedge approaches its enemy bot, it can either charge and scoop the other bot right off the competition floor, or, if the other bot tries to charge, it will travel straight up the wedge, and most of the time, fall over.
There are a lot more variations on the wedge that make different designs more or less competitive, however, beyond making the scoop lower, sharper, and more steep (all of which are obvious though important). These modifications should come to you more easily by intuition and experience than learning about them by reading.
The ram design is not ideal, since a bad wedge could beat it. However, if you are able to hit it on a non wedged side, you could win.
More wheels mean that less weight rests on the scoop. The scoop does not contribute to pushing power, so weight resting on it is weight wasted. Having more wheels generally means that you will have wheel slip when turning. This happens when the wheel isn't pointing in the same direction as its motion, such as in a four wheeled robot turning about its center. All wheels are pointed forward or backwards, but their direction of travel is tangent to circles centered at the turning point. Either will contribute to significant loss of pushing force; the first for obvious reasons and the second because it will cause the wheel and the ground to engage in kinetic friction instead of the static friction of a wheel rolling on the ground.
It is palpable that you will want to use the most powerful motors you can get that are legal under the rules.
More then any other event, it is key that the person operating the Sumobot has spent lots of time with the Sumobot and is a skilled operator capable of thinking on their feet so they can react to whatever their opponent throws at them. You can have a Sumo bot capable of knocking a person out of the ring and you aren't going to win anything unless you have a driver who knows what they're doing. </spoiler>