It's easy to jump into a solution for minisumo without giving it much thought, they are small enough that most practically sized components will lead to a reasonably competitive 'bot. The problem really comes when you find that something isn't quite up to the job or you find that you need to get that edge over the opponent but can really see how, or don't understand why some things work better than others.
Most of this could be taken for granted but hopefully a little thought will give you an edge over the competition. There are a number of calculations than can be used to assess various mechanical aspects of our minisumo 'bot. Given a few simple calculations we can start to see how our minisumo physically interacts with it's surroundings in a way that will allow us make positive improvements.
The minisumo will be presented in a simple way, looking at the interaction with dohyo and the role of friction, how this may dictate the torque requirements. In addition to this we consider momentum, how it's effected by the 'bots velocity and what benefit it may give.
For all the assessments we will use standard units of measurement in our calculations, this will help keep the maths simple. The Friction, Torque and Momentum calculations have been pulled together into this Spread Sheet by Tom Benedict which may help you ballance the characteristics of your minisumo.
Considerations for MiniSumo Design
Having gained a basic understanding of mechanics you'll start to see where this can benefit the minisumo design, to get you started we've summarised a few pointers here.
While not exceeding the weight set in the rules we want to be as heavy as possible. This improves our ability to push, and to resist pushes, by increasing our frictional force interacting with the dohyo and the momentum behind us. Once we've built the minisumo, weigh it and add ballast to bring it up to weight.
Our examples showed the "Centre of Gravity" central to the 'bot but this need not be the case. By moving the Centre of Gravity back towards the wheels you could increase the frictional component at the wheels and reduce it at the scoop giving an overall improvement in "stiction" and effective push. This might be achieved by moving heavy parts like batteries or ballast, consider keeping the mass low (below wheel centre) as the closer your centre of gravity is to your wheel centre the more likely it is to tip up as the 'bot sets off.
Avoid the suggestion of using Lead Sheet to add ballast to you 'bot. LEAD IS TOXIC, there are many alternative. Due to the toxic nature you will not be allowed to enter MiniSumo.Org.UK events with lead ballast weights. Twenty eight pence in "copper" coins will give you 100g of low cost ballast.
To increase the push your minisumo can exert consider the tyre materials you use on your wheels. It's possible to get materials that have higher than 1 values for μs when in contact with the dohyo, rather than seeing your pushing force as a fraction of the 'bots weight it will be more.
Comparison can be made between tyre materials by using a spring balance, level with the dohyo surface, to drag a sample of the material, or your 'bot, across the dohyo - remember that you must keep the weight acting on the tyres or sample to get a true picture. Alternatively you can fix the spring balance and have the minisumo pull against it.
Avoid making the tyres too sticky, many competitions use the "paper test". Your 'bot is placed on a sheet of paper, if the sheet is lifted by the 'bot when picked up then the tyres are deemed to be too sticky.
When you get to the competition keep your tyres as clean as possible. You want the friction between the tyre and dohyo, rather than dust and dohyo.
Think about drives that have enough torque to make the most of the pushing force you can achieve through your wheels. The faster your 'bot can traverse the dohyo the more momentum you'll have for impact force on the opponent - provided neither are wedge shaped!