Sumo 3K - Post 02 - Physics in action

This post talks about torque, friction, 4WD rationale, and the static vs kinetic friction lesson from testing. 

We first needed to understand what physics actually governs combat robot performance. Two concepts dominate: torque and friction. Getting these right is the difference between a robot that wins and one that gets pushed around.
















Friction

The maximum force your robot can exert on the ground before its wheels slip is given by the formula 


f = μ × N = μ × mg


Where μ is the coefficient of friction between tyre and floor, m is the robot's mass, and g is gravitational acceleration.


This means there are exactly two ways to increase traction: add mass, or increase the friction coefficient of your tyres. We did both. The metal chassis adds mass naturally — steel is dense. And high-tread rubber tyres are useful specifically for their high coefficient of friction on arena floor surfaces.


Robot mass - 2.35 kg

Weight limit - 3.00 kg

Drive wheels - 4 (4WD)

Tyre type - High-tread rubber


Torque


Torque (τ = F × r) is the rotational force the motor applies to the wheel. A higher torque motor at the same wheel radius produces more linear force at the tyre contact patch. DC Johnson motors running at 12V and 500 RPM were selected because they are optimised for high torque at moderate speed, exactly the profile needed for pushing rather than racing. You need to hold your ground and push.


4WD over 2WD


This was one of the most important early decisions. With 2WD, only two wheels contribute traction. Under heavy lateral load (pushing an opponent), the non-driven wheels become dead weight. With 4WD, all four wheels contribute traction force simultaneously. The total grip is effectively doubled compared to 2WD at the same mass. The robot is less likely to spin out when the opponent pushes back. Turning in a tight arena is more controllable with independent drive on all corners.


Static vs. kinetic friction 


This distinction showed up unexpectedly during testing. After fitting the top chassis plate, the robot wouldn't move from standstill — it needed a small push to get going. The cause was twofold: the battery voltage had depleted slightly, and the added weight of the top plate had increased the normal force and therefore the static friction the motors needed to overcome at startup.


Static friction (μs) is always greater than kinetic friction (μk). Once the robot was in motion, the required force dropped; kinetic friction is lower. Recharging the battery restored enough motor current to overcome μs from rest. This is a real-world physics lesson that no textbook quite matches.

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