Traction & pushing force

A drivetrain can only deliver as much force to the floor as friction allows. Past that point the wheels spin and extra motor torque is wasted.

The traction limit

F_traction = μ × N

• μ — coefficient of friction between wheel and arena floor.
• N — normal force on the driven wheels (not total weight, unless all wheels are driven and weight is evenly distributed).

Wheel material on steel floor	μ (approx)
Hard plastic / printed PLA	0.3–0.5
Polyurethane (skate wheel)	0.6–0.9
Silicone / soft rubber (“gription”)	0.9–1.4
Foam / sponge (lightweight, low grip)	0.4–0.7

Arena floors vary — bare steel, painted steel, textured, or coated. Soft silicone wheels grip best but wear and tear off in hits.¹

Getting normal force onto the driven wheels

• All wheels driven (4WD or 2WD with only 2 wheels total): N = full robot weight, best case.
• 2WD with undriven casters/skids: only the driven axle’s share of the weight counts. Bias the CG toward the driven wheels.
• Weight transfer: under forward drive torque the robot pitches back, loading rear wheels. A weapon’s gyro reaction and impacts also shift N moment to moment.

Design implications

• A control bot maximizes μ (soft wheels) and N (heavy, low, driven wheels carrying most of the mass) because pushing is its weapon.
• A spinner accepts lower traction — it needs the weapon’s mass high and often can’t fit 4 large soft wheels — and wins by damage instead.
• Match drive gearing to the traction limit: there is no benefit to tractive force well above μ × N.

Front wedge interaction

A wedge that gets under the opponent transfers their weight onto your driven wheels, raising N and your effective pushing force while removing it from theirs. This is why wedge geometry matters even on weapon robots — see Control bots & wedges.

References

Footnotes

1. Meggiolaro, M. A. RioBotz Combat Robot Tutorial, v2.0, PUC-Rio, 2009 (CC BY-NC-ND) — traction, friction coefficients, and weight-transfer analysis. ↩