Robot Pushing Force, Traction & Torque

What determines how much traction a robot will have?

**A robot’s traction is determined from two factors, the coefficient of friction of its wheels (or treads), and the normal force exerted on them.

This goes back to the basic physical equation:
F = mu * N, where “F” is the force of friction (traction, in this case) “mu” is the coefficient of friction (grippiness of the wheels) and “N” is the normal force applied to the wheels (the weight of the robot resting on the wheels).

One may increase a robot’s traction simply by adding robot weight, or increasing the “grippiness” of it’s wheels.

Note:
This is a simplified approximation of friction between two perfect (flat, smooth) surfaces. It is a good bench-mark for understanding traction, but more detailed models for traction exist.**

Will adding additional motors increase the pushing force of my robot? Will gearing down the motor to increase the output torque of my robot increasing its pushing force?

**Not directly, this would only increase the pushing force if your robot does not have sufficient torque output to match it’s current traction.

Robot pushing force is limited by two quantities, output torque of the wheel, and traction of the wheel.

To increase pushing force, one must increase both of these quantities.

(i.e. a drag-racer may have all the torque in the world, but it does no good on ice. Or alternately, a tank may have great traction, but if it is powered by a hampster in a wheel it won’t push very hard.)**

How do I increase the torque output of my robot drivetrain?

**There are two simple ways to increase the torque output.

  1. Add additional motors.
  2. Gear the final output speed slower.

(Refer to basic gear theory, found in the “Motion” section of the Vex Inventor’s Guide).**