Robot chassis and drivetrain design covers the frame, wheels, motors, gears, and layout that let a robot move reliably. Students need this cheat sheet to connect mechanical choices to speed, pushing force, turning ability, and balance. A strong drivetrain helps a robot drive straight, climb obstacles, protect components, and perform tasks consistently.
Good design starts with clear requirements, such as floor type, payload, turning space, and competition rules.
The most important ideas are motor torque, gear ratio, wheel size, traction, center of mass, and chassis stiffness. Gear reductions trade speed for torque, while larger wheels usually increase speed but reduce pushing force for the same motor torque. Traction depends on normal force and the coefficient of friction, and it limits how much force the drivetrain can apply before slipping.
Stability improves when the center of mass is low, the wheelbase is wide enough, and the frame resists bending.
Key Facts
- Gear ratio = driven gear teeth ÷ driving gear teeth, and a larger gear ratio increases output torque while decreasing output speed.
- Output torque = motor torque × gear ratio × drivetrain efficiency, where efficiency is usually less than 1 because of friction losses.
- Output speed = motor speed ÷ gear ratio, so a 5:1 reduction makes the wheel turn one fifth as fast as the motor.
- Wheel linear speed = wheel circumference × wheel revolutions per second, and wheel circumference = pi × wheel diameter.
- Available traction force = coefficient of friction × normal force, so a robot slips when drive force is greater than available traction.
- Pushing force at the wheel is approximately wheel torque ÷ wheel radius, so smaller wheels can increase pushing force for the same torque.
- Turning radius depends on drivetrain type, wheel spacing, and wheel control, with tank drive and mecanum drive able to turn in place.
- A lower center of mass and wider support base increase stability and reduce the chance of tipping during turns, acceleration, or collisions.
Vocabulary
- Chassis
- The main structural frame of a robot that supports motors, wheels, electronics, mechanisms, and loads.
- Drivetrain
- The system of motors, gears, belts, chains, axles, and wheels that moves the robot.
- Gear ratio
- The ratio comparing input rotation to output rotation, often used to trade speed for torque or torque for speed.
- Torque
- A turning force that causes rotation, commonly measured in newton-meters or pound-inches.
- Traction
- The grip between the wheel and the ground that allows the robot to accelerate, turn, or push without slipping.
- Center of mass
- The average location of an object's mass, which strongly affects balance and tipping risk.
Common Mistakes to Avoid
- Choosing the fastest gear ratio without checking torque is wrong because the robot may stall, accelerate slowly, or fail to climb obstacles.
- Ignoring wheel diameter is wrong because larger wheels change both speed and pushing force even when the motor and gear ratio stay the same.
- Assuming more motor power always means more pushing force is wrong because traction limits the maximum force before the wheels slip.
- Mounting heavy parts high on the chassis is wrong because it raises the center of mass and makes tipping more likely during turns or impacts.
- Building a flexible frame is wrong because bending can misalign wheels, waste power, damage components, and make the robot drive unpredictably.
Practice Questions
- 1 A motor spins at 6000 rpm and drives a wheel through a 6:1 gear reduction. What is the wheel speed in rpm?
- 2 A drivetrain has a motor torque of 0.40 N·m, a 5:1 gear ratio, and 80% efficiency. What is the output torque?
- 3 A robot wheel has a diameter of 10 cm and spins at 4 revolutions per second. Using pi = 3.14, what is the robot's approximate linear speed in cm/s?
- 4 A robot needs to push heavy game pieces on carpet but also turn accurately in a small space. Explain two drivetrain or chassis design choices that would help.