Whegs are wheel-leg hybrids used in mobile robots that must move across rough ground, rubble, roots, curbs, and stairs. A wheg looks like a wheel with several curved spokes that act like short legs as it rotates. This design matters because ordinary wheels are fast and simple on smooth floors, while legs are better at stepping over obstacles.
Whegs combine some advantages of both, giving a robot rugged mobility without the complexity of full walking legs.
As a wheg turns, one curved spoke contacts the ground, pushes backward, and lifts the robot body over an obstacle. The spoke tips can hook onto step edges or grip uneven surfaces, creating traction where a round wheel may slip or stall. Engineers choose the wheg radius, number of spokes, motor torque, and ground clearance to match the terrain.
Wheg robots are useful for search and rescue, planetary exploration prototypes, inspection robots, and classroom robotics because their mechanism is mechanically simple but physically rich.
Understanding Robotics: Whegs (Wheel-Leg Hybrid)
A wheg does not move with perfectly smooth rolling. Its height rises and falls as each spoke takes the load, so the robot body bounces slightly. This bouncing depends on the shape of the spokes, the spacing between them, and the softness of the ground.
A curved spoke can make contact gradually, which reduces sudden impacts. A straight, rigid spoke may strike the ground harder but can produce a stronger stepping effect.
The useful radius changes during a turn because the contact point moves along the spoke. This means the robot speed can vary even when the motor spins at a steady rate.
Obstacle climbing is mainly a problem of contact geometry. The next spoke must reach a useful point on the obstacle before the robot chassis collides with it. If the spoke tip slides down a vertical face, the robot may remain stuck even with a powerful motor.
Grip matters because the contact force has a forward part that lifts the body and a sideways part that can cause slipping. Rubber covers, textured tips, or flexible materials can improve grip on smooth surfaces. On loose soil, however, a tip may dig in and throw material backward instead of lifting the robot.
Motor torque is especially important at low speed. When a wheg starts to lift the robot, the motor must turn while supporting much of the robot weight. A motor can spin quickly with little load but slow sharply when the required torque rises.
This is why a geared motor is common in a wheg robot. Gears trade rotation speed for turning force.
Students can notice this by comparing a robot on a flat floor with the same robot climbing a book or a small curb. Battery voltage, motor current, gear ratio, and robot mass all affect whether it completes the climb.
The body design matters as much as the whegs. A high body avoids scraping, though it can make the robot easier to tip. A wide stance improves stability, but wide robots may struggle in narrow spaces.
The centre of mass should stay low and near the middle so that the robot does not pitch backward during a climb. Turning can be difficult because left and right whegs may need to rotate at different speeds. When building or testing one, pay attention to repeatable measurements.
Record obstacle height, surface material, battery level, spoke shape, and motor setting. Small changes can produce large differences in traction, bouncing, and climbing success.
Key Facts
- A wheg is a wheel-leg hybrid that uses rotating spokes instead of a continuous circular rim.
- Linear speed for rolling motion is approximately v = omega r, where omega is angular speed and r is effective radius.
- Motor torque must overcome obstacle load: tau = F r, where F is the force at the contact point.
- More spokes usually make smoother motion, while fewer spokes can give deeper stepping action over larger obstacles.
- A wheg can climb an obstacle when a spoke can grip or reach over the obstacle edge and the motor can supply enough torque.
- Ground clearance should be greater than the height of terrain features that would otherwise hit the robot chassis.
Vocabulary
- Wheg
- A wheg is a rotating wheel-leg mechanism with spokes or curved legs that help a robot move over rough terrain.
- Torque
- Torque is the turning effect of a force, calculated as tau = F r when the force acts perpendicular to the radius.
- Traction
- Traction is the grip between a robot's contact surface and the ground that allows it to push without slipping.
- Ground clearance
- Ground clearance is the vertical distance between the lowest part of the robot body and the ground.
- Gait
- A gait is the repeating pattern of contact and motion used by legs or whegs to move a robot forward.
Common Mistakes to Avoid
- Treating a wheg like a perfect wheel is wrong because its effective radius changes as different spokes contact the ground.
- Ignoring torque requirements is wrong because a robot may have enough speed on flat ground but still fail to lift itself over a step.
- Assuming more spokes are always better is wrong because extra spokes can make motion smoother but may reduce the ability to hook and step over tall obstacles.
- Forgetting chassis clearance is wrong because even strong whegs cannot help if the robot body gets stuck on the obstacle.
Practice Questions
- 1 A wheg has an effective radius of 0.12 m and rotates at 8 rad/s on flat ground. Estimate the robot's forward speed using v = omega r.
- 2 A spoke must apply a 35 N force at a radius of 0.10 m to lift the robot over a small step. What torque must the motor provide, assuming the force is perpendicular to the radius?
- 3 Explain why a wheg robot might climb a stair better than a normal wheeled robot, but still be simpler than a robot with fully articulated legs.