Rolling Animals - Disadvantages of Wheels - Obstacle Navigation - Going Over Obstacles

Going Over Obstacles

Wheels are poor at dealing with vertical obstacles, especially obstacles on the same scale as the wheel itself. Assuming a vehicle or animal can shift its center of mass, the limiting height of vertical obstacles for a passive wheel is equal to the radius of the wheel. If the center of mass cannot be shifted, the highest obstacle a vehicle can surmount is one quarter to one half the radius of the wheel. Because of these limitations, wheels intended for rough terrain require a larger diameter.

In addition, without articulation, a wheeled vehicle can become stuck on top of an obstacle, with the obstacle between the wheels, preventing them from contacting the ground. Limbs, in contrast, are useful for climbing, and equipped to deal with uneven terrain.

For unarticulated wheels, climbing obstacles will cause the body of the vehicle to tilt. If the vehicle's center of mass moves outside of the wheelbase or axle track, the vehicle will become statically unstable and tip over. At high speeds, a vehicle can become dynamically unstable, meaning that it can be tipped over by an obstacle smaller than its static stability limit, or by excessive acceleration or tight turning. Without articulation, this can be an impossible position from which to recover.