Many factors are involved when determining the types of terrain a robot is capable of crossing. Two main factors are at play: the type and consistency of the ground, and the potential energy involved.
Ground
The type of ground, from hard pavement to carpet, grass, sand, forest floor etc. relates to the type of tread you chose, as well as the torque available from the drive motors. Deeper treads provide more grip whenever the terrain is uneven or granular (sand, snow or pebbles) but are not effective on flat (pavement, cement, flooring) terrain. A bigger wheel radius results in more clearance and helps avoid situations where the robot's chassis is stuck and the wheel loses contact with the ground. For humans, it is easy to see the impact an incline has on walking, running or biking; it is always harder to move up an incline than to move on a flat surface.
Potential Energy
Potential energy (PE) is defined as:
PE = m * h
m: total robot mass
h: height from selected reference level (arbitrary since we are only concerned with the change)
The change in potential energy can similarly be defined as ΔPE = m * Δh where Δ is "change". The energy needed to raise a mass (in this case a robot) is directly proportional to the distance you wish to raise it. When going up an incline (or rolling over an obstacle), your robot is increasing its potential energy and when it travels down a distance it is decreasing its potential energy (something that is not easily recovered in robotics). If you want your robot to scale steep inclines, it will need significantly more torque (for the same given speed) than if it were moving on level ground. Note that the energy required to move a robot up a certain distance is the same if it moves up a steep or gradual incline though the speed and torque required will most likely differ.
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