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Ground clearance

Ground clearance – definition

Ground clearance is the minimum vertical distance between the supporting ground plane and the lowest relevant part of a mobile robot. In a wheeled UGV, the term usually refers to the lowest point of the chassis, drivetrain enclosure, battery case, sensor bracket, or cable protection located between or near the wheels. Wheels and tyres are normally excluded because they are intended to contact the ground. A technical specification must state this convention explicitly.

For a mobile robot on level, rigid ground, nominal ground clearance can be expressed as:

c = zlowest - zground

where zlowest is the vertical position of the lowest protected robot component and zground is the supporting surface. The value is valid only for a defined configuration: tyre diameter, tyre pressure or wheel compliance, payload, battery position, suspension state where applicable, and robot attitude.

Ground clearance is a geometric mobility parameter. It affects whether a UGV can pass over rocks, crop residues, cables, thresholds, uneven soil, construction debris, or terrain ridges without the underside touching the surface. It does not by itself describe traction, slope capability, obstacle-climbing capability, or waterproofness.

Ground clearance in wheeled UGV design

For differential-drive robots, clearance must be evaluated together with wheelbase, track width, wheel radius, chassis shape, and the location of low-mounted payloads. A robot can have sufficient nominal clearance and still become high-centred when a ridge contacts the underside between the front and rear wheel contact areas.

The most relevant related geometry parameters are listed below.

Parameter Meaning for a UGV Practical consequence
Static ground clearance Distance under the robot on level ground without dynamic loads. Useful for comparing chassis layouts.
Loaded ground clearance Clearance with the intended payload and battery configuration. More relevant for field deployment.
Breakover angle Maximum crest angle that can be crossed without the underside contacting the terrain. Important on berms, ramps, furrows, and ridges.
Approach and departure angles Angles between the ground and lines from wheel contact areas to front or rear overhangs. Determine whether the robot strikes an obstacle before the wheels climb it.
Lowest-point location Position of the limiting component relative to the wheelbase and centreline. Determines the terrain feature most likely to cause contact.

Ground clearance should therefore be treated as a configuration-dependent specification. Adding a LiDAR mount above the chassis does not reduce clearance. Adding a low battery enclosure, skid plate, GNSS antenna cable loop, or sensor payload below the chassis can reduce it significantly.

Measurement method and specification requirements

A reproducible measurement requires a level reference surface and a defined robot state. The robot should be placed on its intended wheels, with the specified battery and payload installed. The measurement must identify the component that establishes the minimum distance.

A complete UGV specification should document at least the following items.

  • Measurement condition: unloaded, nominal payload, or maximum rated payload.
  • Ground condition: flat rigid plane, not grass, gravel, or deformable soil.
  • Wheel and tyre configuration, including wheel diameter and inflation state where relevant.
  • Whether protective skids, cable covers, and optional sensor brackets are included.
  • The location of the lowest point: front, centre, rear, left, or right side.
  • Whether the value is static or accounts for suspension compression and structural deflection.

In field robotics, the operational value is often lower than the catalogue value. Soft soil can increase sinkage. Payload can compress compliant tyres. Acceleration, braking, slopes, and wheel traversal over obstacles change robot pitch and roll. For this reason, terrain tests should use a safety margin rather than treating nominal clearance as an exact obstacle height limit.

Ground clearance in ROS 2 navigation and simulation

ROS 2 has no standard message field called ground clearance. The parameter belongs to the robot’s physical model and must be represented indirectly in its URDF, collision geometry, navigation footprint, terrain assumptions, and operational constraints. ROS coordinate-frame conventions are defined by REP 105, which specifies common frames such as base_link, odom, and map.

For a ground vehicle, base_link is commonly attached to the robot body. Its height above the ground can be encoded through the transforms between the body, wheel links, and collision links. The origin of base_link is a modelling decision, not a guaranteed representation of the lowest point of the chassis.

<link name="chassis_link">
  <collision>
    <origin xyz="0 0 0.12" rpy="0 0 0"/>
    <geometry>
      <box size="0.45 0.32 0.10"/>
    </geometry>
  </collision>
</link>

In this example, the collision box extends 0.05 m below its origin, so its underside is 0.07 m above the chassis_link origin. If the chassis_link origin coincides with the ground plane, the modelled underside is 0.07 m above ground. This is a geometric example only. It is not a hardware specification.

A standard 2D Nav2 costmap models obstacles in a planar occupancy representation. It can inflate obstacles around a robot footprint, but it does not inherently reason about underbody clearance, terrain height, or high-centring. A low obstacle may be safe for a robot with adequate clearance but still appear as occupied in a 2D LiDAR costmap. Conversely, a terrain ridge may be hazardous even if it is poorly observed by a horizontal 2D scanner.

Terrain-aware navigation requires additional perception and planning components, such as elevation maps, voxel representations, depth-camera point clouds, 3D LiDAR data, or traversability analysis. The planner must then combine obstacle height with the known underside geometry of the platform.

Leo Rover and Raph Rover context

Leo Rover is a four-wheel, differential-drive mobile platform with an integrated Raspberry Pi compute unit and ROS 2 software support. Its nominal clearance must be assessed with the installed wheel set, battery, and accessories. It should not be assumed that a Leo Rover is ready for autonomous terrain navigation without integrating perception, localisation, collision modelling, and a navigation stack.

When a Leo Rover carries a depth camera, LiDAR, compute expansion, or custom enclosure, engineers should inspect the underside and cable routing after installation. A payload mounted below the chassis can become the limiting clearance feature even when the standard chassis geometry remains unchanged. The IP55 rating describes protection against ingress under defined test conditions; it is not a substitute for clearance, obstacle, or terrain capability data.

Raph Rover is intended for applications requiring a larger payload capacity. Its larger chassis and payload envelope make clearance verification especially important because installed batteries, compute units, agricultural sensors, inspection equipment, and protective structures may alter the lowest-point geometry. The correct procedure is to create or update the URDF collision model and validate the loaded physical configuration on representative terrain.

Limitations and engineering trade-offs

Increasing clearance can improve underbody obstacle tolerance, but it changes other vehicle properties. Raising the chassis or payload increases the centre-of-mass height and can reduce static stability on slopes. Larger wheels can increase clearance, but they also affect odometry calibration, torque demand, wheel inertia, and the transform geometry used by localisation and simulation systems.

Clearance must therefore be evaluated with wheel traction, centre of mass, sensor field of view, payload mass, and the intended terrain profile. For indoor laboratory floors, thresholds and cable ramps are typical constraints. For agriculture, furrows, soil sinkage, crop residue, and vegetation can dominate. For inspection and construction environments, debris and uneven transitions may require both clearance margins and reliable near-field perception.

Normative references and technical sources

The following sources define relevant ROS modelling and coordinate-frame conventions. They do not define a universal ground-clearance measurement method for every UGV, so the physical measurement convention should be documented in each platform specification.

See also