Quick engineering summary
Differential drive and drive-steering units can both be used for AGV and AMR chassis, but they generate vehicle motion in fundamentally different ways. The right choice depends on turning behaviour, wheel load, floor conditions, installation space and the required control strategy.
conventional forward/reverse motion and zero-radius rotation are sufficient, lateral translation is not required, and a mechanically straightforward drivetrain is preferred.
active wheel-angle control, unusual chassis geometry, precise directional alignment or coordinated multi-wheel steering is required.
1. How the two architectures generate a turn
A differential-drive chassis changes direction by controlling the left and right drive-wheel speeds independently. Equal wheel speeds produce straight travel. A speed difference produces a curved path, while equal and opposite wheel speeds can produce an in-place rotation.
A drive-steering unit separates traction and steering. The wheel generates drive force while another controlled axis changes wheel direction.
2. Turning behaviour and tyre scrub
Differential drive can achieve a very small turning radius without a separate steering mechanism. During an in-place turn, the left and right drive paths operate in opposite directions around the vehicle's instantaneous centre of rotation.
This does not mean tyre scrub disappears. Passive support wheels and tyre contact patches still have to change direction relative to the floor. On a heavy vehicle, pivot turns can therefore create significant lateral scrub.
With drive-steering units, each driven wheel can be oriented toward the required trajectory. In a multi-module chassis, the steering angles should correspond to a common instantaneous centre of rotation. Otherwise one or more wheels may be forced to slide laterally.
3. Wheel load and tractive force
Both architectures rely on tyre-floor friction to transmit traction. A first-order traction estimate normally includes rolling resistance, grade resistance and acceleration force.
Total vehicle mass should not simply be divided by the number of wheels. Actual wheel reactions depend on centre-of-gravity position, support geometry, frame stiffness, floor unevenness and dynamic load transfer.
A differential-drive vehicle may need suspension or floating supports to maintain sufficient normal force on its driven wheels. A drive-steering chassis has the same fundamental requirement: every driven wheel needs enough vertical reaction to transmit the commanded traction force.
Calculate wheel reactions, required tractive force and wheel torque before selecting the drive module.
AGV/AMR Tractive Force & Wheel Load Calculation Guide β4. Floor condition, tyre scrub and wheel material
Turning behaviour should be evaluated together with floor friction and tyre material. Frequent pivot turns on a high-friction floor can create additional scrub and heat in a differential-drive chassis, especially when wheel loads are high.
Drive-steering units can reduce unnecessary lateral tyre motion when the steering angle follows the required trajectory, but this depends on correct steering geometry and wheel-speed coordination.
In either architecture, polyurethane hardness, tyre width, wheel diameter and floor condition influence traction, rolling resistance, deformation and wear. These factors should be checked using the actual operating environment rather than only the nominal vehicle payload.
5. Installation height is a packaging problem
It is common to assume that differential drive is always the lower-profile solution and that a drive-steering unit must be tall. That comparison is too simple.
A conventional vertical steering-drive assembly may require space above the wheel for the steering bearing, motor and transmission. A horizontal drive-steering layout can place more of the drive structure beside the wheel and may fit a much lower chassis envelope.
For either architecture, compare the actual mechanical envelope:
- overall module height and mounting-plane position;
- wheel diameter and tyre width;
- motor and gearbox envelope;
- steering sweep, where applicable;
- ground clearance;
- cable and connector routing space.
Compact horizontal arrangement for chassis where installation height and service access are important.
6. Control and commissioning requirements
Differential drive
The basic kinematic model is relatively straightforward because vehicle heading is controlled through left/right wheel velocity. In practice, accurate motion still depends on encoder feedback, wheel-diameter consistency, tyre deformation, controller tuning and floor slip.
Small differences in effective wheel diameter or wheel speed can create heading error over distance. This is particularly relevant when polyurethane tyres deform differently under unequal wheel loads.
Drive-steering
A drive-steering unit adds a controlled steering axis. The controller must manage steering angle, steering zero position, traction speed and the relationship between multiple wheel modules.
With two or more independently steered drive units, transition behaviour becomes important. Wheel angles and wheel speeds should be coordinated so that the chassis does not command incompatible trajectories during steering changes.
7. Maintenance should be evaluated at module level
Differential drive generally has fewer steering-specific components. Typical service items include drive motors, gearboxes, tyres, bearings, encoders and suspension or floating mechanisms where fitted.
A drive-steering unit additionally includes steering bearings, steering transmission, steering feedback and cable-management considerations.
Component count alone does not determine service cost. For industrial vehicles, module accessibility and replacement time can be equally important. A removable drive assembly may be easier to service than individually embedded drivetrain components.
8. Differential drive vs drive-steering: engineering comparison
| Design consideration | Differential drive | Drive-steering unit |
|---|---|---|
| Turning principle | Left/right drive-wheel speed difference | Controlled wheel angle plus traction speed |
| Dedicated steering axis | No | Yes |
| In-place rotation | Yes, with suitable chassis geometry | Possible with suitable module arrangement and control |
| Sideways translation | Not with a conventional two-wheel differential chassis | Possible in suitable multi-module architectures |
| Basic control | Relatively straightforward kinematics | Requires steering and traction coordination |
| Pivot-turn tyre scrub | Can become significant on heavy vehicles | Can be reduced when wheel angle follows the trajectory |
| Installation envelope | Can be compact | Depends strongly on vertical or horizontal layout |
| Wheel-load sensitivity | Driven-wheel normal load must remain adequate | Wheel load and steering geometry both matter |
| Mechanical complexity | Lower steering-related component count | Additional steering mechanics and feedback |
| Typical design priority | Compact, mechanically straightforward traction architecture | Directional flexibility and active wheel orientation |
Worked example: a 6,000 kg AGV
Consider an indoor AGV with a gross vehicle mass of 6,000 kg, rolling-resistance coefficient of 0.02, target acceleration of 0.15 m/sΒ² and a maximum 2% operating grade. This is an illustrative calculation, not a final module-sizing recommendation.
Rolling resistance: 6000 Γ 9.81 Γ 0.02 β 1,177 N
Acceleration force: 6000 Γ 0.15 = 900 N
Grade force at 2%: 6000 Γ 9.81 Γ 0.02 β 1,177 N
The combined straight-line requirement is about 3,254 N. With a 25% engineering allowance, the target becomes approximately 4,068 N.
This calculation shows why a β6-tonβ label alone is not enough. Pivot turning can add resistance beyond the straight-line calculation, while driven-wheel load and available tyre friction must also be checked.
For a vehicle that mainly travels forward/reverse and needs compact pivot turning, differential drive may be appropriate. If independently oriented wheels, coordinated multi-wheel steering or crab motion are required, drive-steering units may be the better architecture.
See the AGV/AMR tractive-force and wheel-load calculation guide for the full calculation method.
9. A practical chassis-selection workflow
Forward/reverse only, zero-radius rotation, narrow-aisle alignment, crab motion or true lateral translation.
Include payload, centre of gravity, acceleration, slope, rolling resistance and an appropriate safety margin.
Compare mounting height, module width, wheel diameter, motor envelope, ground clearance and service access.
Consider floor friction, joints, ramps, turning frequency, operating hours and continuous thermal load.
Motor power and rated payload should be verified after the vehicle geometry and motion requirements are clear.
10. Where we normally start the evaluation
When active wheel-angle control is not required
- defined forward/reverse routes;
- zero-radius rotation is sufficient;
- no lateral translation requirement;
- compact integrated drivetrain preferred;
- simple mechanical architecture is a priority.
When active control of wheel direction is important
- restricted manoeuvring space;
- special chassis geometry;
- precise docking orientation;
- multiple independently steered modules;
- directional flexibility is required.
11. Project data to prepare before module selection
Before selecting either architecture, prepare the basic project data below. This allows the drivetrain and chassis layout to be checked together.
Vehicle mass, maximum payload, centre-of-gravity position and support-wheel layout.
Maximum speed, acceleration, turning requirement, aisle width and lateral-motion requirement.
Floor material, joints, ramps, maximum slope and expected unevenness.
Available installation height, mounting space, wheel diameter and ground clearance.
DC bus voltage, feedback requirement, controller interface and communication protocol.
Operating hours, starts per hour, route cycle and continuous or intermittent loading.
Compare both layouts before the chassis interface is finalized
Send the vehicle weight, payload, speed, slope and available installation space. We can review whether differential drive or drive-steering is the more suitable starting point and provide relevant CAD data for module selection.