How to Calculate Tractive Force and Wheel Load for an AGV/AMR

When an AGV or AMR drive system is being selected, two questions have to be answered separately: how much load is carried by each wheel, and how much force the driven wheels must transmit to the floor.

They are related, but they are not interchangeable. A drive module can have enough mechanical load capacity and still be unsuitable    if the required wheel torque is too high, the driven wheels carry too little vertical load, or the duty cycle exceeds the continuous    capability of the motor and gearbox.

For preliminary sizing, it is more useful to separate wheel load, tractive-force demand andavailable wheel-to-floor traction, then check them together at vehicle level. The calculations below are intended    for engineering screening. Final values should be confirmed from the actual chassis, wheel, floor and operating cycle.

1. Do Not Confuse Rated Load, Wheel Load and Tractive Force

Rated product load

Rated load is the mechanical load that a drive unit, wheel or caster is designed to carry under its specified conditions.    It does not tell you how much longitudinal force that wheel can generate at the floor.

Actual wheel load

Actual wheel load is the vertical reaction carried by a particular wheel on the finished vehicle. It depends on the centre of gravity,    payload position, battery location, lifting mechanism, support-wheel arrangement, chassis stiffness, suspension or floating mountings,    manufacturing tolerances and floor flatness.

Tractive force

Tractive force is the longitudinal force transmitted by the driven wheels to move the vehicle. It is used to overcome acceleration,    rolling resistance, gradient resistance and other resistance that is actually present in the operating condition being checked.

This is why a drive module should not be selected from vehicle weight or product load capacity alone.

2. Start With the Fully Loaded Vehicle

Use the maximum operating mass of the complete vehicle:

Vehicle self-weight should include the chassis, batteries, drive system, lifting mechanism, electrical equipment and other hardware    that remains on the AGV or AMR during operation.

Convert mass to weight force when required:

where W is vehicle weight in newtons, m is vehicle mass in kilograms, andg is gravitational acceleration, approximately 9.81 m/s².

3. How to Calculate AGV/AMR Static Wheel Load

For a vehicle with clearly defined front and rear support lines, the initial reactions can be estimated from force and moment equilibrium.

Let:

• L = wheelbase
• x = longitudinal distance from the rear support line to the centre of gravity
• W = total vehicle weight

This check is useful because front and rear support lines can carry very different loads. A battery pack, lifting mechanism or payload    that is offset from the geometric centre can move a significant portion of the reaction force to one end of the chassis.

A rigid four-point chassis needs extra caution

Front and rear reactions can be estimated from overall equilibrium, but the exact load at each corner of a rigid four-point chassis    cannot always be determined from a simple planar calculation.

Actual corner reactions can also be affected by:

• chassis torsional stiffness;
• mounting tolerances;
• wheel and tread compliance;
• spring or floating-module preload;
• floor height variation.

If individual wheel load is important for traction, bearing life or structural verification, use the actual suspension model,    structural analysis or wheel-load measurement rather than presenting an equal-share calculation as the final result.

4. Static Wheel Load Is Not Always the Operating Wheel Load

Wheel reactions change when the vehicle starts moving. Acceleration and braking transfer load longitudinally. A lifting mechanism can    move the centre of gravity. Floor joints and thresholds can temporarily increase the load on one support point.

A simplified estimate of longitudinal load transfer is:

where a is longitudinal acceleration, h is centre-of-gravity height, and L is wheelbase.

In practice, two operating values matter especially during early drivetrain review:the lowest driven-wheel load, because it limits available traction, andthe highest support-wheel load, because it governs mechanical loading.

5. Define the Operating Case Before Calculating Tractive Force

There is rarely one useful “total resistance” value that represents every AGV or AMR manoeuvre. Define the operating cases that the vehicle    actually has to perform, then calculate them separately.

Do not automatically add the worst value from every case and call it the continuous requirement unless the vehicle genuinely performs    those conditions at the same time.

6. How to Calculate AGV/AMR Tractive Force

Acceleration force

Maximum vehicle speed alone does not determine acceleration force. Two vehicles with the same top speed can require very different peak    torque if their acceleration times are different.

Rolling resistance

The rolling-resistance coefficient should not be treated as a universal material constant. Wheel diameter, tread construction, hardness,    resilience, bearing type, wheel load, floor surface, temperature and speed can all influence the actual resistance.

For a real project, use validated wheel-and-floor data or vehicle measurement where possible. A preliminary coefficient can be useful for    concept sizing, but it should not become a guaranteed vehicle parameter simply because it appeared in an early spreadsheet.

Gradient resistance

If the route is level, the gradient term is zero. If the route includes a ramp, calculate that ramp as its own operating case.

Turning and other mechanical resistance

Caster swivel, skid steering, tyre scrub, bearings, seals, scrapers, cable chains and auxiliary mechanisms can create additional resistance.    These effects are difficult to reduce to one reliable generic coefficient for every vehicle.

On a prototype or existing vehicle, push-force measurement, motor-current logging and testing at representative payload and floor conditions    are often more useful than adding an arbitrary “turning resistance factor”.

7. Convert Vehicle Tractive Force to Drive-Wheel Torque

Once the required vehicle-level tractive force has been established for an operating case, determine how that force is shared by the driven wheels.

For a symmetric two-wheel drive, equal force sharing can be used as a preliminary estimate:

Wheel-side torque is then:

Use the effective rolling radius where accuracy matters. A compliant tread can produce a loaded rolling radius that differs from the nominal catalogue radius.

For preliminary motor-shaft checking:

where i is gearbox ratio and η is total mechanical drivetrain efficiency.    Final motor selection still requires the torque-speed profile, acceleration requirement, gearbox rating and thermal duty.

8. Check Whether the Drive Wheels Can Actually Transmit the Force

Calculated motor torque is not automatically usable traction. The approximate adhesion limit at a driven wheel is:

where μ is the available wheel-to-floor adhesion coefficient and N_drive is the vertical load on that driven wheel.

A useful reverse check is:

This lets the engineer compare the required adhesion with the actual tread and floor condition instead of inserting one generic friction value into every project.

If one driven wheel carries less vertical load than the other, check the lower-loaded wheel individually. A larger motor does not solve a traction-limited chassis;    it may only make the wheel reach slip sooner.

9. Peak Capability and Continuous Capability Are Different Checks

Starting, acceleration and some manoeuvres create short-duration peak demand. Long travel cycles and frequent turning create a different problem: heat.

Review at least:

• continuous wheel torque;
• permitted peak torque and peak duration;
• continuous and peak motor current;
• gearbox continuous and transient limits;
• starts, stops, turning frequency and travel time;
• motor, gearbox and wheel temperature during representative duty.

A single “maximum power” number cannot describe the complete duty profile.

10. Worked Example: Preliminary Wheel-Load and Tractive-Force Calculation

The following numbers are used only to show the calculation sequence. They are not JY Robot design recommendations and should not be copied into another vehicle project.

Step 1 — Front and rear support-line reactions

W = 1,800 × 9.81 = 17,658 N

R_front = 17,658 × 0.9 / 1.6 ≈ 9,933 N

R_rear ≈ 7,725 N

The two front driven wheels therefore carry about 9.93 kN in this preliminary static model. Their individual loads still need to be verified from the actual chassis arrangement or measurement.

Step 2 — Acceleration force

F_acc = 1,800 × 0.4 = 720 N

Step 3 — Straight-line force requirement

F_straight = F_roll + F_acc

F_straight = 360 + 720 = 1,080 N

No gradient term is included because this example assumes a level route. If the real vehicle has a ramp, calculate that condition separately.

Step 4 — Preliminary force per drive wheel

F_wheel ≈ 1,080 / 2 = 540 N

Step 5 — Wheel-side torque

T_wheel = 540 × 0.10 = 54 N·m per drive wheel

This is the wheel-side torque for this straight-line acceleration case before any project-specific allowance. It is not yet a final motor selection.

Step 6 — Adhesion check

If the front support-line reaction were shared equally, each driven wheel would carry approximately:

9,933 / 2 ≈ 4,967 N

The minimum adhesion coefficient required to transmit 540 N would then be:

μ_required = 540 / 4,967 ≈ 0.11

The equal split is only a screening assumption. Final traction validation should use the actual lower-loaded drive wheel and verified wheel-to-floor performance.    The purpose of the example is to show what needs to be checked next, not to prove that a particular motor or drive module will work from a few assumed inputs.

11. Common Calculation Mistakes We Try to Avoid

• Treating product load capacity as vehicle gross weight. The drive unit carries only part of the vehicle load, and drive capability is a separate check.
• Dividing vehicle mass equally by all wheels without checking the centre of gravity. This can hide a heavily loaded support point or a lightly loaded drive wheel.
• Using one generic rolling-resistance coefficient for every floor and wheel. Use actual project data when it becomes available.
• Adding worst-case ramp, acceleration and turning resistance when those events do not occur together. Define the real operating cases first.
• Checking motor torque but not traction. More motor torque does not help if the drive wheel is already adhesion-limited.
• Checking peak torque but ignoring duty cycle. Repeated starts, long travel and frequent pivoting can become thermal problems even when the peak calculation passes.

12. What We Need Before Reviewing a Drive Configuration

For an AGV or AMR drivetrain review, the most useful application data are:

• vehicle self-weight and maximum payload;
• centre-of-gravity position, where available;
• chassis drawing and support-wheel positions;
• drive-wheel load measurement, if available;
• normal and maximum travel speed;
• acceleration and braking requirements;
• wheel diameter and tread material;
• floor material and condition;
• turning or pivot-rotation requirement;
• ramps or gradients, if present;
• operating cycle;
• installation space;
• battery voltage and control interface.

The calculation is used to narrow the drivetrain options. Final selection should be checked on the actual vehicle through wheel-load measurement,    representative floor testing, current and temperature monitoring, and prototype operation at the required payload and duty cycle.

Frequently Asked Questions