How to Calculate AGV Drive Wheel Torque and Motor Sizing

Torque sizing errors are a common root cause of AGV overheating, unstable launch behavior, and underperforming pilot builds.

This guide gives a practical engineering method buyers can use before RFQ or sample review.

1) Inputs You Must Lock First

Use real operating values, not nominal brochure numbers.

• m = gross vehicle mass including payload and safety margin (kg).
• a = target acceleration on flat floor (m/s2).
• theta = maximum slope angle (rad or deg).
• Crr = rolling resistance coefficient (floor + wheel dependent).
• r = loaded wheel radius, not nominal radius (m).
• n = number of driven wheels sharing traction.
• i = gearbox ratio.
• eta_g = gearbox efficiency.
• v = target travel speed (m/s).

2) Core Force Equations

Use the traction force model below for worst-case operating points.

F_acc   = m * a
F_grade = m * g * sin(theta)
F_roll  = m * g * Crr * cos(theta)
F_total = F_acc + F_grade + F_roll

Where g = 9.81 m/s2.

Then convert force to wheel torque:

T_wheel_each = (F_total * r) / n

Motor-side continuous torque:

T_motor_cont = T_wheel_each / (i * eta_g)

Power check at target speed:

P_wheel = F_total * v
P_motor = P_wheel / eta_system

3) Worked Example (Procurement-Friendly)

Assume:

• m = 1200 kg
• a = 0.5 m/s2
• slope = 3% (approx theta = 1.72 deg)
• Crr = 0.02
• r = 0.10 m (loaded)
• n = 2 drive wheels
• i = 20
• eta_g = 0.90
• v = 1.5 m/s

Step 1: force components

• F_acc = 1200 * 0.5 = 600 N
• F_grade ~= 1200 * 9.81 * sin(1.72 deg) ~= 353 N
• F_roll ~= 1200 * 9.81 * 0.02 * cos(1.72 deg) ~= 235 N
• F_total ~= 1188 N

Step 2: wheel torque per driven wheel

• T_wheel_each = (1188 * 0.10) / 2 = 59.4 N*m

Step 3: add engineering margin

• continuous margin factor: 1.35 (typical range 1.25-1.50)
• T_wheel_cont_target = 59.4 * 1.35 = 80.2 N*m

Step 4: motor-side continuous torque

• T_motor_cont = 80.2 / (20 * 0.90) = 4.46 N*m

This value is your minimum continuous design point before thermal validation.

4) Peak Torque and Thermal Rules

Do not size only for peak torque.

Recommended checks:

• Peak wheel torque factor: 1.8-2.2x continuous for short transients.
• Confirm peak duration and recovery window in duty profile.
• Verify winding temperature rise at ambient upper limit.
• Validate continuous operation at target cycle, not only bench burst.

5) Typical Crr Reference (Starting Point)

Always measure or calibrate with field data before freezing final motor selection.

6) Frequent Buyer-Side Mistakes

1. Using nominal wheel diameter instead of loaded radius.
2. Ignoring slope torque because ramps are "short".
3. Treating all drive wheels as equal traction contributors under load transfer.
4. Selecting motor by peak torque only, without continuous thermal validation.
5. Missing gearbox efficiency and reflected inertia impacts.

7) RFQ Fields to Request From Supplier

When asking for wheel-module recommendation, require these outputs:

• Continuous and peak wheel torque rating.
• Continuous and peak motor torque at specified ratio.
• Rated and max wheel speed under load.
• Thermal test condition and resulting temperature rise.
• Declared efficiency assumptions.

8) Buyer Decision Gate Before Sample Approval

Approve sample only if all are clear:

• Calculation basis and assumptions are documented.
• Torque reserve and thermal reserve are both quantified.
• Test condition matches your real duty profile.
• Motor, gearbox, and wheel are validated as one system.

For a manual sizing review, send payload, target speed, slope, duty profile, and wheel envelope to [email protected]. Jimmy Su will return a calculation sheet with assumptions and recommended torque range.