Why Mid-Drive Motors Need Professional Test Benches
Mid-drive motors — mounted at the bottom bracket between the cranks — are the dominant architecture for high-performance electric bicycles. Brands like Bosch, Shimano STEPS, Bafang, and Tongsheng have standardized the mid-drive format for pedelecs in the 250W–750W range, while performance e-bikes and light electric motorcycles push into the 1–5 kW range with the same basic architecture.
Unlike hub motors, mid-drive units work through the bicycle’s existing gear transmission, making torque sensing, gestion thermique, and efficiency testing more complex. A professional test bench — rather than a chassis roller or field test — is essential for isolating motor and controller performance from drivetrain variables, and for generating the reproducible data needed for type approval, OEM qualification, and continuous quality control.
Mid-Drive Motor Architecture and Test Considerations
What Makes Mid-Drive Testing Different
Mid-drive motors present specific test bench challenges not found in hub motor or industrial motor testing:
- Integrated torque sensor: Most mid-drive units include a built-in crank torque sensor for pedal-assist control. The test bench must either replicate the torque input signal or bypass the pedal-assist logic to command the motor directly
- High reduction ratio: The motor shaft runs at 3,000–6,000 rpm while the output shaft (chainring) runs at 60–120 rpm. The test bench must accommodate this on the output side or connect at the motor shaft
- Controller integration: The motor controller is typically embedded in the motor housing — the test bench connects at the battery terminals only
- Narrow speed range at output: Output shaft testing (at the chainring) is low-speed / high-torque — requiring a different torque sensor range than motor-shaft testing
- Thermal sensitivity: Compact motor housing with limited cooling surface means thermal saturation occurs quickly under sustained load — thermal testing is critical
Two Test Bench Configurations
Motor-shaft configuration: Connect the test bench dynamometer directly to the motor’s internal shaft (before the reduction gear). Tests motor-only performance at high speed / low torque. Requires disassembly or a special adapter port. Gives the highest measurement accuracy for motor efficiency.
Output-shaft configuration: Connect the test bench at the chainring output (after reduction gear). Tests the complete motor + gearbox system at low speed / high torque. This is the more common configuration for production testing and type approval — it matches how the motor interfaces with the bicycle.
Diagram comparing motor-shaft vs
Key Performance Tests for Mid-Drive Motors
1. Continuous Torque and Power Test
For EN 15194-compliant pedelecs (250W, 25 km/h limit), the motor must sustain rated continuous power for 30 minutes without exceeding thermal limits. For higher-power motors (750W–5 kW), the Chinese GB standard specifies continuous duration at rated torque before thermal validation.
Test procedure:
- Mount motor on output-shaft test fixture
- Connect battery simulator to motor controller input (rated voltage)
- Command motor to rated speed via controller signal or torque input simulation
- Apply rated torque via load dynamometer
- Run for 30 minutes; log winding and housing temperature every 30 secondes
- Check temperature rise does not exceed insulation class limit at test end
2. Peak Torque Test
For light electric motorcycle classification (under GB/T 36979), the peak torque test duration is:
- Electric bicycle motors (≤4 kW): Peak torque sustained 30 seconds minimum
- Light electric motorcycle motors (4–11 kW): Peak torque sustained 60 seconds minimum
The load dynamometer must respond and apply load within 2 seconds of test start. Peak torque values for mid-drive motors typically range from 80 N·m to 250 N·m at the output shaft (after reduction), equivalent to 15–50 N·m at the motor shaft.
3. Efficiency MAP at the Output Shaft
The efficiency MAP for a mid-drive motor at the output shaft captures the combined efficiency of motor + internal gearbox. Test points cover the realistic riding envelope:
| Vitesse (output shaft rpm) | Torque range (N·m) | Represents |
|---|---|---|
| 40–60 | 20–80 | Urban cycling, flat road |
| 60–90 | 40–120 | Moderate assist, slight gradient |
| 30–50 | 80–200 | Steep climb, full assist |
| 80–120 | 10–40 | High-speed low-assist cruise |
Peak system efficiency for well-designed mid-drive units (moteur + gearbox) at optimal operating points: 80–88%. This is lower than motor-only efficiency because internal gearbox losses (typically 5–12%) are included.
4. Thermal Map and Derating Test
Mid-drive motors in compact housings reach thermal limits quickly under sustained steep-gradient climbing loads. The thermal derating test characterizes how the controller reduces power as temperature rises:
- Apply 120% rated load at output shaft
- Log motor housing temperature and controller output power every 10 secondes
- Continue until controller begins thermal derating (output power reduction)
- Record derating threshold temperature and derating rate (% power reduction per °C)
- Verify derating curve matches controller specification
5. No-Load Speed and Drag Torque
The motor’s internal drag torque (measured at the output shaft with no electrical power applied) characterizes drivetrain losses when pedaling without assist. Riders in manual mode experience this as increased pedaling resistance — a key quality parameter for premium e-bike brands.
Drag torque test: motor disconnected from battery, output shaft rotated by the dynamometer at rated output speed. Measure resisting torque. Target: <0.5 N·m for premium units, <1.5 N·m for economy mid-drive motors.
Test Bench Specification: Mid-Drive Motor Series
| Paramètre | Spécification |
|---|---|
| Motor power range | 150 W – 5,000 W continuous |
| Output shaft speed range | 0 – 200 RPM |
| Output shaft torque range | 0 – 500 N·m |
| Torque sensor accuracy | ±0,1 % FS |
| Power analyzer accuracy | ±0.2% (DC input to controller) |
| Motor performance analyzer | Électrique + mechanical simultaneous capture |
| Torque sensor type | Reaction torque (flange-mounted, no slip ring needed at low speed) |
| Battery simulator | 24 V – 72 V, 0 – 50 A bidirectional |
| Temperature channels | 8 thermocouple inputs |
| Pedal assist signal simulation | CAN or analog signal injection (controller-dependent) |
| Conformité | DANS 15194, GB/T 36979, GB/T 34660 |
Contrôle de qualité: End-of-Line (EoL) Essai
Beyond R&D validation, mid-drive motor test benches play a critical role in production quality control. An end-of-line (EoL) test bench runs each motor through a short acceptance test (typically 5–15 minutes) before it leaves the factory:
- No-load run: Checks for abnormal noise, vibration, or current draw (bearing defects, winding shorts)
- Rated load point: Efficiency and temperature rise at a single representative operating point
- Peak torque check: Brief peak torque application to verify controller current limit and motor integrity
- Drag torque check: Unpowered drag measurement within tolerance
- Automated pass/fail: All parameters compared to acceptance limits; report generated and barcode-linked to motor serial number
A well-designed EoL test bench for mid-drive motors processes 30–60 motors per 8-hour shift.
Foire aux questions
Can a mid-drive motor test bench also test hub motors?
Oui, with a different coupling adapter. Hub motors connect via the axle flange to the torque sensor, while mid-drive motors connect via the output shaft sprocket or chainring interface. Many manufacturers test both motor types on the same base bench, switching adapters between product families.
How do you simulate pedal-assist input on the test bench?
Most mid-drive controllers accept a torque sensor signal (from the crank) and a speed signal (from a cadence sensor). The test bench generates both signals electronically using a signal generator or programmable interface. Some controllers also accept CAN commands for direct power/torque control, bypassing the pedal-assist input entirely — preferred for lab testing.
What is the typical efficiency of a mid-drive motor system?
At the optimal operating point (mid-speed, 60–80% of rated torque), a well-designed mid-drive system achieves 82–88% overall efficiency from DC battery input to output shaft. This compares to 85–92% for hub motor systems, which have no internal gearbox losses — the mid-drive’s advantage over hub motors comes from accessing the bicycle’s external gearing, not from motor efficiency.
Does testing in Chinese GB standards also satisfy European EN 15194?
Partially. GB/T 36979 and EN 15194 have overlapping test items (continuous power, couple de pointe, thermal validation) but different acceptance criteria and procedural details. For European market type approval, DANS 15194 testing must be performed by an accredited European test laboratory. Chinese GB test reports can support the technical file but generally cannot substitute for EN 15194 test reports in European certification procedures.
Conclusion
Mid-drive motor testing on a professional test bench is the foundation for both engineering development and production quality control. The combination of output-shaft torque measurement, simulateur de batterie, thermal monitoring, and automated test sequences gives manufacturers the data they need for type approval, OEM qualification, and continuous improvement programs.
EconoTest designs mid-drive motor test benches for motors from 150 Dans ceci 5 kw, covering EN 15194 et GB/T 36979 compliance with full EoL automation capability.
→ Discuss your mid-drive test requirements with our application engineers.
