Understanding E-Bike Assist Sensors
Every electric bike conversion kit relies on a sensor to tell the motor controller when to activate. Without a sensor, the motor would have no way of knowing you are pedaling, forcing you to rely entirely on a hand throttle. The two dominant sensor technologies used in DIY conversions are cadence sensors and torque sensors. These technologies approach the task of measuring rider effort in completely different ways.
A cadence sensor acts like a simple binary switch, detecting whether your pedals are spinning. A torque sensor measures the actual physical force you are applying to the pedals. This difference in measurement results in two entirely distinct riding behaviors, power delivery curves, and battery consumption rates.
Cadence Sensors: The Cruise-Control Spinners
Cadence sensors (like the ones included with Bafang BBS02/BBSHD kits) consist of a magnetic disc mounted on the bottom bracket spindle and a stationary Hall-effect pickup sensor. When you pedal, the magnets spin past the sensor. Once the controller detects a few magnet pulses, it delivers a pre-determined amount of power matching your selected assist level.
It does not matter if you are pedaling hard or "ghost pedaling" (spinning the pedals with zero effort); the motor delivers the same output. This is excellent for riders who want a low-sweat commute, have knee issues, or prefer a relaxed, moped-like experience. However, it can feel jerky because power starts and stops like an on/off switch, often requiring the rider to tap the brake levers (which have cut-off switches) to shut off the motor during tight maneuvers.
Torque Sensors: The Proportionate Amplifiers
Torque sensors (used in the Tongsheng TSDZ2, CYC Photon, and premium factory e-bikes) utilize strain gauges to measure the tiny deflection of the bottom bracket spindle or chainring spider under rider pedal pressure. The sensor measures how hard you push. The controller instantly amplifies your physical effort, multiplying it by a factor determined by your assist level (e.g., Eco matches your force 1:1, Turbo multiplies it 3:1).
If you push lightly, the motor assists gently. If you push hard to climb a hill, the motor instantly delivers peak power. The moment you stop pressing hard, the power subsides. This creates a highly intuitive, natural riding experience, making you feel as if you have superhuman leg strength while preserving the athletic, acoustic feel of riding a bicycle.
| Aspect / Feature | Cadence-Sensing Kits | Torque-Sensing Kits |
|---|---|---|
| Sensor Measurement | Pedal rotation speed (RPM) | Physical pedaling force (Nm) + Cadence |
| Ride Character | Moped-like, switch-like power delivery | Natural, athletic, extension of your legs |
| Rider Effort Required | Very low (supports ghost-pedaling) | Proportional (requires you to apply force) |
| Battery Efficiency / Range | Lower (blasts full power per assist step) | Higher (only assists when force is applied) |
| Response Delay | Slight lag (requires 1/3 to 1/2 turn to start) | Instantaneous (detects force before spindle rotates) |
| Typical Kits | Bafang BBS02B, BBSHD, most hub kits | Tongsheng TSDZ2, CYC Photon, Lingyi controllers |
| System Cost | Affordable and simple to replace | More expensive due to strain gauge calibration |
Choosing the Best Sensor for Your Riding Style
If your goal is to get to work without sweating, or if you plan to carry heavy cargo on flat terrain, a cadence-sensing mid-drive like the Bafang BBS02 with a thumb throttle is highly effective. If you enjoy trail riding, mountain biking, or want to maintain your physical fitness while extending your riding range, a torque-sensing mid-drive like the Tongsheng TSDZ2 is superior. It ensures you still get a workout, but allows you to climb steep hills with ease.
Tuning Bafang Cadence Lag
If you have a cadence-sensing Bafang motor and find it too jerky, you can connect a programming cable and reduce the "Start Current" and "Start Degree" settings. This makes the motor engage sooner but with a gentler ramp-up, simulating some of the smoothness of a torque-sensor system.