Understanding Techno Magnets on E Bikes
Techno magnets are the small permanent magnets embedded in e-bike motors, pedal-assist sensors, and speed sensors. They work with Hall-effect sensors to send position and speed data to the controller, which determines motor output, assist level, and regenerative braking behavior. Without them, your e-bike can’t tell how fast you’re pedaling or where the rotor is, so the motor either won’t run or will behave erratically.
Important: Not every e-bike uses the same magnet layout. Hub motors, mid-drives, and cadence sensors differ in magnet count, placement, and failure modes. A Bosch Performance Line CX has 48 magnets on its rotor ring; a Bafang G310 hub motor has 12. Before ordering replacement parts, check your specific motor model – a generic “12-pole magnetic ring” may not fit the rotor circumference or inner diameter of your hub.
Where These Magnets Live on Your E-Bike
The magnets appear in three main areas, each with a different job and typical failure mode.
Hub Motor Rotors
In geared hub motors (e.g., Bafang G310, Mac, or many Rad Power hubs), a ring of magnets is glued to the inside of the rotor shell. These magnets spin past stationary Hall sensors on the stator to report rotor position. Common count: 12 magnets in a 12-pole geared hub, though some designs use 16 or 18.
Example: A 350W Bafang G310 uses a 12-magnet ring. If one magnet breaks loose, the motor may stutter or throw an “abnormal current” error.
Mid-Drive Motor Rotors
Mid-drive units like the Bosch Performance Line or Yamaha PW series use a rotor with many more poles. The Bosch Performance Line CX, for instance, uses 48 magnets on the rotor ring (24 north-south pairs). That fine resolution allows smoother torque control.
Example: On a Bosch motor, a cracked magnet housing (due to a hard crash) can cause the motor to vibrate loudly at low speeds. Riders often mistake it for a bearing issue.
Pedal Assist Sensors
Two common sensor types rely on techno magnets:
- Cadence sensors – A disc with 6, 8, or 12 magnets mounts on the crank axle. As the magnet passes a fixed reed switch or Hall sensor, the controller registers one pedal revolution segment.
- Torque sensors – Some designs use a magnet ring bonded to the bottom bracket spindle. When you pedal under load, the ring moves axially past a sensor, changing the magnetic field strength. The controller interprets that change as torque.
Example: Many Bafang mid-drive conversion kits use a cadence sensor with 12 magnets. If the disc shifts (common when the crank arm is loose), the sensor may miss pulses, causing the motor to cut in and out.
How They Work with Hall-Effect Sensors
A Hall-effect sensor outputs voltage when a magnetic field passes perpendicular to its face. Techno magnets are typically neodymium (N35 or N42 grade), arranged with alternating polarity (north-south-north-south). As the rotor spins, the sensor sees a changing field and produces a square-wave signal.
- Speed calculation: The controller counts pulses per second and divides by the number of magnets per revolution.
- Direction detection: By using two Hall sensors offset by 60° or 90°, the controller can tell forward from backward rotation (important for pedal‑reverse lockout).
Concrete example: In a standard cadence sensor with 12 magnets at 60 rpm pedaling speed, the controller sees 12 pulses per second. That gets converted to assist level 1–5 based on a pre‑set map.
Common Problems and How to Spot Them
| Symptom | Likely Cause | How to Confirm |
|---|---|---|
| Motor cuts out under load | One or more magnets loosened in the rotor ring | Spin wheel by hand; listen for clicking (magnet dragging) |
| No pedal assist, but throttle works | Cadence sensor magnet disc slipped or broken | Remove crank arm; check if disc is missing magnets or out of round |
| Motor runs backward intermittently | Hall sensor misaligned with magnet ring | Measure voltage at each Hall pin while slowly rotating the wheel |
| Jerky acceleration / surging | Magnet debris (rust or metal filings) obstructing the sensor gap | Inspect gap with a feeler gauge (should be 1–2 mm) |
Example: On a 2019 RadRover hub motor, a single magnet came unglued and stuck to the stator. It caused the Hall sensor to read a constant field, locking the controller into a false default position. The motor hummed but wouldn’t spin. Removing the loose magnet and re‑gluing it solved the issue.
A 2021 RadRunner owner reported magnet rust causing a “bad Hall sensor” error. Visual inspection showed rust flakes bridging two adjacent magnets. Cleaning the rust with a soft brush and applying a thin coat of clear epoxy to the magnet surface prevented recurrence.
Practical Fixes and Verifications
Checking Magnet Health Without a Multimeter
- Rolling test: Place a steel screwdriver near the magnet ring. If the screwdriver is not attracted at the expected pole positions, the magnet has likely demagnetized or broken.
- Visual check: Remove the motor side cover (or crank arm for cadence sensors). Look for cracked, chipped, or discolored magnets. Rust spots on neodymium magnets indicate moisture ingress – the magnets still work but may continue to corrode.
Verification step for cadence sensor discs: Remove the crank arm and count the magnets on the disc. Most 12-magnet discs have a small notch at the 12 o’clock position; if you count fewer than 12, the disc is damaged and needs replacement.
Re-gluing Loose Magnets
Use a two‑part epoxy rated for high vibration (e.g., JB Weld or Loctite 648). Clean both surfaces with isopropyl alcohol. Apply a thin layer, press the magnet into its slot, and ensure the polarity orientation matches the neighboring magnets (alternating north‑south). Let cure 24 hours before reassembly.
Adjusting Sensor Gap
For cadence sensors, the air gap between the magnet surface and the Hall sensor is critical. Most manufacturers specify 1.5 mm ±0.5 mm. Too wide, and the sensor may miss pulses; too narrow, and the magnet can physically hit the sensor. Use a plastic feeler gauge (non‑magnetic) to set the gap.
Practical implication: If your e-bike motor starts cutting out under load, the most likely cause is a loose magnet. Knowing how to inspect and re-glue the magnet ring can save you an unnecessary motor replacement – often under $10 in epoxy instead of $200 for a new hub.
Replacement Parts – Beware of Mismatches
- Hub motor magnet rings – Available from Bafang, MAC, and generic Chinese sources. Often labeled “12‑pole magnetic ring for 48‑slot stator.” But check the ring’s inner diameter and magnet grade.
- Cadence sensor discs – Sold as “PAS disc with 12 magnets” or “8‑pole PAS disc.” Match the hole pattern to your crank axle.
- Sensor boards – If the Hall sensor itself is dead, replace the whole motor position sensor PCB. Check connector type (JST‑ZH, Molex, etc.) before ordering.
Trade-off to watch for: Aftermarket magnet rings often use weaker N35 grade instead of OEM N42. That can cause intermittent sensor triggering, leading to surging or cut-out under load. Also, cheap epoxy may not handle heat – in a hub motor running at 70°C (158°F), weak glue can fail within months. For critical applications like mid-drive motors, stick to OEM parts.
Frequent Questions About Techno Magnets
Can I replace a missing magnet with any neodymium magnet of the same size?
Not easily. Grade and residual flux density matter. If you use a weaker magnet (e.g., N35 instead of N42), the sensor may not trigger reliably. Also, the magnet must have the same dimensions and polarity orientation – a 1mm too‑tall magnet can grind against the sensor. It’s safer to buy a matching ring.
Why do e-bike magnets rust?
Many cheap hub motors use uncoated neodymium magnets. Moisture inside the motor (from riding in rain) causes surface corrosion. The magnet still functions, but rust flakes can jam the gap. Coated magnets (epoxy or nickel) are more reliable.
Do techno magnets ever lose their strength over time?
Neodymium magnets lose about 0.1% per decade at room temperature. Heat above 80°C (176°F) can accelerate demagnetization. In a typical hub motor running at 70°C under load, noticeable degradation takes thousands of miles. You’ll likely see mechanical damage first.
Can a magnet be too strong?
Yes. A stronger magnet can saturate the Hall sensor, causing it to output a continuous high signal instead of a pulse train. The controller then sees no speed change and may shut down. Stick to the OEM magnet grade.