How do Garmin Speed Sensors Measure Speed?

The Garmin Bike Speed Sensor and Speed Sensor 2 both contain a magnetometers that measures the three dimensional components of the ambient magnetic field (similar to an accelerometer measuring acceleration). As the wheel rotates, the magnetometer measures its own rotation within the surrounding earth's magnetic field. The earth's magnetic field does not vary much (if at all) over the course of a bike ride. This means that the sensor doesn't need to concern itself with the variation of the earth's magnetic field. However, since the product is available in numerous countries around the world, the product has several consistency checks to ensure that it is observing the earth's field and not another magnetic feature.

Will Riding Around Metal Objects Such as a Steel Fence or Manhole Cover Affect the Accuracy of the Sensor?

While these materials are magnetic and will contribute to the raw signal observed by the magnetometer in the sensor, Garmin speed sensors have been designed to track the wheel's rotation correctly while passing these types of magnetic features. Testing has confirmed that these magnetic features very rarely generate errors or tracking issues.

What About Indoor Trainers That Feature Magnets?

Garmin speed sensors will operate correctly when used with a magnetic fluid trainer.

Magnets and magnetized steel near the sensor could be particularly troublesome as they would introduce error and noise into magnetometer data. For this reason, the BSS performs an automatic calibration routine whenever it is motionless for more than 3 minutes to minimize the affect of any magnetized components on the bike.

Are There Any Other Potential Cases of Interference?

Despite the auto-calibration process mentioned above, the sensor will likely have challenges if mounted over a dynamo hub (i.e. a hub that generates power for a light or other device). These hubs typically have magnetic components which will interfere with the sensor. Since these hubs are much larger than the typical road/mountain bike hub, users will generally mount the sensor on the other wheel.

In addition, magnets are often used in other wheel speed, cadence or power sensors. Use of any magnets near the sensor should be limited. If a magnet is mounted too close,  it could cause the Garmin speed sensor to malfunction. Stronger magnets must be kept a greater distance from the sensor. Ideally, all magnets should be removed from a bicycle when using the sensor. If necessary, the sensor can be moved to the other wheel.

Occasionally, a component on a bicycle will exhibit unnaturally strong magnetism that will interfere with the sensor yielding less accurate results. These magnetized components could be the wheel hub, the free wheel inside the hub, the cassette, chain or other component on the bike and may have become magnetized inadvertently by storing the components in very close proximity to a strong magnet. In this case, it is advised to mount the Garmin speed sensor on the other wheel of the bike.