SB20 - Resistance Failure

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The SB20 has an electronic eddy-current brake inside the flywheel. It produces the pedalling resistance that you feel as you ride.  



Electronic Eddy-current Brake

An eddy-current brake is a near magical device. The aluminum flywheel shell spins around the SB20's electro magnets. This creates resistance thanks to eddy current waves. Resistance increases when the flywheel spins faster or the electro-magnetic field strength is increased. That's somewhat counter-intuitive when you consider that aluminum is not magnetic.

Virtual Shifting and Zwift

There are two basic ways to change resistance:

1. Use the shifter buttons to pick a different virtual gear.
2. Ride in a app like Zwift. As terrain and conditions vary, the resistance that you pedal against will go up and down.

How to Isolate Failures 

You will feel inconsistent resistance - or no resistance - when SB20 electro-magnetic brake has failed. The physical flywheel is rarely at fault.  It is the controlling components that fail most often.

Here are the most common scenarios:
  1. Your bike has some resistance when you first power up and start riding. Some shifter buttons change resistance. Some do not. This is a shifter button problem. See SB20 - Shifter Failure.

  2. Your bike has some resistance when you first power up and start riding. As you increase resistance by shifting to large virtual gear, resistance suddenly drops. You likely have a power supply failure.  See SB20 - Power Supply Failure.

  3. Your bike has inconsistent resistance when you first power up and start riding. Shifting has no effect. You likely have a controller failure. See SB20 - Upper or Lower PCB Failure.

Other Possible Fixes

Here is some basic resistance debugging tips: SB20 - Bike Resistance Issues.

In rare cases the optical sensor in the Flywheel has failed.  Obtain a new sensor from Stages support.  Replace the optical sensor as shown in: SB20 - Flywheel Bearing Replacement.



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