SB20 - Flywheel Bearing Failure

The flywheel has 3 pairs of deep-groove bearings. While complete bearing failure is uncommon,  we should expect problems to increase as bikes age past 5 years old. 

The bearings in the SB20 flywheel do not typically fail overnight. They initially degrade with subtle increases in friction and noise. Heat is the next symptom. This can be followed by lubricant leakage and then obvious mechanical looseness.

How to Isolate Failures 

Overall flywheel bearing health is easy to assess. 

Turn bike power off. Spin the flywheel shell by hand.

You should hear nothing. With the power off, the flywheel should be silent when spun by hand.  

Put your hand on the stationary part of the flywheel and then the frame next to the axle.  You should feel no vibration of any kind. 

With the bike powered off, start pedalling. Wear just socks if possible. Spin up to high cadence. You may feel minor vibration when accelerating (that's the belt).  When pedalling at a constant cadence, everything should be buttery smooth.  Listen carefully and try to feel rumbles, growls and grinding that match flywheel speed. 

Spin the cranks in unison with the flywheel.  Once again, everything should be smooth and quiet.  

Stop pedalling and coast. At high speed you will hear a slight whine from the roller clutch in the cog freewheel. You should feel no vibration.  Everything should become silent as the flywheel begin spinning at lower RPMs.

🔬 If you feel significant vibration and rumbling while coasting, this could indicate failure of the drive cogs. This is very rare, so don't count on it 😀.

Finally, turn the power on and start pedalling again  Like with the power off, you may feel minor vibration when accelerating and then nothing as you pedal at a constant rate. When coasting, everything should be smooth and quiet. You may hear very minor slight electronic whine.  That's good.  That's the indication that your electronic brake it doing its job. It also signifies your flywheel is running quietly enough.  

Other Warning Signs

• The flywheel shell gets hot when working normally. That's how the eddy-current electronic brake creates resistance. When the bearing are starting to fail, the flywheel may start getting hotter. 
• Grab the stationary flywheel with your hands. Try to move the shell, up and down, and then side to side.  There should be no perceivable movement of any kind.
• Watch for grease oozing out of any of the bearing around the axle, drive cogs or flywheel shell.

How to Fix 

You must remove and disassemble the flywheel to accurately determine which bearing has failed.  

With the flywheel removed from the bike, you can spin the cogs, axle and flywheel shell by hand to feel for bad bearings.

 
To service bearings that are starting to fail: SB20 - Flywheel Cog and Axle Bearing Service.

If you think your bearings have failed or simply want to work on your bearings once-and-for-all: SB20 - Flywheel Bearing Replacement (Recommended) .

When performing either of the above, also consider doing: SB20 - Simplifying Flywheel Bearing Service and SB20 - Advanced Flywheel Bearing Servicing.

Other Possible Fixes

Vibration that follows pedal speed could be a sign of poor drive belt tension or alignment.  See: SB20 - Drive Belt Adjustments (2025).