SB20 - Flywheel Bearing Replacement

Previous posts describe how to add grease to the large Flywheel Bearings in the SB20. 

This post shows how to replace them.

I used 6910ZZ bearings that cost less than $15 each.  

See the FAQ below for a discussion on using name brand versus generic bearings.

Video

Here's the video.

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Flywheel Bearing Removal Steps

Replacing Flywheel Bearings requires the following steps:

1. Perform Flywheel disassembly as described in SB20 - Advanced Flywheel Servicing. 
2. Remove Coils and Optical Sensor.
3. Remove and replace large Flywheel Bearings.
4. Replace Coils and Optical Sensor .
5. Perform Flywheel re-sassembly as described in SB20 - Advanced Flywheel Servicing. 

Difficulty Level

Servicing the Flywheel bearings requires some advanced DIY skills. I rate the difficulty to be 9 out of 10.

While nothing is technically extreme, I rate this harder than similar tasks for the following reasons:

1. Risk of damaging the Stator - particularly the coils. It is difficult to get replacement parts but easy to hurt these components.
2. Removing large bearings can tricky. Separating the bearings from the Spacer and Stator Hub requires some adept chisel work. You will likely damage the original bearings. This is not a problem if you have new bearings ready to install. Keep this in mind.

For comparison: I rate Belt tensioning and alignment and Simplifying flywheel re-assembly to be 5 out of 10.  Servicing the cog and axle bearings is 7 out of 10.  

Prerequisite

Follow the flywheel removal instructions in my previous post: 

SB20 - Advanced Flywheel Bearing Servicing 

✅  I assume you have extracted the Stator from the Flywheel Shell.

What if the Stator is not easy to extract?

Some people have commented that lifting the Stator is not as easy I showed in my previous video. Surfaces get rusted. Parts can seize. The Flywheel Shaft may also have some Loctite on the inner ring.  All can make the Stator difficult to lift out of the Flywheel.  

If the Stator gets stuck, turn the Flywheel over, and tap the entire assembly against a wood plank.

You may have to tap fairly hard, several times. The Stator should eventually slide out.

⚠️ You can damage the flywheel bearings with this type of pounding. This is not a problem if you are replacing the bearings. Think twice if you are just doing maintenance or re-greasing 🤔.

🔬 Using Loctite on the outer ring of flywheel bearings is fine. Never use it on the inner rings. The SB20 design expects "interference" on the outer rings only. 

Remove Coils

Bearing removal often requires some heady-duty hammering to dislodge the bearings. To be safe, I recommend removing the Coils from the Stator Hub.

Remove the six hex bolts that fasten the Coils to the Stator Hub.

Lift Coils

With all fasteners removed, the Coils can be separated from the hub.  

🔬 You may need to gently tap the hub with a hammer.

Remove Optical Sensor

Flip the hub over and remove the small Optical Sensor PCB.  There are two screws holding it in place.

Remove Bearings 

Carefully insert a chisel into the gap between the Spacer and inner ring of the Bearing. 

This gap is very small since the inner ring and spacer flush with each other.

Support the Stator Hub between a pair of wood blocks.

Use a hammer to dislodge the Bearing and Spacer.  

⚠️ You may need to hit the chisel fairly hard.

Flip the hub over to remove the second Bearing. 

 

With the Spacer out of the way, removal of the this Bearing is easier.  I was able to us blunt socket extension instead of a chisel.

Replace Bearings 

Clean and apply Loctite to the outer ring of each replacement bearing. 

Lightly tap into place with a hammer, taking care to strike the outer ring only.

 Stake the outer ring with a metal punch to help seat the Bearing and hold it in place.
 

Repeat at four opposing locations around the outer ring.

Flip the hub over and insert the Spacer.  

🔬 Your must ensure the Spacer is properly aligned and flush with the inner ring of both Bearings.

Once again, tap into place with a hammer taking care to strike only the outer ring.

Verify that each Bearing is aligned with the Stator hub.  

🔬 The Bearings and Spacer should be flush and turn together when you spin them with your finger.

Finish up by staking (pinning) the Bearing with a metal punch.  

For more information on bearing replacement see Stages Service Bulletin:

     TSB-02202025 – SB20 Flywheel Repair Kit

Replace Coils

Align the hub with the tab as shown.

🔬 Use a hex wrench or similar tool to help line-up the bolt holes.

Re-install the six hex bolts.

Re-install Optical Sensor

Thread the cable through the hub and re-install the two screws on the Optical Sensor PCB.

You can now complete assembly. Follow the instructions at:  SB20 - Advanced Flywheel Servicing.

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FAQ

Q: Name brand SKF and NTN bearings can be expensive? What about cheap bearings for sale on Amazon?

A: It is hard to say. There is obviously a wide variety of choices. But let's be fair: Generic, Asian-manufactured bearings are common on exercise equipment and many outdoor bikes. Countless people are happy.

A good followup question might be: Are 4 year old OEM bearings better than new low-priced bearings?

I used ACROPIX 6910ZZ bearings from Amazon.  They cost $25 for the pair.  

Time will tell if $150-$250 name brand bearings would have a been a better choice.  After 4 weeks of daily use, my generic replacement bearings remain silky smooth and silent. I am very happy.

Q: Why do you do these blog posts?

A: It’s been 2 years since Stages stopped producing new SB20 bikes and started making bankruptcy noises. Around the same time, I started doing blog posts and then repair videos to deal with the fact that Stages will eventually end SB20 support. I have covered just about every major SB20 component. This post adds the last major piece: Fully disassembling the flywheel to replace the large bearings that hold the electronic brake (a.k.a., stator).

I trust that many of you have seen that the SB20 is well constructed and is serviceable by owners with DIY skills. Good news: Spare parts can still be purchased from Stages. That makes self-service more viable. Not-so-good-news: It is unclear how long the Stages parts inventory will last.

I wish there were better answers for owners who cannot do their own repairs. The SB20 was sold in many countries. It's hard to devise an after-market service strategy that works for everyone. For now, DIY rules the day.

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References

Servicing the Large Flywheel Bearings: SB20 - Advanced Flywheel Servicing. 

Making Flywheel Bearing Service Easier: SB20 - Simplifying Flywheel Bearing Service.

Axle Bearing and Cogs Removal: SB20 - Flywheel Cog and Axle Bearing Service.

Drive Belt Tensioning and Alignment: Stages SB20 - Drive Belt Adjustments (2025) 

Stages Service Bulletin: TSB-02202025 – SB20 Flywheel Repair Kit