SB20 - Advanced Flywheel Bearing Servicing

This post focuses on disassembling the SB20 flywheel to extract the internal electromagnet stator assembly from the flywheel shell. 

 

You can then add grease to the large Flywheel Bearings or replace them. You can also replace the Optical Sensor should you need to.

Video

Here's the video.

Who Should Do this Service?

1. Anyone who wants to extend their flywheel life. Sealed bearings can last a lifetime without servicing - but they can also fail after a few years. One way to maximize lifespan is to add grease once or twice a decade.
2. If you have grinding, clicking and thudding sounds that indicate bearing failure. All SB20 flywheel bearings are inexpensive. The service procedure shown can also be used to replace bearings.
3. You read the Stages service bulletin on quieting clicking flywheel noises, and want to do a better job.

In short, servicing sealed bearings is optional. Most bearings never fail. Extreme temperatures, and exposure to humidity can be a negative factor. Just keep in mind: Spraying lubricants like WD-40 or PTFE into your bearings (like mentioned above), will shorten their lifespan. Repacking your bearings with grease is a good maintenance step especially after a few years when you start feeling minor vibrations or hearing a few rumblings. You must balance this with your mechanical skills and confidence. If you are comfortable doing the work described here, go for it! Otherwise, consider just waiting for a bearing to fail. The effort needed to replace a bearing is about the same as servicing one.

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 if you are not careful.
2. Special tool required. A 55 mm socket makes this job almost easy. I am afraid that most people will use adjustable wrenches (as suggested by Stages). Be prepared for some cursing and swearing if you go this route.
3. Large bearings that are not well secured. Stages glues these bearing in place and pinches them with a metal punch. Knock a bearing out of alignment and you may never know you made things worse.

For comparison, I rate Belt tensioning and alignment to be 5 out of 10.  Servicing the cog and axle bearings is 7 out of 10.  

Prerequisite

In a previous post, I described how to service the Stages SB20 smart bike cog and axle bearings. 

SB20 - Flywheel Axle and Cog Bearing Service 

Please review these instructions before considering the maintenance and repairs described in this post.

✅ I assume you have removed axle bearings and cogs before starting the work described below. 

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Servicing Details

Here are the internal flywheel components we will be working on:

Flywheel Bearing Service Steps

Servicing Axle and Cog Bearings requires the following steps:

1. Complete axle bearing and cogs removal as described in SB20 - Flywheel Cog and Axle Bearing Service. 
2. Secure Flywheel Shell so it does not spin.
3. Remove Flywheel Nut and plastic cover. 
4. Lift out Stator.
5. Add grease or replace Flywheel Bearings .
6. Replace Stator.
7. Replace Flywheel Nut and plastic cover. 
8. Slide in Flywheel.
9. Complete axle bearing, cogs and bike reassembly as described in SB20 - Flywheel Cog and Axle Bearing Service.
   

Secure the Flywheel

Removing the Flywheel Nut requires significant twisting force (from 30 to 100 Nm).  

Stages suggests having someone hold the flywheel.

No one in my family likes holding things 🤣, so I built a clamp using a 5 dollar piece of 2x4 lumber.

Flywheel Clamp (truly optional but nice-to-have)

My flywheel clamp secures the Flywheel between 2 pieces of lumber, tightened with a threaded rod.

The clamp itself is held in place with a pair of utility clamps. 

Removing the Flywheel 

⚠️ Stages suggests using an adjustable wrench after removing the flywheel cover.

I am not a fan of this approach. My Flywheel Nut required more removal torque than could be leveraged with an adjustable wrench. This method also exposes the delicate electromagnet coils to damage. One slip of the wrench could destroy your stator coils. 

Get the Right Tool

I recommend investing in a 55 mm (2 ⅛") socket. The cost at an auto supply store is around $20. It's only $15 at a discount retailer like Harbor Freight. 

Removing the Flywheel Nut is then easy...

Keep the flywheel cover on for this step.  It will protect the internal components.

Remove the Flywheel Cover

Remove the 3 mm hex bolts and then the flywheel cover.

Lifting out the Stator

Use a pair of locking pliers to firmly grip the stator fins as shown. 

You can then lift up and extract the stator from the flywheel shell. 

⚠️ This step may not be as easy as it looks.  Many Stator Bearings have Loctite (and sometimes rust) on their inner rings. You may have to shake the assembly a few times to extract the Stator.

If this doesn't work, flip the flywheel over and tap the flywheel against a wood block. This should loosen the Stator so you can flip it back over and lift it out.

Adding Grease to the Drive-Side

You can now add NLGI 2 grease to the drive-side Flywheel Bearing.

These 6910z Flywheel Bearings have metal dust covers that are not removable. Fear not. You can force grease into the bearing by pushing lubricant into the opening around the inner ring.  

🔬 In the video, I show a few seconds of effort. You want to add enough grease to pack the bearing about half full. This should take you about a minute.

Adding Grease to the Non-Drive-Side

Flip over the Stator to add grease to the other bearing.

⚠️ Cover the optical sensor before starting.

Replacing Bearings 

Rather than servicing the bearings, you can replace them.  

1. Insert a chisel or screw driver between the spacer and bearing.   

Hammer outward to release the bearing and spacer. You are replacing the bearing so don't worry if they get damaged during removal.

⚠️ There is some risk to the flywheel coils during this step. Consider removing the 6 screws that hold the plastic coil assembly to centre hub. Lift off the coils and set them aside.

2. 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.



Finish up by tapping tapping outer ring in several places with a hammer and screwdriver to ensure the bearing is fully set and aligned.  

3. Ensure the spacer is properly aligned on the inner ring of both bearings.

Check the each bearing is flush with the Stator hub.  The bearings and spacer should turn together when you spin them with you finger.

 
4. Stake the outer ring of each bearing with a metal punch to help hold it in place.
 

Repeat at 4 opposing locations around the bearing.

You can now reassemble the flywheel.  Don't ride for 24 hours to allow the Loctite to cure.

For more information on bearing replacement see Stages Service Bulletin:

     TSB-02202025 – SB20 Flywheel Repair Kit

Optical Sensor Replacement (if needed)  

🔬 If you need to replace the Optical Sensor, now is the time.

Replace Stator

Bearing service / replacement is now complete. Let's put everything back together,

Use your locking pliers to replace the Stator.  Ensure everything spins easily and silently.

Replace Cover

Replace the cover and 3 mm hex bolts

Replace Flywheel Nut

Replace and fasten the Flywheel Nut. 39 Nm of torque is recommended by Stages.  

I found I get quieter operation by tightening to 30 Nm with a drop of blue Loctite thread locker. 

That's it

You have successfully serviced the Flywheel Bearings 🏆.  

Finish by reassembling the axle, cogs and bike as detailed in:  SB20 - Flywheel Cog and Axle Bearing Service.

Cleaning Up

Use isopropyl alcohol for cleanup. It will not affect bearings if you splash them.  Solvent properties disappear after the alcohol evaporates in a few minutes. 

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FAQ

Q: Where should I use Loctite?

A: The design of SB20 Flywheel expects 'interference' on the outer ring of all bearings.  This means the outside of the bearing must be fixed solidly in place.  

Anytime you replace a bearing in the SB20 flywheel, you should apply Loctite to the outer ring. The exception is the non-drive-side axle bearing. Its outer ring is clamped firmly by the Cap Nut against the Stator shoulder.

In the SB20 design, bearing inner rings are less constrained. They can be allowed to move slightly or can be fixed in place. No special treatment is usually needed.
 

Q: Where type of grease do you for use bearings?  What about bike grease?

A: I use NLGI 2 automotive grease for bearings in exercise equipment and bicycles. Automotive grease is independently tested for performance across a wide variety of conditions. 

Machine grade NLGI 2 is commonly used in automobile bearings. It has the consistency of petroleum jelly. It works at high and low speeds. It can withstand higher heat than can be produced by any bike, indoors or out.  FWIW: I skip the lithium varieties. Bike bearings are not subjected to enough moisture to require it.

I never use any type of bicycle grease. It could be made out of anything. You have no idea what you are getting - and you won't know until it is too late.

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 decide to find out. In January 2026, I replaced my OEM 6910ZZ bearings with ACROPIX 6910ZZ bearings from Amazon.  

They are initially smoother than my re-greased 4 year old bearings. I will report back if things ever go bad. So far so good.

For those who care: The stator interference fit on the outer ring was perfect. So was the lateral connection with the inner ring to the bearing spacer. The inner ring (non-interference) fit with the flywheel shaft was also good - better than the OEM bearings that you can see spinning in the video above (3:24) . 

  

Q: How much load is placed on the bearings?

A: The Flywheel Bearings don't do much. They simply hold the 11 kg Stator steady as the Flywheel shaft spins inside it. There are almost zero lateral forces (axial load) on the Flywheel Bearings and only about 12 kg of radial load (6 kg per bearing). These 6910ZZ bearings are rated for 1450 kg (3200 lbs) radial and 225 kg (500 lbs) axial loads. I would say we are safe.

FWIW: I think the Flywheel Bearings are problematic because they can click and squeak as their outer rings move against the Stator hub. The interference fit with the outer ring of the Flywheel Bearing is not great. Stages asks its service people to glue it with Loctite.

The relatively small Axle Bearings suffer a bit more. They support the complete 23 kg (50 lbs) Flywheel + Stator assembly. Axial (side-to-side) load is low. The Cap Nut holds the non-drive side bearing against the Flywheel shaft shoulder.  The highest force is likely exerted when servicing the flywheel. Placing the flywheel on its side creates up to 45 kg of axial force on this bearing. In normal use, radial force is approximately the same, split across both bearings. Once again, no big issue here. These 6000 2RS bearings are rated for 450 kg (1000 lbs) radial and 225 kg (500 lbs) axial loads.

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References

Axle bearing and cogs removal: SB20 - Flywheel Cog and Axle Bearing Service.

Drive belt tensioning and alignment: Stages SB20 - Drive Belt Adjustments (2025) 

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Replacement Bearing Numbers

Flywheel Bearings: 6910ZZ bearing

Axle Bearings: 6000 2RS bearing

Cog Bearings: 6807 2RS bearing

Stages Service Documents

Stages SB20 Service Bulletins 

TSB-02202025 – SB20 Flywheel Repair Kit