SB20 - Advanced Flywheel 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. 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. None costs more than $5. 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-20 or PTFE into your bearings (like mentioned above), will shorten their lifespan. Repacking your bearings with grease is a good maintenance step. 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 with use adjustable wrenches (as suggested by someone at 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 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 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 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 to 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 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 Stators Bearings have Loctite (and sometimes rust) on their inner rings. You may have to shake the assembly a few times to extract the Stator.

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.  It will take several minutes to add enough grease.  You can't really add too much grease into this type of bearing. Any excess will leak back out in the unlikely event that you over-saturate the bearing.

Adding Grease to the Non-Drive-Side

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

⚠️ Cover the optical sensor before starting.

Optical Sensor Replacement (if needed)  

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

Replace Stator

Bearing service 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.   

That's it

You have successfully serviced the Flywheel Bearings 🏆.  

Finish by reassembling the axle, cogs and bike as detailed in:  SB20 - Flywheel 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 expect 'interference' on the outer ring of all its 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 on 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 move or be fixed in place. You can literally use Loctite or grease and get similar performance with similar tradeoffs.
 

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 ot 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.
  

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References

For axles bearing and cogs removal, see: SB20 - Flywheel Bearing Service

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🔺 If you need to replace the flywheel shell bearings, see Stages Service Bulletin:

     TSB-02202025 – SB20 Flywheel Repair Kit

These instructions are good - just a bit complicated 😀.  You can follow this blog post to do most of the work.

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Replacement Bearing Numbers

Flywheel Bearings: 6910Z bearing

Stages Service Documents

Stages SB20 Service Bulletins 

TSB-02202025 – SB20 Flywheel Repair Kit