SB20 - Simplifying Flywheel Bearing Service
Making Your SB20 Flywheel Easier to Service
When replacing or servicing the Stages SB20 Flywheel Bearings, a few questions are commonly asked:
- Loctite adhesive is recommended to glue bearings in place. Where should it be used? Can you over do it? Can you skip the Loctite?
- Reassembling the drive-side axle can be difficult. There is not enough room to insert a cone wrench to properly tighten the flange and lock nuts. This means the flange nut can come loose allowing the entire flywheel to potentially move. What can you do ?
Video
Here's the video.
This post expands on my previous Flywheel Servicing posts and videos:
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SB20 Flywheel Bearing Design
The design of SB20 flywheel relies on some basic principles:
- The axle and flywheel bearings expect an interference fit on their outer rings. The means the outer edges of the bearing must be kept stationary to prevent strange sounds and ultimately avoid excessive wear. While the risk of wear is minimal, clicking noises are common when the outer ring is allowed to move.
- The inner rings are less constrained. They are designed to allow small amounts of movement. In practice, the locking nuts on the axle and flywheel prevent axial (side-to-side) movement. Radial movement (in the load-bearing direction) relies on a looser fit than the outer rings.
Loctite or Not?
Loctite adhesive is now recommended by Stages used to hold bearings in place. When properly applied, it prevents movement and therefore stops most clicking and creaking noises. Just remember: Loctite will also making bearings harder to remove in the future. As such, Loctite should be used sparingly.
Axle Bearings
The non-drive axle bearing is held firmly in place by the Cap Nut that secures the outer ring against the flywheel shoulder. No Loctite is needed.
The drive-side bearing is different. Its outer ring is not held against the flywheel shoulder. This means it can move slightly. Loctite is a good idea on this bearing.
The inner ring of these bearings are held laterally by the lock and flange nuts on either end of the axle. They do not rely on gentle pre-load like other common bearing designs. The lock and flange nut firmly clamp the inner rings against the axle shoulders.
🔬 No adhesive is needed on the inner rings of either Axle Bearing.
🔬 No adhesive is needed on the inner rings of either Axle Bearing.
Flywheel Bearings
The flywheel bearings use a press friction fit into the stator hub. No lateral fastening or preload springs are used. The large Flywheel Nut secures the bearing inner rings but does touch the outer rings. Movement can result in clicking noises. Loctite is recommended on these outer rings.
🔬 No adhesive is needed on the inner ring of either Flywheel Bearing.
Waiting for Loctite to Cure
It takes about 24 hours for Loctite to dry. During that time you should completely reassemble the flywheel. This will help ensure all bearings are fully seated and aligned. Wait until the next day before riding.
Red or Blue Loctite? Or none?
I prefer using medium strength Blue Loctite (609 or 242). It makes future bearing removal much easier.
If you use high strength Red/Green Loctite (640 or 680), you will get a more secure bond but bearings may be less serviceable. You increase the risk of damaging the bearings when removing them in the future. This is fine if you expect to always replace the bearings. Not so good, if you want to do servicing.
Note: It appears most SB20 bikes were shipped from the factory without Loctite on any bearings. Loctite was adopted in 2024 by Stages service people to deal with clicking and rattling noises. You can consider using no adhesive if the bearing interference fit is good and you are not hearing any unexpected noises.
Drive Side Axle Nut Tightening
On most SB20 bikes, the drive side axle lock nut cannot be tightened with a cone wrench. The flywheel housing is too close to the flange nut.
We can fix this issue by adding one or two washers between the drive-side axle lock nut and the axle bearing.
We can do this thanks to the gap between non-drive side flange nut and the frame hanger.
🔬 Because this gap can vary - especially after servicing the Flywheel - you should always the tighten the drive-side Axle Nut first when working on the flywheel or drive belt. This makes aligning the drive belt easier and more consistent.
What Kind of Washers Can I Use?
You need an outside diameter (O.D.) of 16-20 mm (9/16"-3/4") and inside diameter (I.D.) of 10 mm (3/8"). Thickness should be around 1.5-2 mm (1/16"-3/32").
Visually, the washer should cover the bearing inner ring and a portion of the plastic dust shield. The washer must never touch the outer ring.
Thickness can vary. Test the total thickness of the washers you choose by sliding them into the gap between the non-drive side flange nut and the frame. You want to see about 1-2 mm of gap remaining.
Metal or Vinyl?
I experimented with both metal and vinyl washers. Metal washers work best when placed against the bearing inner ring. This eliminates axial (sideways) pressure on the plastic seal.
Vinyl washers can prevent excessive axial force that can happen if you over-tighten the axle lock nuts and push the inner rings too far inward.
Adding Washers to Drive Side
You can add washers one at a time and then replace the lock and flange nuts. If you have enough room for a tight fitting cone wrench, you probably have the right number of washers.
⚠️ Too many washers means the effective axle width will be too long and the flywheel will not fit between the hangers on the SB20 frame. Too few washers, and you won't have enough room for a cone wrench.
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FAQ
Q: Did you really destroy a set of bearings secured with red Loctite like shown in the video?
A: Yes. They were the 4 year old bearings that I re-greased on previous video. They were not silent after servicing so they had to go in any event.
FWIW: After on month, new bearings are still super smooth and silent. And yes, I used blue Loctite with these bearings.
Q: Glue seems like a primitive way to hold bearings in place. Why does the SB20 need it?
A: It is not the bearings. It is the design. In a devices like motors, you will often clamp the outer ring to keep it stationary. In an automobile, you can have an outer ring interference fit like the SB20. The difference is, the machined fit is so tight that you need several tons of pressure to mount and remove the bearings. The SB20 interference fit tolerances are less stringent. They are more in line with bicycle bearings. You can mount and remove bearings with less force.
Stages bikes routinely start making clicking noises after a few years of riding. Stages service bulletins began recommending Loctite to improve long-term fit. It is particularly important for the large flywheel bearings since the stator hub is not machined.
The good news is: the large flywheel bearings carry the low axial and radial loads. They only support the 11 kg stator that floats above the spinning flywheel without almost zero lateral force. The much smaller axle bearings work harder to support the entire 23 kg flywheel. Their mounting surfaces are machined and provide a significantly better fit. Loctite is less critical here.
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