Birdy AXS: Chain Length, Freehub Grease and QR Axle Changes

Final post on the Birdy AXS 12 Speed folding bike! At least for now, until some new project or ideas pop into my head.

Throughout the whole upgrading process from the stock 9 speed Birdy City, I have learnt a lot about the Birdy folding bike. Especially for the folding design, where it is different from any other bike.

There were a few issues which were not fully resolved earlier, but they have now been fixed. 3 of the problems have been highlighted below, along with how I solved them.

First issue was the chain length on the Birdy drivetrain. As I transplanted the SRAM AXS 12 speed drivetrain from the Fnhon DB12 folding bike, I reused the existing chain as the mileage was quite low.

However, the chain stay length of the Birdy is longer at 430 mm, compared to the 400 mm of the Fnhon DB12. Therefore, using the same chain length with the same cassette and chain ring, on a longer chain stay, makes the rear derailleur cage stretch out too much.

At the largest sprocket (lowest gear), the rear derailleur cage is very stretched out and looks over the limit.

Technically it still works, but the rear derailleur cage looks to be over rotated, along with a rough pedaling feeling in the low gear due to the high chain tension.

Easy way would be to extend the chain slightly to reduce the chain tension, as there is still spare chain capacity when in the smallest sprocket (top gear).

Shown here are the extra chain links leftover from the original 12 speed Flat Top chain, along with a new chain link connector.

Adding 4 links to the chain, which adds 9 grams to the chain weight.

With the additional chain links, the rear derailleur cage is no longer over rotated, while pedaling is smoother due to lower chain tension.

Another improvement I wanted to make was to use a better quick release axle at the rear. I was using a lightweight Mixim titanium axle at the rear, but it didn't have sufficient clamping force to secure the kickstand properly. Whenever I put the kickstand up or down, the kickstand bracket will rotate around the axle, which is quite annoying.

Kickstand is clamped onto the rear dropout with a fixing screw and the QR axle. If the clamping force from the QR axle is not sufficient, the kickstand bracket will move.

I know that Shimano quick release axles with an internal cam design provides really good clamping force, which is why I decided to get one to try.

A new Shimano QR axle rear among the small bike goodies that I got during a recent trip to Japan.

Good looking Dura-Ace rear QR axle! Hefty at 66 grams though.

Unfortunately, the Dura-Ace QR axle was just a little bit too short for the Birdy. I did some rough calculations as shown below, and it did seem that the 163 mm Dura-Ace QR axle would be long enough.

Rear hub OLD: 135 mm

Birdy rear dropout thickness: 7.2 mm x 2

TiParts kickstand bracket thickness: 3.6 mm

Total width: 153 mm

If I use a 163 mm QR axle, there would be 10 mm of exposed thread which seems to be sufficient. However, I found that the actual length of the QR axle was slightly shorter at 160 mm. Also, the thread on the QR nut is recessed by about 2 mm. All these meant that I only had about 5 mm of thread engagement, which is not quite enough.

Only way is to get a longer Shimano QR axle. This time it is a Deore XT QR axle which is longer at 168 mm.

FH-M8000 rear QR axle, rare item nowadays.

Comparing the Dura-Ace QR axle on the left with the Deore XT QR axle on the right.

Deore XT QR axle on the right is 5 mm longer.

Curiously, the longer Deore XT QR axle (64 grams) is lighter than the Dura-Ace QR axle (66 grams).

Healthy amount of exposed QR axle thread for the QR nut to tighten onto!

Testing out the studio lighting functions on the iPhone

With the Deore XT QR axle, the kickstand bracket does not move when I swivel the kickstand up or down. This is due to the superior clamping force of the Deore XT QR axle.

As the kickstand is in the way, I cannot reposition the QR axle lever to block the handlebar grip from touching the rear brake rotor.

Lastly, the other issue that I wanted to fix was the LOUD freewheeling sound from the rear hub. This is the Ascent Cirrus Elite carbon wheelset, which uses Hubsmith hubs. I didn't realize that the freewheeling sound was so loud.

Some people like loud freehubs, but I prefer quiet ones. In fact, I previously got the Zenith Elite wheelset which had a really interesting magnetic freehub runs very quietly.

To quieten down a loud freehub, one thing to try is to add more grease into the pawl and ratchet  mechanism, so that there is more damping for the pawls.

Packed lots of Dura-Ace freehub grease into the Hubsmith rear hub!

Unfortunately, despite packing the hub full of grease, the loud freewheeling sound came back after a while. The grease just was not thick enough to dampen the sound.

I highlighted this concern to Ascent Bikes, which kindly offered to help me out with this. They had prior experience with making the Hubsmith freehubs quieter, and that is with a different grease.

This is the grease they used to make the freehub quieter.

Motorex White Grease 300773 is the product code.

This is a more viscous grease which was able to reduce the freewheeling sound. When freshly applied, the sound is greatly reduced. I was initially worried that adding a grease that was too viscous may cause the pawl and ratchet mechanism to skip, but I did not encounter this issue.

After running in the hub for some distance, the freewheeling sound gradually got louder, but still much softer than the stock Hubsmith condition. Eventually the sound level seems to have stabilized at about half of the original loudness, which is good enough for me.

With that, the Birdy folding bike upgrade has been fully completed, along with fixing some of the issues encountered along the way. All the articles on the Birdy will be useful for anyone attempting to do some modifications of their own, as I have probably encountered most of the issues.