Birdy AXS: Hubsmith Freehub Construction and XDR Freehub Body

In the previous post, I wrote about how I wanted to transfer the compatible components over from the Fnhon DB12 to the Birdy folding bike. The SRAM AXS groupset will be transferred over, but this is not so straightforward.

The SRAM Force 10-36T 12 speed cassette requires a XDR driver, which is not commonly found on small wheeled bikes. On the custom 451 wheelset by Ascent Bikes, I was lucky the Novatec rear hub has a compatible XDR freehub body which enabled the modification.

On the new Cirrus Elite 406 carbon wheelset, I checked to make sure that the Hubsmith rear hub has a compatible XDR driver, so as to use it with a SRAM cassette. The default freehub body is the Shimano HG spline design, which may be useful one day, so I will keep it around.

XDR freehub body for the Hubsmith rear hub. This is an additional part which I purchased together with the Cirrus Elite wheelset.

Check out the special pawl design, with 3 pawls that has 3 engagement points each.

Before I can install the SRAM Force AXS cassette, I will need to swap the freehub body. This is usually quite straightforward on most hubs.

Using Size 5 Allen Key on both ends of the hub axle to loosen the end caps.

Non-drive side axle end cap unscrewed. Note that the large hexagon socket requires a Size 12 Allen Key which is not commonly found.

Also note the 6 bolt rotor mounting on this hub, which I usually avoid as Centerlock rotors are so much easier to install and remove. However, the Birdy wheelset uses hubs that only have a 6 bolt rotor mounting. Later, I realized that the Birdy front hub actually benefits from having a 6 bolt rotor mounting, as the brake rotor can be flipped to the correct orientation. This is required due to the unique Birdy front brake caliper position, which is on the drive side instead of the non-drive side.

To remove the other axle end cap, the Size 12 and Size 5 Allen Keys are used together.

Drive side axle end cap removed. Check out the large internal diameter, which is way bigger than needed for quick release axles.

The hub axle internal diameter is slightly larger than 12 mm, as it is actually designed to be common use with 12 mm thru axles. Only the axle end caps need to be changed to swap between thru axles and quick release axles.

Freehub body removed, with very little grease inside. There are many small ratchet teeth on the hub shell for reduced engagement angle.

On the freehub, there are 48 engagement clicks per rotation. This translates into 7.5 degree of engagement, which is very good. It is not as short as the 5 degree found on the Hunt 650B Adventure Carbon Disc wheelset, but still more than enough.

As the ratchet teeth are small, the pawls are designed such that there are 3 simultaneous engagement points on each pawl instead of just 1. This distributes the load to ensure sufficient strength and durability with the small ratchet teeth.

Freehub body comes with a spacer to ensure that the freehub body contacts only the inner race of the hub bearing. It also prevents the pawls and pawl springs from moving sideways.

HG spline freehub body on the left, versus the XDR driver on the right. For a more detailed comparison check out this post.

Centre area is similar, but the XDR driver is shorter, which provides space for the small 10T sprocket.

HG spline freehub body is 66 grams.

XDR driver is practically the same, at 67 grams. No weight change from swapping freehub bodies.

In order to reduce freewheeling noise, I added a generous amount of Shimano freehub grease to dampen the ratchet and pawl area.

One thing that I didn't expect from this wheelset is the super loud Hubsmith rear hub! The freewheeling noise is too loud for me when I am freewheeling, as it annoys me as well as other people nearby. I will add more grease to see if it reduces the noise.

I found that the drive side axle end cap does not fit properly onto the XDR driver, as it tends to interfere with the rubber seal.

After discussing with Ascent Bikes, we concluded that this rubber seal on the XDR driver can be safely removed without having a big effect on the sealing quality. This is because the original HG spline freehub body does not have this seal anyway (check out the pictures above). Therefore, removing the seal merely makes the sealing performance on par with the HG spline freehub body, and not worse than that. The sealing quality will depend mainly on the integrated seals (dark green colour) on the sealed bearing.

Rubber seal removed from the XDR driver. Sealing will depend on the green seal, which is similar to the design on the HG spline freehub body.

Just to illustrate the problem, the seal interferes with the axle end cap when both are fitted. Removing the seal solves this issue.

Axle end cap installed on the XDR driver, no interference now!

Successful swap to the XDR driver on the Hubsmith rear hub. Now it is ready for the SRAM cassette.

Weight of the Cirrus Elite rear wheel with the XDR driver is 657 grams, same as before the freehub body swap.

I like the flexibility of having interchangeable freehub bodies. It allows me to change cassettes in the future if I want to, without having to get an entire new rear wheel. Most big hub brands offer rear hubs with interchangeable freehub bodies, especially on better quality hubs.