Birdy AXS: Folding Considerations for Birdy

The Birdy is a full suspension folding bike, which means that other than offering a comfortable ride through the suspension, it also has to fold down to a reasonable size.

In the earlier post, I documented how the stock Birdy City folds, and what are some of the considerations to take note of during folding. I also compared the folded size to the Fnhon DB12, as well as some of the bike geometry.

After the modifications, I have learnt a lot more about the quirks of the Birdy, especially during folding. Here are some of my findings and observations, for you to take note of if you plan to get a Birdy or do any modifications.

Starting with the front suspension fork, one of the challenge is to route the front brake hose neatly and robustly, so that it can withstand folding without damage or interference. I experimented with many different routes and also consulted many online sources to see how other people do it.

Most dealers use mechanical brake calipers or hybrid mechanical/hydraulic brake calipers, where the hydraulic caliper is actuated by a standard steel brake cable. This solves the problem of bending or stretching the brake outer casing when the fork is folded.

On the other hand, there are a few successful examples where a full hydraulic hose routing is done, and seemingly with no major issues. This is definitely my preference as I much prefer the feel of the hydraulic brake levers.

The objective is to route the hydraulic hose in a way that it does not get kinked or twisted excessively when folded, during the folding process and when fully unfolded.

After many trials with a spare hydraulic hose, I finally found something that works. The free length, fixed length (with cable tie) and curvature of the hose are all factors that determine whether the hose can move freely during folding.

Final routing, with the hose running on the inside of the fork, using a cable tie to fix the position. The direct hose connection to the XTR brake caliper matches this routing well.

Birdy AXS: Assembly Part 2

Even before I got the Birdy City, I already planned to make major modifications on it. In fact, the modification ideas have already been floating around in my head for quite some time, just that I didn't proceed with it.

For full bike modification projects, I think it is a good idea to test out the bike in its stock condition first, so that you know the baseline performance. Afterwards, when the modifications are complete, the performance can be compared so that you can appreciate the upgrades and improvements.

As planned, many of the drivetrain and components will be moved over from the Fnhon DB12, as the Birdy will be replacing the Fnhon as my go-to folding bike. As such, I won't be sharing the details of the existing components, as the information can be found over at the Fnhon DB12 blog page.

For those components that are new, the details have already been shared in the earlier posts, or you can find the links at the relevant picture captions below.

Let's start with a picture of the completed bike, before zooming into each component for more detailed pictures.

Birdy upgrade complete! The new model is now Birdy AXS 12 Speed.

Birdy AXS: Assembly Part 1

In the previous posts, the stock drivetrain and suspension setup of the Birdy City was studied, followed by disassembly of the frameset and the suspension fork.

The Ascent Cirrus Elite carbon wheelset is new, as the Birdy requires a special front hub design. Otherwise, most of the other components will come from the Fnhon DB12 folding bike, as the Birdy will replace the Fnhon as my main folding bike. That said, the Fnhon frameset will not be abandoned, as it will be used for another project. 

I got a new set of hydraulic brake calipers for the Birdy, as the existing XTR brake calipers on the Fnhon will continue to be used for the new Fnhon build. The Birdy still uses the conventional Post Mount brake mounting, both at the front and the rear, which does not allow me to use the newer Flat Mount brake calipers. As I prefer a direct hose connection to the brake caliper instead of having a banjo bolt connection, this means that I will choose the XTR M9100 brake caliper that I have used previously. Alternatives are the non-series BR-R785 and the Deore BR-M6100.

XTR M9100 hydraulic brake calipers! However, the version that is currently in stock comes with metal brake pads, and not resin brake pads.

Birdy AXS: Tire and Rotor Setup

Since I will be changing the wheel size on the Birdy from the stock 18" 355 to the new 20" 406, I will also need new tires and inner tubes. For 20 inch 406 tires, there are lots of choices as it is a very common tire size.

The limiting factor here is the frame clearance during folding, and the rear derailleur cage clearance during riding. I understand from others' experience that for this larger 20" 406 wheel size, I cannot use a tire that is too wide, as it also makes the tire outer diameter too big for proper folding.

My preferred tire for 20" 406 is the Schwalbe Kojak 35-406 tire that is 35 mm wide, as it provides the best balance between comfort and rolling resistance. However, it seems that this tire width may be too wide to be used on the Birdy. The next best option is thus the Panaracer Minits Lite tire, which comes in a slightly narrower 32-406 tire size.

I have used this tire previously in the narrower 28-451 size, on the Dahon MuSP and also the Fnhon DB12. The 32-406 size was briefly used on the Dahon MuEX many years ago, but it was replaced as it did not feel suitable at that time.

Panaracer Minits Lite folding tires.

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.

Birdy AXS: Ascent Cirrus Elite Carbon Wheelset (20" 406)

Finally, after thoroughly studying the Birdy City in its stock configuration, and disassembling the frameset to study the individual frame parts, I can now move on to share about the new components!

Prior to getting the Birdy City, I already had a rough idea of what I wanted to modify on the Birdy. Most of the components are decent, and would be good enough for casual riding. However, since I wanted to use the Birdy to replace my existing Fnhon DB12 folding bike, I plan to transfer most the compatible components such as the SRAM AXS drivetrain over to the Birdy.

The main major component that cannot be transferred over would be the custom 451 wheelset on the Fnhon DB12. For a start, the 451 wheel size is too big to fit on the Birdy, as it can only accept a maximum wheel size of 406, and that is with slim tires.

Next, the Birdy requires a special front hub, with inward offset hub flanges and rotor mounting. Therefore, a new wheelset is needed for the Birdy, due to the special hub design.

Whenever I want to get a new wheelset, I will always consider getting one from Ascent Bikes. I have actually gotten quite a few wheelsets from them, with some of them custom built.

1) Extra strong 20" 406 wheels custom built for the Bike Friday Haul-A-Day cargo bike.

2) Low profile lightweight 20" 451 wheels custom built for the Fnhon DB12 folding bike.

3) Zenith Elite 700C road wheelset with a quiet rear hub for the Cervelo Aspero, and later the Focus Paralane.

Therefore, I decided to get a high end carbon wheelset from Ascent Bikes for the Birdy. The model is Cirrus Elite, which comes in either 406 or 451 rim sizes, and various other hubs. In this case mine is a pretty standard configuration for Birdy bikes.

Birdy Wheelset Specifications:

Birdy compatible Hubsmith front hub and rear hub.

Quick release axles.

100 mm front OLD, 135 mm rear OLD.

Shimano HG spline on the rear hub.

Disc brake mounting (only 6 bolt is available, not Centerlock).

50 mm tall 406 carbon rims with black bladed spokes and spoke nipples.

3M reflective decals (Darklight).

The wheelset is actually hand built in Singapore, with some of the components flown in from overseas. So you could actually customize it if you wanted to, subject to part compatibility and availability. I did not have to wait long for the wheelset, it took just slightly over a week.

New wheelset fresh from Ascent Bikes!

Birdy AXS: Birdy Front Suspension Fork

I will be dedicating a whole post to write about the front suspension fork of the Birdy folding bike, as it is unique and very interesting. Also, I managed to disassemble the entire suspension fork, so there are lots of parts for me to study.

As shown in the earlier introduction post, the hook of the front latch needs to engage the coil of the front suspension spring. The spring coil needs to be rotated to a specific angle to match the hook shape of the latch.

Birdy AXS: Birdy Bike Frame and Handlepost

In the previous few posts, I have studied the stock setup of the Birdy City, followed by comparing the geometry and size, and finally dismantling the frameset. Now, I will study these individual frameset parts in more detail, by checking out the design and the weight.

Frameset parts laid out in an exploded view.

Birdy AXS: Dismantling the Birdy Bike Frame

In order to study the Birdy frameset, all the stock components need to be disassembled. Most of them are not a problem, except for the square taper crankset and bottom bracket. I have a slight phobia of square taper cranksets, due to having stripped the crank arm threads when extracting one a long time ago. Same for the bottom bracket with multiple small splines that are hard to engage properly.

For removing the square taper crankset, I make sure that the square taper extractor tool is well greased on the threads, so as to maximise the thread engagement to avoid damage. As for removing the square taper bottom bracket, I made a simple tool modification to make sure the bottom bracket tool does not slip off during usage.

As shown below, I used an actual crank arm bolt plus a few large washers to keep the tool on the bottom bracket spindle, when applying torque on the tool. This virtually eliminates tool slippage and spline damage. Just take note that as you loosen the bottom bracket cups, you will need to release the crank arm fixing bolt slightly to enable the bottom bracket to back out.

Using a spare crank arm fixing bolt and a few washers to keep the tool in place. The crank arm fixing bolt is screwed into the thread of the bottom bracket spindle.

Birdy AXS: Stock Birdy Bike Ride Before Modifications

Now that I have studied the stock Birdy City in more detail, it is time to go for some test rides, to see how the stock setup performs. This will give a baseline feel before I disassemble and modify the Birdy folding bike.

During the test rides, the main areas that I will pay attention to would be the shifting performance, braking performance, rolling resistance, and suspension feel. Later on, after the bike has been upgraded, I will compare and see how much or little these areas have changed.

Before even riding, I have to adjust the saddle and seat post. As shown in the first post, the default seat post setting has a rearward offset, which means that the saddle position is rather far back. This is a problem for me as I need a zero offset seatpost in order to position my knee properly over the pedal spindle.

As I have previously done bike fitting at LOUE Bicycles, I am able to use the same setup to adjust the saddle position and height on the Birdy for the optimum riding posture. With a rearward offset seat post, it is not possible.

Luckily, this Birdy seat post is cleverly designed such that the saddle rail clamp can be reversed. Basically just rotate the seat post 180 degrees, then slide the clamp to compensate. This effectively gives the seat post a zero/forward offset, which is what I needed.

Birdy stock seat post rotated 180 degrees, with the saddle rail clamp adjusted to give a zero/forward offset. Pardon the messy workbench behind.