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.