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.

20" 406 size, 32-406 ETRTO spec. These are lightweight tires, with a claimed weight of just 170 grams each.

32-406 size, Made in Japan

For this size and width, recommended tire pressure is 65-100 PSI.

Actual weight is just 167 grams each, even lighter than the claimed weight.

Other than new tires, I will need to get new inner tubes as well. I have always used Schwalbe inner tubes previously, but this time I decided to try something more lightweight.

As first trialed on the Specialized Aethos, I used lightweight TPU inner tubes which are much lighter than conventional butyl inner tubes. After using these TPU inner tubes for close to a year, I am quite confident that they are reliable enough for normal usage. It is one of the most cost effective way to save weight on the bike.

Size comparison between a 20" 406 TPU and butyl inner tube. Huge difference here!

Weight of 20" 406 butyl inner tube is 104 grams.

The TPU inner tube is only 25 grams, at less than one quarter the weight! Part of the low weight is due to the plastic Presta valve.

I needed the long valve version as it needs to be used with the high profile Ascent Cirrus Elite carbon wheelset.

New Panaracer Minits Lite tires mounted on the Ascent Cirrus Elite carbon wheelset!

Actual tire width is about 30 mm, smaller than the claimed 32 mm due to the narrow rims with an internal width of just 18 mm.

Finally, this new wheelset also requires disc brake rotors. I could have used the stock 6 bolt rotors that came with the Birdy City, but decided to just get new Shimano rotors instead. The Birdy-specific front hub is not available in Centerlock version.

As the disc brake caliper and rotor for the Birdy is placed on the drive side instead of the non-drive side, the rotor is spinning in the opposite direction relative to the brake caliper. Some Centrelock rotors are designed to take load in a specific direction, as the curved arms are stronger in one direction than the other. Therefore, if you install the rotor the wrong way, it will not be as strong.

For Centrelock rotors, it cannot be flipped around, unlike 6 bolt rotors which are completely flat. In this case, on the Birdy front hub, the disc rotor needs to be flipped, such that the braking load is transmitted correctly through the arms.

I selected the Shimano RT66 6 bolt rotor as it looks the nicest out of the limited selection available. There are many third party rotors which are lighter but also look more flimsy. Some of them seem to have more holes than brake track!

Shimano 6 bolt rotor. Note the directional arm design to stiffen the rotor without adding too much weight.

SM-RT66-S, where the S stands for a diameter 160 mm rotor.

This 160 mm rotor weighs 112 grams, which is not too bad. 

Each rotor requires 6 mounting bolts and the anti-loosening shims.

Rotor and SRAM Force AXS 10-36T 12 speed cassette mounted! Note that the front rotor has been deliberately flipped to work correctly on the Birdy.

Showing the other side of the wheelset. Diameter 160 mm rotors look big compared to the rims!

With the new Birdy wheelset ready, it is time to re-assemble the Birdy! I am moving many of the components over from the Fnhon DB12, which is why I did not write about the other components.