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.

Front suspension parts weigh 106 grams, which is pretty lightweight. Wonder how much aftermarket suspension parts weigh?

Here is the top cap which goes under the fork crown, used to preload the headset bearings. It is just a standard stem top cap, not a special part.

Birdy front suspension fork, including the pivot bolts but without the suspension parts, weigh 950 grams. This is a lot heavier than I expected! It essentially means having two forks on the bike.

Front suspension pivot bolts disassembled. Loctite is used to prevent self loosening, since there is no other anti-loosening mechanism in place.

On the female side of the pivot bolts, there is a protrusion that fits into a key slot on the fork, to prevent rotation during pivot bolt tightening.

Weight of stock Birdy front suspension pivot bolts is 22 grams, or 11 grams per side.

This is the rear section of the fork that attaches to the handlepost steerer tube, and is thus fixed to the main frame.

It has special plastic bushings which rotate around the pivot bolts seen earlier. Not as smooth as using sealed bearings, but should be good enough in this application.

Stem-like clamp at the top of the fork, which wraps around the handlepost. As mentioned earlier, it is slightly too tight to fit over the steerer tube without widening it manually.

The rear section of the suspension fork, with the plastic bearings, weigh 441 grams. This is essentially the weight and shape of a Dahon/Fnhon aluminium fork.

Here is the front section of the suspension fork that moves, to provide the suspension feature.

Here is the front brake mount, which uses the Post Mount standard.

Hydroformed/forged parts look impressive and saves a lot of work needed to weld many small parts together, as seen on the previous generation of Birdy. Probably saves a bit of weight as well.

Pivot area with a key slot for the pivot bolts to slot into. Not sure if the holes serve any purpose.

Front section of the suspension fork, without any hardware weighs 489 grams.

From this design, I understand that the two prongs of the front section will rotate around the pivot location of the rear section. There are plastic bushings to reduce the friction between these two sections of the fork, while I am sure the mating diameters can be closely controlled to minimize radial free play.

However, the width of the pivots is harder to control. When tightening the pivot bolts, we need to eliminate the lateral free play, but yet not so tight that the suspension joint seizes. It is hard to adjust the pivot bolt tightness such that we hit the sweet spot of smooth rotation with minimum lateral free play.

There is quite a bit of stiction in the pivots, as there is no stopper to ensure that the bolts are not overtightened, causing over compression of the plastic bushings. What I understand is that excessive stiction makes the suspension unresponsive to small and quick bumps. In MTB speak, having excessive stiction causes poor small-bump sensitivity. This is not ideal on a bike such as the Birdy, as the use case for a Birdy is to smoothen the frequent small bumps that we encounter on less-than-perfect pavements and paths.