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.

During folding, when the fork is fully extended, the hose length inside the fork leg needs to slide out.

When folded, there is a slight kink in the hose. This is not ideal, but it is the best I can do now. After a few months of usage, there are no cracks on the outer layer, but I have to keep monitoring.

Another point which I overlooked earlier is the front tire stopper. This black plastic knob sticks out from the inside face of the fork, and presses against the tire when the front wheel is folded up. It is a crude design to prevent the front wheel from swinging down when the front suspension fork is folded.

Previously this plastic knob was pressing against the stock 18" 355 CST tires, but now that I have changed the wheel size to 20" 406, the rim is larger and the plastic knob will touch the rim instead. Unfortunately there is no way around it for now (let me know if you have ideas!) so I have to let the plastic knob rest against the carbon rim of the Ascent Cirrus Elite carbon wheelset. I have adjusted the knob protrusion such that it rests lightly on the rim to prevent the front wheel from dropping. Hopefully this is enough to prevent scratches from developing over time.

Fork fully folded such that the front tire is resting on the crown of the fork, while the black plastic knob presses against the side of the carbon rim.

Folding the front suspension fork is the first part of the Birdy fold. The second step is to unlatch and fold down the rear triangle.

Prior to that, you have to make sure that the chain is at the correct sprocket, and the crank arms rotated to the correct angle. When you change the cassette, chain length, rear derailleur or chain ring size, all these affects the folding. I would say that the biggest effect comes from the rear derailleur design and chain length.

On the stock Birdy City with the Sora RD-R3000 9 speed rear derailleur, the cage was long enough such that the recommended gear to fold the bike would be at the top gear, where the sprocket is the smallest. This also puts the chain more in line with the chain tensioner under the bottom bracket.

However, with the Force AXS rear derailleur, the derailleur behaves very differently due to the shorter cage. This meant that folding the bike with the chain in the smallest sprocket does not work, as there is too much chain slack. 

Eventually, after much trial and error, I found that putting the chain at gear 5 works reasonably well. The larger 21T sprocket at gear 5 absorbs some of the chain length, while the chain is not too far offset from the chain tensioner to prevent it falling off the tensioner or the cage pulley.

How it looks when the rear triangle is folded with the chain in gear 5.

Chain tensioner capacity is maximized with the roller nearly at the chain stay, while the chain is wrapped around the rear derailleur as well.

The chain tensioner had to be adjusted as well, mainly to move the starting and ending position of the roller to maximize the chain capacity. It is no easy task, with so many variables (chain length, sprocket position, chain tensioner position) that are inter-related.

In gear 5, I can fold and unfold the rear triangle repeatedly, without the chain falling off, the rear derailleur cage getting over-stretched, or interference somewhere. Sometimes the chain gets thrown over to another sprocket when unfolding, but that is easy to sort out just by pedaling forward.

When folding the bike, it is important to make sure that the non-drive side crank arm does not interfere. with the rear triangle. The original Birdy crankset has a rounded crank arm head to avoid interference, unlike the Shimano non-drive side crank arm that has two clamp bolts and a more squarish head.

Crank arm needs to be rotated to a precise angle to maximize the clearance with the rear triangle.

The clearance can be controlled by the plastic stopper seen here, which rests against the bottom of the bottom bracket shell.

When folded, the rear brake hose is wrapped around the joint, but there is sufficient slack to do so.

Note that the offset front hub tucks in neatly underneath the SRAM Force AXS rear derailleur and battery when folded. 

See how the offset front hub tucks in under the rear derailleur.. However, a Shimano shadow rear derailleur will not fit as it will interfere with the front hub.

Note the black plastic stopper (middle of picture) that stops the front fork against the rear triangle. 

In this setup, the front wheel spokes just manages to avoid the rear derailleur.

If folded properly, both the tires and the seat post will touch the ground. The front suspension will lightly touch the ground as well, but not be bearing any load.

The third step of the folding (after folding the front suspension fork and rear triangle) is to place the front and rear wheels side by side with each other, in a parallel position. The fourth step is to put down the seat post all the way, so that it touches the ground. The fifth and final step is to fold down the handlepost.

It takes a bit of practice to fold the Birdy properly, due to the many areas with tight clearances. Folding it slowly will prevent the parts from knocking into each other. On the other hand, if you don't mind the parts hitting each other, you can fold or unfold the Birdy very quickly. Check out the Youtube videos showing speed folding of the Birdy.

One problem unique to this Birdy with a fixed height handlepost is the inability to adjust the handlebar position when folded. As shown below, the handlebar grip (left side) will touch the rear brake rotor when the handlepost is folded down.

Left handlebar grip touching the rear brake rotor when the handlepost is folded down.

On most other Birdy where the handlepost height is adjustable, the top half of the handlepost can be rotated or raised such that the grip can avoid the rear brake rotor. One trick that I have seen is to adjust the lever position of the rear quick release axle, such that the lever will stop the grip from touching the rotor.

Unfortunately that trick does not work for me due to the fixed nature of the handlepost and handlebar position on my Birdy bike. I don't think this is a big problem, as I don't intend to roll the bike when it is folded. To prevent the grip from touching the rotor, I just slip a sock over the handlebar grip. This prevents the grip (with oils from the hand) from contaminating the rotor, and at the same time, prevent brake dust on the rotor from dirtying the grip.

View of the folded Birdy, with 20" 406 wheels. As mentioned earlier, the fixed handlebar position causes the grip to contact the rotor.

View from the other side. Still pretty compact compared to a bifold.

Folded view of Birdy with 18" 355 (top) vs 20" 406 wheels (bottom). Note that the front suspension spring position is slightly different due to the wheel size.

Compared to a bifold, there are many more areas to take note of when folding the Birdy, since there are tighter clearances at many areas. Thankfully most of them only need to be adjusted once during initial setup, and not every time I fold the bike. Only the gear and the crankarm position need to be adjusted every time during folding.

If you are having problems with the folding of your Birdy bike, this article might be useful to show you where adjustments can be made and what are the effects.

Update in September 2024:

There is a stopper on the rear triangle of the Birdy that is used to prevent the rear triangle from dropping down when folded. This stopper rests against the seat post when the seat post is fully inserted.

Previously I had to manually grind down the stopper to make it flatter, as changing the wheel size from 18" to 20" required the stopper to be adjusted. In the end I removed the entire stopper and replaced it with a piece of rubber, as the stopper could not be made thinner any more.

Now, I have found a piece of stopper that is of the ideal thickness. Ideally, the stopper should have minimum clearance to the seat post, to allow the seat post to slide past, and yet avoid a big gap that will let the rear triangle drop down too much when lifting the folded bike.

Improvised stopper to rest against the seat post.

Thickness is ideal for minimum clearance with the seat post when the bike is folded.