Birdy AXS: Folding the Birdy Bike

Although the Birdy is a folding bike, foldability is not the main feature, unlike a trifold like the Brompton or similar clones like the United Trifold. Rather, it is marketed as a full suspension mini velo that happens to fold as well. The unique Birdy folding method comes from the dual purpose folding joints, which double up as the suspension linkage as well. As such, the folding joints are not seen as a weaknesses in the Birdy frame design, but rather a design that enables full suspension.

I tend to prefer the traditional bifold method, such as those seen on Dahon or similar bikes. The bifold method is easy and does not affect the chain stay length, unlike the trifold method on the Brompton or the Tyrell. This reminds me of the Java Neo 2, which is a unique trifold bike that has a fixed chain stay length, due to the concentric bottom bracket design.

Based on this definition, the Birdy is a trifold folding bike, with the front fork and rear triangle folding inwards. However, the folding process is trickier, while the folded package is not as neat or compact as a Brompton. Anyway, the Brompton is the gold standard and benchmark when it comes to having a compact folded size.

Anyway, let's take a look at the folding process of the Birdy. Although I have tested the bike before, I have not had the chance to study the folding design closely, until now.

A half fold, where the front fork and rear triangle are folded in. Not well balanced though as the seat post has not been put down.

Full fold with the seat post down and the handlepost folded. If you add a rear rack or roller wheels, it can be rolled around like the Brompton.

View of the other side. Rear derailleur and chain are well protected within the fold, although the disc rotors are exposed on both sides.

Top view showing that the wheels are not parallel to the main frame when folded.

The right side pedal can be used to support the bike when folded, although it is not very stable. Sometimes the front suspension coil spring will touch the ground instead.

When the rear triangle is folded, the chain tensioner is activated and wraps the chain around the chain ring.

Another view showing how the chain tensioner takes up the chain slack between the chain ring and the rear derailleur cage. Chain slack occurs as the effective chain stay length reduces when the rear triangle is folded.

Based on advice, the chain should be in the top gear (smallest sprocket) so as to minimize the chain slant angle from the sprocket to the chain tensioner.

The inward offset front hub flange creates space for the rear derailleur to tuck into when folded, as you can see here. This also means that some rear derailleurs may not be compatible.

Front brake cable requires some slack and flexibility, as the length and routing changes when the front suspension fork is folded. There is a plastic knob that presses against the front tire or rim to stop the front wheel from dropping down when the folded bike is lifted.

The handlebar grip on the left side will touch the rear brake rotor when the handlepost is folded down. This can be alleviated somewhat if a telescopic handlepost is used instead, which allows the handlebar position and angle to be adjusted.

As I am using a fixed height handlepost, this grip to rotor touching issue cannot be avoided.

When the handlebar grip touches the rear disc brake rotor, it is not a good design. The brake rotor will transfer brake dust or dirt to the grip, while the grip can also transfer some oil to the rotor. To minimize this issue, I have decided to put a sock over the left handlebar grip when the handlepost is folded down, so as to avoid this transfer of dirt and oil.

The folding steps for the Birdy is not difficult, but practice is needed to make the process smooth. If you have not folded a Birdy bike before, it is very difficult for you to figure out the proper folding method without any guide, unlike a bifold folding bike which is more straightforward.