Before I disassemble the stock Birdy City folding bike for modifications and upgrading, I thought it would be a good idea to study the stock configuration, separated into Drivetrain Overview, Suspension and Brake System, Folding the Birdy Bike, and this post, which is a comparison post.
In this post, I will compare the geometry and size of the Birdy bike (unfolded and folded) with the Fnhon DB12. Other previous folding bike comparisons I have done are:
The reason I am comparing the Birdy to the Fnhon DB12 is because I plan to use the Birdy to replace my Fnhon DB12 as my go-to folding bike. Instead of adding a bike which makes it more difficult for me to choose, I prefer to do a 1-to-1 replacement. It will become my ideal bike for casual riding and also when bringing it to other places by car.
As such, the bike does not need a Brompton's compact fold, nor need to be wheeled around when folded. As long as it fits in a standard car boot it is fine. I do want the bike to be lightweight and fast, with high quality shifting and braking performance.
Overview of the bike shape and dimensions. Fnhon is using 20" 451 wheels, while the Birdy is using 18" 355 wheels (stock condition).
I realized that the stock wheel size on the Birdy City is actually the same as on the Tyrell IVE, at 18" 355. This size is not as common as the Brompton-type 16" 349 or the Dahon/Fnhon 20" 406 and 451 found on most folding bikes. The choice of tires for 18" 355 is thus more limited.
Another view showing the different wheel sizes. Seat tube angle is also different but that should not be a problem as it can be compensated by seat post and saddle adjustments.
Wheelbase of Birdy is about 101 cm.
Wheelbase of Fnhon is slightly longer at about 102 cm.
Birdy handlebar (with fixed height handlepost) is 920 mm from the ground, lower than the Fnhon DB12, which is 955 mm from the ground. This will be raised slightly after I change the Birdy wheel size to 20" 406.
Before comparing other geometry, the bottom brackets are first aligned to each other, as this is the datum to use when comparing different bikes.
The Birdy has a longer chain stay, but the rear wheel sticks out less as it is smaller in diameter.
Birdy chain stay length is 430 mm, which is even longer than that of my Cervelo Aspero gravel bike.
I guess this adds a bit of stability but makes it less agile.
Fnhon chain stay length is about 400 mm, which is pretty much the standard for 20" bifold folding bikes and road bikes.
Bottom bracket height from the ground is 280 mm for this Birdy. Will vary depending on tire choice and wheel size.
Bottom bracket height from the ground is about 300 mm for this Fnhon. Will vary depending on tire choice and wheel size.
Based on these geometry numbers alone, the Birdy should feel a bit more stable (not accounting for front fork geometry) due to the longer chain stay and lower bottom bracket height. In actual fact, it probably doesn't make a noticeable difference for folding bikes as there are so many other factors that have more effect on the ride stability (tire width, handlebar height, front fork geometry).
Next, I compared the size and dimensions of the folded Birdy and Fnhon, to see if the Birdy will end up taking up more or less space. I tried to find some dimensions of the folded Birdy online for reference, but this is unreliable as the dimensions vary from source to source. Some measure it with or without the saddle sticking out, with the pedal folded or unfolded, with different wheel and tire sizes, etc. Only reliable way is for me to measure it myself, in my actual bike configuration.
Overview of the Fnhon DB12 with 20" 451 wheels, next to the Birdy with 18" 355 wheels.
Top view of the bikes. The Birdy saddle looks like it sticks out a lot at the rear.
Using the wall as a line datum, I aligned both the front of the Birdy and Fnhon to it, and measured the length from the front tip to the rear of the bike.
Folded length of the Birdy is 66 cm when measured to the end of the chain ring...
Actual length measured to the end of the saddle is 74 cm. Depending on the shape of your storage space, this saddle protrusion may or may not make a difference.
Folded Fnhon with 451 wheels measure 74 cm from front to rear. Same size as the Birdy when it is measured to the rear end of the saddle.
Comparing the folded width, with the wall as the datum. The Fnhon sticks out more due to the rear derailleur and the right side pedal.
Birdy has a folded width of just 33 cm which is narrow and compact. Folding pedals are not even required.
Folded Fnhon with the SRAM Force AXS rear derailleur sticks out more, with a width of at least 39 cm. This is assuming the rear derailleur is at the middle of the cassette, while the right side pedal is foldable so that it does not stick out even more.
Folded height comparison. Highest point of the folded Birdy is the handlebar grip, while on the Fnhon it is the folding joint of the handlepost.
Height of folded Birdy when measured to the tip of the handlebar grip is 70 cm. This will vary based on your handlebar width.
Height of folded Fnhon, when measured to the joint of the handlepost, is slightly shorter at 67 cm.
In summary, the folded size of the stock Birdy City (18" 355 wheels) compared to the folded size of the Fnhon (20" 451 wheels) is:
Length x Width x Height
Birdy City: (66 or 74) x 33 x 70 cm
Fnhon DB12: 74 x 39 x 67 cm
Based on this comparison, the Birdy seems to fold down to a smaller size, as it is 6 cm narrower, while the length would be similar or shorter depending on how you measure it. Although the Birdy is 3 cm taller when folded, I think this additional height would usually not be a problem unless you are putting the bike under a desk or in a shelf.
This was a bit of a surprise to me, as I expected the folded width of the Birdy to be wider due to the awkward fold with the slanted wheels. I guess this actually makes the Birdy fold more compact, as compared to the Fnhon bifold which has more space in between the wheels when folded.
The Birdy also has a smaller wheel size which no doubt helps to keep the folded size compact, in terms of length and height. I expect the Birdy folded size to increase slightly when I change the wheels from the stock 18" 355 to the new 20" 406. This would be similar in size to the Birdy R20 racing model.
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