Birdy AXS: Wheel Size Comparison - 18 Inch (355) vs 20 Inch (406)

One of the major modifications that you can do on the stock Birdy folding bike is to change the wheel size. For most bikes, when you upgrade the components, the wheel size remains unchanged, since the frame is only designed to fit one wheel size comfortably. In the case of the Birdy frameset, the stock 18" 355 wheelset can be swapped for a larger 20" 406 wheelset. This means that the rim diameter is increased from 355 mm to 406 mm, a 51 mm increase.

However, if the wheel outer diameter (inclusive of tire) is directly increased by 51 mm, it will not fit in the frame properly when folded. Therefore, using a larger rim diameter needs to be complemented by a reduction in tire height. In other words, swapping from an 18" 355 rim to a 20" 406 rim requires the tire to be changed to a slimmer type (which is also shorter in sidewall height).

Therefore, if you are looking for a more comfortable ride, using a smaller 18" 355 rim will allow a wider tire to be used for more air volume and comfort. On the other hand, if you are looking for a sportier ride with lower rolling resistance, go for a 20" 406 rim with slim and slick tires.

This dual wheel size compatibility concept is the same as that found on the Ascent Bolt/Intrepid mini velo bikes, where you can go for a smaller 406 wheel with very wide tires, or a larger 451 wheel with slim tires.

In my case, I wanted a larger 20" 406 wheelset for faster road riding, which is why I changed to the Ascent Cirrus Elite carbon wheelset. I also chose the Panaracer Minits Lite tires for low rolling resistance and reduced weight.

Stock 18" 355 wheelset on the left, new 20" 406 wheelset on the right. The difference in wheel diameter is quite obvious. Rotor diameter is the same at 160 mm.

Stock 18" 355 wheelset with CST tires has an actual outer diameter of 440 mm.

On this stock wheelset, the tire width is about 36.7 mm, which is a nice width for light off-road or gravel riding.

On the stock tire and Sora rear derailleur, tire to RD cage clearance at the lowest gear is just over 9 mm which is very safe.

Changing to a new wheelset, tires and 12 speed drivetrain will affect all these dimensions and clearances. Let's see how they differ after the modifications.

Ascent Cirrus Elite 20" 406 carbon wheelset with Panaracer Minits Lite 32-406 tires has an outer diameter of 470 mm. An increase of 30 mm over the stock wheelset.

Actual tire width is 30 mm, a bit smaller than the claimed 32 mm. Full slick for minimal rolling resistance.

Small clearance between the rim/tire and cage of the SRAM Force AXS rear derailleur.

Actual clearance is just 4 mm, which is small but still OK. The carbon rim width is 27 mm, while the tire width is 30 mm.

As you can see from the new setup above, the rim/tire clearance with the rear derailleur cage is pretty much on the limit. In other words, you should not use a rim (27 mm) or tire (30 mm) that is wider than what I am using.

Take note that this is with a 12 speed drivetrain, where the lowest gear (largest sprocket) goes quite inboard and close to the centreline of the bike. If you are using a 9 or 10 speed drivetrain, you might be able to use a rim or tire that is slightly wider than what I am using, as the lowest gear does not go so inboard.

Just for reference, the pictures below shows how it looks like if you use a 18" 355 wheelset with the wide Billy Bonkers tires. This is available in some stock configurations, such as the gravel spec by Mighty Velo. These tires have a seriously large width of 2 inches, or 50 mm! This is even wider than the Venture 47 gravel tires that I have on the Cervelo Aspero gravel bike.

Birdy with 18" 355 wheelset and 18 x 2.00" Billy Bonkers tires!

Only about 4 mm of clearance between the tire and the rear derailleur cage, despite it being a 9 speed drivetrain.

Hopefully this post gives you some good information on what to watch out for when changing wheel sizes or tires on the Birdy folding bike.