Disadvantages
It is expensive, with some models costing as much as custom made bikes. Component selection can sometimes seem illogical, with lower level Capreo components on the top-of-the-line monocoque frame, and higher quality SRAM X7 components on the less expensive model. However, the Capreo groupset is specifically designed for small-wheel bicycles as its smallest 9-tooth sprocket compensates for the small wheel size. Tire selection is adequate, and Kevlar-beaded tires, which can be folded for carriage in panniers, are available (as of late 2007) as the Schwalbe Marathon Racer.
Birdy uses less common 18-inch-diameter (460 mm) wheels instead of the more common 20-inch wheels found on BMX bikes. This allows for a more compact folded size but makes finding replacement wheels, tubes and tires more difficult. It is easy to fit more common 17-inch and 16-inch wheels if 18-inch wheels are unavailable. Some have even fit 20-inch wheels to the bike.
The unique front folding mechanism is long, weaker, and more flexible than others; the tolerance of linkages amplifies the flex, causing the leading arms to wander; thus an old Birdy may be unstable at speed. The Birdy's leading arm design provides anti-dive suspension, unlike trailing arm designs such as that used on the 1946 Vespa.
The Birdy, like most folders, has a long seatpost and stem. These components increase the weight to the bike, and the longer stem limits the exent to which the rider can pull on the handlebars. Thousands of miles of this activity can damage the headset or weaken the stem mount. The larger head tube on the Moulton gives it better hill climbing and sprinting abilities. However, with proper cycling shoes, Birdy riders can be found in non-UCI racing events and in pacelines of fast group rides.
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