US$500 for a Mark 1 model, and more commonly up to $700. The 100% aluminium components used to get stress fractures in the early years, particularly when ridden by heavier riders or very energetically. New models have much stronger components. The Birdy uses 18 inch (355mm ETRTO) wheels. This allows for a compact folded size but makes finding replacement wheels, tubes and tires more difficult. Gears and brake components are standard. But the availability of some spare parts for the Birdy is rare, even in places in which sell folding bicycles, due to their unique design. For example: The Birdy uses a unique rear rack, a "spoon" clip to hold the rear in place, and unique front suspension bearings. Mudguards, however, can be made from any plastic equivalent and are a cheaper option than the Birdy models. Criticisms noticed by some riders: Component selection on the Mk 2 sometimes seemed 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 was specifically designed for small-wheel bicycles as its smallest 9-tooth sprocket compensates for the small wheel size. The unique front folding mechanism - while popular and distinctive - is long, weaker, and more flexible than a standard design; the tolerance of linkages amplifies the flex, causing the leading arms to wander up to the Mk2 design (now much strengthened); thus an old Birdy may be unstable at speed. Bushes on the front suspension can be replaced. The Birdy, like most folders, has a long seatpost and stem. Thousands of km of use can damage the headset or weaken the stem mount - old models could experience cracking after about ten years. The longer handlebar stem limits the extent to which the rider can pull on the handlebars, but a stronger adjustable version was introduced in the early 2000s that is more sturdy, and a further improvement in 2015. 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. The back folding mechanism, which folds beneath the frame, doesn't retain chain tension when folded if dérailleur gears are fitted (unlike the Brompton for example). Again, this has been solved on the Mk 3. See also Outline of cycling References