A folding bicycle is a bicycle designed to fold into a compact form, facilitating transport and storage. When folded, the bikes can be more easily carried into buildings and houses or on public transportation (facilitating mixed-mode commuting and bicycle commuting), and more easily stored in compact living quarters or aboard a car, boat or plane.
Folding mechanisms vary, with each offering a distinct combination of folding speed, folding ease, compactness, ride, weight, durability and price. Distinguished by the complexities of their folding mechanism, more demanding structural requirements, greater number of parts, and more specialized market appeal, folding bikes may be more expensive than comparable non-folding models. The choice of model, apart from cost considerations, is a matter of resolving the various practical requirements: a quick easy fold, compact folded size, or a faster but less compact model.
Military interest in bicycles arose in the 1890s, and the French army and others deployed folding bikes for bicycle infantry use. In 1900, Mikael Pedersen developed a folding version of his Pedersen bicycle for the British army that weighed 15 pounds and had 24 inch wheels. It included a rifle rack and was used in the Second Boer War.
The British WWII Airborne BSA folding bicycle was used from 1942-1945 in the Second World War by British & Commonwealth airborne troops, Commandos and some infantry regiments. Some were also used as run-abouts on military bases. A folding bicycle was developed as a small size was needed to enable it to be taken on parachute jumps from aircraft or in small gliders. The War Office in 1941 called for a machine that weighed less than 23 lb (this was not achieved – the final weight was about 32 pounds) and which would withstand being dropped by parachute. When parachuted, it was rigged so that the handlebars and seat were the first parts to hit the ground as bent wheels would disable the bike. BSA abandoned the traditional diamond design as too weak for the shock and made an elliptical frame of twin parallel tubes, one forming the top tube and seat stays and the other for the chainstay and down tube. The hinges were in front of the bottom bracket and in the corresponding position in front of the saddle, fastened by wing nuts. The peg pedals could be pushed in to avoid snagging and further reduce the space occupied during transit. The bicycle was used by British paratroopers, Commandos and second-wave infantry units on the D-Day landings and at the Battle of Arnhem.
The 1970s saw increased interest in the folding bike, and the popular Raleigh Twenty and Bickerton Portable have become the iconic folders of their decade. It was however the early 1980s that can be said to have marked the birth of the modern, compact folding bicycle, with competing tiny-footprint models from Dahon and Brompton. Founded in 1982 by inventor and physicist Dr. David Hon and his brother Henry Hon, Dahon has grown to become the world’s largest manufacturer of folding bikes with a two-thirds marketshare in 2006.
Folding bikes generally come with a wider range of adjustments than conventional bikes for accommodating different riders, because the frames are usually only made in one size. Seatposts and handlebar stems on folders extend as much as four times higher than conventional bikes. For even greater range of adjustment, longer after-market posts and stems are available. While folding bicycles are usually smaller in overall size than conventional bicycles, the distances between center of bottom bracket, the top of the saddle and the handlebars, the primary factors in determining whether a bicycle fits its rider, are usually similar to that of conventional bikes. The wheelbase of many folding designs is also very similar to that of conventional, non-folding, bicycles. Some manufacturers are producing folding bikes designed around folding systems that allow them to use 26″ wheels, e.g. Montague, KHS, Dahon Bicycles, and Tern Bicycles.
Advantages of smaller wheels include potential for more speed, quicker acceleration, maneuverability and easier storage. The A-bike is similar to the Strida but has tiny wheels and compacts a bit smaller. Bikes with smaller than 16″ wheels are often called portable bicycles. They forgo the performance and easy ride benefits of their larger counterparts, acquiring characteristics similar to those of an adult folding kick scooter. Regardless of how each folds, the result is easier to transport and store than a traditional bicycle.
Folding mechanisms are highly variable.
Half or mid fold Many folding frames follow classic frame pattern of the safety bicycle’s diamond frame, but feature a hinge point (with single or double hinges) allowing the bicycle to fold approximately in half. Quick-release clamps enable raising or lowering steering and seat columns. A similar swing hinge may be combined with a folding steering column. Fold designs may use larger wheels, even the same size as in non-folders, for users prioritizing ride over fold compactness. Bikes that use this kind of fold include Dahon, Tern, and Montague.
Vertical Fold Instead of folding horizontally, this style of bike has one or two hinges along the main tube and/or chain and seat stays that allow the bike to fold vertically. The result leaves the two wheels side by side, but is often more compact than a horizontally hinged design. The Brompton and Dahon Speed Uno both feature vertical folding.
Triangle hinge A hinge in the frame may allow the rear triangle and wheel to be folded down and flipped forward, under the main frame tube, as in the Swift Folder, Mezzo Folder and Bike Friday. Such a flip hinge may be combined with a folding front fork as in the Birdy. Swing and flip hinges may be combined on the same frame, as in Brompton Mezzo Folder and Dahon, which use a folding steering column. Folding mechanisms typically involve latches and quick releases, which affect the speed of the fold/unfold. Bike Friday offers a model, the Tikit, featuring a cable-activated folding mechanism requiring no quick releases or latches, for increased folding speed.
Magnet folding and suspension system A magnet combined with a rear shock absorber forms the folding mechanism. The magnet connects and locks the back wheel section to the frame. To fold the bike in half, the magnet disconnects with one movement and in a second, and without having to use one’s hands, the rear wheel rotates forward and the bike folds vertically. This mechanism also enables to roll the half-folded bike on its rear wheel.
Break away and other styles Bikes may partly fold and partly disassemble for packing into a standard or custom sized suitcase for air travel (e.g., Airnimal and Bike Friday). Other variations include the bicycle torque coupling is a proprietary connector system that can be retrofitted to a standard frame. The Giatex folds and retracts, adjusting to the size of the rider. The Gekko folds from the seat tube like an upside down umbrella. The iXi literally breaks into 2 halves. The Strida has a triangular frame and folds to resemble a unicycle.
Folding mechanisms may incur more cost and weight, allow folding smaller, and tend to use smaller wheels. 24 inch wheels are the largest for which flip hinges are generally used, but smaller wheels, typically 16 or 20 inches, are more common. Smaller size does not mean lighter weight, as most of these designs forgo the bracing benefits of the diamond frame, and must compensate as a step-through frame does, with thicker metal. The step-through design is a boon to a wider range of rider size, age and physical ability. Another system found on folders such as Montague Bikes utilizes the seat tube as a pivot point for the frame to fold. This system uses a tube within a tube design to give the bike more torsional stiffness. It allows the user to fold the bike without “breaking” any vital tubes down, preserving the structural integrity of the diamond frame. This system is operated by a single quick release found along the top tube of the bike.