A folding bicycle or folder is a type of bicycle that incorporates hinges or joints in the frame and handlebar stem that permit it to be broken down into a more compact size. As well, most folders have wheels of 20 inch (51 cm) diameter or less. Folding bikes can be taken on public transport and into apartment buildings or workplaces where conventional bicycles are not allowed. This facilitates mixed-mode commuting, because a folding bike does not need not be chained in the street or train station. Folding also makes it easier to transport a bicycle in an automobile.
Folding bicycles often cost more than non-folding bicycles with the same performance-related features, because they have more parts, to allow folding and to lock the frame when unfolded. This results in a more complicated design, which is more complex to manufacture; as well, there is a smaller market for this type of bike. As an alternative to folding, some bicycle models achieve similar results by separating into two or more parts for more compact storage or ease in transporting. This type of bicycle is sometimes grouped in the same category as folding bicycles but are also referred to as break-away, disassemble-able, or separable bicycles.
Most folding bicycles are intended for commuter and utility purposes, and hence they emphasize ruggedness, comfort and convenience over speed. A small number of high-end folding bikes are made for speed. In the interest of compact folding, certain trade-offs are common: Most folding bicycles use small (20" or less) diameter wheels. All else being equal, smaller diameter wheels give a rougher ride than larger wheels. Folding bicycles often have a shorter wheelbase, which also contributes to a rougher ride. To avoid losing any more comfort, narrow tires are rarer than for non folders. Suspension systems can be incorporated into the design of a bicycle to give a smoother ride. The suspension, however, will also absorb some of the pedaling energy that would otherwise be used to move the bicycle forward in a more rigid bicycle.
Folding bicycles often separate or fold in the middle of the frame, which, depending on the design, can weaken the frame and cause more energy-absorbing flexing. Folding bicycles often have elongated seatposts and stems. These longer components, which project above the frame like masts, experience greater bending stresses where they meet the frame, compared to the shorter components of regular bikes. There have been sporadic reports of failure in these components in online message forums, and at least one recall due the failure of the steering mechanism. Folding bicycles necessarily have more parts, to allow folding and to lock the frame when unfolded. This results in a more complicated design, with more parts that can potentially fail.
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. However, full-size (26") wheeled bikes that fold are becoming more prevalent in the folding bike world. Many manufacturers are producing folding bikes designed around folding systems that allow them to utilize 26" wheels. For example, Montague Corporation bases all its folding bicycles on the 26" wheel.
The wide variety of folding bicycles reflects the different methods to allow a bicycle to fold. The simplest folding frames have one hinge which allows the bicycle to simply fold approximately in half. Bicycles built on this pattern usually also have quick-connect clamps to allow raising and lowering the steering and seat columns quickly. A similar swing hinge may be combined with a folding steering column. Simple fold methods tend to use larger wheels, sometimes the same size as in non-folders, for users for whom maximum compactness is less important than the virtues of larger wheels. These larger folders are compact enough for a car trunk or boot, though usually too unwieldy to carry onto a bus. Most follow, at least approximately, the classic pattern of the diamond frame safety bicycle. These types were commonly made as a sideline by regular bike manufacturers in the middle 20th century, and refined late in the century.
A hinge in the frame may instead be used to allow the rear triangle and wheel to be folded down and flipped forward, under the main frame tube, as in the Swift 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 and Dahon, which use a folding steering column. Folding mechanisms typically involve latches and quick releases, which determine the overall speed of the fold/unfold - a consideration when using the bicycle for multi-modal commuting. In 2007 Bike Friday released a model with a cable-activated folding mechanism requiring no quick releases or latches, claiming a "hyper" fast fold.
Folding mechanisms generally 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.
Bikes such as those from Airnimal and Bike Friday partly fold and partly disassemble for packing into a standard or custom sized suitcase for air travel. 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. The A-bike is similar to the Strida but has smaller wheels and compacts a bit smaller. Bikes smaller than a Brompton 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 bikes are often associated with urban commuters. Multi-method commuters, who fold and unfold cycles several times per day, and then carry them in cramped public transport, typically want a bike that is easy and quick to fold. Marine users, such as sailboaters who use them to scoot around on land when they dock, more often prefer a bike that folds into a compact package that fits easily into the hold of their boat. Some folder riders use their bikes for recreation, and may only unfold the bike several times a month for a jaunt in the park, so they are more often willing to compromise ease and speed of foldability to get better riding speed and comfort.
While folding bikes have a reputation as slow-moving vehicles, in an urban street setting, high-end folding bikes can go at good speeds. A Moulton Bicycle (whose frame separates in two) was used in a criterium race on the streets of Toronto, Canada. According to the Bike Friday FAQ, "...tests have shown that up to 16 mph the small wheel is more efficient than a big wheel. Between 16 and 33 mph there is little difference. Over 33 mph the gyroscopic effect of the big wheel makes it more effective. Most folks do not go over 33 mph...