Bicycle suspension is the system, or systems, used to suspend the rider and bicycle in order to insulate them from the roughness of the terrain. Bicycle suspension is used primarily on mountain bikes, but is also common on hybrid bicycles.
Bicycle suspension can be implemented in a variety of ways, and any combination thereof: Front suspension Rear suspension Suspension seatpost Suspension saddle Suspension stem (now uncommon)
The suspension stem is now uncommon with the ongoing trend of short stems which limit the suspension size and the "slacker" head tube angle for stability. Bicycles with only front suspension are referred to as hardtail and bicycles with suspension in both the front and rear are referred to as dual or full suspension bikes. When a bicycle has no suspension it is called rigid. Bicycles with only rear suspension are uncommon although the Brompton folding bicycle is equipped with rear only suspension.
Although a stiffer frame is usually preferable, no material is infinitely stiff and therefore any frame will exhibit some flexing. Bicycle designers intentionally make frames in such a way that the frame itself can absorb some vibrations.
Besides providing comfort to the rider, suspension systems improve traction and safety by helping to keep one or both wheels in contact with the ground.
History
As early as 1885, the Whippet brand of safety bicycle was notable for its use of springs to suspend the frame. In 1901 the famous author on bicycles Archibald Sharp patented air suspension for bicycles. He was involved with Air Springs Ltd and they developed an air sprung motorcycle in 1909, but they also loaned an air sprung bicycle to the correspondent to show how good the suspension was.The A.S.L. Spring-Frame Motor Bicycle, The Motor Cycle, 7 Jul 1909, p502
In 1983 Brian Skinner designed and implemented the first suspension system for a mountain bike with a simple rear shock and a single pivot called the MCR Descender. This bike still had a rigid front fork.oldschoolracing.ch - vintage Mountainbikes race ready! “MCR Descender.” Accessed 8 May 2020. http://www.oldschoolracing.ch/archiv/descender/. Front suspension appeared on bikes in 1990 when the founder of Manitou, Doug Bradburry, designed and built the a front suspension fork in his garage. The fork used elastomers and had no dampening effect, just a spring. However, the stiffness was able to be controlled and functioned well except in cold weather.The Pro’s Closet. “Vintage Full-Suspension Mountain Bikes: The Good, the Bad, and the Ugly.” Accessed 1 March 2020. https://www.theproscloset.com/blogs/news/five-vintage-full-suspension-bikes.
The first full-suspension folding bike from German engineers Riese and Müller was developed in 1992 and was launched in 1995 as the Birdy model. It worked perfectly and was equipped with a joint-free aluminum main frame and 18-inch rims.
Front suspension
Telescopic Front suspension is often implemented using a telescopic (i.e., telescoping) fork. The specifics of the suspension depend on the type of mountain biking the fork is designed for and is generally categorized by the amount of travel. For instance, manufacturers produce different forks for cross-country (XC), downhill (DH), freeride (FR) and enduro (ND) riding which all have different demands for amount of travel, weight, durability, strength, and handling characteristics.
Telescopic suspension forks have become increasingly sophisticated. The amount of travel available has typically increased. When suspension forks were introduced, 80–100 mm of travel was deemed sufficient for a downhill mountain bike. This amount of travel is now common for cross-country disciplines, whereas downhill forks typically offer 200 mm or more of travel for handling the most extreme terrain.
Other advances in design include adjustable travel, allowing riders to adapt the fork's travel to the specific terrain (e.g. less travel for uphill or paved sections, more travel for downhill sections). Many forks feature the ability to lock out the travel. This eliminates or drastically reduces the fork's travel for more efficient riding over smooth sections of terrain. The lockout can sometimes be remotely controlled by a lever on the handlebars via a mechanical cable, or even through electronics.
As with all shock absorbers it usually consists of two parts: a spring, and a damper. The spring may be implemented with a steel or titanium coil, compressed air, or even an elastomer. Different spring materials have different spring rates which have a fundamental effect on the characteristics of the fork as a whole. Coil-sprung forks keep an approximately constant ("linear") spring rate throughout their travel. The spring rate of air-sprung forks however increases with travel, making them progressive. Titanium coils are much lighter but much more expensive. Air-sprung forks are generally lighter still.
Air springs work by using the characteristic of compressed air to resist further compression. As the spring itself is provided by the compressed air rather than a coil of metal it is much lighter; this makes their use popular in cross country designs. Another advantage of this type of fork design is that the spring rate can easily be adjusted by changing the air pressure within the fork. This allows a fork to be effectively tuned to a rider's weight. To achieve this in a coil sprung fork, one would have to swap out different coils with different spring rates. However air pressure naturally controls both spring rate and preload at the same time, requiring air forks to have additional systems to adjust preload separately, adding to its complexity. Another disadvantage of air-sprung forks is the difficulty in achieving a linear spring rate throughout the fork's action. As the fork compresses, the air held inside is compressed. Towards the end of the fork's travel, further compression of the fork requires ever greater force. This results in an increase in spring rate and gives the fork its progressive feel. Increasing the volume of the air inside the spring reduces this effect but the volume of the spring is ultimately limited by the need to be contained within the fork. The use of two air chambers within the system has allowed a more linear feel to air suspension, this is achieved by having a 'reserve' chamber that becomes connected to the main chamber when it reaches a certain amount of compression. Once achieved, a valve opens and effectively makes the chamber larger. By linking the two, the force needed to compress the air in the chambers is reduced which reduces the exponential spring rate feel traditionally associated with air systems when approaching the end of the suspension's travel.
The amount of preload on coil-sprung forks can generally be adjusted by turning a knob on top of one of the fork legs. Air sprung designs have various ways of dealing with preload. Several systems have been designed to influence preload, such as separately pressurizing different chambers, or systems that automatically set sag after changing the air pressure.
A damper is usually implemented by forcing oil to pass through one or more small orifices (also called ports) or shim stacks. On some models the damper may be adjusted for rider weight, riding style, terrain, or any combination of these or other factors. The two components are often separated by housing the spring mechanism in one of the fork's legs and the damper in the other. Without a damper unit the system would rebound excessively and would actually give the rider less control than would a rigid fork.
To prevent water and dirt from damaging the suspension, gaiters have been used to cover the fork's stanchions. However even when properly sealing the stanchions and sliders, the gaiters have to have small openings in them to allow air to move in and out of the cavity between gaiter and stanchion as the fork moves through its travel. Some water and grit may find its way in through these holes, staying trapped inside and accumulating over time. Since modern dust wipers and seals keep out water and dirt adequately enough by themselves, and since gaiter-less stanchions are generally regarded as more aesthetically pleasing, gaiters have fallen out of favor.
Alternatives Some manufacturers have tried other variations to the telescopic fork. For example, Cannondale designed a shock absorber build into the steerer tube called HeadShok, and a single-sided fork with just one leg, called Lefty. The stanchions of both systems are not round but have flat faces machined onto them which slide on needle bearings instead of bushings, this prevents the wheel from rotating in relation to the handlebars. Both of these systems claim to offer greater rigidity and better feel, with lighter weight. Others such as Proflex (Girvin), Whyte and BMW, have made suspension forks that employ four-bar linkage systems instead of relying upon telescopic fork legs, much like BMW's Duolever. The Suntour Swing Shock fork is based on a coil cantilevered swing set-up, and suspension is given by a coil spring which is located within the steerer tube and can be accessed from the top, technology which was originally used for suspension on early motorcycles.
Rear suspension Bicycles with rear suspension typically also have front suspension. Recumbent bicycles with suspension are an exception and often employ rear-only suspension.
Mountain bike suspension technology has made great advances since first appearing in the early 1990s. Early full suspension frames were heavy and tended to bounce up and down while a rider pedaled. This movement was called pedal bob, kickback, or monkey motion and took power out of a rider's pedal stroke — especially during climbs up steep hills. Input from hard braking efforts also negatively affected early full suspension designs. When a rider hit the brakes, these early suspensions compressed into their travel and lost some of their ability to absorb bumps. This happened in situations where the rear suspension was needed most. When braking efforts cause the suspension to compress it is referred to as brake squat,M. Sasaki, Kenneth (2001) Path Analysis a bicycle rear suspension analysis method (PDF) when braking causes the suspension to extend it is called brake jack.
Problems with pedal bob and brake jack began to be controlled in the early 1990s. One of the first successful full suspension bikes was designed by Mert Lawwill, a former motorcycle champion. His bike, the Gary Fisher RS-1, was released in 1992. Its rear suspension adapted the A-arm suspension design from sports car racing, and was the first four-bar linkage in mountain biking. This design reduced the twin problems of unwanted braking and pedaling input to the rear wheel, but the design wasn't flawless. Problems remained with suspension action under acceleration, and the RS-1 couldn't use traditional cantilever brakes since the rear axle, and thus rim, moved in relation to the chainstays and seatstays. A lightweight, powerful disc brake wasn't developed until the mid-1990s, and the disc brake used on the RS-1 was its downfall.