Bicycle fork
Encyclopedia
Bicycle
A bicycle, also known as a bike, pushbike or cycle, is a human-powered, pedal-driven, single-track vehicle, having two wheels attached to a frame, one behind the other. A person who rides a bicycle is called a cyclist, or bicyclist....
that holds the front wheel and allows the rider to steer and balance the bicycle. A fork consists of two fork end
Dropout (bicycle part)
A bicycle dropout is a type of fork end that allows the rear wheel to be removed without first derailing the chain. The more inclusive term fork end refers to a slot in a frame or fork where the axle of the wheel is attached....
s which hold the front wheel axle, two blades which join at a fork crown, and a steerer or steering tube to which the handlebars
Bicycle handlebar
Bicycle handlebar or often bicycle handlebars refers to the steering mechanism for bicycles; the equivalent of a steering wheel. Besides steering, handlebars also often support a portion of the rider's weight, depending on their riding position, and provide a convenient mounting place for brake...
attach (via a stem) allowing the user to steer the bicycle. The steerer of the fork interfaces with the frame
Bicycle frame
A bicycle frame is the main component of a bicycle, on to which wheels and other components are fitted. The modern and most common frame design for an upright bicycle is based on the safety bicycle, and consists of two triangles, a main triangle and a paired rear triangle...
via a set of bearings
Bearing (mechanical)
A bearing is a device to allow constrained relative motion between two or more parts, typically rotation or linear movement. Bearings may be classified broadly according to the motions they allow and according to their principle of operation as well as by the directions of applied loads they can...
known as a headset
Headset (bicycle part)
The headset is the set of components on a bicycle that provides a rotatable interface between the bicycle fork and the head tube of the bicycle frame itself. The short tube through which the steerer of the fork passes is called the head tube. A typical headset consists of two cups that are pressed...
mounted in the head tube
Head tube
Most bicycles, tricycles and motorcycles have a tubular frame. The front fork pivots within the head tube . On a motorcycle, the "head tube" is normally called the steering head...
.
Dimensions
Offset
Bicycle forks usually have an offset, or rake (not to be confused with a different use of the word rake in the motorcycle world), that places the fork ends forward of the steering axis. This is achieved by curving the blades forward, angling straight blades forward, or by placing the fork ends forward of the centerline of the blades. The latter is used in suspension forks that must have straight blades in order for the suspension mechanism to work. Curved fork blades can also provide some shock absorption.The purpose of this offset is to reduce 'trail', the distance that the front wheel ground contact point trails behind the point where the steering axis intersects the ground. Too much trail makes a bicycle feel difficult to turn.
Road racing bicycle forks have an offset of 40-45mm. For touring bicycle
Touring bicycle
A touring bicycle is a bicycle designed or modified to handle bicycle touring. To make the bikes sufficiently robust, comfortable and capable of carrying heavy loads, special features may include a long wheelbase , frame materials that favor flexibility over rigidity , heavy duty wheels , and...
s and other designs, the frame's head angle and wheel size must be taken into account when determining offset, and there is a narrow range of acceptable offsets to give good handling characteristics. The general rule is that a slacker head angle requires a fork with more offset, and small wheels require less offset than large wheels.
Fork offset influences geometric trail, which affects a bicycle's handling characteristics. Increasing offset results in decreased trail, while decreasing offset results in increased trail.
Length
The length of the fork is usually measured parallel to the steerer tube from the bottom of the lower bearing race to the center of the front wheel axle. A 1996 survey of 13 700c road forks found a maximum length of 374.7 mm and a minimum of 363.5 mm.Width
The width of the fork, also called spacing, is measured colinear with the front wheel axle between the inside edges of the two fork ends. Most modern adult sized forks have 100 mm spacing. DownhillDownhill bike
A downhill bike is a full suspension bicycle designed for downhill cycling on particularly steep, rocky trails. Unlike a typical mountain bike, durability and stability are the most important design features, compared to lighter, more versatile cross-country bikes...
mountain bike forks designed for through axles have 110 to 200 mm spacing.
Steerer tube length
The steerer tube is sized either to just accommodate the headset bearings, in the case of a threaded headset, or to contribute to the desired handlebar height, in the case of a threadless headset.Steerer tube diameter
When sizing a fork to a frameBicycle frame
A bicycle frame is the main component of a bicycle, on to which wheels and other components are fitted. The modern and most common frame design for an upright bicycle is based on the safety bicycle, and consists of two triangles, a main triangle and a paired rear triangle...
, the diameter of the fork steerer or steer tube (1" or 1⅛" or 1½") must not be larger than that of the frame, and the length of the steerer tube should be greater than but approximately equal to the head tube length plus the stack height of the headset
Headset (bicycle part)
The headset is the set of components on a bicycle that provides a rotatable interface between the bicycle fork and the head tube of the bicycle frame itself. The short tube through which the steerer of the fork passes is called the head tube. A typical headset consists of two cups that are pressed...
. Adapter kits are available to enable use of a 1" fork in a frame designed for a 1⅛" steer tube or a 1⅛" fork in a 1½" frame.
General sizing issues
The blades must be the proper length to both accommodate the desired wheel and provide the approximate steering geometry intended by the frame designer. The functional length of the fork is typically expressed in terms of Axle-to-Crown race length (A-C). Also, the axle on the wheel must fit in the fork fork ends (usually either a 9mm solid or hollow axle, or a 20mm thru-axle). Some manufacturers have introduced forks and matching hubs with proprietary standards, such as Maverick's 24mm axle, Specialized 25mm thru-axle and Cannondale's Lefty system.Threading
Fork steerer tubes may be threaded or unthreaded, depending on the headsetHeadset (bicycle part)
The headset is the set of components on a bicycle that provides a rotatable interface between the bicycle fork and the head tube of the bicycle frame itself. The short tube through which the steerer of the fork passes is called the head tube. A typical headset consists of two cups that are pressed...
used to attach the fork to the rest of the bicycle frame
Bicycle frame
A bicycle frame is the main component of a bicycle, on to which wheels and other components are fitted. The modern and most common frame design for an upright bicycle is based on the safety bicycle, and consists of two triangles, a main triangle and a paired rear triangle...
. An unthreaded steel steerer tube may be threaded with an appropriate die
Taps and dies
Taps and dies are cutting tools used to create screw threads, which is called threading. A tap is used to cut the female portion of the mating pair . A die is used to cut the male portion of the mating pair . The process of cutting threads using a tap is called tapping, whereas the process using a...
if necessary. The thread pitch is usually 24 threads per inch except for some old Raleigh
Raleigh Bicycle Company
The Raleigh Bicycle Company is a bicycle manufacturer originally based in Nottingham, UK. It is one of the oldest bicycle companies in the world. From 1921 to 1935 Raleigh also produced motorcycles and three-wheel cars, leading to the formation of the Reliant Company.-Early years:Raleigh's history...
s which use 26.
Suspension
On most mountain bicycles, the fork contains a set of shock absorbers, in which case the blades typically consist of upper and lower telescoping tubes and are called "legs." The suspension travel and handling characteristics vary depending on the type of mountain bikingMountain biking
Mountain biking is a sport which consists of riding bicycles off-road, often over rough terrain, using specially adapted mountain bikes. Mountain bikes share similarities with other bikes, but incorporate features designed to enhance durability and performance in rough terrain.Mountain biking can...
the fork is designed for. For instance, manufacturers produce different forks for cross-country (XC), downhill, and freeride riding. Forks designed for XC racing are typically lighter, less robust and have less suspension travel than those designed for rougher terrain and more extreme conditions.
Popular makers of suspension forks include Cannondale, Fox, Manitou, Marzocchi, and RockShox.
Suspension fork design has advanced in recent years with suspension forks becoming increasingly sophisticated and diverse in design. The amount of suspension travel available has increased over time. When suspension forks were introduced, 80–100 mm of travel was deemed sufficient for a downhill mountain bike. Typically this amount of travel is now used in XC disciplines, with downhill forks now offering 150 to 200 mm of travel for handling extreme terrain. This is not the limit of suspension fork travel; for example, the Marzocchi Super Monster T offers 300 mm (12 inches) of travel.
Other advances in design include adjustable travel allowing riders to adapt the fork's travel to the specific terrain profile. Typically, less suspension travel is needed for uphill or flat terrain than for downhill terrain. Advanced designs also often feature the ability to lock out the fork to eliminate or drastically reduce the fork's travel for more efficient riding over smooth sections of terrain. This lockout can sometimes be activated remotely by a cable and lever on the handlebars.
The shock absorber usually consists of two parts: a spring
Spring (device)
A spring is an elastic object used to store mechanical energy. Springs are usually made out of spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...
and a damper or dashpot
Dashpot
A dashpot is a mechanical device, a damper which resists motion via viscous friction. The resulting force is proportional to the velocity, but acts in the opposite direction, slowing the motion and absorbing energy. It is commonly used in conjunction with a spring...
. The spring may be implemented with a steel or titanium coil, an elastomer
Elastomer
An elastomer is a polymer with the property of viscoelasticity , generally having notably low Young's modulus and high yield strain compared with other materials. The term, which is derived from elastic polymer, is often used interchangeably with the term rubber, although the latter is preferred...
, or even compressed air. The choice of spring material has a fundamental effect on the characteristics of the fork as a whole. Coil spring forks are often heavier than designs which use compressed air springs, but they are more easily designed to exhibit a linear, Hookean
Hooke's law
In mechanics, and physics, Hooke's law of elasticity is an approximation that states that the extension of a spring is in direct proportion with the load applied to it. Many materials obey this law as long as the load does not exceed the material's elastic limit. Materials for which Hooke's law...
response throughout their travel. Replacing steel coils with titanium coils in a design can decrease the weight of the design but leads to an increase in expense. Air springs utilize the thermodynamic property of gases that their pressures increase as they are compressed adiabatically. As the "spring" is provided by air rather than a coil of metal, forks with air springs are often lighter. This makes their use more common in XC designs. Another advantage of this type of fork design is that the spring constant can be adjusted by adjusting the air pressure. This allows a fork to be tuned to a rider's weight. One disadvantage of this design is the difficulty in achieving a linear response, as pressure varies approximately inversely (not linearly) with volume in a gas. As the fork compresses, the effective spring constant of the fork increases. Increasing the volume of the air inside the spring can reduce this effect, but the volume of the spring is ultimately limited, as it needs to be contained within the dimension of the fork blade.
The damper usually forces oil to pass through one or more small openings or shim stacks in the suspension fork. On some models, the spring, the damper, or both may be adjusted for rider weight, riding style, terrain, or any combination of these or other factors. The two components may be separated with the spring mechanism in one blade and the damper in the other.
Variations
There have been many fork design variations tried over the years. Several are still in use today.Suspension
Some have employed linkages to provide the mechanical action instead of relying upon telescoping elements. Others, notably CannondaleCannondale Bicycle Corporation
The Cannondale Bicycle Corporation, is an American bicycle manufacturer, headquartered in Bethel, Connecticut with manufacturing and assembly facilities in Taichung, Taiwan...
use a single shock built into the steering tube above the crown.
Single blade
Cannondale Bicycle Corporation
The Cannondale Bicycle Corporation, is an American bicycle manufacturer, headquartered in Bethel, Connecticut with manufacturing and assembly facilities in Taichung, Taiwan...
and Strida
Strida
Strida is a portable, belt-driven, folding bicycle with a distinctive 'A'-shaped collapsible frame, designed by UK engineer and designer Mark Sanders...
, offer a fork with a single leg. Cannondale's has a suspension shock built into it; Strida's does not.
Attachment to the frame
Not all forks steer on bearings installed in a head tube. The forks on StridaStrida
Strida is a portable, belt-driven, folding bicycle with a distinctive 'A'-shaped collapsible frame, designed by UK engineer and designer Mark Sanders...
s and Pedersen bicycle
Pedersen bicycle
The Pedersen bicycle, also called the Dursley Pedersen bicycle is a bicycle that was developed by Danish inventor Mikael Pedersen and produced in the English town of Dursley. Though never hugely popular, they enjoy a devoted following and are still produced today...
s pivot about an axis external to the fork, supported at two different points on the frame. A new trend in triathlon
Triathlon
A triathlon is a multi-sport event involving the completion of three continuous and sequential endurance events. While many variations of the sport exist, triathlon, in its most popular form, involves swimming, cycling, and running in immediate succession over various distances...
bikes is similar, but the pivot bearings are at the top and bottom of something that still resembles a head tube.
Materials
Forks have been made from steelSteel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
, aluminum, carbon fiber
Carbon fiber
Carbon fiber, alternatively graphite fiber, carbon graphite or CF, is a material consisting of fibers about 5–10 μm in diameter and composed mostly of carbon atoms. The carbon atoms are bonded together in crystals that are more or less aligned parallel to the long axis of the fiber...
, titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
, magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
, and various combinations. For example, a fork may have carbon fiber blades with an aluminum crown, steer tube, or fork ends.
In rigid forks the material, shape, weight, and design of the forks can noticeably affect the feel and handling of the bicycle. Carbon fiber forks are popular in road bicycles because they are light, and also because they can be designed to lessen and absorb vibrations from the road surface.
There have also been examples of wooden forks. However, these have not been common in the modern era.
Attachment points
Forks may have attachment points for brakes, racks, and fenders. These may be located in the crown, along the blades, and near the fork ends. These are often holes, threaded or not, and may be located on tabs that protrude.See also
- Bicycle frameBicycle frameA bicycle frame is the main component of a bicycle, on to which wheels and other components are fitted. The modern and most common frame design for an upright bicycle is based on the safety bicycle, and consists of two triangles, a main triangle and a paired rear triangle...
- Bicycle and motorcycle dynamicsBicycle and motorcycle dynamicsBicycle and motorcycle dynamics is the science of the motion of bicycles and motorcycles and their components, due to the forces acting on them. Dynamics is a branch of classical mechanics, which in turn is a branch of physics. Bike motions of interest include balancing, steering, braking,...
- Bicycle and motorcycle geometryBicycle and motorcycle geometryBicycle and motorcycle geometry is the collection of key measurements that define a particular bike configuration. Primary among these are wheelbase, steering axis angle, fork offset, and trail...
- Bicycle suspensionBicycle suspensionA bicycle suspension is the system or systems used to suspend the rider and all or part of the bicycle in order to protect them from the roughness of the terrain over which they travel...
- Fork tube
- Motorcycle forkMotorcycle forkA motorcycle fork connects a motorcycle's front wheel and axle to its frame, typically via a pair of triple clamps. It typically incorporates the front suspension and front brake, and allows the bike to be steered via handlebars attached to the top clamp....
- Stem (bike)