Bicycle fork
The fork is a component that is rotatably attached to the bicycle and connects the front wheel to the frame; it is therefore sometimes referred to as the front fork or front wheel fork. At the lower end of the fork legs (fork blades, fork tubes) there is a receptacle for the axle of the bicycle hub , the front dropout (fork dropout , fork dropout ). The area where the fork legs meet and are connected to the fork stem (fork stem tube) is called the fork bridge (fork crown, fork head). By means of a headset , the steerer tube is rotatably mounted in the head tube of the bicycle frame, which on conventional bicycles assumes an angle ( steering head angle ) of about 65 to 75 degrees to the horizontal.
Specific dimensions
- Installation width , also referred to as the clamping width or hub width.
- Clear distance between the dropouts. Depending on the type of bike, it can be between 74 and 150 mm, the standard size that is predominantly found today (2018) is 100 mm.
- Installation height
- Vertical distance between the contact surface of the lower control bearing (fork cone) and the center of the hub axle. For example, the installation height of a classic racing bike with a rigid fork is around 370 mm. For rigid forks that are to replace a suspension fork (28 inch), the installation height is roughly 460 mm.
- Fork advance , also referred to as return (according to DIN EN 15532) or fork offset
- Vertical distance between the axis of the steerer tube and the center of the hub axis. In the case of rigid forks with bent ends, this is also referred to as fork bend or fork pre-bend. The fork lead together with the steering head angle and the diameter of the wheel determine the caster . A frequently encountered fork lead is around 45 mm.
Steerer tube
Steerer tubes are designated according to the outer diameter in inches of the upper steering bearing. The following sizes are carried out:
| 1" | (25.4 mm) | original standard size, e.g. B. classic racing bike with steel frame, city bike | |||
| 1-1 / 8 ″ | (28.6 mm) | z. B. mountain bike, e-bike | |||
| 1-1 / 4 ″ | (31.8 mm) | ||||
| 1-1 / 2 ″ | (38.1 mm) |
Steerer tubes for Ahead headset (with stem external clamp) are threadless, those for the traditional threaded rate (with stem with internal clamping) have at the upper end a zölliges external thread and a tube wall thickness of 1/16 "(1.6 mm). Thus the inner diameter for the size 1 ″ is 7/8 ″ (22.2 mm) and for the size 1-1 / 8 ″ it is 1 ″ (25.4 mm).
The outside diameter at the bearing seat (fork cone) of the lower steering head bearing is approx. 1.5 mm larger than the nominal size. With a threadless steerer tube, the steerer tube at the lower steering head bearing can be expanded to a larger nominal size. This is known as a tapered steerer (tapered or stepped steerer tube ).
The historical versions of the 1 ″ steerer tube have deviations from the current (2018), so-called ISO version. For example, there was a French, purely metric version with 25 mm outside diameter, 22 mm inside diameter and a thread pitch of 1 mm (25.4 tpi) instead of 24 tpi. In Japan, the JIS design was common, with slightly different dimensions on the lower steering head bearing. There was also a special version for the BMX bike.
Attachment of the brake and other attachments
As a rule, there is a through-hole for an M6 screw connection in the middle of the fork bridge. A side brake, a lamp holder and a mudguard bracket can be attached here at the same time. Depending on the type of bicycle brake , the fork leg has brake sockets (canti sockets), an IS or PM mount for the disc brake, a U-shaped bracket for the torque arm of the roller brake or special threaded sleeves / mount adapters for a rim brake that can be attached directly. Further mounting options (eyelets, threaded sockets, soldering parts) for attaching side-running dynamo, luggage rack (lowrider), fender struts or cables can be available.
When using hub brakes such as B. a disc brake, the fork is loaded much higher than with the rim brake. (For further information see bicycle brake , section Disc brake). With disc brakes in particular, forces occur during braking which want to pull the hub axle out of the slot in the dropout. The washer of the axle nut or the support of the quick release can lie in a precisely fitting recess in the dropout so that the axle remains positively fixed even when it is loose. Alternatively, two elevations, so-called “Lawyer Lips”, are attached to the outside end of the dropout.
Rigid fork
The rigid fork is the traditional version of the bicycle fork and, in contrast to the suspension fork, is not equipped with suspension elements. It consists of steel or aluminum pipes or is made of carbon fiber reinforced plastic . The advantages over the suspension fork are lower mass, freedom from wear, no maintenance and usually also lower costs. Nowadays (2018) the rigid fork is mainly used on racing bikes and in BMX sports, but there are also versions with a rigid fork on everyday bikes.
Materials
Until the 1980s, bicycle forks were made almost entirely of steel. With the advent of aluminum bicycle frames, this material was increasingly used for rigid forks. Racing bikes with fork parts made of carbon fiber reinforced plastic ("carbon", CFRP) reached the end customer market from the 1990s. Aluminum and CFRP have the property that the notch effect can lead to sudden breakage without warning. In the case of steel, failure is noticeable, even with previous injuries such as notches or deformations. With comparable strength, a fork made of aluminum weighs slightly less than that of steel, while those made of CFRP are even lighter, but also more expensive. The fork legs of a steel fork can tend to be designed to be more elastic, since this material is more tolerant than aluminum alloys or CFRP when it comes to dynamic bending loads ( fatigue fracture ). More exotic materials such as titanium are also occasionally used.
Types
With the classic rigid fork made of steel, the straight upper ends of the fork legs are soldered into the sleeves of the fork bridge (fork head sleeve). At the fork bridge, the cross section of the fork legs is mostly oval and tapers continuously downwards to a round cross section with a small diameter. To achieve the fork advance, the fork in the lower third, where the cross-section is smaller, is bent forward; at the same time, this fork bending makes the fork less rigid against impacts from uneven road surfaces. The dropout was made by flattening and slitting the tube at the bottom. In today's (2018) versions, a cast steel dropout is usually soldered into the round tube end.
The unicrown fork is characterized by fork legs made of aluminum or steel, which are bent at the top in the direction of the fork shaft and there are usually welded to a short pipe or fitting; For steel, a soldered version with a specially shaped fork bridge is also possible. With the advent of this type and also due to the increased requirements when using the disc brake, the design of the fork legs has changed. The tube cross-section at the dropout is noticeably larger than that of the classic steel fork, right up to the “Big Fork”, in which the diameter of the fork leg remains constant over its entire length. Due to this more massive design and also because of the material aluminum, the fork bend with a larger radius starts higher up or is omitted entirely (straight fork). In this case, the fork legs are mounted at an angle to the axis of the head tube in the area of the fork bridge in order to obtain the necessary fork advance or the hub mount is placed in front of the fork leg. The unicrown fork tends to be stiffer than the original “classic” steel fork, especially if there is a mount for the disc brake.
Suspension fork
In contrast to a rigid fork, a suspension fork is equipped with springs and shock absorbers to improve ground contact and comfort . Although suspension forks have been known since the beginnings of the modern low wheel , they only found their way into the mass market with the spread of mountain bikes in the 1990s.
Suspension forks normally have 200 millimeters of travel in " Downhill " sport, 140 to 180 mm in "All Mountain (Enduro)" and around 80 to 100 mm in Cross Country sport. Increasingly, however, suspension forks are also being used in everyday bicycles.
Designs
Telescopic fork
The suspension consists of a standpipe and a sliding tube, which slide into each other during compression. The standpipe is the fixed pipe in relation to the frame, i.e. always the upper one. The lower, movable tube to which the hub is attached is the sliding tube. The stanchions are held together under the steerer tube by a fork bridge that carries the steerer tube in the middle.
The inner tube is usually made of chrome-plated steel or, for more modern or more expensive suspension forks, of coated aluminum. Exotic materials like titanium or carbon have hardly been used so far. The outer tube is usually made of magnesium alloys, aluminum only in cheap models; very cheap forks even use steel.
Right side up
The most common design of the suspension fork is the "right-side-up" suspension fork, which means something like "right side up". In this version, the thinner stanchions that are attached to the fork bridge are immersed in the thicker immersion tubes, at the lower ends of which the hub axle mounts are arranged. The upper end of the dip tubes is connected to one another so that the torsional forces that occur during steering do not have to be diverted via the axle. The dip tubes and their connection usually consist of a component cast in one piece, the so-called "casting". The standpipes are mounted in the immersion tubes by means of simple plain bearing bushings, which means that the immersion tubes could rotate freely around the standpipes if they were not rigidly connected to one another above the impeller. Since the axial load on the two slider tubes is usually different due to the design (distribution of suspension and damping on each fork tube) and driving dynamics (lateral forces acting on the front wheel), the aim is to make the slider tube unit as stiff as possible in order to ensure a clean response to achieve low distortion, tension and tilting. For additional stiffening, a clamped thru axle is often used in addition to the upper connection in downhill or freeride models in order to achieve an even stiffer connection.
Upside down
In the USD technique (USD for Upside Down), the thinner dip tubes, to which the hub is attached, dip into the thicker stanchions that are attached to the fork bridge. This increases the stability because the thicker tubes are at the top, where the greatest leverage is at work. The unsprung mass (sliding tubes, hub, wheel) is smaller, which leads to a faster compensation movement of the fork, especially with fast impacts. The USD technique is mainly used in downhill and so-called "extreme freeride", since the more complex technique is only worthwhile here. The advantages of suspension forks for off-road driving ("cross country") are hardly noticeable, as the forks are optimized for low weight and have little spring travel.
The connection of the immersion tubes above the impeller does not exist with upside-down forks, since the sliding tubes are immersed in the standpipes here. In order to connect the sliding tubes, you would have to construct a very long bracket from the hub over the wheel to the other side of the hub. That would weigh a lot and still twist a lot. Therefore, the above-mentioned forces with upside-down forks are only absorbed by the hub axle. For this purpose, hubs with particularly thick and therefore stiff quick-release axles are used, which are bolted to the sliding tubes over a large area and often also clamped flat.
There were upside-down suspension forks with a slot in the stanchions through which the sliding tubes could be accessed. Like a right side up fork, you can connect the sliding tubes with a bridge over the wheel and thereby reduce the load on the hub so much that you can use conventional hubs. Due to the slots in the stanchions, there are of course considerable problems in sealing the fork, which is why this design has become very rare.
The asymmetrical single-arm Lefty fork from Cannondale can also be counted among the upside-down telescopic forks . There is only a single fork leg on the left side of the wheel, which is mounted on a special axle stub. At the headset, the fork leg is clamped to a steering shaft with two fork bridges, which create the necessary lateral offset so that the wheel is in the middle plane of the bike.
Double bridge
In contrast to the usual single-bridge suspension fork, the stanchions in a double-bridge suspension fork do not end under the steerer tube, but are guided further up to under the stem, where they end in a second fork bridge. As a result, the steerer tube is hardly subjected to bending stress and the entire construction is much more stable, but of course also heavier. Disadvantage: Due to the high stiffness of the fork, the frame can break on the head tube if it is not designed for double crown forks. If you want to retrofit such a fork, you should always ensure that the frame is suitable for this type of fork.
Parallelogram forks
A parallelogram fork consists of a rigid fork that is suspended from the steerer tube by at least two struts. The struts are rotatable on both sides, so that the fork can move up and down. With their recordings, the struts form a parallelogram .
The advantage of parallelogram forks is the rapid response behavior and the possibility of keeping the compression low when braking by aligning the struts accordingly. If the struts and their receptacles are dimensioned in such a way that they only approximately produce a parallelogram, their corresponding arrangement can also keep the caster virtually constant during compression.
The problem is the large number of wear-sensitive joints in the area of the greatest leverage, which results in a rather poor stability.
Suspension in the steerer tube
This type of suspension fork is mainly installed by the US company Cannondale under the name "HeadShok" . The suspension technology is not in the stand and sliding tubes, but in the center of the steerer tube. In principle, such a fork is an ordinary rigid fork in the lower part, the steerer tube of which is, however, mounted in a telescoping manner coaxially with complex linear roller bearings in a second steerer tube, which in turn is mounted in the head tube of the frame and connected to the handlebar. The part of the fork below the suspension mechanism corresponds in principle to a simple rigid fork. With such forks, the steering force is transmitted either through an angular cross-section of the two steerer tubes or via a scissor joint that connects the two fork steerer tubes, which run into one another, in a torsionally rigid manner. With HeadShok bikes, the travel to work can be identified by a rubber bellows between the lower steering head bearing and the fork crown.
This construction offers a number of advantages. The biggest one is the greater rigidity compared to normal telescopic forks, as two parts slide into each other at only one point (in the head tube). As a result, these forks are almost as rigid as rigid forks. In addition, each type of brake can be easily attached to the fork, which is not necessarily the case with the other fork designs.
The main disadvantage is the need for a steerer tube that does not conform to the standard dimensions. That is probably the reason why this construction, which was patented in the 1950s, only found a certain distribution from around 1990.
Suspension and damping
The tasks of a suspension fork are divided into suspension and damping. The springs first absorb the impact energy. However, this energy is only stored in the springs which, without damping, would spring back over the original position during rebound and would start to vibrate.
That is why there is damping (mostly through oil), which already helps with compression, but above all controls and brakes the rebound movement. This becomes clear with cheap suspension forks without damping, which tend to jump wildly and snap back, especially at high speeds.
As with all physical processes, only one energy conversion takes place here, i.e. H. the fork cannot simply make the impact energy disappear into nothing. Specifically, kinetic energy is converted into thermal energy. At the beginning there is the kinetic energy of the compression. The damping oil is pressed through thin channels, heats up and releases the energy in the form of heat.
suspension
Steel springs
The classic and simplest solution is the suspension with coil springs made of steel. They are robust and have a linear spring characteristic. For weight reasons, however, air suspension is also very common. High-quality dampers, which are provided with steel springs, are mainly used in freeride or downhill areas. Alternatively, springs made of titanium can also be installed. This results in a significant weight reduction, but is also a lot more expensive.
Air suspension
With forks with air suspension one encounters the problem that the compression of air results in a very progressive characteristic curve with a high initial value, often by installing a second air chamber, the pressure of which counteracts the actually resilient air and thus reduces the necessary initial force. Some suspension forks have steel springs built into them as a counter, but this is only common in the lower price segment. The characteristic curve of the suspension can then be adjusted by a suitable choice of the volume ratio of the two air chambers. In the case of an air-sprung fork, the hardness of the suspension can be adjusted very easily using the air pressure. In the early days of air suspension, there were still problems with the air in the damper heating up due to friction. The air expands when it is heated and thereby affects the cushioning. This then became harder and ultimately uncontrollable. However, these problems have largely been avoided through modern technology.
Elastomers
Elastomer suspension is only found in the lower price segment because of its strong temperature dependency and lack of durability. Although these have a very good effect, permanent deformations occur after a while.
Others
Other possible suspensions, for example by means of resilient carbon fiber elements (for example Remec), never got beyond the experimental stage, even if there were attempts by some manufacturers to sell something like this. Sometimes carbon and titanium frame manufacturers (e.g. Cannondale, Moots) try to take the flexibility of the material into account in their constructions in such a way that they shape the material on the rear chainstays like a leaf spring and thereby create a springy effect that only applies to the seat stays must be performed. For reasons of weight, titanium springs are occasionally used in spring dampers instead of steel springs.
damping
An oil damper is almost always used for damping. Few air-cushioned forks achieve a kind of cushioning by letting the compressed air flow through very small bores. Friction damping has died out due to poor function and excessive wear. In the case of very inexpensive suspension forks, there is usually no damping whatsoever.
In contrast to automotive technology, there is no predominant type of shock absorber for bicycle forks. Rather, new ways to dampen the fork are constantly being devised. Oil is routed through (often adjustable) bores in all dampers in order to achieve the dampening effect, but the process differs from manufacturer to manufacturer and often also from model year to model year. Three types of dampers can be roughly classified:
Encapsulated dampers
Completely closed (encapsulated) dampers that are built into the fork as a complete, finished component. This comes closest to the dampers used in the automotive sector. These dampers are also known as closed damper cartridges or "cartridge" dampers. The problem with these dampers is that they cannot absorb a lot of energy because they usually have a very small oil volume. In addition, there is usually no expansion tank for the oil which expands when heated. For these reasons, there have been great reliability problems with such dampers in the past.
Open cartridges
Damper cartridges are dampers that contain the necessary pistons and bores, but rely on getting their oil from a surrounding oil bath. These dampers are also known as “open cartridge” dampers or “open bath” dampers. Their advantage is the simpler structure compared to a closed damper and the generally much larger oil volume. Due to the larger amount of oil, such dampers can absorb more energy, which is transferred to the oil in the form of heat. In addition, the surrounding oil is often used to lubricate the bearing bushes of the standpipes.
Integrated dampers
These are damper constructions that are integrated into the fork. In this case, the damper is not available as a separate part, but the damping effect is achieved through the special design of some parts that are necessary for the construction of the suspension fork. This saves weight, which is why this construction is used for almost all weight-optimized forks. However, such a construction raises several problems, some of which are technically difficult to solve, in particular with regard to the adjustability of the damping and functional reliability. This is why this type of damping is rarely found in inexpensive forks.
See also
Footnotes
- ↑ Headset to Bicycle Fit Specifications (9/11/2008). Cane Creek Web site, accessed June 3, 2018.
- ↑ Description of the Lefty fork at Bike-Magazin.de , accessed on June 3, 2016.
literature
- Fritz Winkler, Siegfried Rauch: Bicycle technology repair, construction, production. 10th edition, BVA Bielefelder Verlagsanstalt GmbH & Co. KG, Bielefeld, 1999, ISBN 3-87073-131-1