Bicycle suspension
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A full suspension Mountain Bike
An elastomer suspension stem.
A
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. Bicycle suspensions are used primarily on
mountain bikes, but are also common on
hybrid bicycles, and can even be found on some
road bicycles.
Bicycle suspension can be implemented in a variety of ways:
- Suspension front fork
- Suspension stem (although these have fallen out of favor)
- Suspension seatpost
- Rear suspension
- Suspension hub
or any combination of the above. Bicycles with suspension front forks and rear suspensions are referred to as
full suspension bikes. Additionally, suspension mechanisms can be incorporated in the seat or saddle, or the hubs.
Besides providing obvious comfort to the rider, suspensions improve both safety and efficiency by keeping one or both wheels in contact with the ground and allowing the rider's body mass to move over the ground in a flatter trajectory.
Contents
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Front suspension
Suspension fork of a Trek Fuel 90
Front suspension is often implemented with a set of
shock absorbers in the front fork. The suspension travel and handling characteristics vary depending on the type of mountain biking the fork is designed for. For instance, manufacturers produce different forks for cross-country (XC), downhill (DH), and freeride riding.
Suspension fork design has advanced in recent years with suspension forks becoming increasingly sophisticated. The amount of travel available has typically increased. When suspension forks were introduced, 80â100 mm of travel was deemed sufficient for a downhill mountain bike. Typically this amount of travel is now more normal for cross-country disciplines. Downhill forks can now offer in the region of 170 to 203 mm
[1] 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 profile (e.g. less travel for uphill sections, more travel for downhill sections). Advanced designs also often feature the ability to lock out the fork to completely eliminate or drastically reduce the fork's travel for more efficient riding over smooth sections of terrain. This lockout can sometime be activated remotely by a cable and lever on the handlebars.
The shock absorber usually consists of two parts: a spring and a damper or
dashpot. The spring may be implemented with a steel or titanium coil, an elastomer, 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, however they are more easily designed to keep a linear spring rate throughout their travel. Substituting titanium coils in place of steel coils in a design can decrease the weight of the design but leads to an increase in expense. Air springs work by utilizing the characteristic of compressed air to resist further compression. As the "spring" is provided by the compressed air rather than a coil of metal they can often be made lighter; this makes their use more common in cross country designs. Another advantage of this type of fork design is that the spring rate can easily be adjusted by adjusting the pressure of the air in the spring. This allows a fork to be effectively tuned to a rider's weight. One disadvantage of this design is the difficulty in achieving a linear spring rate throughout the fork's action. As the fork compresses, the air held inside the air spring also compresses; towards the end of the fork's travel, further compression of the fork requires ever increasing compression of the compressed air with the spring. This results in an increase in spring rate. 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 leg. In addition, to prevent damage to the suspension mechanism,
gaiters may be used to cover the spring cylinder.
The damper is usually implemented by forcing oil to pass through one or more small openings or shim stacks. 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 leg and the damper in the other.
Some manufacturers, especially Cannondale, have tried other variations including a single shock built into the steerer tube above the crown (also called a "HeadShok"), and a fork with just a single leg (also called a Lefty). Both of these systems claim to offer greater stiffness and better feel, with lighter weight - by having only one leg, and using Needle Bearings instead of bushings, as well as special forging techniques. Others, namely Proflex (Girvin), Whyte and BMW, have made bikes that utilize suspension forks that employ linkages to provide the mechanical action instead of relying upon telescopic fork legs.[
citation needed]
Rear suspension
Perhaps because front suspension has been easier to implement and more readily adopted, it is often assumed, and
rear suspension is sometimes synonymous with full suspension.
Suspension Categories
A 2002 Rigid 21 speed Trek 800 Sport
No Suspension, also called a Rigid, is a mountain bike with no suspension.
Hardtail, Most modern mountain bikes have front suspension but no rear suspension, these are referred to as
hardtails.
Full suspension mountain bike 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 stoke â especially during climbs up steep hills. Input from hard braking efforts (known as brake jack) also negatively affected early full suspension designs. When a rider hit the brakes, these early designs lost some of their ability to absorb bumps â and this happened in situations where the rear suspension was needed most.
The problems of pedal bob and brake jack began to be solved 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 1990. It adapted the
A-arm suspension design from sports car racing, and was the first
four-bar linkage in mountain biking. This design solved 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. A lightweight, powerful disc brake wasn't developed until the mid 1990s, and the disc brake used on the RS-1 was its downfall.
Horst Leitner began working on the problem of chain torque and its effect on suspension in the mid 1970s with motorcycles. In 1985 Leitner built a prototype mountain bike incorporating what became known later as the "Horst link". Leitner formed a mountain bike and research company, AMP research, that began building full-suspension mountain bikes. In 1990, AMP introduced the Horst link as a feature of a fully independent linkage rear suspension for mountain bikes. The AMP B-3 and B-4 XC full-suspension bikes featured active Horst link/
Macpherson strut rear suspensions and optional disc brakes. A later model, the B-5, was equipped with both the Horst link and a four-bar active link suspension featuring up to 125 mm (5 inches) of travel on a bicycle weighing around 10.5 kg (23 pounds). For 10 years AMP Research manufactured their full-suspension bikes in small quantities in
Laguna Beach, California, including the manufacture of their own cable-actuated-hydraulic disc brakes, hubs, shocks and front suspension forks.
[2]
Soft tail
The
Soft tail (also Softail) relies on the flexing of the rear triangle and a rear shock or elastomer placed in line with the seat stays. Soft tails are a variation of the original Amp Research Mac-Strut design (technically a 3 bar suspension design). Soft tails have no
moving parts, besides the shock/elastomer, making it extremely simple. It maintains pedaling efficiency and power delivery because of the solid
chainstays. They tend to be extremely light compared to other rear suspension types. Soft tails are out of favor now because of the limited rear axle travel of these designs - typically around 1 inch. Some examples include the KHS Team Soft Tail, Trek STP and the Moots YBB. The Cannondale Scalpel is an exception with 4 inches of travel.
A full-suspension mountain bike with a single-pivot suspension.
Single pivot
The Single pivot is the simplest type of rear suspension. It simply consists of a pivot near the bottom bracket and a single swingarm to the rear axle. The rear axle will always rotate in a part-circle around the pivot point. Some implementations use linkages to attach the rear triangle to the rear shock for a progressive spring rate. Other implementations directly attach the rear triangle to the rear shock for a more linear rate. Santa Cruz's Superlight is such an example. The main benefit of this design is its simplicity. There are few moving parts, relatively easy to design and has good small bump compliance. Challenges with this design are brake jacking, and chain growth.
Manufacturers that use a single pivot design are Trek, GT, K2, Morewood, Transition, Orange, Cannondale, Mountain Cycle, Santa Cruz, Haro, small boutique frame builders such as bcd and, due to its simplicity, many inexpensive department store bikes.
Four-bar suspensions without Horst link
Four-bar linkage rear suspension
This kind of suspension performs exactly like similarly placed monopivots under acceleration and chain forces, which means they aren't neutral under acceleration and braking. Placing the pivot on the seat stay (above the drop out) makes the rear axle travel path like that of a single-pivot bike, since the chain stay is the only component that affects the rear axle's arc. The
four-bar suspension utilizes several linkage points just to activate the shock. Having the pivot in front of the drop out (i.e. on the chain stay, where it is called Horst link) allows the linkage components to affect the path of the rear axle, thereby allowing for a more complex arc of the axle path. However, because of patent problems with Horst link, some manufacturers choose this visually similar design.
[3]
One manufacturer well known for their long-time use of the seat-stay pivot four-bar link suspension is Kona, who incorporate the design on their entire line-up, along with other manufacturers such as Infiza and Icon.
Four-bar suspensions with Horst link
FSR rear suspension
A Horst link suspension has one pivot behind the bottom bracket, one pivot mounted at the chain stay, in front of the rear wheel drop-out (this pivot being the venerated "Horst link"
[1] ), and one at the top of the seat stay. Some examples of Horst link four-bar designs include the now-discontinued AMP B-5, the Specialized FSR and related bikes, Ellsworth, KHS, Titus, and Merida.
The bike company
Specialized worked with Leitner Technologies to develop a heavier-duty version of the four-bar/Horst link suspension which was marketed as the Specialized FSR (Future Shock Rear). The FSR patent describes a four-bar bicycle suspension system with the rear wheel mounted to the seatstay. The rear pivot though, is located on the chainstay both in front of and below the rear axle. Through this pivot positioning, the popular FSR system works by providing a wheel path that helps prevent the suspension preload or unload (squatting and locking) during acceleration and braking. The design is regarded by some[
who?] as superior to single-pivot/four-bar system due to other designs having a wheel path that either squats or "locks", depending on the position of the swingarm. The FSR system uses a wheel path that is in the middle of either squatting and lockout throughout most of the travel (circular, like single pivots). The FSR proved popular, and became a standard for full suspension designs, although recent innovations from competitors have set the company back. Specialized bought several of Leitner's patents in May 1998 and other manufacturers must now pay license fees to Specialized for the use of the 'Horst link' suspension design. The Horst link suspension design is the most leased or "borrowed" suspension design. It is very popular with companies such as Norco, Ellsworth, Chumba, KHS, and Fuji.
[4]
In 2003 Specialized introduced the
Brain, an external inertia valve designed to effectively eliminate pedal bob. The system utilizes a brass weight inside a cylinder situated atop the non-drive-side chainstay, near the rear dropout, and connected to the shock directly or through a hose. The weight closes the shock valving and deactivates the rear shock at rest. Upward force from rough terrain displaces the weight, opening the valve and engaging the suspension. In the original Brain mechanism, when the terrain evens out, the weight returns to its original position through a return spring, and deactivates the shock again. The position of the weight near the rear axle is designed to prevent downward pedaling force from affecting the mechanism while optimizing response from terrain. A newer version of the Brain was developed that utilizes the rebound hydraulic fluid flow to return the weight to its rest position instead of relying on a return spring. This was developed to address a noticeable delay in the shock activation/deactivation.
[5]
Unified rear triangle
The "Unified rear triangle" or "URT" for short, keeps the bottom bracket and rear axle directly connected at all times. The pivot is placed between the rear triangle and the front triangle so that the rear axle and bottom bracket move as one piece, and the saddle and handlebars move as another piece. This simple design uses only one pivot, which keeps down the number of moving parts. It can be easily modified into a
single-speed, and has the benefit of zero chain growth and consistent front shifting. On the other hand, when the URT rider shifts any weight from the seat to the pedals, he or she is essentially standing on one end of the swingarm, resulting in an increase in unsprung weight which varies according to the length of the swing-arm and distance between the bottom bracket and the pivot, and as a result the suspensions effectiveness is reduced to some extent. During braking, riders naturally brace themselves on the pedals,[
citation needed] and combined with
brake dive leads to more severe pitching, sometimes called "stinkbugging".[
citation needed] Because of lockout and pitching, along with persistent suspension bob in low-pivot URTs, and a constantly changing saddle-to-pedal distance, the URT design has fallen out of favor in recent years.
[6]
Examples of bike with this kind of suspension include the Castellano Zorro, Catamount MFS, Ibis Szazbo,
Klein Mantra, Schwinn S-10, Trek Y, and Voodoo Canzo.
Virtual Pivot Point
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(November 2008)The
Virtual Pivot Point or VPP, is a linkage designed bike frame that is built to activate the suspension differently depending on what inputs the suspension has received. The "Virtual Pivot Point" system owned by
Santa Cruz Bicycles, Inc is protected by four US patents, three of which were originally issued to Outland Bicycles. The four patents cover a specific linkage configurations that are designed to aid the pedaling performance of a rear suspension bike without negatively affecting the overall bump absorption capabilities. The Santa Cruz Blur and V-10 models introduced in 2001 popularized "dual short link" type suspension systems, but have the unique characteristic of having links that rotate in opposite directions. VPP suspension is also licensed to Intense Cycles.
DW-link
Diagram of the dw-link suspension, as implemented on an Iron Horse Sunday, showing the location of the instant center at top-out
Main article:
DW-link
Dave Weagle's
dw-link suspension is claimed by many cycling media and user reviews at consumer sites like MTBR.com to be the pinnacle of cycling suspension performance today. The dw-link design is protected by patents in the USA and Europe, with patent coverage in more countries than any other bicycle suspension in existence today. The dw-link is licensed to
Ibis,
Independent Fabrication,
Turner Suspension Bicycles, and Pivot Cycles.
[7]
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