Bicycle suspension

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A full suspension Mountain Bike

Bicycle suspension refers to 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 suspension are used primarily on mountain bicycles, 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 seat post
Rear suspension

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 rider comfort, suspensions improve both safety and efficiency by keeping one or both wheels in contact with the ground and allowing the rider's mass to move over the ground in a flatter trajectory.

[edit] Front suspension

Suspension fork of a Trek Fuel 90

Main article: Bicycle fork

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, 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 80-100mm 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 200 to 230mm of travel for handling the most extreme terrain.

Other advances in design include adjustable travel allowing riders to adapt the forks travel to the specific terrain profile. eg less travel for uphill sections more travel for downhill sections. Advanced designs also often feature the ability to lockout 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 favor of steel coils in a design can decrease the weight of the design but leads to an increase in expense. Air springs work by utilising 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 disadvantages 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.

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 steering tube above the crown and a fork with just a single leg that has a shock built into it. Others have marketed suspension forks that employ linkages to provide the mechanical action instead of relying upon telescoping elements.

[edit] 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.

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 — 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 — 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 1970's 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 125mm (5 inches) of travel on a bicycle weighing around 10.5kg (23 pounds). For 10 years AMP Research manufactured their full-suspension bikes in small quantities in Laguna Beach, Caifornia, including the manufacture of their own cable-actuated-hydraulic disc brakes, hubs, shocks and front suspension forks.^[1]

[edit] 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 Trek STP Moots YBB.

A full-suspension mountain bike with a single-pivot suspension.

[edit] 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 (see Faux Bar). 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. Its flaws, however, are that the design suffers from bob, brake jacking, and chain growth. Manufacturers that use a single pivot design are Fisher, Morewood, Orange, Cannondale, Santa Cruz, Mountain Cycle, small boutique frame builders such as bcd, and many inexpensive department store bikes.

[edit] 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 is essentially standing on the swingarm, resulting in a massive increase in sprung weight, and as a result the suspension tends to stop working. Because of this effect, along with persistent suspension bob and a constantly changing saddle-to-pedal distance, the URT design has fallen out of favor in recent years.^[2]

Examples of bike with this kind of suspension include the Klein Mantra and Schwinn S-10.

[edit] Four Bar Suspensions and the Horst Link

The Four Bar active suspension utilizes several linkage points to activate the shock. A 'true' four-bar, 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"Image:Http://www.azfreeride.com/files/news images/nicolai ion/ion.jpg ), 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.

A four-bar, seat-stay pivot suspension is similar, having a pivot above the drop out instead of in front of the drop out (ie no Horst Link and no patent problem). Having the pivot in front of the drop out (i.e. on the chain stay) allows the linkage components to affect the path of the rear axle, thereby allowing for a more vertical travel path. Placing the pivot on the seat stay (above the drop out) effectively makes the rear axle travel path more like that of a single-pivot bike, since the chain stay is the only component that affects the rear axle's arc. Seat-stay four-link pivot bikes perform exactly like similarly placed monopivots under acceleration and chain forces, which means they aren't as neutral under acceleration as Horst-link, four-bar bikes, and in general do not pedal, climb, accelerate or brake as well as a Horst Link four-bar suspension. However, when brakes are mounted on the seat stays, seat-stay four-link bikes have an advantage while braking over rough ground.^[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.

[edit] FSR System

Specialized FSR rear suspension with AFR Brain shock.

The bike company Specialized worked with Leitner to develop a heavier-duty version of the four-bar/Horst Link suspension which was marketed as the Specialized FSR. The popular FSR system works by providing a wheel path that helps prevents the suspension to preload or unload (squatting and locking) during acceleration and braking. The design is regarded by some 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 squating and lockout throughout most of the travel. The FSR proved popular, and became a standard for full suspension designs. 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]^[6]

[edit] Virtual Pivot Point

The VPP (or Virtual Pivot Point) is a linkage designed bike that is built to activate the suspension differently depending on what inputs the suspension has received. When most people think "Virtual Pivot", they think of the VPP™ bikes by Santa Cruz™ and Intense™. These bikes use a specific type of Virtual Pivot design that was originally developed and patented by Outland Bicycles™ about 10 years ago. The patents cover a specific linkage configuration and rear wheel travel path that is designed to aid the pedaling performance of a rear suspension bike without negatively affecting the overall bump absorption capabilities of the suspension. Yeti Cycles has created a unique rail system to eliminate pedal jacking. The DW-Link is another design licensed to Iron Horse and Independent Fabrications. Giant's Maestro is yet another design that works very well. The Maestro suspension design is just as efficient as the FSR system and the VPP. It also has the benefit of having a progressive spring rate. Patents have drawn definite lines among the manufacturers.

To define a VPP/DW-link/Maestro suspension design, imagine a line that follows the path the rear axle takes as it moves through its travel. Continue it into a complete circle. The center of the circle will be the pivot point. It will be a virtual point in space and not an actual pivot on the bicycle. It can either be a fix point or one that moves around as the suspension compresses (floating), depending on the design.

The VPP family of suspension systems are in fact four bar designs. They have short links instead of the longer links on a conventional four bar (chainstay and rocker).

The VPP (Virtual Pivot Point) system used by Santa Cruz and Intense, also claims to have reduced the problem of pedal bob. Soon after the VPP was introduced, the creation of the Progressive Suspension 5th Element rear shock (based on Currnut's platform damper) near the beginning of the Millennium allowed riders to adjust almost any frame, regardless of design, to be pedalled without the pedal bob that plagued earlier designs. Other companies have followed Progressive's lead (mainly Manitou with its SPV system based on the 5th Element and Fox's ProPedal which uses a shim stack rather than an air pressurized valve) and a revolution in suspension design is under way. However, these 'intelligent' shocks always have to compromise between their resistance to bob and performance with small bumps. VPP designs help reduce pedal bob by virtue of their design and thus can use a rear shock with little or no platform damping. This allows the suspension to be "fully active" and provides great small bump performance while also remaining plush on high speed descents.

Many think the tried and trusted technology of the four bar linkage with a comparatively simple shock still offers the best performance. However, the four bar linkage causes pedal bobbing, lockout, and brake jack. With the rise of more complicated shocks and a larger market share of full suspension bikes mountain bike suspension tuners have now arrived. It is now possible to have shocks tuned to individual rider's desires.

[edit] Active Braking Pivot

The Active Braking Pivot (ABP) has just been invented and put into use by Trek. The ABP system uses a rear pivot concentric to the rear axle, and the brake is mounted on the seatstay. This allows the suspension to pivot freely under braking, without brake-induced suspension stiffening (sometimes called brake jack). Other suspension systems (FSR, VPP, Maestro) stiffen under braking.
Trek also introduced a Full Floater system to go along with the ABP. The Full Floater system mounts the rear shock to two moving points in the suspension (rocker link and an extension of the chainstay). Other systems mount the shock to one end on the swing-arm, and the other to a fixed mount on the frame. This means as one part of the suspension compresses the shock, the other end of the shock moves as well. This allows Trek engineers more freedom to more accurately and precisely tune the system's leverage ratio.
The significance of ABP and Full Floater is that while other designs focused on the highly over-hyped axle path, Trek's innovation addressed brake-induced suspension movement and shock leverage ratio -- areas that improve performance.

[edit] Monolink

The "Monolink" made by Maverick Bikes uses 3 pivot points and place the bottom bracket on a floating linkage between the front and rear triangle. It was designed by Paul Turner. The monolink design is unique in that is uses a shock body that is integrated into the rear triangle, and that the saddle to bottom bracket distance changes as the suspension is compressed, although not as large as a URT design. The suspension is more active when in the saddle, as pressure on the cranks actively works against the suspension. However, because of this property, there is less bob in out of the saddle sprints. The monolink design is also unique in having a rearward axle path, which is similar to the angle of attack of the front suspension. Examples are the Maverick ML7/5, ML8, Klein Palomino, and Seven Duo.

The Optima Stinger recumbent with rear suspension

[edit] Equilink

The "Equilink" suspension system was developed by Felt Bicycles for their full suspension line. The system is a "four bar" suspension with 2 pivots behind the bottom bracket and 2 behind the seat tube. The key feature of the system is an "equilink bar" that connects the upper pivot link and the lower pivot link. Felt contends that this system "equalizes" movement of the suspension in response to pedalling forces by linking the motion of the upper and lower linkages.^[7]

[edit] Terminology

Several terms are commonly used to describe different aspects of a bicycle suspension.

[edit] Travel

Travel refers to how much movement a suspension mechanism allows. It usually measures how much the wheel axle moves.

[edit] Preload

Preload refers to the tension applied to spring component before external loads, such as rider weight, are applied. More preload makes the suspension stiffer and less preload makes it softer.

[edit] Rebound

Rebound refers to the rate at which the suspension component returns to its original configuration after absorbing a shock. The term also generally refers to rebound damping or rebound dampening adjustments on shocks, which vary the rebound speed.

[edit] Sag

Sag refers to how much a suspension moves under just the static load of the rider.

[edit] Lockout

Lockout refers to a mechanism to disable a suspension mechanism to render it substantially rigid. This may be desirable during climbing or sprinting to prevent the suspension from absorbing power applied by the rider. Some lockout mechanisms also feature a "blow off" system that deactivates the lockout when an appropriate force is applied to help prevent damage to the shock and rider injury under high unexpected loads.

[edit] Bob

Bob refers to how a suspension, usually rear, responds to rider pedalling. Bob is usually an undesirable characteristic as it robs power from pedalling. Many suspension systems incorporate anti-bob or "platform" dampening to help eliminate bob.

[edit] Compression Damping

Compression damping refers to systems that slow the rate of compression in a front fork shock or rear shock. Compression damping is usually accomplished by forcing a hydraulic fluid (such as oil) through a valve when the shock becomes loaded. The amount of damping is determined by the resistance through the valve, a higher amount of damping resulting from greater resistance in the valve. Many shocks have compression damping adjustments which vary the resistance in the valve. Often, lockouts function by allowing no compression.

[edit] Mountain bikes

Rear suspension of a Trek Fuel 90

Many newer mountain bikes have a full suspension design. In the past, mountain bikes had a rigid frame and a rigid fork. In the early 1990s, mountain bikes started to have front suspension forks. This made riding on rough terrain easier on a rider's arms. The first suspension forks had about 1½ to 2 inches (38 to 50 mm) of suspension travel. Soon after, some frame designers came out with a full suspension frame which gave riders a smoother ride throughout the ride.

Newer suspension frame and fork designs have reduced weight, increased amount of suspension travel, and improved feel. Many lock out the rear suspension while the rider is pedaling hard or climbing, in order to improve pedaling efficiency. Most suspension frames and forks have about 4 inches (100 mm) of suspension travel. More aggressive suspension frames and forks made for downhill racing and freeriding have as much as 8 or 9 inches (200 or 230 mm) of suspension travel.

Many riders still prefer to ride a hardtail frame, and almost all mountain bicycle riders use a suspension fork. Well-known suspension fork manufacturers include Manitou, Marzocchi, Fox Racing Shox, Rock Shox, and (to a lesser extent) Magura, White Brothers and Maverick. Some Cycle manufacturers also make their own suspension systems to fully complement the bike set-up.

[edit] Road bikes

Although much less common, some road bicycles do incorporate suspensions, particularly the Soft Tail variety mentioned above. One current example is Trek Bicycle Corporation's s.p.a (Suspension Performance Advantage) rear suspension available on some of their Pilot models. Virtually all bicycles produced by Alex Moulton bicycles also have full suspension, which is used to compensate for the small wheels and high pressure tires characteristic of the unconventional design of these bicycles.

[edit] Recumbent bikes

Many recumbent bicycles have at least a rear suspension because the rider is usually unable to lift themselves off of the seat while riding. Single pivot is often perfectly adequate when the pedalling thrust is horizontal - that is, forwards rather than downwards. This is usually the case provided the bottom bracket is higher than the seat's base height. Where the bottom bracket is significantly lower than the seat base, there may still be some pedalling-induced bounce.

Short wheelbase recumbents benefit from front suspension, not least because the front wheel is commonly smaller than the rear wheel and bumps are unduly felt without it.

[edit] Softride

The Softride Suspension System was launched at the Interbike 1989 bike show. The original SRS systems consisted of two foam filled fiberglass boxes bonded together with a viscoelastic layer. Originally intended for the use in mountain bikes, Softride produced its first full-fledged mountain bike, the PowerCurve, in 1991. During 1996 Softride released its first aluminum frame road bike, the Classic TT. The Softride Suspension System is used almost exclusively for triathlon racing.^[8]