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Mountain bike suspension

A full suspension Mountain Bike

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.

mtb sus category

• Front suspension
• Rear suspension
• Bicycles with only front suspension are referred to as hardtail and bicycles with suspension in both the front and rear are referred to as full suspension bikes. When a bicycle has no suspension it is called rigid. Bicycles with only rear suspension are uncommon.

Besides providing able to cope with terrain suspension systems improve traction helping to keep one or both wheels in contact with the ground.

Front suspension

Suspension fork on a Trek Fuel 90

Cannondale Lefty fork

Front suspension is often implemented using a telescopic 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, structural strength and handling characteristics.

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, 80–100 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 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 fork. This completely 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 actuating the lockout 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 spring rate throughout their travel and act linear. The spring rate of air sprung forks however increases with travel, making them progressive. Titanium coils are much lighter but also come at a significantly increased cost. 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 bike.

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 bikes utilizing suspension forks that employ four-bar linkage systems instead of relying upon telescopic fork legs, much like BMW's Duolever.

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 'Naked' stanchions are generally regarded as more aesthetically pleasing, gaiters have fallen out of favor.

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, 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 1990. 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.

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". The Horst Link is a type of four-bar suspension. 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 optional disc brakes and Horst link rear suspension very similar to the Macpherson strut. Note that the sliding piston in the shock absorber represents the fourth "bar" in this case. A later model, the B-5, was equipped with a revolutionary four-bar front suspension fork, as well as the Horst link in the rear. It featured 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 hubs, rear shocks, front suspension forks and cable-actuated-hydraulic disc brakes which they pioneered.

Soft tail

A soft tail (also softail) relies on the flexing of the chainstays of a regular diamond frame to create suspension travel, sometimes incorporating a specific flexing member within the chainstays. A shock absorber (or elastomer) is placed in line with the seat stays to allow the chainstays to move up and down, and for shock absorption. As the suspension moves through its travel the seat stay and shock absorber move out of alignment. This misalignment creates a mechanical lever for suspension forces, causing torque on the joint between chain- and seatstays. This is an inherent structural disadvantage of the soft tail design and severely limits the amount of travel possible, typically around 1 to 2 inches. Soft tails have few moving parts and few pivot points making them simple and requiring little maintenance. Some notable examples include the KHS Team Soft Tail, Trek STP and the Moots YBB. The Cannondale Scalpel is a four-bar suspension design where one of the pivots is replaced by a flexing link and has 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. The rear axle is held by a swingarm which is connected to the frame via a single pivot located near the bottom bracket. When the suspension moves through its travel, the path the rear axle describes is a circle around the pivot point. The rear triangle can be directly attached to the rear shock for a fairly linear leverage ratio between wheel travel and shock absorber travel.

The main advantage of the single pivot design is its simplicity. It has few moving parts, few pivot points, is relatively easy to design and has good small bump compliance. Challenges with this design are brake jack and chain growth.

Due to its simplicity, many inexpensive department store bikes use this design.

Linkage driven single pivot

Another implementation of the swingarm attaches the swingarm to the shock via additional linkages, typically creating a four-bar linkage actuating ("driving") the shock to create a more progressive leverage ratio between wheel travel and shock absorber travel. This designs is referred to as linkage driven single pivot, colloquially called faux-bar. It employs a four-bar linkage but the rear axle is held in a swingarm and still describes a circular axle path. The four-bar linkage serves only to actuate the shock and has no role in governing the axle path. Manufacturers of the linkage driven single pivot often use the word "four-bar" in their marketing campaigns, which gave rise to the design's nickname "faux-bar".
Notable manufacturers well-known for their long-time use of this suspension design include Kona and Jamis.

Linkage driven single pivot rear suspension

High single pivot

This variation of single pivot suspension places the pivot in front of and above the bottom bracket, at a height above the smallest chainring or higher. This gives the design a significant amount of anti-squat when pedaling in smaller chainrings which helps counteract suspension bobbing particularly of importance on steep climbs, when one would use the smaller chainrings. However this is a trade-off since the pivot placement causes the design to suffer more from pedal kickback. Notable examples are Santa Cruz' Superlight and Heckler frames.

Four-bar

Four-bar rear suspension

A Horst link suspension has one pivot behind the bottom bracket, with one pivot mounted at each of the chain stays, in front of the rear wheel drop-out (this pivot being the venerated "Horst link"), and one at the top of the leveraged shock linkage that connects to the seat stay. Some notable examples of Horst link four-bar designs include the Specialized FSR and related bikes, Ellsworth, KHS, and Merida.

The Horst Link patent system proved popular since its debut, becoming a standard for rear suspension designs using an 'active' model. Specialized bought several of Leitner's patents in May 1998, and other manufacturers in U.S. now license the Horst link design from Specialized for the use of the 'Horst link' or FSR suspension patent. It is used by notable companies such as Norco, Ellsworth, KHS, and Fuji. European manufacturers, such as Cube, do use the same suspension design, but can not import it to the United States. The FSR patent system uses a wheel path that attempts to position suspension compression between a preloaded and an unloaded condition throughout most of its travel.

Split pivot

The split pivot design is a special case of linkage driven single pivot in which one of the four-bar's pivot points coincides with the rear axle. This allows for the disc brake caliper to be mounted on the floating linkage (also called coupler) instead of on the swingarm. As a result of this the braking torque now interacts with the suspension via the floating linkage. The linkages can be designed such that this has a positive effect on suspension performance under braking, typically reducing brake jack. Furthermore, the relative rotation between brake disc and brake caliper as the suspension goes through its travel is different from that in single pivot designs. The four linkages in a split pivot design influence how braking torque is transmitted, how the brake caliper moves in relation to the disc and influence the leverage ratio between wheel travel and shock travel. Since these influences may have a different optimum linkage design, the bike's design has to strike a balance. Trek Bicycle Corporation released a version of the split pivot design called Active Braking Pivot (ABP) in early 2007. After a patent war between Dave Weagle's "Split Pivot" design and Trek's ABP, Dave Weagle was awarded its first patent in the USA on May 18, 2010, US Patent 7,717,212. Cycles Devinci has released a licensed implementation of Dave Weagle's design.

Virtual Pivot Point

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 uses two co-rotating short links - unlike counter-rotating links in the VPP design - configured to optimize suspension behavior to avoid compression under acceleration (anti-squat). 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.

Switch link

Another variation of two short links design is a Dave Earle designed Yeti SB-66 and SB-95 "Switch" link. It is uses an eccentric upper pivot. Upper link switches its rotation direction mid-travel, which is contrasted with co-rotating and contr-rotating links of VPP and DW-link designs. Patent application for this design is pending. Santa Cruz Bicycles is suing Yeti Cycles over aspects of this design in relation to VPP patent.

Independent Drivetrain

The Independent Drivetrain (AKA IDrive) Pat # 6,blablablabla099,010 / 6,073blablablabla,950, was the 4th commercialized suspension design developed by pioneering MTB suspension designer Jim Busby Jr. The independent drivetrain system was a direct result of the limitations encountered with the GT LTS (links tuned suspension) 4 bar linkage design used by GT Bicycles from 1993 to 1998. The defining feature of Independent Drivetrain is the isolation of the bottom bracket (crank) from the front or rear triangle. This isolation allows the BB to move in such a manner as to neutralize the unwanted characteristics of chain growth at the pedal. Some may call this a "modified URT" but in reality it is a highly reconfigured 4 bar if examined theoretically. By using this isolated BB construction, pedal forces do not induce undesired suspension compression or extension nor does suspension activity produce pedal actuation through chain growth.

Monolink

The "Monolink" made by Maverick Bikes, is a variation of a MacPherson strut. blaaaaaalbalalbla It uses three pivot points and the sliding action of the shock to provide the fourth degree of freedom. Monolink is unique in placing the bottom bracket on a floating linkage between the front and rear triangle. While this increases unsprung weight and reduces cushioning on the feet, it also minimizes chain growth, allowing for more efficient pedaling. It was designed by Paul Turner. It is a licensed variant of the Independent Drivetrain suspension system Pat # 6,099,010 / 6,073,950. The monolink design varies from the Independent Drivetrain original design in that it 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.

Equilink

The "Equilink" suspension system was developed by Felt Bicycles for their full suspension line. The system is a "Stephenson-style six-bar" suspension system: the Equilink ties the lower link (between the rear triangle and main frame) to the upper rockers. Felt contends that this system "equalizes" movement of the suspension in response to chain forces by linking the motion of the upper and lower linkages. Some, however, argue it works on the same principle of the dw-link; that is it creates a dropping rate of chain growth as it moves through its travel.

Terminology

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

Travel

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

Preload

Preload refers to the force applied to spring component before external loads, such as rider weight, are applied. More preload makes the suspension sag less and less preload makes the suspension sag more. Adjusting preload affects the ride height of the suspension.

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 damping adjustments on shocks, which vary the rebound speed. More rebound damping will cause the shock to return at a slower rate.

Sag

Sag refers to how much a suspension moves under just the static load of the rider. Sag is often used as one parameter when tuning a suspension for a rider. Spring preload is adjusted until the desired amount of sag is measured.

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.

Bob and squat

Bob and squat refer to how a suspension, usually rear, responds to rider pedalling. Squat usually refers to how the rear end sinks under acceleration, and bob refers to repeated squat and rebound with each pedal stroke. Both are undesirable characteristics as they rob power from pedalling. Many suspension systems incorporate anti-bob, anti-squat, or "platform" damping to help eliminate bob.

Pedal feedback

Pedal feedback describes torque applied to the crankset by the chain caused by motion of the rear axle relative to the bottom bracket. Pedal feedback is caused by an increase in the distance between the chainring and rear cog, and it can be felt as a torque on the crankset opposite to forward pedalling.

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.

Unsprung mass

Main article: Unsprung mass

Unsprung mass is the mass of the portions of bicycles that is not supported by the suspension systems. At one extreme are road bicycles with no suspension in the frames, very little in the tires, and none in the saddles. By raising themselves off their saddles, riders may provide suspension with their knees, making their mass be sprung mass, but all of the mass of the bicycles remains unsprung mass. At the other extreme are full suspension mountain bikes. With front and rear suspensions the only parts unsuspended are the wheels and small parts of the front forks and rear chain-stays. Even then, as mountain bikes have large low-pressure tires which allow much more travel than small high-pressure road tires, the wheels are sprung to some extent as well.

In general, bikes are so light compared to their riders that travel is a much bigger motivator than unsprung mass in determining where to put the suspension and how much to use. The exception to this is that on recumbent and tandem bicycles where the riders are either unable to lift themself out of their seat or unable to see in advance when that will be needed, the riders' mass can no longer be expected to be supported by their knees over road irregularities. These bicycles generally have some sort of suspension system to reduce unsprung mass.

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 4-6 inches (100–150 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 XC riders still prefer to ride a hardtail, and almost all mountain bike riders use a suspension fork. Notable suspension fork manufacturers include RockShox, Fox Racing Shox and Marzocchi.

See also

• Bicycle and motorcycle dynamics
• Bicycle fork
• Motorcycle fork
• Mountain bike
• Suspension (motorcycle)
• Suspension (vehicle)
• Swingarm
• Four-bar linkage
• Shock absorber