février 01, 2013

MTB rear suspensions | explanations

Perhaps because front suspension has been easier to implement and more readily adopted, it is often assumed that a bicycle with rear suspension has 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 stroke — 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 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.(...)

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.


AMP Research B4

Soft tail

A 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 them extremely simple, and maintain pedaling efficiency because of the solid chainstays. They tend to be lighter compared to other rear suspension designs. Soft tails are uncommon now because of the limited rear axle travel - typically around 1 inch. Some notable examples include the KHS Team Soft Tail, Trek STP and the Moots YBB. The Cannondale Scalpel is an exception with 4 inches of travel.

2011 Moots YBB
Single pivot

The single pivot is the simplest type of rear suspension. It simply consists of a pivot near the bottom bracket or higher up on the down tube and a single swingarm to the rear axle. The rear axle will always rotate in an arc 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. 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.

Proflex 856

Notable manufacturers that use a single pivot design are Trek, GT, K2 Sports, Cannondale, Haro and, due to its simplicity, many inexpensive department store bikes.

Mountain Cycle Fury - single pivot

High single pivot

This kind of a single pivot suspension places the pivot in front of and above the bottom bracket at a height above the smallest chaining or higher. Notable trait of this suspension is that pedaling in a smaller chain rings results in the suspension becoming "semi-active" by exerting an extending force on the suspension. Notable examples are Santa Cruz Superlight and Heckler frames.  

Santa Cruz SuperLight

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, and combined with brake dive leads to more severe pitching, sometimes called "BJs, or brake jacking".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.

Notable examples of bike with this kind of suspension include the Ibis Szazbo, Klein Mantra, Breezer Twister, Schwinn S-10, and Trek Y.

Linkage driven low single pivot

This is kind of monopivot suspension with additional pivot on the seat stay (above the drop out). This suspension utilizes several linkage points to activate the shock. Because of a visual similarity with US patented Horst link design, this popular design is often derisively referred to as a "faux-bar". Notable manufacturers well-known for their long-time use of this suspension include Ventana, Kona and Jamis.
Kona Cowan

Four-bar linkage

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.
Specialized FSR Enduro Expert

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

A variation of a single pivot design that places a rear pivot concentric with the rear axle. It behaves similar to a low single pivot suspension under acceleration while retaining four-bar design advantages for isolating braking forces, that is typically felt by the rider as suspension stiffening ("brake jack"). Trek Bicycle Corporation released a version of the Split Pivot design called active braking pivot (ABP) in early 2007. Dave Weagle, designer of DW-Link suspension 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 design.
Morewood  using Dave Weagle’s Split Pivot 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.
Santa Cruz Blut LT


Diagram of the dw-link suspension, as implemented on an Iron Horse Sunday, showing the location of the instant center at top-out
Ibis Ripley

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.


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.

Yeti  SB66

Independent Drivetrain

The Independent Drivetrain (AKA IDrive) Pat # 6,099,010 / 6,073,950, 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.
GT's independent drivetrain


The "Monolink" made by Maverick Bikes, is a variation of a MacPherson strut. 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.

Seven Duo


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.