Four Bar Linkage

January 31, 2009 by  
Filed under Frames

Four bar linkage suspension designs were developed to improve upon the downfalls of single pivot designs in terms of stiffness, as well as various other important factors. However these improvements often come with increased weight, and extra maintenance required due to the increased complexity.

‘Horst Link’ Design

The ‘Horst Link’ was developed by Horst Leitner and Karl Nicolai and first patented(a) in 1993. Several of the patented ideas were bought by one of the mountain biking industry giants, Specialized, between 1998-1999. The concept behind the ‘Horst Link’ was to reduce the change in effective chainstay length and hence chain growth. This is achieved by placing a pivot below and in front of the rear axle (as seen in Figure (a)). As a result of this pivot location, the rear axle is mounted on the effective seatstay of the linkage, and is no longer arcing around the mainframe pivot point like single pivot designs.

When calculated correctly this design can achieve a number of positive attributes, such as eliminating pedal kickback and any form of braking induced suspension reaction which are detrimental characteristics. However to achieve this, the axle path becomes less desirable, and so a compromise has to be made to get an optimal design.

‘Lawwill’ Design

A less commonly used four bar linkage design is the ‘Lawwill’ linkage developed by Mert Lawwill in Germany between 1992 and 1994 and patented in 1996(b). The concept of this design is the most advantageous of the basic four bar linkage designs in terms of axle path manipulation and brake isolation. Once again the axle is mounted on the second bar of the linkage; however that link is now much shorter and actuates the shock via a long seat stay mounted rocker as seen in Figure (b). The problem with this design is that, in order to achieve the desired strength, the linkage inherently uses a lot of material to get the desired stiffness characteristics from the extended rocker, and so the weight increases.

horst link design Four Bar Linkage

lawwill linkage Four Bar Linkage

faux bar suspension Four Bar Linkage

‘Faux Bar’ Design

The most basic four bar linkage design is the ‘Faux Bar’ design as seen in Figure (c) above. This is the suspension linkage design Banshee Bikes currently use for its large travel bikes. This linkage still has the axle mounted on a chainstay which is directly mounted to the mainframe via a pivot, exactly the same as a single pivot design, and so will have the same axle path characteristics. However the two extra links that act as an interface between the chainstay and the shock via the seat tube, in this example, make this design much more laterally stiff than a single pivot bike due to there being two frame mounted pivots as opposed to one. The rocker plates that actuate the suspension can be orientated in a variety of geometries and can actuate the shock in a number of different ways (as seen in Figure 3) and can offer more adjustability in terms of leverage ratios experienced by the shock. 

faux bar designs shwoing alternative geomectric orientations Four Bar Linkage

Although the increased number of pivots provides the linkage with a better lateral stiffness, as well as more options for geometrical optimisation of the linkage and frame design, there can be maintenance problems if the bearings are not designed to take the dynamic and static loads demanded of them. This can prove costly for the consumer as well requiring time off the bike for the customer to replace them. This is a potential problem that must be considered when designing all multi pivot bikes; will the benefits outweigh the potential reliability issues associated with more complex designs?

‘Virtual Pivot Point’ (VPP)

The Virtual Pivot Point linkage design was developed by James Klassen of Outland Technologies and was patented(c) in 1995. The VPP linkage is essentially achieved by mounting the axle on a triangulated second link (that encompasses the rear wheel) of a four bar linkage design, (as shown in Figure 4). The VPP linkage directs the rear axle along a predetermined S-shape path as the suspension compresses. A result of this concept is that the suspension will not compress much when pedalling due to the inherent chain stretch designed into the system.  This results in good acceleration characteristics without the loss of small bump absorption. The main drawback of current VPP, other than the potential maintenance problems that are apparent in all multi pivot bikes, is that there is no brake isolation. This means that the suspension will become less reactive under braking unless a floating brake (which will add weight and complexity) is used.

vpp linkage Four Bar Linkage

This design has gained a very good reputation with mountain bikers worldwide, and so has become commonly used. Several variations of the design have been developed to make small improvements to the stiffness and suspension characteristics with care taken to avoid patent infringements.

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