Mountain Bike Frame Geometry

February 5, 2009 by  
Filed under Frames

When designing any basic hardtail mountain bike there are several important geometric aspects to consider. As soon as a decision is made to design a bike with rear suspension, then a number of dynamic factors must also be considered. The relevance and importance of the dynamic constraints becomes even more crucial as the amount of rear wheel travel increases to the amount required to compete in the extreme-freeride and downhill race markets.

The most important parameters such as lengths and angles considered during basic frame design are shown in Figure 5.

mountain bike frame geometry Mountain Bike Frame Geometry

The dimensions of the front triangle on a well designed frame should be governed by the geometry of human bodies, and designed to be ergonomic in order to provide comfort and control. As a result most bikes are designed in three or more different sizes to suit people of all shapes and sizes.

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.  Read more

Single Pivot Suspension Design

January 30, 2009 by  
Filed under Frames

A single pivot design is the simplest possible design for rear suspension system. Essentially this design consists of the rear axle being mounted into a swingarm which actuates a spring damper suspension unit (shock) via leverage on a single pivot. An example can be seen in the image below. This is the most basic and most commonly used suspension linkage available on the market and was directly modelled on the motorcycle industry’s standard design.

single pivot suspension design Single Pivot Suspension Design

Although the concept is essentially very simplistic, consideration must be given when deciding the location of the pivot and the shock mounting points. For example an apparently insignificant change in pivot position will alter the axle path, and therefore dramatically change how efficiently the bike pedals and reacts to bumps.

Mountain bikes will experience lateral forces on the wheels when cornering, landing slightly sideways, or going through very rough terrain. It is difficult to make single pivot bikes as laterally stiff as multi pivot bikes due to the moments all being transferred through just one pivot. As a result of these thrust loadings on the bearings they tend to wear out faster. However this is not a big problem due to the fact that the bearings used are normally standard sizes, hence affordable, and easily purchased when requiring replacement.