In overwhelming majority of constructions, the front wheel springing takes place via a suspension fork. The systems applied for the rear wheel springing are then much more interesting, complex and sophisticated. Given the fact the suspension forks are standardized to certain extent, the rear wheel suspension systems can be just what imparts not only the original fashion but mainly the riding properties to the bicycle of a specific manufacturer.
The rear wheel suspension is understood to be a certain mechanism through which the rear wheel can be attached to the frame. This mechanism is usually composed of two (frame + suspension rear stay) members in the single-pivot constructions or more members with one central suspension unit.
Application of various types of rear wheel suspension constructions is based on the endeavour of designers to give the maximum efficiency to the conversion of energy generated by the rider into forward movement. All of this with the very essence of springing retained. As we mentioned above, the essence is predominantly the provision of good riding properties, enhanced riding comfort, and reduced bodily fatigue of the rider.
The systems presently used for mounting and guiding the rear wheel can be generally divided into two groups:
- Single pivot systems
- Four pivot systems
These systems appear in various modifications in all the bicycle types from XC up to DH. The differences in the types of construction are given not only by varying requirements for the amount of travel of the rear structure but also by the quality of the designer’s knowledge in the fields of mechanics, the development background, riding tests, and available production technologies.
Both the frame platforms, A-RAY and ERA, employ the four-pivot frame construction so we will further deal with this method of leverage system only.
Four-Pivot Rear Wheel Suspension Systems
The second large group of the rear wheel suspension constructions are the four-pivot systems. The basis for each four-pivot suspension is a mechanism with one degree of freedom which is called parallelogram. This mechanism (schematically indicated on a figure) consists of the frame, three moving arms and four pivots (hence the name “four-pivots”). Individual elements of the mechanism are called: 1-balance beam (chain stay), 2-pitman (seat stay), 3-linkage.
Balance beam (pos.1) and linkage (pos.3) are rotary-connected to the frame. Pitman (pos.2) is rotaryattached to both balance beam and linkage (pos.1 and 3) to perform a normal planar motion. Given the fact the rear wheel is connected to the pitman (pos.2) it is its centre that performs the normal planar motion as well. The rear wheel’s centre trajectory in the spring action is thus always the normal planar curve.
This 4-pivot construction is recently very modern and perhaps owing to its complexity over the singlepivot construction many a myth is circulated about it. For example, it is said in such a myth that when the rear wheel moves along the straight line (or other than circular trajectory which may be formed by the geometry of just the four-pivot constructions) during the travel, such springing is independent of the drive and braking forces. This is absolutely misleading. Since each type of suspension, whether of single-pivot, four-pivot or any other kind, is a mechanism which forms an integral part of a bicycle. During the ride, such mechanism is acted upon by dynamic (driving, braking) forces which, consistently with the rules of mechanics, are distributed into the entire mechanism. These forces then influence behaviour of the whole apparatus the part of which is the rear wheel suspension as well. It is therefore unmistakable that the springing is on all accounts influenced by the acting forces.
FORCES ACTING ON THE SHOCK-ABSORBER (under a straight-line steady motion – ranked by the importance)
- from the terrain the rear wheel moves along
- vertical motion of the rider body’s centre of gravity in standing pedalling
- vertical motion of the centre of gravity of lower limbs – very significant since the actual frequency of wheel springing is usually close to the frequency of pedalling
- the tension of chain in pedalling causes resolution of this force into the shock-absorber (the force induced by chain tension changes its position in dependence on the shifted gear so it is not possible to eliminate this force completely)
- resolution of the braking force into the shock-absorber
- tipping force from the rider’s centre of gravity during acceleration and deceleration in pedalling
FOUR-PIVOT STRUCTURE – CHARACTERISTICS
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The main and crucial advantage is the mounting of wheel and brake on the “seat stay” (pitman) where the braking force is resolved predominantly into the tension in a chain stay and into the pressure in the linkage. The rear structure therefore can virtually spring without any limits. - The susceptibility of springing to the braking process is minimal (it depends on the kinematic arrangement and the amount of the pressure force in a linkage that gets into the shock-absorber).
- The force induced by chain tension resolves predominantly into the pressure in chain stay and linkage. Thus it is not necessary to place the chain stay’s pivot to the level of central chainwheel.
- Springing is influenced minimally by the chain tension (it is dependant on the kinematic arrangement again).
- The disadvantage lies in complexity and, consequently, in a higher price. Four pivots are a must.
