The construction of the rear suspension is clearly different from the front suspension. At the rear suspension, the rear fork is pivotally attached to the frame and a spring element is placed between the rear fork and the frame.
For the spring element there is the same principles for the suspension and damping that are used in the front fork. The big difference is that the spring element is much smaller and much stiffer. Rear spring elements also have the advantage that they are not usually subjected to bending because they are hinged to the rear fork and frame.
The force on the spring is F spring = a / b X F road surface and the displacement of the spring is V spring = b / a X V axis at a ratio of a: b or from 5 : 1, a maximum force on the road surface of 2000 N and a maximum displacement of the axis of 50 mm, the maximum force on the spring is 10,000 N and the maximum displacement 10 mm. The spring stiffness is then an average of 1,000 N / mm (front fork 40 N / mm).
Afbeelding 55. With an improperly positioned pivot the chain wants to be longer or shorter when the spring is moving. The result is that the cyclist will feel a vacuum or suddenly feel a great force when he kicks on the pedals.
Another disadvantage of rear suspension is that the kick forces exert influence on the rear fork.
At a too highly placed pivot point the fork will be pushed down. At too low disposed pivot point the fork will be pulled upward. The spring moves down and the frame and goes down with it.
By placing the pivot point of the rear fork on to the correct place to the frame you can minimize these two effects. The pivot point of the suspension system must belocated approximately at the level of the small gear and above the bottom bracket.
By changing the chain force the fork must not be turning. This can only happen if the chain exerts a moment on the pivot. The rear suspension of the point of rotation then must also be on the chain line. At a too highly placed pivot point will be exerted by increasing the force of a moment on the pivot point. The fork will turn downward and push the frame up. At too low disposed pivot point, the chain pulls down the frame.
For the rear suspension, the following four different constructions are used:
- A fixed rear fork is pivotally attached to the frame. Between the swingarm and frame a spring element is attached. The hinge point must be approximately at the level of the small chainring. This construction has the advantage that it has only a pivot point, see Figure 56b. The disadvantage is that the rear fork is subjected to bending ,thats why it must be executed with a quite heavy construction. Another disadvantage is that the spring element has to come about in which the seat tube is located. The frame shape is often customized.
- The lying and standing rear fork are hinged to the frame and connected together. In the vertical rear fork the spring element is connected, see figure 56. The advantage of this construction is that the frame has the same shape as the classic frame. The disadvantage is that the spring element with bending is loaded (mounted as a brake on the seatstays) by lateral forces on the frame and braking forces.
- The rear fork is made up of four-link mechanism in the shape of a trapezium. The advantage is that the influence of the kick forces are smaller. The disadvantage of this construction are the many pivot points shown in Figure 57.
- The problem of chain and springs in the rear fork when turning the bike can also be solved by attaching the bracket to the rear fork. This construction is used by Trek, Batavus, Giant, etc. The disadvantage of this construction is that the distance between the saddle and pedals varies during cycling and that the pedals are not suspended (especially noticeable when you go out of the saddle).
It is best to place the pivot point of the rear fork close to the bracket for the proper play-free rotation of the rear fork relative to the frame is the point of rotation provided with bearings. These may be either ball bearings as sliding bearings. That bearings have more friction but is not such a problem because the suspension still to be muted. The friction is to be regarded as a form of damping.