Modern or extend whilst riding allowing the

Modern mountain bikes utilise frame linkage systems in
conjunction with a spring and damper shock to improve off road riding
performance. The shock can compress or extend whilst riding allowing the rear
wheel to track uneven ground. The leverage ratio of a bike describes the
mechanical advantage that the rear wheel movement has over the shock during
compression. The instantaneous leverage ratio expresses the ratio of rear wheel
and shock displacement at any point throughout the suspension travel. This is a
critical factor defining the suspension characteristics of a bike.

 A constant leverage
ratio will result in a linear force curve during shock compression. A decreasing
instantaneous leverage ratio as the shock is compressed will result in a
progressive force curve during shock compression. For example, a bike may have
a 3:1 leverage ratio at the beginning of the travel decreasing to a 2:1 ratio
at the end stroke.

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Work is the integral of a force ()
over a displacement ()
defined by the equation
  (the area under a force displacement graph). The
figure below is a graphical representation of the force required to compress
linear and progressive linkage systems through the entirety of the shock
travel.

From
the graph it can be seen that a progressive linkage design may be considered advantageous
due to the shock having to do less work to compress the initial stroke. This
translates to a progressive suspension design having superior small bump sensitivity.

The progressive curve steepens to provide a mid-stroke similar to the linear
ratio but aggressively ramps up requiring the shock to do significantly more
work to compress the end stroke. On the bike this creates a stable pedaling platform
through the middle of the travel and a firm end stroke to prevent harsh bottom
outs.