Penske Racing Shocks 8900 Series - Manual (Page 25)

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Digressive Piston
Digressive Piston
The “high flow” design incorporates larger ports on
the face of the piston to increase the flow of oil
throughout the shocks high speed action. When
the shim stack opens, oil is “dumped” through the
piston in large capacities. The increased flow of oil
reduces the progressive damping characteristics of
the linear side of the piston.
In addition to the larger ports, the face of the piston
is designed to allow adjustments to the preload on
the shim stack. Increased preload delays the opening
of the shim stack, causing an increased damping
force at low shaft speeds. When the shims crack
open, oil is “dumped” at a high rate, reducing the
progressive damping characteristics.
To visually explain piston preload, Figure 3, shows a high flow piston with zero preload on the shim stack. Figure 4, shows a
high flow piston with an exaggerated amount of preload. The preload cups the shim stack, energizing the shims until the instant
high shaft velocity snaps them open. The preload may be varied by adding or subtracting a series of shims under the main shim
stack.
The high flow piston design is offered in two variations. The double digressive piston is preload variable on both the compres-
sion and rebound sides. The digressive / linear piston is preload variable on the digressive side only, leaving the other side with
linear characteristics. In most cases, the linear side of the piston would be rebound.
Digressive/Digressive
The double digressive piston has .050
(55mm), .030 (45mm) of available preload
as shown in Figure 1. Stacking preload
shims between the piston and the shim
stack varies the amount of preload on the
shim stack.
When referring to the amount of
preload on a shim stack, you’re
referring to the amount of preload on
the piston face of the shim stack. For
example; 55mm .010 preload = .050
(total available preload) minus .040 (the
combined thickness of the preload stack).
Digressive/Linear
The linear side of the digressive/linear
piston is treated as a standard linear piston,
using a standard valve stack with (5) backup
shims. Due to the higher flow, it is a rule of
thumb to run (1) step stiffer on the linear
side than what was used on a standard
linear piston (example: A up to B).
.050 (55mm)
.030 (45mm)