Penske Racing Shocks 7300 Series - Manual (Page 16)

Digressive Piston
Digressive Piston
The digressive 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 character-
istics 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 digressive/linear piston with zero preload on the shim stack. Figure 4,
shows a digressive/linear 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 digressive piston design is offered in two variations. The double digressive piston is preload variable on both the
compression 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, however, it can be used
either way.
Digressive/Digressive
The double digressive piston has .050 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; .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. Due to the higher flow when
running the linear side on rebound, it is a
rule of thumb to run (1) step stiffer on the
rebound side than what was used on a
standard linear piston (example: A up to B).
.050
NOTE: USE (3)
BACK-UP SHIMS (.750 X .020) ON
COMPRESSION VALVE STACK.
TORQUE
300 in•lbs