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4. Flow through
5. Piston with apertures
needle valve
Function
The function, in principle, is that fluid is forced
through needle valves at a low rate of flow (Fig.4)
and through a number of apertures in the piston
(Fig.5) at a high rate of flow. The flow through
these apertures is regulated by shims (thin steel
washers) that at high pressure are deflected to
open for the fluid. On most models the needle
valves can be set individually.
By altering the size of the shims-stack (Fig.6)
(ie, number, thickness, diameter) the character-
istics of the damping action can be varied (this
shall only be done by Öhlins authorized service
workshops).
4
6. Shims stack
Compression damping
When movement of the motorcycle causes
compression in the shock absorber, the fluid flows
through the needle valve (combined compression
and return valve) in the piston rod. If velocity of
the piston is high, ie, in the case of rapid
compression, this will not be sufficient and
consequently the shims underneath the piston
will open to allow a greater rate of flow (Fig.7).
The fluid that is displaced by the volume of the pis-
ton rod is forced into the external fluid chamber via a
separate compression valve. The separating piston
is displaced, thus increasing the gas pressure.
7. Flow through piston
Stop washer
Shims stack
Compression flow Rebound flow
Piston
Rebound damping
When the spring presses the shock absorber out
again, the fluid flows back through the needle
valve in the piston rod. The fluid flowing into the
chamber is forced by the pressure of the gas back
into the shock absorber via a separate nonreturn
valve.
If velocity of the piston is high, the shims on
top of the piston will also open to allow the fluid
to flow though (Fig.7).
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