The ‘Gospel’ according to St Öhlins (reproduced/modified
without permission)
All motorcycles are designed with a suspension
geometry that includes height and fork angle. The changing of components can
affect this and it is therefore essential that both the rear and the front ends
match each other. Changing to Öhlins suspension gives
optimum performance only when both the front fork and the rear suspension
interact properly. It is of the greatest importance that the front and rear
loaded height are within the specified values. See
section: Setting the spring preload.
Most of the Öhlins shock
absorbers are of the De Carbon type. The fluid is put under gas pres-sure and
the gas and the fluid are kept apart by a separating piston. The separating
piston is often fitted in a separate fluid chamber, connected by hose, or fixed
direct on top of the shock absorber (piggyback). There are also types of shock
absorbers where everything is fitted inside the main cylinder tube (internal
gas reservoir), and even a small number of emulsion shock absorbers that do not
have a separating piston. Pressurization of the fluid is made with nitrogen.
The pressurization prevents cavitation of the fluid
and the shock absorbing action is therefore more even. The external reservoirs
also contribute to better cooling of the fluid, giving longer service life for
both the fluid and components.
Öhlins shock absorbers
have integrated temperature compensation. As the temperature
in-creases. and the fluid flows more easily the
flow is controlled accordingly. The shock absorbing effect is therefore
independent of the temperature. The more advanced models permit individual
adjustment of compression damping and re-bound damping, and also certain
adjustment of the length of the shock absorber. Öhlins
shock absorbers provide the possibility for adjustment, making them adaptable
to most motorcycles, riders and ranges of use. All of the shock absorbers have
adjustable preloading of the spring action; either mechanically or hydraulically.
In principle, fluid is forced through needle valves at
a low rate of flow and
through a number of apertures in the piston 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 (ie number, thickness, diameter) the characteristics of the
damping action can be varied.
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 the velocity of the piston
is high, as in the case of rapid compression, the rate of flow will not be
sufficient and consequently the shims underneath the piston will open to allow
a greater rate of flow.
The fluid that is displaced by the volume of the
piston rod is forced into the external fluid chamber via a separate compression
valve. Even this valve is fitted with shims that open at high piston velocity.
The separating piston is displaced, thus increasing the gas pressure.
When the spring forces 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 non return valve. If velocity of the piston is
high, the shims on top of the piston will also open to allow the fluid to flow
through.
Always ensure that the basic setting made by Öhlins is correct. It is adapted to the make and model (in
its original state) and for a rider of aver-age weight. Everything must
harmonize. Incorrect spring action can give a fork angle that is too steep or
too flat. This in turn will give a tendency for over-steering or
under-steering, which could seriously affect the handling characteristics of
the motorcycle.
Preload on the spring/springs is very important
because it affects the height of the motorcycle and the fork angle.
Consequently, handling characteristics can be changed, even negatively. Proceed
as follows (it will be much easier if done by two persons):
(a) Place the motorcycle on a stand.
(b) Lift up the rear end to a fully extended position.
© Measure the
distance, eg, from the lower edge of the rear mud
guard or from a point marked by a piece of tape, immediately above the rear
wheel axle, to the wheel axle. (R1)
(d) Make a similar measurement on the front axle,
e.g., from the bottom of the upper fork crown to the front wheel axle. The fork
must also be fully extended. (F1)
(e) Allow the motorcycle (without rider) to apply load
on the springs and repeat the measuring procedure. (R2, F2)
(f) Then take the same measurements with the rider and
equipment on the motorcycle. It is important that the rider has a correct
riding posture, so that the weight is balanced on the front and rear wheel in
the same way as when riding. (R3, F3)
The measurements may not differ from the following
sizes:
Without
rider: Rear: 5-10 mm (R1-R2)
(static sag) Front:
25-30 mm (F1-F2)
With
rider: Rear: 30-40 mm
(R1-R3)
Front:
35-48 mm (F1-F3)
Adjust the preload with the rings on the shock
absorber or by hydraulic preloading. In the first case, hold the upper ring and
adjust the lower one to the desired position. Then lock with the upper ring.
For hydraulic preloading, increase by turning clockwise and reduce by turning
counter clock-wise
On shock absorbers that have mechanical type
adjustment the position of the adjusting/preload rings can be adjusted. On a
shock absorber that has hydraulic setting, the basic position can be adjusted
but should be attended to by an Öhlins authorized
service workshop.
To optimize the road holding qualities of a motorcycle
the front fork must match the rear suspension. The springs must be in good
condition and not fatigued. The fluid in the front fork should be changed at
least once every year. The spring preload affects the ride height,
it does not affect the spring stiffness. Therefore, on models with a
linkage to the shock absorber, the suspension may actually feel harder when you
reduce the preload and the shock absorber gets into the harder range of the
link system. Use adjusters to set the correct preload.
The adjusting possibilities of Öhlins
shock absorbers facilitate fine setting. You can optimize adjustments to suit
your own weight and equipment, your individual way of riding and the condition
of the road. To be able to improve the road holding qualities it is of the
utmost importance that you fully understand the functioning of the shock
absorbers. Then you can learn by trial and error how they affect the
motorcycle. Depending on the model there are adjustments for rebound damping,
compression damping and adjustment of the length of the shock absorber. Damping
is set with the knobs, they have a nor-mal right hand
thread. By turning clockwise they increase the damping action and counter
clock-wise they reduce it. The knobs have definite positions with noticeable
“clicks”, so it is easy to count to the right setting. Rebound damping
action affects the characteristics of the motorcycle most. The setting knob
is located at the bottom on the piston rod. It can be adjusted in about 40
steps. The compression damping knob is located at the end of the reservoir.
This can be adjusted in about 25 steps.
When making new adjustments it is easiest to go back
to fully closed, and then count forward to the new setting. The adjusting knobs
should not be screwed in too hard. If no ”click” is
felt, the shock absorber must be inspected by an authorized service workshop. It
could be due to incorrect or lack of gas pressure.
Always begin with the basic settings recommended by Öhlins. Always make notes, adjust in small steps and make
only one adjustment at a time. Adjustments should be made with two steps (clicks)
at a time. Adjustments should not be more than four steps from the basic
setting. By utilizing the adjustment possibilities you can test by trial and
error, and learn how they affect your motorcycle.
Always begin by test riding the motorcycle with all
adjustments at their i.e. delivery setting. Choose a short run of varying
character, ie, long and sharp bends, hard and soft
bumps. Keep to the same run and adjust only one setting at a time. Start with
the rebound damping. If the motorcycle feels unstable, loose and rather bouncy
then the rebound damping should be increased. Begin by turning the adjusting
knob 4
steps (clicks)
clockwise. Test run again and adjust two steps back if it felt too hard and
bumpy.
If the motorcycle is hard and bumpy, especially over a
series of bumps, then the rebound damping should be reduced. Turn counter
clockwise 4 steps, test run and make any necessary correction to 2 steps.
On those shock absorbers with both high and low speed
compression damping, the low speed compression
adjuster affects ride height, smoothness over small bumps and grip. The high
speed compression adjuster affects stability, firmness in depressions and fast
corners. If the motorcycle has low riding position, the low speed compression
should be increased. Screw clockwise four steps and test run again. If this was too much then turn back two steps (counter clockwise).
If it feels unsmooth over small continuous bumps or has bad grip, the low speed
compression should be decreased. Turn counter clockwise four steps. Test run
and make any necessary correction in two steps at the time.
If the motorcycle feels unstable in fast corners and has a tendency to bottom easily in depressions and chicanes, the high speed compression damping should be increased. Screw clockwise six steps and test run again. If this was too much then turn back three steps (counter clockwise). If it feels harsh and too rigid or has a tendency to hop under braking, the high speed compression should be decreased. Turn counter clockwise six steps. Test run and make any necessary correction in three steps at the time. When you have sufficient feel of the motorcycle you can make further fine adjustments. It is feeling and experience that counts.
Sensitivity of the steering can be adjusted by
altering the length of the shock absorber, without affecting other
characteristics. The length is adjusted using two nuts down and the treaded
clevis at the end of the piston rod. The shock absorber can be adjusted up to
12 mm.
A long shock absorber results in steeper inclination
of the front fork (steeper fork angle) and consequently sensitive, quicker
steering. A short shock absorber gives a bigger angle of the front fork (flat
fork angle) and consequently slower and smoother steering. Each complete turn
of the shock absorber gives one millimeter. The length may never be altered
more than to where the groove that is cut in the thread becomes just visible
under the lower nut of the level bracket. Make small steps and test run.
If the shock absorber has an adjustable end
eye/bracket, this must not be threaded out more than that one groove is fully
visible beneath the lock nut. Make sure that the lock nut is tightened after
adjustment. Ensure that the springs are properly preloaded before attempting to
make any adjustments. A simple rule is that increased preload of the spring
should be followed by an increase of rebound damping by two steps. When you
feel that you have achieved an improvement, go back to where you started and
check once more. Be observant of other relevant factors such as tyres, temperature, etc. Test run to make sure whether
further fine adjustment should be made.
Clean the shock absorbers externally with a soft
detergent. Use compressed air. Lift the bump rubber and clean the area below.
Keep the shock absorbers clean and always spray them with oil (WD40 or CRC 5-56
or similar). Never use detergents that can damage the surfaces of the shock
absorber. Thinners and brake cleaner will dry the surfaces too much.
Inspection points:
(a) Check ball joints for possible excessive play.
(b) Check the piston shaft for leakage and damage
© Check the shock
absorber body and for external damage.
(d) Check the external reservoir for damages that can
restrict the floating piston from moving freely.
(e) Excessive wear of rubber components.
(f) Mounting points. Check the hose equipped models
for leaks in hose and inlet plugs.
Preventive maintenance and regular inspection reduces
the risk of functional disturbance.
Recommended service intervals: race track: Every 10 hours, street/road
every 20,000 - 30,000 km.