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Shock oil - an often overlooked but
important component in suspension units.
Suspension, shock absorber, damper....mechanical devices
used to smooth out bumps and dissipate kinetic energy, the energy is
converted to heat, the heat absorbed in the fluid contained in
a hydraulic cylinder. The overall device aids in cooling the
viscous fluid although the temperature will rise with constant use, and a
change in damper characteristics is to be expected.
The hydraulic damper controls the spring, either steel
coil or high pressure air, and the object of damper pistons, oil
ports and shim stacks submerged in hydraulic oil is to keep the
spring action constant through its compression and rebound stroke.
Additions like external compression and rebound controls, high and low
speed ports with external adjusters, by pass ports and separate
compression or rebound circuits, all aid in fine tuning and improved
performance.
Suspension oil is specifically designed to withstand
the harsh conditions of repetitious cycles inherent in suspension dampers,
confined in an airless oil cavity, often pressurized by a separate piston
charged with air or gas ( to accommodate displacement of penetrating shock
shafts and to some extent aid reabsorption of gas bubbles produced in
the oil by transfusion ) the fluid dissipates heat while attempting
to retain its viscous contribution to performance.
Rear suspension dampers utilizing suspension fluid
require an airless medium, under a certain amount of pressure tiny gas
bubbles will appear in the oil as it passes through a
port at high velocity, and the reasorbtion time dependent on
the amount of pressure behind the floating piston. Not to be confused
with foaming, although foaming does not appear to be as common
in dampers used on bicycles. When large amounts of air enters
the system, it usually indicates a failed seal, more apparent will be the
leaking suspension fluid from the shock, another common cause of an
aerated bicycle shock is the escape of gas from behind the floating
piston, emulsification occurs and it becomes quite audible that
something is wrong.
Bicycle dampers are high tech components with added
advancements to improve performance, manufacturers and suspension gurus
will promote upgrades and custom applications to assist with an
improved damper, modifications aside, the shock absorber will
always rely on the suspension fluid for consistent and reliable
performance, and replacing the oil at regular intervals is
advised.
- Changing oil viscosity will effect both
compression and rebound.
- Changing oil viscosity can effect platform valves
and thier characteristics to overall damping.
- Increasing the oil height in a suspension
fork will aid compression resistants but will increase rebound speed due
to the increase in pressure from a reduced air cavity.
- ATF fluid ( Auto Trans fluid ) is not
recommended as a suitable substitute for suspension oil, apart from the
inconsistent viscosity between manufactured batches, viscosity is not
often relevant to whats stated on the container, also ATF appears to
foam, encouraging aerated internals.
- Do not use suspension fluid with additives
such as seal swell, the additives in the oil will destroy some internal
parts in bicycle suspension damper internals.
- Oil viscosity will vary with temperature,
consider changes to damping characteristics with normal extended use
and climate temperatures.
- Suspension fluid can be effected by heat, although
it is more likly to be effected by constant use as the repetitious cycle
gradually wears internal damper parts, if the shock is left unserviced (
at regular intervals ) the contaminated oil will accelerate wear to the
stage of premature shock failure.
- Not all new suspension units are supplied with
a quality suspension fluid, if you do decide to upgrade
your shock oil, flush the internals with the intended fluid before
refilling and/or setting oil heights.
Whose oil do bicycle suspension manufacturers use
?
- Fox Suspension Fluid 7wt - Torco RSF-
Medium.
- Marzocchi Bomber Factory Fork Oil 7.5wt - Golden
Spectro Cartridge Fork Fluid - Light.
- Manitou Suspension Oil - Motorex 7.5wt fork oil.
Manitou Semi Bath Oil - Motorex SAE 5W40 ( Synthetic Motor Oil
)
- Rockshox Suspension Oil - Torco RFF ( Synthetic
& Mineral Blend ) RS 2.5wt = Torco RFF 5wt, RS 5wt = Torco RFF 7wt,
RS 10wt = Torco RFF 15wt, RS 15wt = Torco RFF 20wt,
Red Rum = Torco MPZ Assembly Lube.
Custom Blends.
Oils of the same brand and type can be mixed - 10 wt. and
5 wt. at a 50/50 mix is 7 1/2 wt. Oil viscosity is rated two ways: SAE
weight ( Society of Automotive Engineers ) and VI or viscosity index. SAE
ratings are 5 weight, 7 1/2 weight, 10 weight etc. VI ratings are 80/150,
80/170, etc. and are usually classified as Light, Medium, etc. Both
ratings involve measuring oil flow through a fixed orifice for a set
period of time. The SAE rating is at a fixed temperature whereas the VI
rating is over a specific range of temperatures.
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