How Often should I
service my Suspension?
One question every suspension tuner
has to answer is “How often should I service my suspension?” The answer depends on several factors. Do you race? Do you log a lot of miles? How
critical is consistency of performance? These and other factors must be
considered when implementing a suspension service schedule. Let’s start with what happens to the
components of the suspension during use.
The
main components of the suspension on a motorcycle are the rear shock(s) and the
front forks. Both have a spring(s) and a
damper. The springs support the bike and allow the wheels to move independently
of the chassis. The damper controls the extraneous wheel and chassis movement
by converting the unwanted kinetic energy into heat energy and dissipating it
to the atmosphere.
Modern springs are made either of
Chrome-Silicon steel wire or Titanium wire and last for millions of
cycles. They require almost no
maintenance except for some possible setting or sacking, as it’s called. When a
spring sacks it loses a few millimeters in length, but does not lose its rate.
So about the only maintenance is to check the preload after the first few duty
cycles. For race bikes one race weekend, for street bikes by the 600 mile break
in is fine. In most cases, the springs are pre-set when manufactured and will
not lose any length. Any loss in length is compensated for by adjusting the
preload. Once this initial setting, or sacking is corrected, there is no
additional maintenance needed for the springs.
The other component of the
suspension system is the Damper. (Not Dampener) The damper (also called a shock or
fork cartridge) is the part that needs the most attention. The job of the damper is
convert kinetic energy, or motion, into heat energy and dissipate it the air. While
there are many different systems implemented to accomplish this, they all share
the same basic physics principles. By using hydraulic oil and a piston moving
through the oil, the resistance of movement of the piston through the oil
changes the kinetic energy into heat. It is similar to trying to run through
waist deep water at the beach. The resistance is varied by utilizing a piston
with ports that are covered by a stack of thin flexible washers called shims.
These shims come in various materials, thicknesses and diameters. By using different diameters and thicknesses,
the resistance to flow can be increased or decreased. The resistance force is
called the Damping force. Damping forces are velocity sensitive. That means
that the faster the piston tries to move through the oil, the higher the
resistance force, or more Damping force. It is what happens at the edge of the
piston where the oil is squeezed in between the piston port lip and the shim
stack is what’s most abusive the oil. 
As the oil passes between the
piston port lip and the shim stack, the spring coefficient of the shim stack is
trying to close off the port and squeezes the oil with a shear force. This
shear force then breaks up the molecular chain of the oil and, in layman’s
terms, “Spaghettifies” the oil. When
this happens the oil flows more freely. It does not lose viscosity (resistance
to flow), but instead the long strings of spaghetti like molecules pass between
the piston port lip and the shim stack easier than the interlocking molecular
clumps that the oil was designed and manufactured to have. This phenomenon happens with all oils, but is
more pronounced in heavier weight and non-synthetic oils.
Here is a real world example of how
I learned this. In 1998 we were experimenting with different piston designs in
Penske Racing shocks. Their standard 1/1 or one degree/one degree linear piston
was and still is a great all around piston. It gives good drive grip, good bump
compliance and has very even pressure balancing. They also had a piston called
a Digressive Piston. This piston had a much steeper Low Speed Nose. That is the
piston developed much more low speed damping force and therefore gave
additional drive grip. The digressive nature of the damping curve made it very
plush, as the damping force was reduced at higher shaft speeds. This, however,
resulted in the shock bottoming out too easy. To solve this Penske combined the
best attributes of the 1/1 linear piston and the Digressive to create the VDP
Piston.
VDP stands for Velocity Dependant Piston. As
piston speed changed, so did the nature of the damping curve. So now we had better drive grip, good
bottoming resistance and a very plush feel. The problem was that after about
4-5 laps the shock would lose its damping force. The back of the bike would
pump up and down through the stroke like pogo stick. After trying everything I
could think of to solve the problem I called Penske for help. I was referred to
Jeff Ryan (now the JR in JRi Shocks) and he suggested three things to help
solve the fading problem. First was to switch from the standard Penske Shocks
oil of Silkolene PRO RSF 5 weight to the 2.5 weight. Second was to make sure
there was absolutely no air bubbles in the oil at all. Last was the most
interesting. The VDP piston had separate high and low speed shim stacks. The
low speed stack was three, 8 thousandths shims. He had me change to 24, four
thousandths shims. The spring coefficient of the shim stack remained the same,
that is to say the “spring rate” of the stack was the same and tried to close
off the ports with the same force. 
Penske Racing Shocks 1/1 Linear (Silver), Digressive/Linear (Blue) and VDP/ Linear Pistons (Red)
Right from the first lap of the
next outing the performance of the shock was dramatically improved. First,
drive grip increased as the chassis was more geometrically stable. Second the feel
of the rear tire contact patch was increased. The rider reported a better
connection between the tire and the throttle due to increased feed back and
feel. Last was the fade issue. Whereas with 3 .008” shims, the shock would fade
after 4-5 laps, now with 24, .004” shims there was no noticeable fade after 12
laps, and after a 30 minute Endurance race the shock felt the same as it did on
lap one.
So the question is, why? Why did
changing to a greater number of thinner shims result in reduced fade and
increased performance? To keep it simple, it was the shear forces within the
shims. As the thicker shims bent, the force they exert on the oil is “harsher”
than the reduced shear forces within the thinner shims. This resulted in less of the aforementioned
“spaghettifiying” of the oil. That means that the oil was left in more of the
molecular clumps state and maintained its resistance to flow.
Another reason to service your
suspension regularly is cleanliness. As the piston moves up and down inside the
shock body (or the fork cartridge) it scrapes way a little of the anodizing off
the body, loses a little of the Teflon off the piston band, and leaves those
deposits in the oil. There are also bushings in the fork and shocks that have a
bronze base and Teflon coating that also wear and deposit in the oil. Some of
these chunks can become wedged in between the piston and shim stack and keep
the shim stack from closing off the piston port. Effectively this is like
turning the external adjusters wide open. In forks, where the springs are
inside the damper, the coils can scrape on the inside of the fork tube and
leave even more deposits in the oil. 
We have no idea what this is, or how it got in the fork. It did however, smell like dead fish
Lastly, regular service of
suspension components allows the tech to find problems before they can destroy
expensive components. Over the years we have found thing like, the nut that
holds the piston and shim stack on the damper rod coming loose. Is some cases
we have found it after it came off and the piston and valving gouged the
cartridge or shock body beyond repair. We have seen shims bend beyond the
breaking point and have shattered into little pieces. In a very rare case we
found a broken fork spring.

Broken shims that destroyed the cartridge and damper rod
So there are many reasons to have
your expensive suspension components serviced at regular intervals. Maintaining
consistent damping performance and avoiding expensive repairs are two of the
most important. Many of the After Market suspension companies we represent call
for 20 hour service intervals of their forks and shocks. This is purely a race
schedule. For the street we recommended every 10,000 miles. A
regular maintenance schedule will allow your tech keep track of wear intervals
of moving parts and prevent unnecessary damage.
Modern suspension systems are better than ever, but no system is exempt
from the perils of neglect and the cost of maintenance is much less that cost
of replacement.
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