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By Rick Webster and Jody Campbell
The first article in a series that will teach you *just* about everything you need to know on a specific topic... This
month, SHOCK ABSORBERS!
How do shock absorbers work and why do I need them?
      
This may seem like an overly simple question to ask, I mean heck, they’re on
every car, truck and SUV on the planet. And, they’ve been around darn near since
the dawn of automobiles themselves and a shock absorber is a shock absorber
right? Ummm… nope, not really! Regardless, before we answer these questions,
let’s identify some scientific facts that cannot be disputed and identify some
things you’ll need to know.
Science 101
- Potential energy – this is stored energy. As it applies here, this is
the energy that is stored within the suspension’s springs. An example you may
remember from grammar school is that a ball sitting atop a ladder has
"potential" energy, because it is waiting to fall, due to gravity or some
other force that will be applied to it. Once that force is put into motion,
potential energy is converted into kinetic energy.
- Kinetic energy
– this is energy in motion. As it applies to this
article, this is force of motion within your suspension as it cycles up and
down.
- Law of conservation of energy
– Galileo and a few other scientists
older than dirt came up with the theory that energy cannot be created nor
destroyed, it can only change forms. It applies to our article because while a
suspension is cycling, we need to control this cycling by converting the
kinetic energy into some other form, namely heat.
OK, with these undisputable scientific laws out of the way, we can
intelligently investigate how a shock absorber works and why we need them.
Simply put, a Shock absorber’s sole purpose is to dampen the compression and
rebound of any suspension system by controlling the speed at which a suspension
cycles. Without them, your truck would continue to bounce up and down until the
kinetic energy is finally dissipated from the suspension’s springs (e.g. leaf
springs, coil springs, torsion bar, etc.). Now, let’s think about the law of
conservation of energy… with this law in mind, shocks will perform two
functions. The first function is to slow the suspension’s cycling of compressing
or rebounding. Secondly, since energy can’t be destroyed, the shock transforms
the kinetic energy into heat as it dampens the "bouncing" of the springs. That’s
it… that’s what a shock does. Now you ask, how the heck does it do
that?
How a shock works:
Shock absorbers (a.k.a. shocks, dampers, etc.) work on the principle of fluid
displacement and heat convection. By forcing a piston through oil, shocks
develop the hydraulic friction necessary to oppose the unwanted bouncing in your
suspension. The hydraulic fluid located in the damper body, is forced through
tiny holes (Orifices) in the piston head as it travels (compresses or rebounds).
However, the orifices let only a small amount of fluid through the piston, which
in turn slows down spring and suspension movement. More importantly though,
every shock absorber is a velocity-sensitive damping device. That means the
faster a suspension cycles, the more resistance the shock absorbers provide.
Think of the rowing machine that your Dad bought but never used back in the mid
80’s. You could quite easily pull the handles back if you applied very little
force and did it slowly. Pull hard and fast, and it became much more difficult
to move, hence velocity-sensitive. These rowing machines used basic twin-tube
shock absorbers as their means of providing resistance to the user. As a result,
shock absorbers not only slow the compression and rebound of your springs, but
can also reduce bounce, roll or sway, brake dive and acceleration squat to some
degree.
Geometry
Now that we know how a shock works and why we need them, there is one other
important factor to keep in mind to ensure the adequate effectiveness of this
dampening device. This other factor is the geometry. If we could have our
druthers, each shock would be mounted as close to the wheel as possible, be
exactly perpendicular to the travel of the suspension cycle and be about 8 feet
long. If you could do this 100% of the time, you would be able to reap 100% of
the shocks benefits, with no loss and have unlimited axle articulation. However,
more often than not, this isn’t always the case.
So, if your suspension travels straight up and down (typically only seen on
Ford Twin Traction Beam or the Chevy Independent Front Suspension), then you
would want to mount the shock really far outboard, near the ball joints, and as
close to vertical as possible. This is how both Ford and Chevy mount their shock
absorbers. For those of us with leaf springs, there are a couple different ways
to effectively mount your shock absorbers.
Leaf Sprung, Front Axle
If you have a leaf spring, solid front axle with the shackles mounted in the
rear, your shock absorbers should be mounted as far outboard as possible, but
with a slight lean to the rear (About 1 to 2 degrees of rearward rake for every
2 inches of lift above stock, compounded geometrically). This is because as the
suspension cycles, it does so with a slight arc backwards. Transversely, a leaf
sprung front axle with the shackles mounted in the front would have a slight
rake forward.
Leaf Spring, Rear Axle
Your rear shock absorbers should be mounted as far outboard as possible as
well, and in as close to perpendicular to the travel of the suspension.
Referring to the location of the shackles above, you’ll want to rake the shock
absorbers forward or aft-ward appropriately.
Contradictions
We know that we don’t live in a perfect world and that the rules of thumb
above may not work on your rig depending on a series of factors, typically the
most prevalent being available space and needed droop (rebound). Regardless, if
you try to follow the rules of thumb above as close as possible, you’ll be able
to gain the most benefit from the shock absorber as possible.
Angle of the Dangle
 Mounting shocks at angles reduces
the overall dampening effect of the shock. Reason being; the shock’s mechanisms
will travel geometrically, less of a distance than that of the suspension
system. Some vehicles (early model Land Cruisers, etc.) have their rear shocks
mounted at about a 30-degree inward (inward = leaning toward the differential,
not forward or aft-ward) angle, while others have their shocks mounted at a 20
degree angle or so forward and/or aft ward of the rear axle (e.g. Chevy, Jeep
CJ’s, etc.). There are several reasons why this might be done. First, available
space… regardless, if this is something you are going to do yourself, you’ll
need to increase the static pressure of shock to mimic the shocks effectiveness
of it being in a perpendicular location. Secondly, you can gain more suspension
articulation than would normally be limited by the overall travel of the shock
absorber if it were located perpendicular to that of mounting your shocks at an
angle, if you don’t have room for a taller shock absorber. The charts here show
the overall estimated reduced effectiveness of a raked shock absorber. However,
these numbers should only be used as a rule of thumb as other factors such as
the arc of the suspension cycle can factor in.
Alternate locations
We won’t get into a lot of details here because it will get way too
complicated, but we do want to mention that there are alternatives to the
standard rules of thumb. For those of you who watch monster trucks or SODA/SCORE
racers, you’ll notice that some shock absorbers are mounted behind the solid
axle, onto the lower locating arms. This can be an effective method for mounting
your shock absorber as well, but too many dynamics fall into place for this
article. For example, things that must be taken into consideration are distance
rearward from the axle, compression pressure within the shock, rebound
resistance from within the shock, compression/rebound travel in relation to the
locating arm, arc of travel to the locating arm and so much more.
How long?
Size really does matter here. It is very important that you use a shock that
is the right length and has enough travel in both compression and rebound to
dampen the axle it is connected to. In the easiest of all situations, the shock
is mounted straight up and down. The measurement is fairly easy. Measure the
distance from the suspension bump stop to surface that it makes contact with,
and add a ½" for compression of the bump stop. This measurement is your
compression travel. Now measure from your upper shock mounting point, to the
lower mounting point. For explanation purposes, lets say that the distance from
the bump stop to the contact surface is 5.5" and we add a ½" we now have 6".
Lets also say that the distance from the top mounting point of the shock to the
lower mounting point is 14". Given these two measurements it is easy to see that
you have a difference of 8". This 8" measurement is the length of the shock body
you would need to control travel, measured from the mounting eye to the top of
the shock body, and not limit suspension travel. In this situation you would
actually have approximately 8" of rebound or droop travel in the shock and 6" of
compression travel.
Measuring Shocks at an Angle
This is when things get tricky, essentially what you need to establish first
is the angle you are going to mount the shock. This angle then needs to be
compared to the angle of the suspension when it cycles. Again for explanation
purposes we will say that the suspension cycles nearly vertically. Now we will
say that due to space limitations you need to mount the shock at a 30 degree
angle leaning forward of the axle. First lets say that the suspension travels 6"
vertically until it contacts and compresses the bump stop as stated in the first
example. Next you will need to measure your two mounting points, for explanation
purposes lets say this measurement is 12". Your difference is now 6". Now is
where things get a bit tricky. The easiest way to determine the length of shock
you need is to cycle the suspension from its loaded resting point to the point
were it compresses the bump stop. With the suspension compressed again measure
the distance from the upper and lower shock mounting points. Again from
explanation purposes only lets say that the total distance between these two
points is now 9". You can now see that as the suspension cycles through its 6"
of compression travel you are only using 3 inches of shock travel, 12" original
measurement minus the 9" you now measured. This means that a shock with a
measurement from the lower shock eye to the top of the shock body of 9" would
not limit suspension compression or rebound for this application.
Types of shocks
Twin-tube shocks
 Twin-tube shocks, are, for the most
part, the definition of a standard shock. Nearly all of the text above defines
how this shock works, so we won’t get into much more detail. What we will say is
that a twin-tube shock is the "entry level" shock absorber if you were to
compare all shock absorbers against each other. These are considerably cheaper
to manufacture, but offer the least consistent dampening in comparison.
Twin-tube shocks are much more susceptible to fade, aeration and heat
dissipation.
Coil-Over Shocks
 Coil-over shocks are fairly simple
by design. Simply put, a coil spring is placed over and around the shock body,
adding an additional spring rate to the shock absorber. These coils can be
placed over just about any type of shock absorber depending upon the
manufacturer. Unless you have a specific need for these shocks, or if you plan
on using this design in lieu of leaf or coil spring suspension altogether, don’t
bother.
Gas / Pressurized Shock Absorbers
First, let’s dispel an old wives tale that gas shocks are much more stiff
than regular shocks, offering a harsher ride. Gas shocks can be valved
differently to offer a ride just as 
smooth as a twin tube shock, while still providing far superior shock-damping
consistency than any regular shock on the market. Now, with that said, let’s
define what a gas shock is and how it works. Let’s say your driving your rig at
a good clip down a washboard road. Your suspension will be cycling at a
tremendous rate, thereby forcing the piston within the shock absorber to move at
a tremendous rate as well. When this happens, the oil within a regular
shock absorber gets air bubbles forced into it, forming a frothy, foamy goo.
When this happens, the oil will flow through the orifices of the piston at
unpredictable rates and decrease the performance of any standard shock.
Gas pressurized shock absorber works a bit differently and are not nearly as
vulnerable to the oil aeration as a standard shock absorber. Reason; gas
pressurized shock absorbers are built with pressurized nitrogen inside the shock
body. The pressure can range anywhere from 80 to 350 or more p.s.i. This keeps
the oil from aerating because nitrogen does not mix with the shock oil, and
forces the oil molecules to stay packed together much more closely, thereby all
but preventing the oil from getting any air bubbles within.
Mono-Tube (Single Wall) Shock Absorbers
 These shock absorbers types use a
single-wall shock tube to enclose the piston, the shock oil and (sometimes) the
pressurized gas. These shock absorber types are much more precise at dampening
than the standard shock absorber because they are made with considerably more
precise standards during the manufacturing process. Additionally, in most cases,
the single-wall shock absorber is considerably stronger than the twin-tube shock
absorber because they typically use a larger diameter piston. Further, the
single-wall absorber is more resilient to shock fade because it can divide the
shock’s oil from the air space far better than a twin-tube shock. With this type
of construction comes the benefit of better heat dissipation as well.
Shocks with Reservoirs
 Contrary to popular belief, the
external reservoir on a shock of this type isn’t made to hold extra shock oil.
Its purpose is to house the extra needed air space during a shocks compression
cycle. Typically this is not air at all, but nitrogen. It will hold some
additional fluid as needed, but this shock is designed differently from most
other shocks in that the entire main shock body is completely drowned in shock
oil. All shock absorbers, regardless of the type, need some amount of dead air
space to allow them to work properly. Standard shocks have dead air at the top
of the valve body or utilize a twin-tube model for the needed expansion.
As mentioned previously, the external reservoir is used for storing the extra
needed dead air space. They are typically connected to the main shock body via a
reinforced flexible hose or a metal tube of sorts. The trick here is that as the
shock compresses, the extra oil is forced through the connecting tube, into the
reservoir body and forced against the pressurized air or nitrogen. In theory, if
the oil and the air are not allowed to mix (that’s the way the engineers
designed this), the shock will dampen at a far more consistent rate regardless
of the frequency of the shock compression/rebound cycles, because the oil cannot
aerate. Not to mention they look cool.
Bypass Shocks
 The dampening provided by
standard shock absorbers is provided by the valving system being located at the
head of the shock piston, which determines the dampening rates. Bypass shock
absorbers aren’t all that different in that aspect, but they do add to this
standard method of dampening via valving. How? Bypass shock absorbers add the
component of external metering valves that are completely adjustable with
spanner wrench for changing the rebound and compression of the shock. The other
major aspect of bypass shocks is their oil-looping design. As the piston is
compressed into the body of the absorber, the oil is pushed through the external
bypass tubes and looped back underneath the head of the piston. Transversely,
under rebound, the fluid does the same thing, only in reverse. This entire
process is metered and dictated at an adjustable rate defined by the external,
adjustable check valves. Depending upon make and model, some bypass shocks can
offer multiple tubes to the shock body, typically one for rebound and one for
compression. Some of which have multiple, adjustable check valves to control the
metering of compression and the metering of rebound.
Adding fuel to the fire, yet another reason why bypass shocks are the best of
all dampeners is because they’re not only velocity-sensitive like all other
shock absorbers, but they are also position-sensitive as well. What does this
mean? Simply put, these shocks can use a variable metering system that allows
the shock to offer a much softer rebound and/or compression rate initially, and
increase the dampening effect as the compression or rebound increases, similar
to progressive coil springs. The really cool part? If you have the cash, all of
these aspects of a bypass shock can be built to your needs and adjusted based
upon the type of wheeling you do!
Dual Shocks?
One piece of advice… don’t run
dual shocks just because they look cool, OK? However, if you get frequent
heat-induced shock fade and don’t have the budget for reservoir or bypass shock
absorbers, you may benefit from running a dual or triple shock setup. However,
this doesn’t mean that you just slap another set (or two) of shocks in addition
to your existing ones. You should get a set of more lightly valved shock
absorbers to replace the ones you have now. Do the homework and figure out how
much absorbing your shocks need to do before you add some more, that is unless
you don’t like the fillings in your teeth.
Upside down?
Unless your shocks are specifically designed to be mounted upside down or
designed to be mounted in either direction, please follow the rule stated above
for dual shocks. As a rule, dual tube shocks should never be mounted upside
down. Some people say that monotube or gas pressurize shocks can be mounted
upside down, however in time they will develop and extra inch or more of piston
travel that has little to no dampening effect whatsoever. Ultimately: don’t
mount shocks upside down just because it looks cool. Sometimes a shock must be
mounted upside down due to space limitations, or to protect the shock body, if
this is the case, make sure you use a shock designed to be mounted upside down.
How can I tell if I need to replace my shocks?
While a leaking shock is an obvious sign of a shock-gone-bad, many shocks
wear out without losing any oil. One of the best ways to determine if a shock
needs replacement is to perform the jounce test. Simply bounce the front or rear
end of your rig by jumping or pushing up and down on it for a few seconds, then
let off. If your rig continues to pogo for more than 1 to 1.5 bounces, you may
need to replace your shocks.
To boot or not-to-boot
 A while back this use to be yet
another one of those campfire arguments… do I run shock boots or don’t I? While
some manufacturers recommend that you do, some do not. The general consensus
throughout the 4-wheel drive world now is that you should run shocks without a
boot. Reason; when 4-wheeling, the amount of dust, dirt, grime, mud and grit
generated is far more than that of a normal car, driving on the road. With a
shock boot in place, the nastiness will get caught inside the boot and can’t be
removed. The grit and grime will load up on the piston rod because of the thin
oil coating. This grit will ultimately score not only the seals of the shock,
but the shock piston rod as well, causing oil seepage and ultimately, the demise
of the shock itself. Running without a boot will allow you to blast that crud
away with a garden hose and a soft cloth.
What kind of shock do I need?
Only you can answer the question of the type of shock you need, and this is
based upon your budget and the type of wheeling you do. You should also talk to
some people who have a similar vehicle as your and do the same types of
off-roading. However, here are some very basic guidelines.
- Day to day driver, infrequent off-roader
– Try running a twin-tube
shock. These shocks offer good bump absorbing benefits while keeping your
tires planted firmly on the road and are much cheaper than their more-advanced
cousins.
- Dedicated rock crawler
– This is where some people’s opinions will
differ. In our opinion, a dedicated rock-crawling vehicle doesn’t require
fancy-schmancy shock absorbers. Reason being; your rig is traveling at a few
miles per hour and the rate of suspension cycle is incredibly slow. Save your
money for some other cool gadgets and go with an inexpensive shock.
Contradiction – If you find that you frequently need to travel at
higher speeds, possibly over some washboard roads to get to your rock-crawling
trails, you may consider upgrading to a gas-pressurized or a mono-tube shock
absorber.
- Medium/High-speed trail runner and/or daily driver
– In this
instance, if you find yourself traveling at speeds beyond 20 miles per hour on
the trail and do a lot of daily driving, you would benefit from upgrading to a
mono-tube or reservoir type shock absorber.
- High-speed racing and/or mudder and/or extreme off-roader
– If you
find yourself falling within these categories above you should seriously
consider a set of bypass shocks or at a minimum a set of reservoir type shock
absorbers.
- Money is no object and/or I want to impress my friends
– Go for the
gusto and get a set of bypass shocks!
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Rancho (Tenneco Automotive)
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1 International Drive
Monroe, MI 48161
Phone: 1-734-384-7804
Web Site: http://www.gorancho.com/
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