[Chris]

I found this a year or so ago and thought is was some great info that needed to be shared.

It's from 4x4review.com

Be A Genius - Shock Absorbers
By Rick Webster and Jody Campbell



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.

· 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.




Shock Science 101
Okay, 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.


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?

Regardless of what your girlfriend told you, 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.

 

[Chris]

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.



Air Shocks

Let’s not confuse these air shocks with those old load-carrying air shocks that your Mullet-wearing step brother installed on his ’72 Camaro. These shocks are a combination of a shock and a spring, allowing you to ditch your coil springs or leaf spring as well as your shock absorber and replace it with one unit. They can be identified by their large 2” or larger shafts, and look a lot like coil-over shocks without the coil springs. Generally speaking, the larger the shaft, the more load-carrying capacity this shock has.
Air shocks are cheaper than coil-over shocks (about half the cost, somewhere between $200 and $400 each), but require a link suspension to locate the axle. So, if you are considering a leaf-to-air shock (or coil-over for that matter) conversion, you will need to factor in those costs too. Currently, all air shocks are emulsion style shocks (the body is filled and charged with both oil and nitrogen in the same cylinder) and not a floating-piston style, which is ultimately a superior shock design. Cost and complexity are the big inhibitors here and the reason why they don’t exist today.



For the most part, air shocks are intended for use with light and medium weight vehicles, and you will need to consult an expert on determining what load carrying capacity air shock to run on your rig.

 

[Chris]

Air Bump Stops

Air-charged bump stops are the little brothers of shock absorbers, but play a very different role all together. The full time job for an air bump stop is to assist with the smooth deceleration of a fast compressing suspension, like those on an off-road or desert racing truck. This helps smooth out the ride, keep the axle from slamming to the top of its upward-most travel, and finally helps to control the vehicle too. In nearly every case, the air bump stop replaces the poly or rubber bump stop that mounts to the frame or the axle. They look cool, and we’ve seen them on a lot of rock crawlers, but we have always considered them to be bling, unless they are installed on a race vehicle. For most rock crawlers, our suspensions travel pretty slowly, and air bump stops are frivolous. They ring in at about $150-$200 each.




Shock Absorber Do's and Don'ts



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!

 

[RikRotorhead]

I'm shocked how long of a read that is.