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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.
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.