How's this for some ACAD work?
The shocks are the most important part of how the car reacts with the ground. If the springs are too stiff the car wants to bounce and ride stiff - notice I don't say "doesn't bottom out"? If the springs are too soft, the car dips and wallows around. If the shock damping (not dampening) is too stiff on compression the suspension doesn't have a chance to do it's job and gives a harsh ride that chops and bumps, if the compression damping is too soft, the car bottoms out too easily. If the rebound damping is too stiff, the car can't reach full rebound and eventually the shocks "pack" and loose valuable travel. If it's too soft, the car will bounce off the ground in whoops and after landing off jumps. You have to tune all three systems - springs, compression damping and rebound damping to work together.
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Shock Theory - What Are These Things?
For this section I'm only going to address Coil Over shocks the type that come standard on Pilots and with long travel kits. There are many different types of shocks - Coil Over, Air and Bypass to name a few. The shocks we use on our Pilots are two systems combined into one. The hydraulic damping section contained in the shock body and the load carrying section, known as the spring. I'll start with the hydraulic section first.
The damping provided by a shock is to control the suspension movement in relationship to the chassis of the vehicle and the ground. Without damping of any sort, the car would bounce uncontrollably and the wheels would have no control in keeping contact with the ground. To achieve damping, a shock uses a tube filled with fluid, (usually a light oil) a shaft that has a piston with holes in it attached to one end and shims, which are basically thin washers, on each side of the piston.
As you push the piston assembly through the oil, the oil provides friction and "fights" the shaft's movement. The harder you push the piston through the oil, the harder the oil resists. This is the very basics of what the shock does. The piston has two sets of shims stacked on each side of it. These shims are nothing more than thin washers that can vary from .015" down to .002" in thickness and from 1.500" to .625" in diameter. One stack for compression damping and one stack for rebound damping with the diameter of each washer getting smaller as it moves away from the piston. Also you will find two sets of holes in the piston for the oil to flow through. Once again, one set for compression damping and one set for rebound damping. To control the resistance of the piston as it is pushed through the oil, the shims, which cover the holes in the piston, are allowed to flex. The thicker the shims, the less flex and the more damping. So, just opposite, the thinner the shims, the more flex, less damping. By adjusting the thickness and diameter of the shims in the stacks you can adjust the rate they flex for different shaft speeds. Small bumps that only produce small amounts of shaft travel only flex the edges of the largest shims, but as the shaft travel increases in both travel and speed, the more flex comes in play and the more shims are involved. Higher damping then comes with more shaft/piston speed and shim interaction with the oil. The piston doesn't care (or know for that matter, we're talking about a piece of metal, so it can't think!) whether it's in compression stroke or rebound stroke. it's just being forced through the shock fluid to produce friction and slow the shaft travel speed down - Hence, "Damping".
Shock Adjusting And Tuning
The shock compression damping should be set so that the car just bottoms out on the largest - or should I say hardest landing? - jump. There are some smaller jumps that you hit faster and harder off of than some of the bigger jumps on some tracks, hence hardest landing. By setting the compression damping up for the "biggest hit" the car should now be able to handle everything else on the track with ease. If you get the compression damping too stiff, the car will be very hard feeling and you'll notice every bump, ripple and rut on the track. If you don't get the compression damping stiff enough the car will bottom out on too many things like whoops and jumps. One mistake often made (me included here) is thinking that if the car's bottoming out that the springs need to be stiffer - WRONG!!! The truth is that the compression damping is what needs to be stiffer. You can quite often actually go to SOFTER springs!
Rebound damping is similar in that you want the shocks to expand as quickly as possible off the biggest hit without letting the car bounce back off the ground. Spring rates can make a big difference here because if you go up on spring poundage, your rebound damping becomes less. Stronger springs need higher rebound damping. If you are running ATV style tires on your Pilot, be careful here. It's easy to confuse shock "rebound bounce" with the bounce that the balloon style ATV tires can produce. I noticed a huge difference in rebound feel just by switching between my 10" ATV tires and the 13" DOT tires. If you set the rebound damping to this "biggest hit", the rest of the damping should feel quite smooth and supple through out the shaft travel. If you get the rebound damping too light the car will bounce off the ground and feel "springy". If you get the damping too stiff, in certain areas - such as whoops and stutter bumps - the suspension will never have a chance to completely rebound to full stroke and it ends up "packing" and feeling very stiff like it's gone into hydraulic lock.
Hey, What About That Reservoir (darn French word!) Thing?
The reservoir - oh heck, I'm just gonna call it a res - uh, now where was I? Oh yea. The res serves one main function - It's a chamber for the oil displaced by the incoming shock shaft to go. On a shock with an external res, there's the bonus of adjustable compression damping. Let me explain.
As I said, the res is really nothing more than a chamber for the oil that's displaced when the shock shaft enters the shock body on compression. If you didn't have a "give" area, the shock would be as solid as a piece of steel until you blew the seals. Say you have a shock with a shaft that's .625" in diameter and it has a stroke of 12". The total volume of oil that will be displaced is approximately 5.6 cubic inches of fluid - which I think works out to about 90ml. This has to go somewhere.
On a shock with a built in res, it's nothing more than a bladder or chamber in the top of the shock body. Quite often it's just a floating piston that separates the oil and compressed gas of the chamber. Nitrogen is the preferred gas, as it's completely inert, doesn't react to temperature changes as drastically as straight air and doesn't have the tendency to hold moisture like air. The nitrogen is usually compressed anywhere from about 150 psi to 300 psi. The reason for this pressurization is to keep air that's saturated in the oil from coming out as it's beaten by the moving piston.
Physics time!!! Think of a bottle of Pepsi (No Coke! Pepsi! Cheese burger, cheese burger, Pepsi, cheese burger - anyone over 30 should get this one) - Keep the cap on the bottle, shake it and the bubbles go away quickly, but take the pressure off (remove the cap) and the gas that's saturated in the fluid wants out and you get bubbles. This all goes back to one of the wonderful laws of physics. It's a form of the partial pressure laws - The boiling point (surface tension) of a fluid can be changed with pressure. Let's look at water. Boils at 212F at 14.7 psi. - or in essence, releases the gas that's saturated in it. If you put water under pressure, the boiling point will go up. I'm not sure of the numbers, but for argument's sake I'll say add 5 psi and the boiling point will go up 10 degrees. Same holds true going the other way - put a vacuum on it and the boiling point goes down. You can get water to boil at room temperature if you put a strong enough vacuum on it. Pretty cool, eh? Gee, does that mean we would boil if we were put under a strong vacuum? After all, we're about 90% water. YUCK! So, how does this apply to the oil inside our shocks? It's all relevant to the surface tension of the fluid. If you "beat" the oil hard enough you can coax the naturally saturated air out of it at normal atmospheric pressure. This leads to cavitation of the piston and deterioration or loss of damping. Put this oil under pressure and the air stays in the oil and the damping stays consistent. An added bonus to the pressurized gas behind the piston is that in a remote res it helps push the oil back into the shock body as the shock extends.
Speaking of remote reservoirs - Remote res's are a real advantage over a res built into the top of a shock. First, they increase oil capacity AND are out in the air away from the main body with helps in cooling and they allow for more tuning ability on a shock. The res is attached to the shock body with a section of braided steel line. This line is restrictive to flow of the oil as it's forced from the shock body so there's one compression damping section and, the res itself has a valve system where the oil enters that's adjustable to control the incoming oil flow even more. By varying the orifice size with each "click" of the adjuster knob the damping characteristics can be externally adjusted. This is not to say that the adjust knob is the over all controlling adjustment for compression damping - you have to get the valving and shim stacks correct first. It's more of a fine tuning adjustment for the compression damping. Also, the size of the res can be bigger than one built into a shock. This allows for for a larger gas chamber which helps combat heat and the pressure increase of the gas as the shaft enters the shock body doesn't increase as drastically so the rebound isn't effected as much