Shock Tech and Theory


Active member
Jan 16, 2023
Valving, shock tuning, shims, free bleed screws? Most of us have heard of them but what are they, what do they do, and when do you use which? We hope to at least give you some basic knowledge to allow you to make some changes to your shocks if you are brave enough to try to improve your dampers performance.

Lets start off with the basic components that control fluid in a basic smooth body or coil over style shock.

The Piston

The shock piston is a ported puck that shims are placed against creating a directional check valve allowing us to control the fluid flow independently on the compression and rebound events. Most high-performance pistons will be specific in orientation generally more flow on compression than rebound, (SOME pistons are “equal” or “even flow” allowing the same fluid volume both directions) for our purposes the rebound side will be facing the tenon nut. You will see everything from 4 ports to 9 ports and many other variations from manufacturers. We use a 9 port, 6 compression 3 rebound in all our products.1688743624295.png

Shims are round discs of spring steel in various diameters and thicknesses. How the shims are stacked in relation to each other and in relation to the piston face will alter the flow characteristics or ride quality.
Bleed holes and bleed screws

On some pistons you will find small holes that are drilled all the way through these “bleed holes” control what is called “free bleed”, fluid flow that is not controlled by the shims or main piston ports. Some pistons are threaded to allow you to plug the “leaks” to change the flow characteristics. We no longer use free bleed holes at ADS in our piston designs, there is only a single bleed port to aid in removing air from the system during assembly.1688743692181.png
Now that we know the basic components, we can discuss how they work together.

As you travel across terrain the suspension is doing its best to conform to it, as you drive the shaft of the shock moves in and out over obstacles the tires cannot absorb on their own, if the suspension is too firm you will have lots of feed back into the chassis and tires may bounce off of obstacles loosing traction which leads to a loss of control, if too soft it can bottom out causing damage and loss of control.

Shaft speed

Shaft speed is the speed the piston rod “shaft” travels as it meets obstacles. Knowing when this speed is fast or slow will help you determine what adjustments should be made to your shim stack or free bleed ports.

We have all felt RZR chop in our adventures, this is some of the fastest shaft speed stuff out there. Tightly spaced bumps that are about 5-12” tall normally. This is the stuff that shakes the doors off and ruins your dash, extreme wash board is one way to look at it. This is hard to get rid of in a single coil over application. Hitting a tall “curb or ledge” straight on at 60+mph would be some of the fastest shaft speeds, as the suspension would need to conform immediately to the obstacle.

Largely spaced rollers say 2 to 3 times the wheelbase at a moderate speed say 50 mph would be a lower shaft speed event, since the terrain rolls gently up and down so does the suspension even though you are traveling at speed the shaft speed is relatively low.

Single coil over or smooth body tuning is a game of compromise. The goal is to make as few compromises as possible. We want good slow speed control, so we don’t get the bouncing boat feeling but we don’t want harshness or bottoming out.

Straight stacks

We would describe a straight stack of shims as one of each diameter of shim that the given piston size allows you will hear guys say a straight 15 stack, what they are saying is they have one of each diameter from your fulcrum to your cover shim, and they are all .015” thickness.

You may have heard about flutter stacks but what is it? To break it down simply there is a cover shim with a spacer between itself and the main stack allowing the shim to have some free movement before the other shims help support it.1688743767439.png
A flutter allows fluid flow with less crack pressure than the rest of the stack, light cover shims and small diameter spacers allow for easy opening, thicker/ heavier cover shims and large diameter spacers create more crack pressure, you can use these on compression or rebound, (NOTE: using too thick of a flutter spacer can allow the cover shim to crack over time.)

Flutters can be tuned as a free bleed would be tuned but, with the advantage of direction control between rebound and compression independently, where free bleed is not direction sensitive.

For instance, if you want the effect of “free bleed” on compression you would run a light cover shim thickness to allow for low crack pressure, the thickness and diameter of the spacer will control how much volume the flutter will flow. If you want a lot of flow, go up on thickness and smaller on diameter with your spacer. Keep in mind you are using the piston ports which are very large compared to a small bleed hole so you can get a lot of flow potential quickly.

The same theory works on rebound you can do a flutter stack to get flow with lower crack pressures and simulate adjustable “free bleed”.

(On systems that have a lot of spring rate a fluttered rebound can be tricky to control use with caution and test your changes)

Keep in mind that when running a flutter, you may need to build up the rest of the stack to compensate for the flow you have created from the flutter. Test your changes and document what you have done so that if you go too far one direction you can go back or in most cases split the difference on your adjustment. When getting close to an ideal setup changing one shim can make a noticeable difference.

What shims function at different shaft speeds?

As shaft speed increases the volume of oil passing thru the piston increases, the shims will need to move out of the way to allow for this volume increase the further they bend the more flow we have.

The “heavier” or “thicker” you stack near the fulcrum the higher the shaft speed resistance you will build up, this will increase the force required to open the shims as they bend open further.

The heavier you stack near the piston face the more crack pressure you will develop which can be considered low speed or low fluid volume flow.

For fun lets imagine we put a .020” thick shim against the piston and then a stack of .008” right behind it we would have a bunch of crack pressure and then little high speed resistance. This could feel harsh and then bottom out super easy on a quick hit. This is a less than ideal set up and isn’t the approach I would make for an offroad toy.

Lets go the other way lets do a .020” stack with a flutter cover shim of .006” and a small diameter spacer, this would allow for low crack pressure giving us small bump compliance from the flow of the flutter but when we hit a quick sharp edge it may lock up causing a harsh feeling on fast shaft speeds. (this is purely an example many trucks would actually not have a lock up feeling with this stack)

This is where the playing and testing needs to happen. Knowing what to change for each scenario is important. Knowing how much to adjust will come from experience. Don’t be afraid to try things, don’t be afraid to make multiple adjustments to your valving and set up.

Some tips and tricks

Each shim thickness reacts different you can feel this in your hand by flexing them, 2 .010” shims are not the same as a single .020”, 2 .008” are still much softer than a single .015” even though it adds up to .016”

Covers shims are tuned for chatter bumps and low shaft speeds as they are the first thing to open even though they are two different shaft speed events they begin to work together on your initial hits.

You must always put a cover shim against the piston or you will have no fluid control, it will have huge free bleed effects in both directions.

Generally if you are running a free bleed type piston and your also going to flutter the stack a thicker cover shim will be needed to keep your low speed under control. A non free bleed piston will like a lighter cover shim as it will need to open easier for your low speed bleed.
Nitrogen/ Reservoirs

Let’s talk about nitrogen pressure for a moment as this is a debated concept by many. The main function of our nitrogen charge is to prevent cavitation, that is it. In my opinion playing with nitrogen pressure is a very advanced level of tuning and should not be the first thing to play with. Nitrogen charge pressure should be constant in our system. We need enough nitrogen pressure to prevent cavitation anymore and we increase rod pressure, which can be used in advanced tuning, but everything needs to be correct in the system before playing with this if you haven’t re-valved your shocks and dialed in your spring rates just keep your nitrogen pressure at a safe level. 150-200psi will get 90% of you where you need to be, don’t worry about 175 145 165 psi its all snake oil, your gauge is likely not that accurate to begin with.

Enough rambling back to theory, so how much pressure is enough to prevent cavitation? It’s a trick question we first need to look at how our reservoir is mounted to our shock to understand how it would affect cavitation. We have 3 basic ways of mounting a reservoir to a shock. Top mount, Bottom mount, and Mid mount. Each has its advantages and downsides, let’s look at each.

Top Mount

In a top mount reservoir connection, the reservoir feed port is at the top of the fluid column, you will see this on coil overs and smooth body designs and really old bypasses which is not good lets explain. As the shaft travels into the shock body the volume of the shaft needs to be displaced, the reservoir allows this to happen via an air chamber, air compresses fluid does not. If our nitrogen pressure is too low the oil will begin to cavitate below the shock piston on a compression stroke, cavitation is tiny air pockets that develop from a low-pressure area under the piston, charging the system with nitrogen helps keep this pressure differential in a stable state keeping cavitation under control. The more force the valving is trying to develop the greater the chance we will see cavitation, the goal is to have no cavitation and allow the piston and valving to pass through the fluid column. The more valving you run on a top mount resi shock the more nitrogen pressure you will need to run to prevent this cavitation issue, the problem with this is that the reservoir starts to become the weak link in the system they are generally made from aluminum and will burst if we go too far with pressure. On some setups we have run in excess of 300psi but again this is super advanced, and you MUST know what you are doing or you will hurt things! Bottom mounting the reservoir solves this issue. Packaging becomes an issue quickly when you have a coil spring wrapped around the shock body, that is why you see coil-overs with top mount reservoirs.
This style resi port is not desired on a bypass style shock because we run much more valving, when the piston reaches the bump zone and we have no more bypass, cavitation is nearly inevitable. General rule of thumb the more valving you are running in a top mount resi the more nitrogen pressure you will need to run to prevent cavitation. (tip for top mount resi bypass guys, get a high quality resi with screw in caps, or a steel body resi and crank up that nitrogen pressure, when doing this you may need to drop spring rate a little as the shock will start to act as a spring with higher pressures.)

Bottom Mount

In a bottom mount resi system the fluid column has a dead end that it pushes against and the resi pressure pushes on the bottom side of the piston this creates less of a pressure differential which keeps the cavitation under control, you will find this style on smooth bodies and bypasses.
Mid Mount

Mid Mount has a good compromise between packaging and cavitation control you will find this style on application specific setups like UTVs and Custom Builds, as the piston passes over the resi feed port it transitions from top feed to bottom feed this can give the sensation of a bump zone similar to a bypass.