Shock absorbers have been around in one form or another since the early 1900s, but those drivers wouldn’t recognize the sophisticated dampers used on street or off-road vehicles today. A shock absorber is a fairly simple device, yet it can also be extremely complicated (depending upon style) and commonly misunderstood. First, we’ll cover some of the basics, myths, and truths about shocks. Then we’ll introduce you to some theory about shock valve tuning and provide a few examples of tunes and the shock dynamometer results of those examples. In the end, we hope you will have a deeper understanding of shock absorbers, and how you can tune a rebuildable shock absorber to suit your specific needs.
For the most part, the springs don’t damp (control) the movement of the vehicle, its axles, and its tires and wheels. Instead, the springs support the vehicle on its axles, wheels, and tires. A shock absorber damps spring movement (oscillation). As your Jeep goes along the trail and encounters an irregularity in the trail surface, the tire and wheel move upward and push the spring up (or in the case of a coil, compress it). The spring reaches its maximum upward movement and then pushes back down on the tire/wheel combo, allowing it to follow the surface of the road. Without a shock absorber, the tire/wheel and spring (along with the vehicle) would continue bouncing up and down uncontrolled until the force was finally damped by the spring. If you’ve ever driven a vehicle with bad shocks, you know what this uncontrolled spring oscillation feels like.
Testing of each valving disc combination was performed on Biltsein’s in-house shock absorber dynamometer. The graphs it produced revealed how each valving change we made affected the compression or rebound performance of the shock absorber.
The new Bilstein 8125 is a coilover remote-reservoir damper designed for custom installations. However, what we learned from working with the 8125 can be applied to just about any rebuildable shock absorber.
Before performing any work on a rebuildable remote-reservoir shock absorber, such as the Bilstein 8125 (delivered to the customer with 200 psi of nitrogen pressure), you need to relieve the reservoir of its pressurized nitrogen gas. This can be done through the schrader valve located on the reservoir end cap.
There are a wide variety of shock absorbers today, but most are all designed based upon two basic types: monotube and twin-tube. Monotube shocks have a working piston attached to the piston rod inside the shock body, a portion of which is filled with hydraulic oil. A second, free-floating piston separates that oil-filled section that the primary piston moves through from a high-pressure nitrogen-filled section. The nitrogen gas helps prevent cavitation (air mixing with the oil and creating a foam), which can reduce the shock’s effectiveness. Twin-tube shocks have a piston in an oil-filled internal tube that’s inside the main shock body, and a base valve at the bottom of the internal tube. The exterior shell contains oil and low-pressure nitrogen to help reduce cavitation. Twin-tube shocks have some advantages such as greater piston travel in a compact space, and more oil (help with cooling). However, that extra body shell can also trap heat. Other than outright physical damage, heat is a shock’s worst enemy.
There are also internal bypass, multi-stage externally adjustable, remote reservoir, coilover, and other designs that can affect the performance of a shock absorber. For the purposes of this article, we are using the new Bilstein 8125 shock absorber, a mono-tube remote-reservoir unit that is rebuildable. Because the Bilstein 8125 is a remote-reservoir model, that reservoir helps manage heat and contains the dividing piston. This allows the 8125 to have a longer stroke (more travel) than a non-reservoir shock of the same body length.
There are two basic shock piston designs: digressive and linear. The main difference is in the hole (port) design. The digressive piston we are using in this example has fluted ports on both sides of the piston that begin near the center on one side and end near the outer rim of the piston on the other side. The 8125’s digressive piston is symmetrical so it doesn’t matter which way it’s placed on the piston rod, and it offers a higher level of control without harshness.
After relieving the pressurized reservoir, removing the rod guide screws, and sliding the wiper cap up, the rod guide can be depressed slightly to reveal and allow removal of the snap ring inside the shock body. Then the piston rod assembly can be carefully lifted out of the shock body.
With the piston rod assembly firmly held in a vise (with a soft buffering layer such as a towel between the piston eyelet and the vise), the rod nut can be removed, allowing removal of the valving components. When reassembling after any valve changes, be the sure this rod nut is tightened to the manufacturer’s specific torque setting.
Anytime you are working with the valve discs of a shock absorber, we strongly recommend that the compression valve stack, piston, and rebound valve stack be laid out in order of removal so that they will all be replaced in the proper order. Shock absorber performance can be compromised if these components aren’t properly replaced.
The Biltsein 8125 can also be easily taken apart so the valving of its piston can be altered. Valve discs (some refer to them as shims or plates) are the key to tuning a high-performance shock like the Bilstein 8125. These discs help control the volume and the speed at which oil can pass through the piston, and that directly influences the damping performance of the shock absorber. The shock absorber’s compression and rebound characteristics can be altered (or tuned) separately to provide just the right ride quality you desire for the conditions you drive in.
Using the Bilstein 8125 as our learning model, we’ll look at each of the different parts of the valve disc stack in order, talk about what they do, and how they can affect shock absorber performance. Our baseline setup included, in this order, a valving nut, a rebound brake washer, five rebound discs, a rebound-side bypass disc, two preload discs, the digressive piston, two more preload discs, a compression-side bypass disc, six compression discs, and a compression brake washer. See the baseline valve disc chart below for specific valve disc sizes in our baseline shock valve setup.
Valve discs are stacked in layers over the piston’s ports to regulate the flow of oil, thus thus creating the rate of compression and rebound. The valve discs have different outside diameters, as well as different thicknesses. Both measurements affect how much oil can pass by them. Differing amounts of oil pass around their outer edges due to their different sizes, and more oil can pass by them depending upon how much pressure the oil exerts on the discs as piston speed changes. For instance, two discs with the same diameter, but different thicknesses, will have different flow characteristics. The thicker disc will flex less under pressure and allow less oil to pass, creating a higher rate of compression or rebound (depending upon which side of the piston it’s placed) than a thinner disc of the same diameter.
Bypass discs are available in a variety of thicknesses and combinations of slot numbers and widths, can be used on the rebound and compression side of the piston, and are helpful in tuning the shock for terrain irregularities like embedded rocks to soften the ride. A bypass disc’s “A” value indicates the bypass area in square millimeters. More bypass area allows more oil flow at low stroke velocities, before the deflected discs start to bend.
The bypass spring and check plate (a patented Bilstein device) can be placed between preload discs and the bypass disc to create more force (make the stock stiffer) overall throughout the shock’s rebound curve, allowing the tire/wheel combo to come back to the terrain more gently and slower after compression.
This was our baseline test valve configuration. From left (compression side of the piston) to right (rebound side of the piston), you can see the brake washer, support disc, five deflected discs, bypass disc, two compression preload discs, bypass check spring, bypass check plate, the piston, two rebound preload discs, bypass disc, four deflected discs, support disc, and another brake washer.
Some valve discs, such as the brake, bypass, check valve, and its corresponding check spring, work in unique ways to control the flow of the shock oil that the piston is passing through. The brake, as it’s called, is the thick, washer-like valve disc (with its rounded edge on one side mounted away from the piston). Its job is to “brake” or resist the amount of flex that the compression or rebound discs can undergo during shock piston stokes. It is especially useful for helping to limit or slow the amount of the piston stroke possible under high-speed piston movements. This can occur when the tire of your vehicle hits a large and abrupt trail surface irregularity. It can also happen if the vehicle is airborne and the tire lands hard on the road surface.
Bypass discs (sometimes called bleed discs) can be used on one or both (both in this case) sides of the piston to allow shock oil to pass through much more easily than normal valve discs of the same diameter because of the openings around their edges. One or more pre-load discs can be used between the bypass disc and the piston to preload the deflected discs various amounts. In addition, bypass discs with larger or more openings can be used to allow more oil to pass more rapidly through the piston. The more oil that easily passes through or around the bypass disc, the more rapid the shock can react to slow speed (piston speed) events. This is most helpful in tuning the shock for terrain irregularities like embedded rocks to soften the ride.
The check valve disc and check spring (a patented Bilstein component) can be placed between preload discs and the bypass disc to achieve independent amounts of rebound and compression bypass. The check valve can create more force (make the stock stiffer) overall throughout the shock’s rebound curve, allowing the tire/wheel combo to come back to the terrain more gently and slower after compression. It also drops the “knee” (the point where force begins to taper off in the curve), to a lower force at a lower piston-speed. In our tests, the check valve added more rebound damping. However, its main purpose is to create separation of bleed between rebound and compression.
This is the baseline test valve build chart. The 12 ID number indicates the inside diameter of the discs and piston in millimeters to match the 12mm OD valving post on the piston rod. Numbers such as 30/50 indicate the outside diameter and thickness of the discs in millimeters. For example, 30mm OD by 0.50mm thick or 46.8mm OD by 0.35mm thick. The brakes are 2.5mm and 3mm thick, so they don’t bend but support the deflected discs that do bend. The bypass discs are available in a variety of thicknesses and combinations of slot numbers and widths. The A values indicate the bypass areas in square millimeters. More bypass area allows more oil flow at low stroke velocities, before the deflected discs start to bend. The thickness combinations of the deflected discs on each side of the piston create damping at higher stroke velocities when the deflected discs do bend. The preload discs vary the amount of preload on the deflected disc stack, which varies their resistance to initial deflection.
For our first changeup to the baseline, we replaced the 12 ID, 32/40 compression preload disc closest to the piston with a 12 ID, 32/20 preload disc, and added a 12 ID 31.5/20 bypass check spring and 22 ID, 40.5/50 bypass check plate between the 12 ID, 32/20 compression preload disc and the piston. This setup provided a good deal more rebound piston speed but didn’t alter the compression speed.
For test Number Two, we replaced the 12 ID 30/50 rebound-side support disc with a 12 ID 28/50 support disc in the test number one setup. That didn’t alter the “knee” of the rebound performance curve, but it did taper off the rebound force performance curve to a point about half way between the baseline and the first changeup test. Again, the compression performance was virtually unchanged.
Working with Bilstein personnel Shane Casad, Dennis Baker, and Juan Alvarez, we started with a baseline valve disc setup much like you would get right out of the box, and then created three different valve disc combinations to achieve different compression and rebound curves. Check out the photos to see some of the basic steps in disassembling the shock for a valve change and putting it all back together again, as well as exactly what changes we made to the shock valving and how it affected compression and rebound performance.
Playing around with the size and number of discs, their locations, and inserting a check valve and its spring can easily change things up in a shock absorber like the Bilstein 8125, and you can (with the proper tools and equipment) do it all yourself. We suggest you make small changes at first and write down (or even photograph your valve disc stacks spread out in order) so you can keep track of what you’re doing. Most importantly, find a driving course that features the type of terrain you will encounter most often (one that is unchanging and repeatable) and use that as your test track. That way you can get a feel for how your valve changes are affecting your shock performance in a variety of driving conditions.
Test number three was performed with the 12 ID 32/20 compression preload disc removed from the test number two setup. That made the rebound performance a little quicker over test two, but significantly softened the compression performance in comparison to all three previous test runs on the shock dyno.
Some drastic changes were made to the third test valve configuration for our fourth and final test. The 12 ID, 32/20 compression preload disc was put back in, and all five of the compression-side deflected disc were changed. The fourth test run was done with a set of 12 ID, 24/50; 12 ID, 30/50; 12 ID 36/50; 12 ID, 42/50; and 12 ID, 46.8/50 (smaller to larger progressing toward the piston) compression-side deflected discs. As could be expected, that change altered the rebound curve nominally but stiffened up the shock’s compression performance over test three, but kept it softer than tests two, one, or the baseline.
Once all your valving changes have been made and the shock has all been reassembled, the reservoir must be recharged with pressurized nitrogen prior to use. Bilstein recommends that the 8125 be filled to a range of between 180 and 200 psi for proper performance.