How To Tune Your Own Shocks ?

Shock absorbers have existed in some form since the early 1900s, but drivers from that era would barely recognize the advanced damping technology found in today’s street and off-road vehicles. On the surface, a shock absorber seems like a simple mechanical device, yet it can be surprisingly complex depending on its design — and is often misunderstood.

In this guide, we’ll start by breaking down the fundamentals, myths, and realities of shock absorbers. Then we’ll explore the theory behind shock valving and tuning, and share a few real-world examples with dynamometer data. By the end, you’ll have a much clearer picture of how shocks work — and how to tune a rebuildable unit to match your vehicle’s needs.

It’s important to understand that springs and shocks serve very different purposes. Springs don’t control or damp the motion of your vehicle, axles, tires, or wheels — they simply support the weight of the vehicle on those components. A shock absorber’s job is to dampen the spring’s oscillation (its repetitive up-and-down movement).

Here’s how it works: when your Jeep (or any vehicle) hits a bump or uneven terrain, the wheel and tire are pushed upward, compressing the spring. Once the spring reaches its peak compression, it pushes back down on the suspension, helping the wheel stay in contact with the surface. Without a shock absorber, however, the spring, tire, wheel, and even the vehicle would continue bouncing uncontrollably until that energy was naturally dissipated.

If you’ve ever driven a vehicle with worn-out shocks, you know the result — a constant, uncontrolled bounce that compromises handling, comfort, and safety. That’s why proper damping isn’t just about ride quality; it’s about maintaining stability, traction, and control.

Each valving disc combination was tested on Bilstein’s in-house shock absorber dynamometer, a specialized tool that measures how shocks perform under controlled conditions. The resulting graphs clearly illustrated how every adjustment to the valving setup influenced the compression and rebound characteristics of the shock.

By studying these graphs, engineers could see in real time how even minor changes to disc size, thickness, or stacking order affected the damper’s behavior — allowing for precise tuning to match specific driving conditions, vehicle weight, or performance goals.

The new Bilstein 8125 is a coilover remote-reservoir shock built specifically for custom applications. But the insights gained from working with the 8125 go far beyond this single model — the same principles of setup, tuning, and performance optimization apply to virtually any rebuildable shock absorber, regardless of brand or design.

Before you begin working on any rebuildable remote-reservoir shock absorber — like the Bilstein 8125, which ships from the factory charged with 200 psi of nitrogen — the first and most important step is to safely relieve the nitrogen pressure. You can do this by venting the gas through the Schrader valve located on the reservoir end cap.

Today’s shock absorbers come in many designs, but nearly all trace their roots back to two fundamental types: monotube and twin-tube.

Monotube shocks feature a working piston attached to a piston rod inside a single tube partially filled with hydraulic oil. A free-floating dividing piston separates this oil chamber from a high-pressure nitrogen chamber, which prevents cavitation (air mixing with oil to form foam) that can severely reduce damping performance.

Twin-tube shocks, on the other hand, use an inner oil-filled cylinder containing the piston and a base valve at the bottom, all housed within an outer shell. This shell contains additional oil and low-pressure nitrogen, which also helps reduce cavitation. Twin-tubes have some advantages — including greater piston travel in a more compact size and improved cooling thanks to the extra oil volume. However, that outer shell can also trap heat, and heat buildup is the number-one enemy of any shock absorber.

Beyond these basics, there are many advanced designs — such as internal bypass, multi-stage adjustable, remote reservoir, and coilover shocks — all of which can influence how a shock performs.

For this example, we’re focusing on the Bilstein 8125, a rebuildable monotube remote-reservoir shock. Its external reservoir not only helps manage heat but also houses the dividing piston, allowing the 8125 to achieve longer suspension travel than a non-reservoir shock of the same size.

Shock absorbers also differ in their piston design, with the two primary styles being digressive and linear:

A digressive piston, like the one used in the 8125, features fluted ports that start near the center on one side and extend toward the outer edge on the other.

Because the 8125’s piston is symmetrical, it can be installed in either orientation on the piston rod. This design provides precise damping control and improved ride quality without introducing harshness, making it ideal for performance-oriented setups.

Once you’ve safely relieved the nitrogen pressure from the reservoir, you can begin disassembling the shock absorber. Start by removing the rod guide screws and sliding the wiper cap upward along the piston rod. Next, gently depress the rod guide just enough to expose the internal snap ring seated inside the shock body. With the snap ring accessible, carefully remove it from its groove.

At this point, the piston rod assembly can be slowly and carefully lifted out of the shock body. It’s important to move deliberately during this step to avoid damaging the piston, seals, or inner walls of the shock, as these components are precision-engineered and critical to the shock’s performance.

With the piston rod assembly securely clamped in a vise — making sure to use a soft protective layer (like a towel or soft jaws) between the piston eyelet and the vise to avoid damage — you can proceed to remove the rod nut. Once the nut is off, the valving components (such as shims, spacers, and the piston itself) can be carefully disassembled and inspected or replaced as needed.

When reassembling the shock after any valving adjustments or changes, it’s crucial to tighten the rod nut to the manufacturer’s specified torque setting. This ensures proper preload on the valve stack, prevents loosening under load, and maintains the shock’s performance and reliability.

Whenever you’re working with a shock absorber’s valve discs, it’s essential to lay out the compression stack, piston, and rebound stack in the exact order they were removed. This simple step helps ensure everything goes back together correctly. If the components are reassembled out of order, the shock’s performance can suffer — sometimes dramatically.

One of the biggest advantages of the Bilstein 8125 is that it’s easy to disassemble, allowing you to fine-tune the piston’s valving. These valve discs — often called shims or plates — are the heart of any high-performance shock. They control how much oil flows through the piston and how quickly it moves, which directly affects damping performance. By adjusting these discs, you can fine-tune the shock’s compression and rebound behavior separately to achieve the ideal ride quality for your driving style and terrain.

Using the 8125 as an example, here’s the baseline valve stack we started with, in order:

• Valving nut

• Rebound brake washer

• Five rebound discs

• Rebound-side bypass disc

• Two preload discs

• Digressive piston

• Two preload discs (compression side)

• Compression-side bypass disc

• Six compression discs

• Compression brake washer

(See the baseline valve disc chart below for specific disc sizes used in this setup.)

These discs are stacked in layers over the piston’s oil ports to control oil flow, which determines the compression and rebound rates. Both the diameter and thickness of each disc affect how much oil can pass through. Larger-diameter discs restrict flow more because less oil can move around their edges, while thicker discs are stiffer and flex less under pressure.

For example, two discs with the same diameter but different thicknesses will behave very differently. A thicker disc will allow less oil to pass through, creating firmer damping — whether on the compression or rebound side — while a thinner disc will flex more and provide a softer, more compliant response.

In short, valve disc tuning is where the real customization happens, and understanding how each disc’s size and stiffness affects oil flow is key to dialing in your shock for peak performance.

Bypass discs come in many different thicknesses and with various slot numbers and widths, and they can be used on both the rebound and compression sides of a shock piston. Their main purpose is to fine-tune the shock’s response to small bumps and surface irregularities, such as embedded rocks or rough terrain, helping to soften the ride without sacrificing overall control.

Each bypass disc has an “A” value, which represents the total bypass area in square millimeters. This number is important because it tells you how much oil can flow through the disc at low piston speeds, before the main valve discs start to flex. A larger bypass area allows more oil to pass more easily during low-speed movement, resulting in a smoother, more compliant ride over small obstacles. Conversely, a smaller bypass area restricts flow and provides a firmer, more controlled response.

In short, adjusting bypass discs is one of the most effective ways to tune a shock absorber’s low-speed damping, giving you more comfort and better traction when driving over uneven terrain.

The bypass spring and check plate — a patented Bilstein feature — can be installed between the preload discs and the bypass disc to further refine the shock’s behavior. When used together, they increase the overall damping force throughout the rebound curve, effectively making the shock stiffer during rebound.

This added resistance helps the tire and wheel return to the ground more gradually and with greater control after compression. Instead of snapping back quickly, the suspension settles more smoothly, improving traction, ride comfort, and stability — especially on rough or uneven terrain.

In short, adding a bypass spring and check plate is an excellent way to fine-tune rebound performance and achieve a more controlled, predictable suspension response.

This was our baseline test valve configuration. From left (compression side) to right (rebound side) of the piston, the components were arranged as follows: a brake washer, a support disc, five deflected discs, a bypass disc, two compression preload discs, a bypass check spring, a bypass check plate, the piston itself, two rebound preload discs, another bypass disc, four deflected discs, a support disc, and finally, another brake washer.

Each of these valve discs plays a unique role in controlling the flow of shock oil as it passes through the piston:

• Brake discs – These thick, washer-like components (mounted with their rounded edge facing away from the piston) act as a resistance layer. Their primary job is to limit how much the compression or rebound discs can flex during piston movement. This resistance is especially valuable during high-speed piston events, such as when a tire strikes a large bump or when a vehicle lands hard after becoming airborne. By restricting flex, brake discs help prevent excessive piston stroke and improve control in extreme conditions.

• Bypass discs (bleed discs) – Positioned on one or both sides of the piston, bypass discs have openings around their edges that allow shock oil to flow more freely than it would through standard discs of the same size. Installing preload discs between the bypass disc and piston lets you preload the deflected discs to varying degrees. Additionally, bypass discs with larger or more numerous openings increase oil flow even further, enhancing the shock’s ability to respond quickly to low-speed piston events. This is particularly useful when tuning suspension for terrain irregularities like embedded rocks, as it helps soften the ride and improve comfort.

• Check valve disc and check spring – A patented Bilstein feature, these components can be installed between the preload discs and bypass disc to enable independent bypass control for rebound and compression. Adding a check valve increases overall damping force throughout the rebound curve, allowing the tire and wheel to return to the ground more gradually after compression. It also lowers the “knee” point (where damping force starts to taper off) to a lower force at a lower piston speed.
  In our testing, the check valve provided greater rebound damping, but its main function is to separate the bleed characteristics between rebound and compression for more precise shock tuning.

This is the baseline test valve build chart. The “12 ID” designation refers to the inside diameter of the discs and piston — 12 millimeters — which matches the 12 mm outside diameter valving post on the piston rod. Numbers like 30/50 represent the outside diameter and thickness of a disc in millimeters. For example, a disc labeled 30/50 is 30 mm in OD and 0.50 mm thick, while one marked 46.8/35 measures 46.8 mm OD and 0.35 mm thick.

The brake discs in the stack are 2.5 mm and 3 mm thick. Because they are so rigid, they do not flex under pressure. Instead, their role is to support the deflected discs, which are designed to bend during piston movement.

Bypass discs come in a range of thicknesses and with various slot numbers and widths. Their “A” value represents the bypass area in square millimeters. A larger bypass area allows more oil to flow through at low piston speeds, before the deflected discs begin to bend.

The thickness and stacking order of the deflected discs on each side of the piston determine the damping characteristics at higher stroke velocities, when these discs start to flex. Meanwhile, the preload discs adjust the initial resistance of the deflected stack by varying the preload pressure, thereby controlling how easily the discs begin to bend when the shock is first compressed or extended.

For our first modification to the baseline setup, we replaced the 12 ID, 32/40 compression preload disc closest to the piston with a 12 ID, 32/20 preload disc. We also added a 12 ID, 31.5/20 bypass check spring and a 22 ID, 40.5/50 bypass check plate between the new 12 ID, 32/20 compression preload disc and the piston.

This revised configuration delivered a significant increase in rebound piston speed, while leaving compression speed essentially unchanged.

For Test Number Two, we modified the Test One setup by replacing the 12 ID, 30/50 rebound-side support disc with a 12 ID, 28/50 support disc. This adjustment did not change the “knee” of the rebound performance curve, but it smoothed and tapered the rebound force curve to a point roughly halfway between the baseline and the first test. As before, compression performance remained virtually unchanged.

Working alongside Bilstein engineers Shane Casad, Dennis Baker, and Juan Alvarez, we began with a baseline valve disc configuration similar to what you would find straight out of the box. From there, we developed three unique valve disc combinations to produce different compression and rebound curves. (Refer to the accompanying photos for a closer look at the disassembly process, the valve changes, and how each modification affected performance.)

Experimenting with disc size, quantity, placement, and the addition of a check valve and spring can dramatically alter the performance characteristics of a shock absorber like the Bilstein 8125. With the proper tools and equipment, these changes can be performed at home.

We strongly recommend making small, incremental adjustments and carefully documenting each change—either by taking notes or photographing the valve disc stack in order—so you can track your tuning process. Most importantly, choose a consistent test course that replicates the terrain you drive on most frequently. Testing on a repeatable surface will help you accurately evaluate how each valving change impacts shock performance across different driving conditions.

Test Number Three was carried out by removing the 12 ID, 32/20 compression preload disc from the Test Two configuration. This adjustment resulted in a slightly quicker rebound response compared to Test Two. However, it also significantly softened the compression damping, producing noticeably lower compression forces than those recorded in all three previous test runs on the shock dynamometer.

Test Number Four involved some of the most significant changes in our entire tuning series. The 12 ID, 32/20 compression preload disc was reinstalled, and all five compression-side deflected discs were replaced. The new setup featured the following configuration (arranged from smallest to largest, progressing toward the piston):

• 12 ID, 24/50

• 12 ID, 30/50

• 12 ID, 36/50

• 12 ID, 42/50

• 12 ID, 46.8/50

As expected, these adjustments only slightly altered the rebound performance curve but notably increased compression damping compared to Test Three. However, the overall compression remained softer than the results from Test Two, Test One, and the baseline configuration.

Once all the valving changes are complete and the shock absorber has been fully reassembled, the reservoir must be recharged with pressurized nitrogen before the shock can be used. For optimal performance, Bilstein recommends charging the 8125 to a pressure between 180 and 200 psi. Proper nitrogen pressure ensures consistent damping characteristics, prevents cavitation, and allows the shock to perform as intended under demanding driving conditions.

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