Digressive shock valving is a valving style where damping force climbs steeply at low shaft speeds, then flattens out once the shock passes a point called the knee. The result is a shock that gives the car firm support during slow chassis motions like braking, turn-in, and roll, while staying softer than a linear setup when the wheel hits a curb or a sharp bump. The shape comes from a preloaded shim stack sitting on a specially machined piston, and Penske builds that piston into its line of digressive and linear racing shock pistons.
Penske has already covered the full family of curve shapes in linear, progressive, regressive, and digressive shock valving. This post goes further into digressive valving specifically: how the knee is built, which tuning changes move it, when digressive valving is the right choice for your car, and how to confirm the build on a shock dyno.
Digressive shock valving produces a damper curve with two distinct slopes. Below the knee, force rises quickly with shaft velocity, and above the knee, force keeps rising at a much shallower rate.
A damper curve is a plot of shock force against shaft velocity, and it is the main way racers describe how a shock behaves. On a linear curve, force rises at a fairly steady rate, so stiffening the low-speed end also stiffens the high-speed end. Digressive valving separates the two ends. You can run a lot of low-speed damping, which governs slow shaft movement from weight transfer and body roll, without carrying that same stiffness into high-speed damping, which governs fast shaft movement from bumps and curbs.
The knee is the transition between the two slopes. Where it sits on the velocity axis and how much force it carries are the two numbers that define most of how a digressive shock feels to the driver.
A digressive piston creates the knee by preloading the main shim stack against a raised sealing surface on the piston face. Oil cannot open the stack until pressure overcomes that preload, so force builds quickly at low speed and then levels off once the stack lifts.
Before the stack opens, almost all of the oil flow goes through the bleed, which is a small bypass path around the shim stack through an orifice, a bleed shim, or an adjuster. Flow through a small orifice builds pressure quickly, which is why the low-speed slope is steep. Once pressure reaches the preload, the stack lifts off the sealing lip, the flow area opens up, and the curve bends over into its flatter high-speed slope.
Penske's own description of its VDP design notes that the main shim stack sealing on the digressive lip is responsible for the low-speed knee portion of the curve. Every Penske piston face is dished to preload the shims flat against it. On the Piston, Digressive Linear, Penske offers Digressive / 1° Linear, Digressive / 2° Linear, and Digressive / Digressive face orientations in 45mm and 55mm bore sizes, so you can choose which direction of travel gets the digressive shape.
Three variables control a digressive curve: preload sets the force at the knee, bleed sets the low-speed slope and where the knee lands on the velocity axis, and the shim stack sets the slope after the knee. Change one and the others shift with it, so it pays to make one change at a time and check it on the dyno.
More preload on the main stack raises the force the shock must reach before the stack opens. That moves the knee higher on the force axis and gives the driver more support through slow chassis motions. Less preload lowers the knee, which softens the shock through braking and turn-in.
More bleed lets more oil bypass the stack at low speed, so force builds more slowly and the knee arrives at a higher shaft velocity. Less bleed makes the low-speed slope steeper and pulls the knee toward zero. Penske sells bleed shims for adding bleed without drilling the piston, and the free Bleed Shim Equivalence Chart translates bleed shims to equivalent orifice sizes.
Once the stack is open, its stiffness determines how quickly force continues to rise. A soft stack after the knee produces the flat top that gives digressive valving its curb compliance. Penske's individual shims are sold by part number for building and revising stacks.
Racers usually ask next what happens to rebound when they change compression on a digressive piston. On a piston with separate compression and rebound faces, each stack is built independently, so a change to the compression stack leaves the rebound stack untouched. Shared bleed paths are the exception. A bleed hole or adjuster that passes oil in both directions will change both sides of the curve, which is one reason to dyno both directions after any bleed change.
Digressive valving is the right choice when the car needs strong low-speed support for platform and driver confidence but also has to ride curbs, seams, or sharp bumps without upsetting the chassis. It suits most modern road racing, asphalt oval, and aero-sensitive cars, and Penske's earlier post notes it has also been adopted on dirt cars, motorcycles, and bicycles.
The practical test is the conflict you face with a linear piston. If adding low-speed damping to fix a lazy turn-in or a soft braking platform makes the car skate over curbs or bumps, digressive valving gives you a way out of that trade. If the car already rides bumps well and only needs more support, digressive valving lets you add that support without paying for it at high speed.
Low spring rates are another strong case. Running softer springs can add mechanical grip, but the chassis will move more under weight transfer. A digressive curve lets the shock carry more of the job of controlling slow body motion, so the car keeps its platform on a softer spring. Club racers in classes sanctioned by the Sports Car Club of America often face this trade on cars with limited spring and aero options.
There are cases where a more linear or progressive curve makes sense. On very rough surfaces where the tire needs compliance at low shaft speeds, a steep low-speed slope can reduce grip, and off-road applications often favor progressive valving for big hits. For the engineering background on why damping and spring choices interact this way, Race Car Vehicle Dynamics from SAE International remains a standard reference.
Penske offers several pistons that produce digressive curves, and they differ mainly in how much control you have after the knee. The table below compares them using details from each product page.
|
Penske piston |
Curve shape |
What it lets you tune |
|---|---|---|
|
Digressive on one face, linear or digressive on the other depending on orientation |
Maximum flow with a digressive face for a flat digressive curve |
|
|
Digressive compression and digressive rebound |
A true digressive stack in both directions |
|
|
Digressive knee with an adjustable linear slope after it |
Low-speed knee and high-speed slope tuned separately |
|
|
Linear, digressive, or VDP-style curves |
The widest range of curve shapes from one piston |
The VDP Linear exists because a flat digressive top can feel like a loss of support when a loaded car hits a bump mid-corner. Its inner stack adds flow and force at higher velocity, so you can tune the slope after the knee instead of living with a flat line. For cars that need even less high-speed force, the Assembly, Regressive Valve takes the idea further.
Yes. Penske shocks use standard Penske internals, so a shock built with a linear piston can usually be revalved with a digressive piston of the matching bore size. The pistons above are offered in 45mm and 55mm bores, and the 8760 Series Triple Adjustable is built in both body sizes.
Adjusters matter here too. On the 8760, the high-speed compression adjuster acts on preloaded shims on the piston face, while rebound and low-speed compression adjust through a needle and seat bleed bypass. That means the external adjusters act on the same features that shape a digressive curve: the bleed adjusters influence where the knee lands, and the high-speed adjuster works on the preloaded stack. The 8300 Double Adjustable Piggy Back or Remote Canister Coil-Over Shock and the 7500 Series Double Adjustable Coil-Over Shock are also revalvable with different pistons and stacks.
Ride height stays where it was after a valving change. Damping only acts while the shaft is moving, so static ride height is set by springs and spring collars, and a digressive build changes how the car moves around that position.
You verify a digressive build by running the shock on a shock dyno and checking the knee location, the knee force, and the slope on both sides of it against your target curve. A shock dyno strokes the shock at controlled velocities and records the force, so the knee shows up clearly as the point where the curve bends.
Compare the low-speed slope and knee before and after every bleed or preload change, and check both compression and rebound. Watch for hysteresis, the gap between the force a shock makes as velocity rises and the force it makes at the same velocity as it falls, because a large gap near the knee can blur the transition the driver feels. Penske explains the testing process in what a shock dyno is and how it works, and its post on predicting a damper curve before you build the shock covers how to set expectations before the first test.
Penske's dyno products and services cover S-Link shock dynos and outside testing. For overlaying and comparing curves from build to build, PASS shock dyno analysis software organizes the data your dyno captures so knee changes are easy to see.
The knee is the point where a digressive curve changes from a steep low-speed slope to a flatter high-speed slope. It marks the velocity where shim stack preload is overcome and the main stack starts to open. Its position and force level define most of how the shock feels to the driver.
Digressive valving works on compression, rebound, or both. Penske's Piston, Digressive Linear comes in orientations that put the digressive face on one side or both, and the Piston, Double Digressive produces digressive compression and rebound together.
Digressive valving feels firmer through slow chassis motions and softer over sharp bumps than a linear setup with the same low-speed force. How harsh the car feels depends on where the knee sits and how much low-speed force you run. Too much preload or too little bleed can make small bumps feel abrupt.
On a digressive curve, force keeps rising after the knee at a reduced rate. On a regressive curve, force drops after the peak, so the shock is softer at high velocity than at the knee. Regressive valving is a further step for cars that need to attack curbs aggressively.
Start from a known baseline, change one variable at a time, and dyno the shock after each change. The free Damper Tuning Guide gives you a structured starting point, and a data-driven suspension baseline keeps your changes tied to results on track.
Digressive valving gives you low-speed support and high-speed compliance in the same shock, but the result depends on where you put the knee and how you shape the slopes on both sides of it. That makes it a process of building, testing, and confirming changes against how the car behaves on track.
If you want help choosing a piston or building a digressive baseline for your car, Penske's S3 program puts you in direct contact with the team for shock selection, setup, and ongoing support. If your shocks need a revalve or a rebuild before you make the switch, start a Penske shock service request and the team will build and dyno them to your target curve.