Digressive dampers: four numbers and a knee

One damping coefficient can't serve both body control and ride, because those happen at different velocities. A digressive damper solves it by changing slope partway up — and the place it changes is the tuning knob that matters.

Last chapter ended on a conflict: body control wants ζ around 0.6, ride wants ζ around 0.25, and one damper has to do both.

The resolution is that those two jobs happen at different shaft velocities, and a damper is free to have a different coefficient at each.

Two speeds, two jobs

Watch what the shock is actually doing:

• Body motions — roll on turn-in, pitch under braking, the car settling after a crest — move the damper slowly. Typically under 0.1 m/s. These are the motions you want firmly controlled.
• Road inputs — a sharp-edged expansion joint, a pothole lip, a stone — move it fast. 0.3 to 2 m/s and beyond. These you want to let through, because resisting them just transmits the impact to the body and unloads the tire.

So the recipe writes itself: firm at low velocity, soft at high velocity. A damper whose curve flattens as velocity rises is called digressive, and it’s the standard answer on nearly everything performance-oriented.

The opposite — progressive, steepening with velocity — exists mainly by accident in cheap dampers, and it’s the worst of both worlds: floaty body, harsh over bumps.

The four numbers and the knee

The app models the curve as two straight lines meeting at a knee, which is how most real dampers are described and close enough to how they behave:

Force is continuous across the knee — it’s the slope that changes. And because compression and rebound get their own pair, you get six numbers in total: Slow Comp, Fast Comp, Comp Knee, Slow Reb, Fast Reb, Reb Knee.

The preset’s front compression side is slow 8000, fast 700, knee 0.1 m/s. Here’s what that does:

Below the knee the two are identical. Above it they diverge violently: at a realistic pothole velocity the linear damper is transmitting nearly six times the force. That difference is the ride quality of the car.

The knee is the real tuning knob

Everyone fixates on the coefficients. The knee position is what actually decides the character, because it sets which motions count as “body” and which count as “road.”

• Knee too high (say 0.3 m/s): road inputs are still on the steep part of the curve, so the car is harsh over sharp bumps even though its body control feels right.
• Knee too low (say 0.03 m/s): body motions spill onto the soft part, so the car feels floaty and vague in roll despite a big slow-speed number on paper.

The preset’s 0.1 m/s is a conventional starting point and a sensible one. Move it before you touch the coefficients — it’s a smaller change with a bigger effect on how the car feels.

One caveat worth knowing: this two-line model is an idealization. Real dampers have a rounded transition rather than a corner, and their curves shift with temperature and with gas pressure. The model is right about the shape and about what each parameter does; treat the exact knee as a design intent, not a measurement.

Go look at it

Open the strut layout on the Springs/Shocks tab.

1. Find the six damper fields: Slow Comp, Fast Comp, Comp Knee, Slow Reb, Fast Reb, Reb Knee.
2. First, fix the over-damping from chapter 14 so you’re not tuning on top of a fault: Slow Comp 2600, Slow Reb 4800. Check ζ lands near 0.40.
3. Now destroy the digression. Set Fast Comp equal to Slow Comp (2600) and Fast Reb equal to Slow Reb. You now have a linear damper. Run ▶ Play over Bumpy Back Road and note RMS Accel (g) and the tire-load trace.
4. Restore digression: Fast Comp 700, Fast Reb 1400. Run the identical road. Body acceleration drops and the tire load trace gets less spiky — the wheel is being allowed to move.
5. Now the knee alone. Leave every coefficient where it is and move Comp Knee from 0.1 to 0.3, then to 0.03. Same coefficients, same road, noticeably different car. That’s the knee doing the work.

What it costs you

Digression is close to free in ride terms, which is why it’s near-universal — but it isn’t free everywhere.

The cost lands in high-velocity control. A very soft fast-damping setting means large, fast inputs are barely resisted, so the suspension can use up its travel quickly and arrive at the bump stops. On a road car with 142 mm of travel that’s an occasional thump. On a long-travel car it’s the difference between absorbing a landing and bottoming through the chassis.

There’s also a diagnosis trap: a car that bottoms out is usually treated as needing stiffer springs, when the actual problem is often too-soft fast damping letting the wheel run out of travel. The spring holds the car up; the fast damping controls how quickly it gets used. Fixing the wrong one makes the ride worse without solving anything — which is chapter 16.

Rules of thumb

• Slow:fast ratio: 2:1 to 4:1 on a road car. The preset’s 11:1 on compression is aggressive even by race standards.
• Knee: 0.05–0.15 m/s for most cars. Lower for very soft suspension, higher for stiff, low-travel setups.
• Rebound knee often sits slightly higher than compression, because rebound velocities run higher when the wheel drops into a hollow.
• Rebound:compression 1.5–2.5:1 at slow speed, as chapter 14 said. At fast speed the ratio usually narrows.
• Rally-raid and desert: this is where the whole idea gets taken furthest. Bypass shocks add position-sensitive damping on top of velocity-sensitive: external tubes let oil past the piston over part of the stroke, so the damper is soft in the middle of its travel and progressively firmer near the ends. Combined with two dampers per corner (chapter 10), a desert truck effectively has a damping curve in two dimensions — velocity and position — which is the only way to be simultaneously supple over washboard and survivable on landings.

Try this

1. Find the knee position that gives the lowest RMS Accel (g) on Bumpy Back Road. Is it also the one you’d choose watching the body trace?
2. Set slow:fast to exactly 1:1 (linear) and then tune only the single coefficient for the best compromise. How close can you get to the digressive result? What did you have to give up?
3. On Rally Gravel, does a higher or lower knee help? Does that match what the chapter predicted for large, fast inputs?

Next: the spring you didn’t know you had, sitting at the end of the travel, doing more work than you think.

Preset spring and damper values are illustrative starting points, not a measured setup for any particular car.