Finally, we are getting on the subject of damping adjustment, many people don’t realize that adjusting the damping isn’t making the suspension itself softer or harder. This is not how damping works. Damping is something that delays weight transfer and or slows down the suspension’s movement, this can be perceived as stiffness or softness but it doesn’t actually change the spring rate of the suspension or the suspension’s steady state characteristics. Most decent single adjustable shocks have the adjustment affect the rebound damping. Lower-quality shocks might have the overall stiffness affected in unpredictable ways or even have fake knobs that don’t do anything! So beware of the super cheap Ebay coil overs! The most important idea behind adjusting shocks is you want to minimize tire shock. Tires are like an undamped spring, think of a bouncing ball. You want to minimize the shock to the tires so they don’t oscillate and unweight the tire patch losing grip. In performance suspension tuning this is the #1 job of the shock.
When we talk about damping, we generally classify shock damping as low speed and high speed. Low-speed damping is around 0-2 inches per second of shock shaft velocity. Mid-speed is about 2-5 inches per second and high-speed is anything higher than this. As far as shock tuning, low-speed damping mostly affects stuff like body roll, nose dive, and squat, things the driver really feels. Mid-speed is what a lot of people perceive as a floaty or boat-like feeling. For rebound adjustment, high-speed adjustment affects how the car recovers from large or sharp movements like bounce. In this above force over velocity graph, you can see different damping traces with more or less low-speed damping via adjustment, the black line is soft on low-speed rebound, the yellow trace is about the middle of this particular shocks adjustment and the blue trace is fully stiff with the most low-speed rebound. You can see how the adjustments blend into the mid-speed range and how the softest setting affects the high speed slightly. You can see that this shock has the adjustment mostly affecting the low-speed part of the damping curve.
You can also see how the damping curve gets more digressive as the shock is adjusted stiffer. Digressive means that low-speed damping increases more quickly with velocity. Linear is that the damping force increases in a straight line with velocity and regressive means that the damping force decreases with an increase in velocity. This particular shock starts off with a pretty linear curve and ends up being pretty digressive.
Modern shocks tend to be on the digressive side, at least for performance-oriented cars. This means the suspension is relatively freer to move at high shaft speeds for good bump responses but have good control forces at lower shaft velocities to keep roll, dive, and squat in check. Digressive curves started to be used in the 90’s.
Here are the damping curves of the KW V2, a typical high-quality single adjustable shock. The blue line is fully soft, the green line, is midway through the adjustment, and the red line is fully stiff. Notice that the adjustment affects mostly the low-speed damping with some blending into the mid-speed range. Also, notice that this shock has digressive compression and rebound damping.