“Make the suspension adjustable and they will adjust it wrong” - Colin Chapman
Damping adjustment is one of the most widely misunderstood topics in racing. It is about fine-tuning how long it takes for the car to settle through a corner, not just stiffness.
Unfortunately, many cheap coilovers only offer one damping adjustment setting that controls compression and rebound simultaneously. This type of adjustment puts the driver at a disadvantage because it does not allow compression or rebound to be dialed in correctly, always compromising in some way. This is why at least 2-way adjustable dampers are ideal. Even single adjustable compression OR rebound adjustment would be preferred because that allows at least one of these aspects to be dialed in correctly.
To begin adjustment of your dampers, it is best to start with both settings fully soft. From there, you can begin with compression adjustment. Upon entering the track, shift your focus to how the car feels over bumps. From there, you can begin to increase compression slowly (¼ turn at a time or so) on all four corners until the car begins to feel harsh over bumps. When this happens, dial back the compression slightly, and your compression tuning is complete.
Now that compression has been set, rebound adjustment can begin. Rebound should be adjusted according to how the car feels upon corner entry, that is, how quickly the vehicle rolls onto one side. If your rebound is too soft, you will feel the car roll onto one side relatively quickly, potentially upsetting the car. If this occurs, increase rebound on all four corners the same way by roughly a ¼ turn. Repeat this process until the car is smooth upon corner entry. Further increasing rebound after you have achieved this may cause excessive oversteer or understeer.
It is important to note that compression and rebound can be used to adjust understeer and oversteer on corner entry and exit, this post is geared towards initial vehicle setup.