Can I start with basics? Please don't let me insult anyone here, it's hard to judge at what level the question is asked, surely we can go into this deeper..
The goal is equalized maximum traction on all 4 tires in all situations. The amount of force of the tire contacting the pavement contributes to traction, contact patch area is traction. Let's ignore suspension geometry (camber, toeing, etc..) and talk about going up and down.
Shocks, well.. at work they are referred to as dampers, they are specifically tuned to spring rate, vehicle weight and dynamics. Regarding adjustment, it would be ride height if it's merely cranking the threaded spring nut, yet to tune a damper, the race style dampers can tune the speed of compression (how fast the damper will compress) and the speed of rebound (how fast the damper can extend) via knobs, these race type dampers usually include a resevoir which it sounds like you are referring to. Basically the knobs control the orifice that the fluid is travelling through. A small hole takes longer for fluid to pass through which slows the speed of extending or compressing, this is why there are 2 knobs, controlling the fluid travel during compression or rebound. The company I work at makes the most advanced dampers for high end race cars, and they are very, very expensive. I cannot quote prices, corporate or proprietary info, etc.. respecting confidentiality.
It was said that a 30% change in spring rate will require damper tuning. The damper will have to come apart for internal tuning adjustment if it doesn't have external / manual tuning capabilites (knobs). Adding a stiffer spring will affect vehicle dynamics. Dynamics include braking, such as the front of the car diving, accelleration, cornering and road contact over bumps, etc..
Typically a stiff spring is used on race cars, this accomodates the added downforce by wings at high speed, as well as less body roll around corners. Generally a race car is very stiff to drive at slow speed, the static weight is less than the mass at high speed due to the downforce of the the wings (providing more traction & stability). A street car is soft in handling for comfort, yet there is the ACR which has a higher spring rate. OEMs tune dampers so the car will dive, giving the driver the sensation that the car is braking, as well as body roll around corners so they feel the car is leaning, they do engineer cars so the normal driver (i.e. my mom) feels the car is reacting to their control, otherwise a unskilled driver doesn't know if they are going too fast around a conrner if it doesn't lean and end up sliding the vehicle. Let's ignore that design intention for the time being..
The compression and rebound knob adjustment is needed maintain a consistent tire patch contact / traction to the pavement due to variable car or course conditions. As I mentioned, the amount of tire to pavement contact area equals traction. Tires are literally balloons. Press down on a balloon and the contact patch area on the pavement increases.
If a damper / shock didn't exist, the car would bounce like a buggy on the springs and the tire contact patch / traction would be less and more as the vehicle mass goes up and down due to intertia. If the car axle was rigid mounted to the car, it would be a stiffer ride and over bumps the tire would lose contact to the ground because the vehicle cannot travel up and down maintaining contact on uneven surfaces at high speed. Therefore it's needed that the tire can maintain maximum contact in all aforementioned dynamic conditions.
If you had weak springs on your car.. the damper would have to accomodate quick travel in rebound (extending) to maintain the tire contact patch. If the damper did not extend quick (extending very slowly) and you drove over multiple washboard type bumps, the car would continually drop in ride height until you were on a flat smooth surface and the damper would finally relieve it's fluid transfer and get a chance to extend the springs back to ride height. In an opposite situation, if you had very stiff springs and dampers extended very quickly, the car would act like a buggy because the damper isn't slowing down the travel and the springs would bounce the car up, reducing the tire contact patch / traction. So the damper must be tuned to the spring rate (and all of the dynamic conditions).
Due to the various pavement conditions, corners, braking (diving) and accelleration (more traction on the rear tires), dampers need to be adjustable to maintain a maximum tire contact patch.
If simply you wanted to drag race, the goal is maximum traction on the rear tires. You would want the front dampers to extend quick and want the rear dampers to compress quick to atain maximum weight transfer to the rear tires. Yet on a car going around a race track there are many more variables, cornering, braking, etc..
Sway bars support the tire contact patch during cornering, so when the car leans the sway (anti roll) bar helps keep some weight on the inside tire and less body roll. Yet those who simply change to a thicker sway bar do not realize they are messing around in a territory of vehicle dynamics, including dive not considering front to rear distribution.
Lot's of babbling eh... I hope it helps. If not, no worries..
Best regards,
Mike
