Quote:
Originally Posted by GregW / Oregon
Higher rate springs exert more maximum force on the dampers. In other words, the damper is working harder to control the motion. Whether there is a cause/effect relationship to failures on EDC cars with aftermarket springs I don't know. How the system deals with shorter travel and the higher rates I don't know either. It would be good to get a technical statement from H&R or some such on this subject.
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Actually, the spring does not exert any force on the damper at all. That would happen if the spring and the damper were to be connected in series. In the suspension, they are connected in parallel to the mass of the car. The force is exerted by gravity acting on the mass of the car, and is distributed between the spring and damper.
Meaning,
Fspring + Fdamper = Ftotal.
or,
Fs + Fd = mass of car x acceleration
where,
Fd = - damping coeff x relative velocity of the two connection points of damper
Fs = - spring rate x displacement of spring
If you were to weld the damper rigidly, the spring would not experience any force as it would not experience any displacement. Conversely, in practice, the spring will experience most of the force and compress and support the weight of the car except when the mass starts to oscillate due to suspension inputs and dynamic loading due to acceleration/deceleration. Then the damper ports will move relative to each other and the damper will experience force as well, dissipating energy and damping the oscillations in the process.
That is a mass, spring, damper system in short. For a given value of mass, and predicted inputs to the system, engineers solve the differential equation relating those 3 variables and inputs, and pick spring rate and damping coeff values to control for the displacement of the mass. Although it is true that EDC can vary the damping coefficient, it can do so within a certain range, which is dictated by the physical characteristics of the damper. If one of the system variables changes, say the spring rate, the damping coefficient will need to change to achieve the same desired mass displacement, or for EDC, vary within a different range than stock. A couple of things can go wrong:
1. The new damping coeff range might not be physically attainable depending on what the damper is capable of producing.
2. The new damping coeff range is physically attainable, but within a range that will produce higher forces on the damper than anticipated on average, resulting in premature wear.