The development of externally adjustable dampers, or shock absorbers, is one of the hottest areas of race car chassis engineering. The adjustments available from these dampers provide a quick, simple method of changing the behavior of the car in transitions and over rough pavement. Adjustable dampers can also provide the combination of a smooth ride for daily driving and firmer damping for competition driving simply by rotating an adjuster. There are two common types of adjustments available. This article will focus on the physics behind the results produced from each type of adjustment.
We will now delve into the deepest, darkest depths of the damper (alliteration accidental). We will start off with a basic description of the guts of modern racing dampers, then discuss the types of adjustments available from them and the results of using those adjustments.
I will assume you have not seen a racing damper disassembled. As with many other things, it's what is inside that counts. There is a piston attached to the end of the shaft inside the damper. The chamber that the piston moves in is filled with (almost) incompressible hydraulic oil. The viscosity of the oil causes resistance to oil passage through small orifices. This resistance produces a pressure differential across the piston when the piston moves, thus producing a damping force.
Near the end of the housing opposite the shaft (or in the canister if there is one) is a sliding piston that separates the oil from a high- pressure gas, usually nitrogen. Some volume of gas is necessary because as the shaft moves into the housing, the volume of oil displaced by the shaft must go somewhere. The oil displaced by the shaft moves the gas/oil separator piston and compresses the gas slightly. The static spring rate of a pressurised damper is almost zero, but the static extension force can be rather large. The gas, and therefore the hydraulic oil, is pressurised in order to reduce the severity of aeration.
Aeration is the formation of gas bubbles in the oil due to very rapid pressure loss immediately after the oil passes through an orifice at high velocity. Increasing the oil pressure increases the rate of reabsorption of the gas bubbles. Because these gas bubbles are compressible, the characteristics of the damper change unless the bubbles are reabsorbed into the fluid. Aeration is not necessarily a bad thing because we can use its special characteristics to our advantage. By the way, aeration happens even in 450psi pressurised dampers. That is why changing gas pressure changes damping characteristics. The location of the gas volume is significant because as piston speed in the bump direction increases, the pressure on the shaft side of the piston decreases. Rebound travel increases the pressure on the shaft side of the piston. This changes the rate of aeration recovery. The pressure on the canister side of the piston is almost constant unless there are additional orifices in the canister.
Movement of the shaft forces hydraulic oil through various orifices in the piston, shaft, and canister. One type of orifice is a deflected shim stack. There is a stack of 4 or 5 thin steel shims covering holes on both the top and bottom of the piston. The holes connect with slots on the top and bottom faces of the piston. Oil can pass around one stack of shims, through the slots and holes in the piston, and through the narrow slot (orifice) that opens up when enough pressure differential is applied to the shim stack to force it away from the top or bottom face of the piston. The slots on the piston faces are positioned so that oil passes through different holes in bump than in rebound.
The other type of orifice is a small hole that bypasses the fluid path through the shim stack. This hole may be in the piston or in the sides and end of the shaft. If there is a hole in the shaft, a tapered needle can be mounted in the hole to vary