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 the orifice area. The tapered needle extends through the top of the shaft to an external adjuster.
A remote reservoir (also called a canister) allows the damper designer to add another orifice or two to the oil flow path. Only the oil displaced by the shaft moves through these additional orifices. In this case, a large shaft diameter is a good thing. By using one-way check valves, it is possible to design a damper with externally adjustable shim stack preload and externally adjustable fixed orifices for both bump and rebound, thus producing a four way externally adjustable damper. Most modern racing dampers are two or three way externally adjustable.
Older Koni racing dampers have external adjustments for bump and rebound shim stack preload. Quantum dampers have two holes in the shaft with one way check valves and two needles to produce individual adjustments for bump and rebound. A third adjuster moves the entire needle/seat assembly relative to the shaft in order to change the rebound shim stack preload. A simpler adjustment offered by Fox and Penske consists of a drum in the canister with several holes of progressively larger diameter drilled in the OD of the drum. Rotating the drum aligns a different size hole with the flow path. Penske 8700 dampers also have an additional bump shim stack with adjustable preload in the canister.
Oil flow case one: fixed orifice flow
If the damper piston moves slowly, the pressure differential across the piston is not large enough to force the shims away from the piston face. So, the only flow path is through the fixed orifice. Basic fluid mechanics tells us that drag is proportional to velocity squared. In this case, the force versus velocity relationship of the damper is parabolic: doubling the shaft speed quadruples the damping force. As long as the piston speed is low enough that the shims do not open, the shape of the force versus velocity curve is a parabola that opens upward. Reducing the area of the orifice by moving the tapered needle farther into the orifice or by rotating the adjuster drum to a smaller orifice increases the damping force for a given shaft speed. That is the only way to adjust low speed damping other than by changing the gas pressure.
Low speed and high speed damping refer to the speed of the damper shaft relative to the damper housing, not to car speed. Low speed damping adjustments affect dynamic weight transfer and the motion of the sprung mass relative to a smooth track surface. High speed damping adjustments affect the motion of the unsprung mass (wheels and tires) relative to a bumpy track surface. We are usually much more interested in low speed damping adjustments than high speed.
Oil flow case two: variable orifice flow
When the piston moves rapidly enough to lift the shim stack off of the piston face, an additional flow path is created. Increasing the piston speed forces the shims farther away from the piston, thus increasing the orifice area. So, the shim stack is a variable orifice. Because the orifice size changes with damping force, the damping force is no longer proportional to velocity squared. The shims all have different diameters, so in side view the shim stack looks like a multiple leaf spring. Typically, the diameters of the shims are chosen to produce a linear force versus velocity characteristic. Changing to thicker shims increases the slope of the force versus velocity line. However, this requires disassembling the damper. Changing the preload on the shim stack (which can be done with an external adjustment) changes the offset of the force versus velocity line.
The piston faces are machined with a slight cone angle ranging from 0.5 to 2.5 degrees. So, the shim stack is deflected when it is forced toward the piston. Adjusting the clamping load on the shim stack changes the preload, and thus the offset of the force versus velocity line. More preload equals more damping force, but only at high shaft speed. Changing shim stack preload has no effect on low speed damping other than changing the force at which the shim stack opens. Most racing dampers open the shim stack at 2 to 5 in/sec shaft speed depending on the preload adjustment.
Oil flow case three: Mixed flow
Obviously, when the shim stack opens, oil is still flowing through the fixed orifice also. Therefore, changing the fixed orifice area affects damping through the entire shaft speed range, but primarily at low speed. On the other hand, changing shim stack preload only affects high speed damping. Most damper manufacturers refer to fixed orifice adjustments as low speed adjustments. Conversely, shim stack preload adjustments are referred to as high speed adjustments.
Now for the fun part: Gas pressure
As you go whizzing merrily around the race track, the damper piston is continuously moving back and forth rapidly through the same small volume of oil. This oil becomes aerated when the piston moves in one direction. This same volume of aerated oil is forced through the piston orifices when the piston moves the other way, causing more aeration. Rebound travel increases the pressure on the shaft side of the piston, increasing the rate of absorption of gas bubbles. Bump travel decreases the pressure on the shaft side, decreasing the rate of absorption.
Assuming equal shaft speed and damping force in bump and rebound, slightly more aeration is produced from bump travel than from rebound travel. Because the gas reservoir chamber is on the opposite side of the piston from the shaft, the pressure and the rate of absorption is relatively constant on that side. Additional orifices in the canister complicate this situation further. Nevertheless, the percentage of aeration stabilizes quickly.
Increasing the gas reservoir pressure has a more significant effect on the shaft side of the piston because the rate of gas bubble reabsorption is generally lower on the shaft side, so the percentage of gas bubbles is generally higher on the shaft side. Because the working volume of oil is less aerated during bump travel, more damping force is produced in rebound. The effect of gas pressure is less pronounced in bump travel because the rate of gas bubble reabsorption is generally higher on the canister side of the piston. This is why increasing the gas reservoir pressure increases bump damping some and increases rebound damping more.
When examining shock dyno results, many people incorrectly conclude that increasing the gas pressure increases bump damping and decreases rebound damping. The error in this conclusion is that the static shaft extension force also increases with gas pressure, thus reducing the force applied to the shock dyno load cell during rebound travel. The measured force has an offset due to the extension force produced by gas pressure times shaft area. This force is almost constant and has no effect on damping.
The fluid that you rely on to control the motion of the car is really a mixture of hydraulic oil and gas bubbles. As the bumps get worse, the shaft speed and therefore the rate of aeration increase, reducing the damping forces. This is only a real problem if you run on superspeedways or if you run very low gas pressure (less than 50 psi or so).
When shaft speed or oscillation frequency becomes very high or gas pressure is very low, the percentage of gas bubbles in the working fluid becomes high enough that the damper starts acting like a spring in addition to a damper. Koni recently released graphs of shock dyno results from a new damper design. These graphs showed force versus shaft position for three different peak shaft speeds. At low shaft speed, the graph was symmetric about the vertical axis. As the shaft speed increased, the graph distorted diagonally towards a line with positive slope. A force versus deflection graph for a spring also has a positive slope. These results showed that the damper was speed sensitive, producing a spring at continuously high shaft speed. The presence of compressible gas bubbles in the working oil volume is the most logical explaination to produce those results.
If the canister has orifices of any sort in it, the length of flexible hose connecting it to the damper housing should be as short as possible. The hydraulic flexibility of the hose produces a spring between the two stages of damping orifices, adding to the confusion. For the last few years, the dampers that Penske made for their own Indycars had a canister bolted directly to the damper housing with no flex hose.
All of the damping energy that is removed from the suspension system is converted to heat in the hydraulic oil. Air cooling is required to prevent excessive temperature buildup which causes the oil viscosity to decrease. Canisters significantly increase the surface area available to dissipate that heat. However, if the engine exhaust system is close to the dampers, or if the dampers are buried somewhere with no cooling air flow, one can expect the handling of the car to change after a few minutes on the track.
Keeping the suspension springs cool is also a good idea. The elastic modulus of spring steel decreases 2.2 percent per 100 degrees Fahrenheit temperature rise. The spring rates and ride height decrease right along with the elastic modulus.
Of course, none of this describes the "ideal" adjustment settings to use for any particular car. I have only attempted to describe what happens when the damper adjusters are adjusted.Text article by Neil "Fireball" Roberts, an Indycar engineer.
Neil Roberts (email@example.com)