|The Evolution of Modern Adjustable Dampers and The Future of Their Application to the Street Performance Market
When a tire encounters an obstacle in its path, the wheel has two options (aside from destruction). It can absorb the obstruction in the spring rate of its sidewall, or it can move up and away from the obstruction. When a wheel is asked to carry the extra force of weight transfer from roll, it must depend on the same geometry and absorb the load in the sidewall or in the compression of the suspension components. Whether the force is generated from the bottom or the top, it is the combination of the spring and shock absorber (damper) that must answer the compression and effect the resultant dissipation of energy. Shocks and springs are a bad compromise solution to the problem. The best solution is an active suspension - a double-action hydraulic ram hooked up to a fast (even by today's standards) computer and very good sensors. But nobody allows such suspensions in competition and it is questionable whether the technology for practical application of such a high maintenance system is even yet available. So we go back, for the moment, to the conventional damper package and start to take a look at the developments that have taken place in the past five years and where the technology - and its wider market applications - are headed.
The purpose of shock absorbers is to increase the road-holding capacity of the wheel. Since maximum grip depends on maximizing the vertical force on the tire (since Ff = Cf x Fv), any suspension movement which removes pressure on the tire when it is at the edge of adhesion (i.e. in a corner) can cause the driver to lose traction, thus control. The motion control of the damper is therefore critical. In fact it is useful, though amusing, to think of dampers as magical devices without which the racecar would float in air. For it is the damper that holds the tire to the road and that assists the suspension in conforming to the road when cornering. In passenger cars, dampers and spring rates are used to produce what the manufacturers call a "comfortable" ride. Any racecar or performance car driver will tell you that the float produced by shock/spring packages that are so soft that the average street driver doesn't notice the difference when his dampers have failed - is far from "comfortable." Shock settings that allow for excessive wheel hop, oversteer and understeer are ,in fact, downright scary. In the case of the racecar or performance sedan, the reaction-curve flattening effect of the damper (especially the high rates possible with today's tires) is what produces the most "comfortable" ride, that is, one that allows for control and quick response to inputs, both driver and otherwise. The damper aids in producing consistent steady-state effects when the driver has to steer through a corner - consistency which is dramatically missing in graphs of steady-state cornering in the passenger car. In wing cars dampers have the added necessity of controlling spring movement to help maintain ride height. Where "the aerodynamic downforce is critically dependent on the pitch attitude relative to the ground and the ride height, particularly in front" the suspension, including the dampers, support the effort to maintain consistent airflow past the underbody. As Milliken notes, "From the standpoint of developing and maintaining the largest amount of aerodynamic downforce it would seem desirable to have the car infinitely stiff in pitch, heave and roll. For a perfectly smooth circuit, this suggests no suspension at all. However, circuits are never ideally smooth and some wheel travel is necessary to realize the highest level of adhesion between the tire and the ground and to damp out the tire spring (which is nearly undamped)." Note: In the case of Grand Prix cars, suspensions have tended for the past few years to become more and more rigid, while tire spring (from the deflection of the sidewall) has come to account for some 60-70% of suspension compliance. So what is to be learned from the development of modern racing dampers? In F1 cars, until very recently, the solution was stiff packages, flat circuits, tires with very high sidewall spring rates and very small suspension movements. In CART cars today, the suspension movement solution according to Ohlins is a very sophisticated, gas-charged, twin-tube with perhaps the largest number of possible external adjustments. Like the Grand Prix teams, however, CART engineers are increasingly moving toward shocks designed for specific applications and away from those that are adjusted at the track. With the introduction of sophisticated four, five, six, seven and now nine-posters as well as hydraulically-driven feedback shock dynos and onboard data logging to run the shock dyno it is possible to profile the needs of an F1 suspension for maximum road-holding in the lab and to match the curves of the bump and rebound needed for any given track ever more accurately on the dyno. In the sportscar racing of the 1960's, '70's and much of the '80's, and in late model racing today, adjusting the shocks meant changing from one set to another with the characteristics one found desirable for a particular track or set of conditions. But for the sportscar racer today, there are several different manufacturers offering adjustable dampers. This paper will examine this area and offer some thoughts about translating the performance to be derived from adjustable dampers to the street. For while there are perhaps a dozen damper manufacturers whose product is currently available, the race market is so small as to have limited the engineering advances in this arena. When manufacturers are driven by the race mentality, which has no bottom-line regard - where the largest manufacturer's revenues are a mere $4 or $5 million per year, and, more importantly, where it takes only a half million dollars and a $20,000 shock dyno to set yourself up in the business of damper design and production, stagnation sets in. Opening the market of performance dampers more widely to consumers would, in this author's opinion, have the double impact of supporting the search for better ways of solving the reaction control problem and help move motorsports away from the nonsensical attitude that shock absorbers are some sort of black art. Those designers who have moved in the direction of external adjustment controls instead of taking the approach that the shock absorber must be taken apart and revalved to produce the desired changes, have opened the more logical approach to street application adjustable shocks. Jan Zuijdijk, Rob DeRijk, and Jeroen van Gool, together and separately, have had a considerable impact on the market. Zuijdijk's pioneering design approach when he was at Koni with Henk Richten is considered by many to be the founding principle of adjustable damper technology. But now, 40 years later, what is the new potential for design and commercial application? Roger Mears is often credited with opening the door to using monotube gas shocks on race circuits in 1983, having applied the benefits he found in off-road Fox Shox to a race at the Atlanta Motor Speedway. Today Penske shocks dominate the race market, thanks to nearly two decades of development at the hands of specialists like Ken Anderson. At the time, the Fox shocks were rightly credited with being an improvement over the Monroe double-tube shocks in general use because the Fox gas-pressurized single-tubes experienced less performance deterioration over the length of the race because they reduced the inherent aeration and cavitation problems of the twin-tube design. Dampers are hydraulic devices that convert the energy of the moving wheel (in bump and rebound) into heat that is dissipated through the hydraulic fluid (the shock oil) and then into the air surrounding the shock tube. Singe-tube shocks have an advantage over double-tubes because they dissipate heat more directly and are less prone to cavitation problems. To put it simply, dampers work as the force on the damper mounting points pushes and pulls the damper shaft and causes a piston to move through the oil in the shock body. Whether the gas in the tube is pressurized or not (pressurizing the oil helps reduce cavitation by controlling the size of the bubbles - and one of the variants manufacturer to manufacturer is the amount of pressure, from 60 psi to over 400 psi for Bilstein), for the piston to move, the oil has to move through the piston. Restriction of the flow of the oil through the piston generates a pressure drop across the piston and thus more pressure on one side than the other, which causes the flow of oil through orifices in the piston. The damping force is measured by the difference in the pounds of pressure times the area of the piston. By controlling the flow of oil, one changes the rate. Allowing the oil to flow beyond the displacement area in the main shock body, to an external canister or reservoir, increases the possible range of adjustments. . Naturally the size of the piston rod has a huge impact on the damping effect, which is why some manufacturers are building rods as large as 25 mm in order to make them big enough to displace enough oil to make the shocks work in the short suspension travel ranges of racing suspensions. But the secret to the flow is the arrangement and composition of washers - the shim stack - that progressively uncover the carefully mapped orifices around the piston. Typically the top shims in the stack are of large enough diameter to touch and deflect the outer edges of shims further down the stack to open a path for the oil at a specific point in the cycle. For example, a 1.60 shim might layer on three 0.95 shims which in turn lie on top of a 1.60, 1.40 and 1.35 series that produce a range of damping force from 10% to 90% as the shaft speed increases. The linearity of the response is determined by the shim stack, with the goal of combining just the right configuration for smooth transitions under pressure. The graphs from the shock dyno map the effects of different shim stack configurations. (And become critical tools in calibrating damper behavior because of the inherent variations in the quality of the shims themselves due to the manufacturing constraints from, both volume and cost as well as unavoidable variation in the material. Because the shims deflect according to the pressure on them, different orifices "open" to the flow of oil through the piston, according to the force placed on the system in bump and rebound. They act as one-way check valves because of the pressure differentials caused by the moving piston. All adjustable shocks allow one to control how large an area the holes provide; thus, as the oil flow remains the same, changing the size of the holes changes the amount of resistance - changing the damping effect. The percentage of the total pressure differential is seen by the actual shim stack. At the same time, the orifice control can have a harsh feel due to the velocity-squared effect of the resistance. To ameliorate this effect, damper designers incorporate a small piston bypass leak hole which limits the steep response of the shock at low speed. The shim stack then determines the shape of the rest of the damping effect. The 0.020 inch to 0.040 inch hole in the piston provides a path for a small amount of oil to bleed by the piston and bypass the shim stack, producing a less dramatic bump at high speed. But some engineers prefer a no-bleed design approach to control the dampers better at the low-speeds encountered in braking and cornering. This is why so much attention is being focused on the range between 0 and 1-inch per second. Penske has done much in the area of field-rebuildable shocks, allowing teams to vary damper performance by changing the shim stacks. This is a viable solution where there is time and the appropriate clean environment for assembly, but is not an option in pit lane. Where more practical tuning is desired - as in the majority of race circuits and for the performance street market, dampers that allow adjustments to be made in the size of the orifices or flow of the fluid, are of interest. Koni, JRZ, ProTrac, Moton and others are following this path. So let us review the progress and extrapolate some wider market applications from these trends. Single adjustable shocks allow external adjustments to move internal parts. Rebound adjustments are made by changing the size of the orifice to produce rebound forces 120%, 150% etc. greater than the zero-setting value. In most hsocks, rebound adjustment has some effect on bump. JRZ's use a combination of a double non-preloaded valving stack and orifice adjustment controlled by a hex nut that reveals holes drilled in the threads. Double adjustables allow one to control high-speed compression by metering the flow of fluid to the external reservoir. JRZ, ProTrac and Moton use a fluted drum controlled by turning a large knob on the top of the reservoir. Adjustments increase or decrease the flow of oil through the ports by rotating different-sized holes into line in the inlet path. The Penske 8500 uses a piston and pre-loaded shim stack in the reservoir. By additionally changing the size of the bypass orifice in the reservoir, one affects low-speed compression. Triple adjustable shocks offer a bypass around the reservoir shim stack, controlled with a needle valve or with preloaded Belleville springs in the case of JRZ (allowing up to 15 adjustment positions). So the adjustments are: single = rebound-only; double = rebound and high speed compression (bump) and triple = rebound plus high and low-speed compression. As we've noted earlier, performance control is increasingly found in the realm of low speed compression (remembering that the significance lies in the small amount of travel in the damper over the available time, which produces a low value - "low speed" -- on the force/velocity continuum). All of the manufacturers seek to produce the smooth effect of the twin-tube gas shock. JRZ, ProTrac and Moton take the approach of opening and closing orifices instead of preloading the shim stacks. Ohlins which has put ten years' research and development into the TT44, uses. according to their material, "a patented concept with a unique double wall design and two adjustable bleed valves to control the flow between these tubes. The valves control the initial compression and rebound damping and are check-valved to be completely independent of each other. They meter the oil flow created by the main piston area, not the flow created by shaft area displacement. This translates into low internal pressure during the compression stroke." Ohlins believe this solution allows for higher damping forces at short strokes because "in practice many strokes never reach a velocity high enough to cause enough pressure drop across the main piston necessary to cause the shims to open." Jim Hamilton made the interesting observation to Paul Haney in an interview in 1993. Referencing the V-squared function of orifice damping he observed that the shims in combination with the holes produce second order differential equations in the interaction of the spring, the mass of the racecar and the damper within the changing viscosity of the oil due to temperature. (The temperature increases with the pressure produced by the damping force, altering the viscosity of the oil.) "Those strange valves called shims," he said "have a profound effect. They basically overwhelm the effect that you usually don't want, which is the V-squared effect
The asymmetrical scaling of the holes has another purpose." It remains true that damper adjustment is an empirically-driven science, hence its classification as a black art. Those seriously committed to understanding and maximizing the effect of damper engineering will do well to remember that shock absorption is not a black art. Repeatability and prediction is the art. As Jan Zuijdjik commented in an article in Racecar magazine, "Sometimes teams report that they have tested different brands of dampers but have not detected any performance differences in the racecar
. This does not indicate that all brands of dampers are the same nor that certain dampers are not up to the task. More often than not, it simply illustrates that the team has been adjusting the damping forces up and down the scale, but that the damping characteristic of a particular set of dampers was unsuitable, with the consequence that the racecar did not respond to the change." Physics is still be physics, but as Jim Hamilton noted the variables in the equation very quickly produce second, third and even fourth order effects. Change the tire temperature on the inside right front tire and you'll see a difference in the output of the left rear damper. Change the driver and the curve which was perfectly balanced before is now an amplified wave that totally upsets the car. So how can one possibly develop dampers for street use - albeit in the performance arena - that the civilian driver will appreciate? The better one understands how adjustable shocks can change the feel of the car to the driver, the better one will appreciate the market application. Take the following graph. If you have a damper producing 250 lbs. of force at 15 inches per second, coupled with 500 lb. springs, you'll experience a noticeable effect on a 3,000 lb. street car. Since performance car drivers tend to prefer the feel of the stiffer springs, an adjustment should be made in the rebound setting, as this is the one that most affects the feel. An increase in low speed damping will make the car feel more responsive without increasing ride harshness over bumpy surfaces. The low-speed adjustment would also allow for softer springs without losing mechanical grip. Adjustable shocks allow the driver to make such real-world damper modifications as: Ø Reducing bump to soften the ride and reduce sideways hop in the corners Ø Softening rebound to overcome jack-down from too-soft springs Ø Increasing bump to eliminate bottoming, nosedive under braking and excessive roll Ø Increasing rebound to tighten up the handling and reduce float and lurching in corners. The real question is not whether there is market interest in these handling modifications, but whether it is possible to reduce the price from $5500 a set. Even a massive education campaign will not overcome the price resistance when street sport setting shocks are readily available from Bilstein for $200 a set. Developing the street market for adjustable shocks produces two very desirable effects. First, the performance car driver gains the benefit of greater control and more effective utilization of his suspension system, including the W-, X- and Z-rated tires that are currently far from their limits. And second, the shock manufacturer earns - aside from acquiring significant depth in his field research data - the resources to push his R&D program, potentially to the escape velocity necessary to free the racing suspension from the spring/damper "solution" which offers such unsatisfactory results
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