Clever new 'smart' electronic suspension control systems from Cadillac and DaimlerChrysler give great ride and handling using very different approaches. Here's how they compare.
Chassis innovations by Cadillac and DaimlerChrysler are finally winning the war against bumps and jolts. Throughout this century engineers have struggled with one of the most vexing trade-offs in car design-whether to sacrifice a smooth ride for agile, confident handling, or vice versa New electronically controlled suspensions introduced by the two automakers for 2000 are highly ingenious solutions to this age-old riddle. Cadillac's DTS (DeVille touring sedan) and the Mercedes CL coupe plot ride/handling data points in a previously unexplored region of the performance map.
Except for their use of electronics to control chassis variables, the two new approaches couldn't be more different Cadillac's sixth-generation Continuously Variable Road Sensing Suspension (CV-RSS) efficiently resolves some of the handling limitations inherent to all large, front-wheel-drive sedans. Mercedes' Active Body Control (ABC) gives the sporting CL ride poise to match its handling prowess. To compare the benefits offered, let's first examine each system in detail.
Cadillac's CV-RSS
Suspension refinement has been an evolutionary process at GM's prestige division. The progression began with speed-sensitive shock absorbers in 1989, continued with the first Road Sensing Suspension (RSS) in 1993, and advanced to Continuously Variable Road Sensing Suspension (CV-RS) in 1996. The latter system selects from an infinite number of settings over twice the dynamic range provided by RSS.
Though CV-RSS hardware is little changed for the 2000 model year, there are some highly creative software advancements. While investigating new control strategies for the Corvette's Active Handling stability system, Delphi engineers found that anticipation was the key to improved performance. Instead of waiting for the body to pitch or roll and then reacting to the disturbance, Delphi engineers formulated 16 distinct strategies that use an early prompt, such as a steering input, to initiate a damper-setting change. As a result of this approach, seemingly simple shock absorber adjustments can have a profound influence over car dynamics. Four of those strategies informally dubbed the WOLF (Wide Open List of Functions) algorithms - have been implemented in the 2000 Cadillac DTS. Patent applications also have been made for 21 original concepts.
The CV-RSS system's major components are shown in the schematic at right Data is exchanged by means of the DeVille's 10.4-kbytes-per-second bus and by a few dedicated wires that deliver critical information at a higher rate of speed. The damper controller recalculates individual damper settings every millisecond.
On the system's mechanical side are Delphi triple-tube dampers, located at each corner of the car. Response time varies from eight to 30 milliseconds depending on piston velocity. At 65 mph, a damper needs only nine inches of travel to shift from full soft to full firm.
One performance upgrade engineered for the 2000 model year applies not only to the DTS but also to DeVilles and DHS models equipped with the StabiliTrak 2.0 option. Until this year, StabiliTrak intervention was triggered by watching for any inconsistency between steeringwheel position and the vehicle's yaw rate. According to Delphi engineer Scott Badenoch, computing the rate of change of the vehicle's side-slip angle and using that parameter to trigger StabiliTrak is a superior approach. "Now StabiliTrak performance is more responsive and usually transparent to the driver," he says. Sideslip is calculated from data supplied by the sensor array.
The new WOLF algorithms apply only to the DTS model that has CV-RSS as standard equipment. In response to a major steering input, the damper controller now commands revised damper settings to enhance cornering response. The second strategy manages transient body roll after the steering input. In the third algorithm, StabiliTrak activity triggers new damper settings to minimize corner "dip" when just one brake is applied, and to avoid "bobbing up" when that brake is released (see driving impression, p. 14).
The most ambitious strategy of all is damper-setting changes capable of influencing understeer and oversteer characteristics. Delphi engineers call this suspension-based yaw control because it keeps the car responding to the driver's steering commands longer, diminishing the need for StabiliTrak (brake-based yaw control) intervention.
DaimlerChrysler's Active Body Control
Engineers at Daimler-Benz began advanced suspension-system research during the 1970s under a project codenamed Aktakon. Top line production models had used air springs beginning in 1964 and combinations of hydraulics and pneumatics beginning in 1977. But pursuit of what finally became Active Body Control (ABC) was stymied by insufficient computing speed and memory capacity.
Tests with a 1987 prototype demonstrated that a full-active suspension one combining hydraulics, electronics and mechanical elements to handle both lowfrequency body motions and high-frequency suspension movements -- consumed too much energy and rode poorly. Experiments with a Group C endurance race car a few years later proved the merits of a partially active system - one that focused on controlling body motions.
The ABC system now entering production for the 2000 S-Class CL coupe integrates three subsystems: a fairly conventional spring, damper and mechanical locating linkage for each wheel; high-pressure hydraulic equipment, and the electronic sensors and computerized controllers needed to make the mechanical components behave. A LuK radial-piston pump driven in tandem with the power-steering pump supplies 2,900 psi (200 bar) of energy through half-inch-diameter steel lines to hydraulic plungers located at each wheel position (see illustration)
Two Siemens 16-bit, 20MHz microprocessors monitor signals from 13 sensors to calculate hydraulic control commands every 10 milliseconds. Servo valves add or release hydraulic pressure to each plunger according to those commands. For example, adding pressure to outboard plungers and releasing it from inboard units helps keep the body on an even keel during hard cornering. ABC's weight penalty is 93 pounds over a conventional suspension and 77 pounds over the S-Class sedan's Airmatic suspension. Mercedes claims a fueleconomy penalty of only about 0.3 mpg in combined city/highway driving.
ABC operates up to 5Hz. That's more than enough to manage undesirable body motions such as pitch and roll. (Fully active suspensions capable of controlling wheel motion operate at 30Hz or greater.)
Demonstration tests at the automaker's new Papenburg, Germany, proving grounds comparing pre-production prototypes with a 1999 CL reference car showed dramatic benefits. Softer than normal spring rates and passive damping calibrations yield substantially less vertical shake over rough road surfaces. The lack of anti-roll bars on the vehicle allows one wheel to roll through a pot hole with a greater degree of independence.
Body roll in tight bends is reduced by two-thirds and pitch during hard braking is down by some 80%. Stabilization after an accident-avoidance maneuver is one-third quicker, according to factory test data. The driver is offered some control in the system's operation. A switch selects between sport and comfort control algorithms. A second button raises ride height by one or two inches for lowspeed travel over rough terrain. At highway speeds the body is automatically lowered by 0.4-inches in the interests of stability and fuel efficiency.
Mercedes makes no claim that ABC improves common handling indices such as maximum cornering grip. AI's timed runs through slalom and double-lanechange courses confirmed the better manners - less body roll and minimal tail wag -without revealing any measurable increase in speed.
The Conclusion
Cadillac's shrewd-software approach is a highly efficient means of teaching a traditional large, luxury front-drive sedan new tricks. With no apparent sacrifice in ride quality, transient-maneuver stability and agility are greatly improved. Intelligent interaction between major chassis systems - powertrain, brakes, steering and suspension -- allows conventional components to deliver benefits that were once exclusive to exotic, fully-active suspension systems. CV-RSS consumes no extra fuel and has minimal impact on system cost and mass. Look for it on Cadillac's upcoming Sigma-platform rear-drive cars.
DaimlerChrysler's ABC strategy takes a premium-priced, impeccably balanced sport coupe to the next level of sophistication. Major gains in ride quality are achieved with no loss of handling dexterity. Over pavement riddled with bomb-cratersized potholes, the car rides like it's gliding on whipped cream The major downside is system cost that is undoubtedly higher than the Cadillac/Delphi approach.
Whats next? Combining DaimlerChrysler's advanced hardware with Cadillac's inter-system networking. Amo Rohringer, Mercedes' head of spring and shock absorber development, acknowledges that such an approach has merit That suggests we've just taken the first couple of steps toward a prosperous new era of ride and handling advancement.
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