Tech Briefs
Passive, reactive suspension systems

Figure 1. One version of Tenneco Automotive's Kinetic suspension system. Figure 2. Ride and handling parameters of a vehicle. Figure 3. According to Tenneco, a Kinetic-equipped two-wheel-drive vehicle can outperform a conventionally equipped four-wheel-drive.

Kinetic suspension system technology re- fers to a range of passive, reactive vehicle suspension systems from Tenneco Automotive that the company says offer improved comfort and handling. All of the systems have some form of passive interconnection, which facilitates load sharing between wheels and de-couples many design and operating parameters and modes, such as cross-axle articulation and single-wheel stiffness, from roll control.

Conventional or passive suspension systems are designed as a compromise between ride comfort and handling performance. Ride is primarily associated with the ability of a suspension system to accommodate vertical inputs. Handling and attitude control relate more to horizontal forces acting through the center of gravity and ground-level moments acting through the wheels. A low bounce frequency for maximum ride comfort normally leads to a low pitch frequency.

Active systems provide independent treatment of road-induced forces from body-inertia forces through active control of some of the suspension system functions. Theoretically this means that the compromise in conventional suspension systems can be eliminated. Active suspension systems, however, usually involve a continuous power requirement, fast-acting devices, complex control algorithms, and closed-loop control systems. The cost of these systems has limited their application on mass-produced vehicles.

Semi-active or adaptive systems are terms usually used to describe suspension systems that have some form of intelligence in the suspension dampers. Typically the damping curves can be altered such that the wheel control over the range of inputs is maximized. These systems also require fast-acting devices and complex control algorithms.

Reactive, passive suspension systems are driven by load inputs from the wheels and are regulated from within the suspension system without the aid of active intervention. An early example is the Citroen 2CV, introduced in 1948. It had the front and back suspension interconnected via two longitudinal mechanical support springs. Citroen also invented the hydropneumatic system in the late 1950s. In it, all four wheels are connected to one high-pressure pump, so there is no requirement for individual dampers. At rest, the car sits low to the ground. When the engine is started fluid is pumped into all suspension points. This raises the vehicle to a driveable height. The system has been further improved to set the car to a level that the driver desires. It will also set the height according to road conditions. Also, the Morris 1100, launched in 1962, used Moulton's Hydrolastic suspension, which had a fluid interconnection between the front and back suspension. The interconnection between front and back systems reduced the pitching moment induced by front and back bumps.

Fundamentally, all suspension systems provide support springs, roll control, and damping. All Kinetic systems currently use some form of hydraulic damper (telescopic or inline). For support springs, Kinetic systems can contain conventional mechanical springs such as leaf, coil, or torsion; hydropneumatic springs that contain nitrogen over oil; air springs; or a combination of any two types above. For roll control, Kinetic systems can contain conventional torsion-type roll control springs with the front and rear torsion springs interconnected by hydraulic cylinders, hydropneumatic springs, or a combination of the two (Figure 1).

Figure 4. Articulation wheel loads: A convention system vs. a Kinetic system.

System stiffnesses are used to describe the handling performance of a vehicle (Figure 2). Roll stiffness is the stiffness of the suspension supporting the sprung mass when the wheels on one side of a vehicle have an increase in load and the wheels on the other side have a decrease in load. Pitch stiffness is the stiffness of the suspension supporting the sprung mass when the wheels on the front of a vehicle have an increase in load and the wheels on the back have a decrease in load. During cornering (vehicle roll) the front wheels and the back wheels experience a separate moment caused by the respective vertical force couple at the wheels. The reaction to these two moments defines the roll stiffness of the front wheels, the back wheels, and the whole vehicle.

Figure 5. A Jeep Grand Cherokee with a Kinetic system has a free articulation limit of 550 mm (22 in) and an ultimate articulation limit of 775 mm (31 in), compared to the standard vehicle's ultimate limit of 475 mm (19 in).

In the past, manufacturers have increased the size of the stabilizer bars to reduce the risk of rollover in vehicles, though the larger bars brought about a stiff uncomfortable ride. The Kinetic technology cuts and reconnects the bars through hydraulic cylinders. The left and right sides of the stabilizer bar rotate relative to one another. Then the front and rear cylinders are connected, allowing fluid to move back and forth in response to wheel motion. This shift redistributes road forces through the entire suspension system to absorb shock and equalize load on the wheels (Figure 3).

The results shown in Figure 4 were produced by measuring the four vertical wheel loads while raising the front right wheel at low velocity, effectively producing static results. The difference between the front and rear load changes is due to the test producing a small roll moment as well as pure cross-axle articulation. As the wheel movement in the conventional vehicle is increased, the support springs deflect and the roll stabilizer bars wind up, and the wheel loads change almost linearly. When the load on the back right wheel reaches zero the wheel lifts from the ground, leaving the vehicle balanced on three points. When the vehicle fitted with the Kinetic system articulates, the support and roll stabilization systems are initially not subject to any wind up and the wheel loads remain almost constant, providing free articulation (Figure 5). This is maintained until the relevant bump stop is compressed, at which point the wheel loads change fairly rapidly. Nearly all the rear suspension travel is used before the rear wheel lifts off the ground.

Kinetic also addresses concerns about rollover accidents, which Tenneco quotes as causing more than 165,000 deaths or injuries per year in the United States, with the death rate in SUV rollovers having doubled since 1988. When a vehicle is cornering or involved in an avoidance maneuver, Kinetic responds instantly to reduce the risk of rollover. Fluid in the hydraulic line stops flowing, causing the stabilizer bars to reconnect for optimum stability.

Jean L. Broge