The Activa suspension—used only on some Xantia models—creates mixed feelings. Drivers requiring sporty handling and roadholding praise it because this car turns into curves without turning a hair: it stays completely horizontal and neutral. However, this comes at the expense of ride comfort.
The Activa system operates in two distinct steps. The first one is controlled mechanically by a roll corrector (the component is identical to the height correctors used in the suspension).
The corrector is connected by an L-shaped spring to the bottom wishbone. When the car takes a sharp left turn, its front left wheel will be forced down by the body roll caused by centrifugal force. As the wheel moves down, so does the end of its wishbone, pulling the linkage to the corrector. The piston inside the roll corrector moves upwards, opening the pressure feed into the stabilizing cylinders. These two cylinders are attached to the wheel suspension differently: in the front, the piston pushes the left wheel upwards while in the rear, the right wheel will be forced downwards. This diagonal correction counteracts the roll of the body.
Turning to the other side result in an inverse operation: the roll corrector opens the connection from the stabilizing cylinders back to the reservoir. The front left wheel moves downwards, the rear right one upwards, once again countering the effect of body roll.
An additional Activa sphere in the front acts as an extra accumulator but the rear sphere can be connected or decoupled electrically. Depending on the position of the piston inside the electro-valve, the high pressure feed is either allowed to reach the piston 2 inside the control block, pushing it up and connecting the sphere 1 to the rest of the circuit (dashed line on the illustration), or the residual pressure in the sphere moves the piston 2 down, isolating the sphere 1.
When the Activa sphere is open to the rest of the system, roll correction is applied through a spring element formed by the accumulator and the Activa sphere. The supply side of the stabilizing cylinder pistons have half the area of the other side, connected to the Activa sphere 1 with the valve 2 open. Changes in the length of the linkage is therefore not transmitted directly to the roll bar. Upon the influence of external forces like body roll, the movement of the piston compresses the gas content in one sphere and at the same time, expands it in the other.
The stabilizing cylinder works as a spring with asymmetrical characteristics: its effective hardness is smaller around the corrected position, but it hardens progressively as the piston is forced out of that position.
The Activa system has two operating modes, depending on the position of the electro-valve 2. In the first mode roll correction is always active because the roll corrector is upset. The resulting flow of fluid will tend to move the active linkage upsetting the balance of presssure in the two extra spheres, and making the coercive force be applied through a spring element which becomes progressively stiffer the more correction is needed.
The ECU controling the electro-valve uses sensors identical to the Hydractive system. The values of vehicle speed, steering wheel rotation angle and speed determine when the second mode of anti-roll behavior has to be enforced. Similary to the operation of the suspension computer, the Activa ECU also uses the driver as the input to determine the motion of the vehicle body: if the roll is caused by the unevenness of the road surface, the steering wheel will not be rotated. In curves, the computer calculates the maximum potential lateral acceleration (vehicle speed is measured by its sensor, the turning radius is communicated by the steering wheel angle sensor, the mass of the car is a known constant—the centrifugal force can be calculated from these values) and decides wether the spring element formed by the two spheres needs to become rigid to make the system compensate for the body roll.
In this mode the Activa sphere is isolated from the rest of the system, the fluid line between the roll corrector and the active linkage is blocked at both ends, making the linkage completely rigid. Even if the roll collector end is open, the linkage remains quite rigid (providing for a very hard spring coupled with high damping); only half of the displacement escapes from the additional accumulator sphere through a restrictive regulator.
The additional damping of the Activa sphere is now switched off, the correction is applied only through the very hard roll-bar. When the possible range of correction is exhausted (strut linkage extends or contracts as far as it can), at about 0.6 g lateral acceleration, only the very hard roll-bar remains functional.
The diagrams showing the kinetic characteristics of an Activa car reveal the details. The first diagram shows the relationship between time and roll angle for a constant lateral acceleration. It can be observed clearly that the Hydractive system can only limit roll damping, not roll angle. Note that the initial slope of both Hydractive curves—the section up to 0.4–0.6 seconds— is practically the same in both soft and hard mode. This slope represents the combined hardness of the roll bar and the associated hydraulic components. Yet, the reaction time is longer in the soft mode (0.8 seconds versus 0.6, indicated by the last bend when the curve turns into a horizontal line). As the corner spheres are isolated and their combined gas volume is less in hard mode, the maximum roll angle stabilizes around 2.5 degrees while in soft mode it reaches 3 degrees.
The second diagram depicts the relation between the lateral acceleration and the roll angle. The hydraulical-mechanical roll bar of the Activa starts the same as the Hydractive system with minimum lateral acceleration. But, while the Hydractive stays almost linear—the sharper you turn, the bigger the body roll angle will be—, the Activa compensates by keeping the body roll angle at a constant below 0.5 degree up to a lateral acceleration of 0.6 g (by providing an effectively infinitely stiff roll bar setup). But even when the limits of the roll bar are reached, having contracted or extended it as far as it can go, the effective roll bar remains quite stiff: the roll angle will increase only moderately, up to a maximum of 1 degree.