Semi-active suspension technology: the inside track
In this article: The secrets behind semi-active suspension technology
The new Citroën C5 Aircross was launched in 2017 with KYB selected as the original equipment supplier for shock absorbers. The initial reviews in the automotive press were highly commendatory and, following a test drive, one particularly enamoured hack gushed: “Even before we’d driven our first mile, it was manifest that the improvements in comfort and overall refinement are little short of astonishing”.
Active suspension is starting to become a reality in passenger cars, and semi-active solutions are conquering more segments. Thanks to a joint development between KYB and PSA, a suspension concept based on passive shock absorbers capable of merging high performance with comparable costs has been developed and applied to the Citroën C5 Aircross. Citroën is calling the system Progressive Hydraulic Cushions.
How the vehicle achieves its ‘flying carpet’ effect
The secret of this revolutionary concept is a double hydraulic stops system. The total stroke of the shock absorbers can be divided into three different parts for which the shock absorber will provide different characteristics. The first part corresponds to the position around the centre of the stroke. In this working area the conventional valving in the piston and the base valve provides the damping forces. The second and third parts correspond to the positions close to the end of the rebound and the compression strokes, with the hydraulic compression and rebound stops responsible for providing additional energy absorption.
This split allows the shock absorbers’ main valves to focus on comfort, and entrusts the hydraulic stops to take responsibility when more demanding situations are encountered. In order to achieve this effectively, both the rebound and compression stops have to be able to provide sufficient energy absorption and to have a very flexible response. The stops provide an unprecedented comfort level and give what Citroën describes as a “flying carpet effect” as the car feels like it’s flying over bumps and holes in the road.
The challenges faced by KYB during the development of this hydraulic stop system were huge. One important point was to keep the main damping law of the shock absorber invariable and this goal was achieved by hydraulic and FEA (finite element analysis) calculations and verified by driving tests. Another key requirement was to design different components with sufficient robustness in order to withstand the high demands of the vehicle even in the worst conditions. In order to achieve the objective, KYB studied different material options and several geometries before reaching the optimal solution. All components had to be built with the highest precision.
The working principle for the rebound stop is based on a reinforced plastic segment that is placed in the inner tube of the shock absorber through a deformation that defines the working area of the hydraulic rebound stop. When the rebound washer stops contracts the segment, a new oil chamber is created, meaning the oil is only capable of getting out of the chamber through the aperture of the segment. This controlled flow generates a hydraulic force that can be tuned with the adjustment of the segment opening. Additionally, the working area of this hydraulic stop can be tuned by changing the inner tube deformation length.
For the hydraulic compression stop a similar principle is used. A new oil chamber is created by the interaction of a polymer component placed in the shock absorber piston and a metallic tube press fitted in the base valve assembly. The polymer part is built with some slots for the oil passage. In order to achieve maximum effort, a pre-compressed additional valve is places in the base valve sub-assembly. The installation of the hydraulic compression stop enables the car manufacturer to simplify other suspension components such as the compression bumper, as well as to redefine some structural parts, because of the lower efforts that will be transmitted to the vehicle chassis.
Just the beginning
KYB was able to develop a system that combines robustness with wide tuning range, which provides the vehicle with a soft damping when comfort is demanded and with excellent handling when control is needed. It is important to highlight that these features are met with a passive system that assures an unbeatable response time and competitive cost. KYB is applying the double hydraulic stop system in other forthcoming vehicles in the European market such as the Citroen C4 Cactus.
This is an edited article from the February 2019 edition of the IMI’s magazine. Need to find out more about suspension system technology? Try this elearning option.