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From the engine to the wheels


A suspension’s task is everything but easy: connecting tires and body combining comfort, driving precision and durability is no joke. Great news, though, loom on the horizon as far as comfort and efficiency are concerned

Nicodemo Angì

suspensions and tires spend an inordinate amount of time coping with hazardous situations, yet this perfectly coordinated “couple” allow us to travel in complete safety. Let's take a look at what we can expect in the near future from the evolution of suspensions.

At least a couple of technological developments are emerging: one, so to speak, more "mechanical", while the other relates to a massive use of electronic systems.

We’ll start from the first, as we are already quite familiar with it: behind it we find one of the most committed brands in terms of comfort, Citroën. Many will remember the legendary DS and its hydro-pneumatic suspensions: already used in 1954 on the Traction Avant; these reached their maximum expression with the DS and its unrivalled driving comfort.



Suspension spheres, divided in half by an elastic membrane, were the heart of the system: the upper portion of the sphere contained nitrogen gas which served as an elastic element replacing the classic coil spring. The lower part was filled with hydraulic oil and a piston connected to one of the suspension arms acted on it. Under pressure special valves would regulate the flow when the suspensions were shaken, thus implementing its shock absorbing functions. The system, in addition to giving the "flying carpet" effect, was also self-levelling (the DS could be driven even after completely removing one of the rear wheels) and the vehicle’s height from the ground was fully adjustable. This technique found its application in many other Citroëns: Xantia and XM, for example, which also had an active anti-roll system, and the CX or the more compact GS; the last one was the C5.

An hydraulic system is also found at the heart of Citroën’s new Advanced Comfort system, a "package" that includes special seats and a structural body bonding which uses adhesive in addition to spot welds to join panels. But we are interested in the suspensions themselves, whose great improvement is in the suspension strut: the rest of the system, in fact, is rather conventional with a classic coil spring as an elastic element.



The solution is actually much simpler, as hydraulic sphere and complicated hydraulic circuits have been done away with, and fits like a glove to a SUV like the C4 Cactus, the car used as a first contact with the press.

The heart of the innovation is the Progressive Hydraulic Cushions, inserted inside the hydraulic shock absorber: oil flows through a series of flow holes so that the piston connected to the stem can follow the movement of the wheel. The movement of the stem towards the end stops progressively closing the flow holes and so the oil flows with increasing difficulty, gradually slowing down the movement of the stem itself. At this stage a system of valves kicks in during the return phase, so as to rebound quickly to a normal position.

The diagram illustrates how the Progressive Hydraulic Cushions work: the horizontal axis is the movement of the stem and the vertical axis represents the hydraulic damping force. Near the "zero point" damping is practically non-existent and the suspension can best cope with the smaller bumps but approaching the end of stroke, whether extending or compressing, the damping force grows as the displacement increases (the red areas of the displacement / force diagram). The vertical section at the end of the graph represents the instant in which the movement stops and the shock absorber rebound back to normal position. This system prevents the suspension from "slamming" against the bump-stops making the ride, according to the journalists who have tried it, comparable to that of top-end cars. This innovation will hit the market in 2018 and is expected to equip several models, given the low cost.



On the other hand, electric suspensions are definitely more expensive but linked to today's electronic and connected world, in the sense that one of the elements, the damper, is an electrically managed device. An attempt in this sense was already made by Bose in the mid-80s: the McPherson strut suspension of a Lexus-laboratory vehicle, contained a linear electric motor that replaced the spring (its mechanism is similar to that of a speaker, so it is no coincidence that it was developed by Bose).

The project was technically a success but the system proved to be heavy, expensive, “power-hungry” and would have required car manufacturers to re-design their vehicles.

The idea is still worthwhile pursuing, as by varying the electrical parameters it is possible to adjust suspension damping as desired, also in view of and a commercial follow-up with Bose Ride, an active suspension system for the truck driver's seats.

Additionally, the electric units used, either linear or rotating, are also able to gather energy, a great advantage in the hybrid and full-electric era. In fact, they are reversible and can convert mechanical energy into electricity.

The Audi eROT system uses suspension jolts to move small alternators thanks to small gears able to turn movement into rotation. Audi claims that this system is very reactive (to vary the damping it is enough to vary the load as "seen" by the alternators) and takes up less space than a classic shock absorber. Furthermore, as it is able to recover energy (if the road is uneven this can reduce CO2 emissions by 5 g/km) the system proves to be perfect to work on future 48-volt mild-hybrid vehicles.



Modern cars, so full of sensors, offer a further evolutionary step that could soon take us from active to predictive suspensions. The former act "in retrospect": the system scans the suspension movements 100 times per second and adjusts the electronically controlled dampers accordingly. Predictive suspensions, instead, use cameras normally used in the increasingly popular drive-assist systems, such as automatic braking and Lane Assist, to detect the condition of the road surface and adjust the suspension settings in advance, thus making the life of the tires easier.

At the moment these systems are reserved for high-end vehicles: the new Audi A8 will have the predictive eRot system while Mercedes’ Magic Body Control has more conventional approach to active suspensions.

However, connectivity offers other groundbreaking developments: communication between vehicles and infrastructures will provide real time information on road conditions in a cloud and this will not only send a signal to the suspensions of other cars but may even go as far as informing the authorities for quick, red-tape permitting, road works.


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