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The design of an automobile suspension is dictated by factors as varied as the size or class of the vehicle (determining, in particular, the space available for installing the suspension), its market positioning (comfort, sporty, all-terrain vehicle, etc.), and the manufacturer's preference (for example, the transmission of power to the front wheels, rear wheels, or four wheels).
This article is a bibliographical review which will present the main known automotive suspension systems in the world and the recent technological improvements of its systems in order to obtain safety, comfort, and satisfactory handling for the driver.
The Main Types of Automotive Suspensions
In this section, we present an inventory of the most used suspension architectures. There are several types of suspension systems. They differ depending on whether they are on the front or rear trains of the vehicle. All of these systems can be divided into two groups: rigid-axle suspensions and independent-axle suspensions. Generally, the front axle is equipped with independent suspensions while the rigid-axle type is used on the rear wheels.
1. MacPherson-Type Suspensions
The system was invented in 1955 and was used in the first Ford Consul. It equips the front and rear drivetrains. It is an independent suspension, consisting of a single suspension arm (spring and damper), an anti-roll bar, and a lower arm. The junction between suspended and unsprung masses is represented out by a ball joint on the wheel side and a chassis-side axle (to allow different suspension settings). The spring-shock absorber system (also known as the strut) is rigidly attached to the rocket carrier. This strut also acts as the pivot of the steering.
The damper then performs two missions: damping the relative movements between the wheel and the body and participating in the kinematics of the wheel plane by means of the anti-roll bar and the lower arm. The damper rod is larger in diameter (25mm) than the shock absorber (14 mm) used in other types of suspensions.
Pseudo MacPherson Suspension
The difference between a "true" MacPherson and a "pseudo" MacPherson is the addition of a lower wishbone to the MacPherson strut (vertical damper combination). Thus, the function of guiding the wheel plane is no longer done via the lower arm and the anti-roll bar but by the lower triangle. The advantage of pseudo-MacPherson is that it allows the carmaker to omit the anti-roll bar since this has no function in guidance. This reduces production costs and axle mass. For light vehicles, the stiffness of the springs may be sufficient to ensure stability of the body in a roll. If an anti-roll bar exists, then the latter is fixed on rods, which are themselves fixed on the strut under the spring.
MacPherson Suspension With Independent Pivot
A pseudo-MacPherson with an independent pivot has the advantage of decoupling the camber angle settings and the pivot axis position. This type of suspension is mainly used in sports vehicles. The downside is higher cost than a typical pseudo-MacPherson.
2. Adaptive Suspension
With an adaptive suspension system, air suspension ensures a constant level to both axles, regardless of load. Four height sensors ensure a continuous identification of the distance between the axles and the body. If the control unit notes a change in the specified distance, it activates a compressor that balances the pressure in the spring elements using solenoid valves. Within a few fractions of a second, the specified distance is found. By preventing the sagging of the rear of the vehicle under the weight of a load, the adaptive suspension increases safety and comfort. Speed-sensitive level adjustments also help to save fuel while improving handling and performance. The stability in cornering is also improved by differential locking in the axle springs.
3. Active Suspension
Used primarily on high-end vehicles (Audi Q7, Mercedes Airmatic), and formerly by Citroën, an active suspension allows the suspension characteristics to be adjusted by supplying energy through an oil pump or air compressor. This energy supply makes it possible, for example, to maintain the body horizontally, regardless of the loading of the vehicle or its phase of acceleration, braking, or turning, and to modulate the height of the vehicle. It is, therefore, an adaptive suspension with broad possibilities.
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The latest evolution in this type of suspension is to anticipate the irregularities in the road and adapt the characteristics of the suspension to them, as in Mercedes's Magic Body Control. The German manufacturer has coupled the suspension to a camera that reads the profile of the road. Up to a speed of 80 mph, the vehicle is able to anticipate the asperities of the road to adapt accordingly to the properties of Airmatic air suspension.
4. Multi-Link Suspension
The multi-link suspension is generally made up of five arms to independently block each degree of freedom of the wheel. Only the vertical displacement is controlled by the damper. The development of this suspension is relatively complex: the unsprung masses are relatively large, and the bulk is quite large.
Indeed, multi-link suspensions are generally only used in both front and rear in sedans. Only a few models of compact cars have a multi-link suspension and that only in the rear (Volkswagen Golf with more than 120 horsepower, Ford Focus).
5. Double Wishbone Suspension
The double wishbone suspension is commonly used in competition, on supercars (Lamborghini Aventador, McLaren 650S), or off-road (Range Rover, Toyota Land Cruiser). It consists of an upper wishbone and a lower one (with two attachment points on the chassis side and a hub-side attachment point) to guide the wheel.
With the double wishbone suspension, the wheel pivot is driven by two triangular arms and a coupling rod. Vertical stability is ensured by the strut that rests on the lower triangle. The advantage of this type of train is its low height and its wide loading width, which allow the adoption of suspensions favoring "sporty" behavior. This construction has a beneficial effect on the 4MOTION drive, the transmission of significant driving forces, and road grip. The double triangulation suspension is very compact, and with a suitable coupling bar, it avoids excessive load transfers. Its variants include the extremely compact double-wishbone longitudinal suspension and the trapezoid-arm rear suspension.
The dual triangle design provides superior ride comfort, dynamics and handling. However, its manufacturing cost is high. This is why it is generally only standard on top-of-the-range models such as Mercedes E-Class and S, BMW 5 and 7 series, and Audi A4.
The primary purpose of automotive suspensions is to alleviate disturbances caused by road imperfections, and, ideally, to eliminate excess vibration of the vehicle and passengers in order to ensure better comfort and well-being for passengers. Passengers may feel discomfort at being subjected to acceleration. For example, at certain frequencies, the vibrations of certain organs of the human body can cause discomfort. Moreover, long-term exposure to vibrations increases the sensitivity of the human body. In this context, in order to quantify comfort, standards define the tolerance thresholds of the human body for vibration as a function of the amplitude, frequency, and duration of disturbances.
Basically, handling is the ability of the vehicle to maintain its traction on a controlled path in spite of acceleration and braking forces, and centrifugal force, regardless of the type of road surface and the loads carried.
Since tires are the only link between the vehicle and the road, adhesion starts with them, and therefore their design and condition. But the ability of the suspension to keep the tires in contact with the ground also matters. In general, the tighter and more flexible the tread, the better the grip of the tire. However, the flexibility of the rubber used must also be limited, so as to ensure a sufficiently long tire life.
The rubber and its carcass, made of fibers and steel, must also withstand the heat generated by the rubbing of the tire on the road and the constant bending it undergoes. When the vehicle is traveling at a constant speed and in a straight line, handling is not a problem. On the other hand, on uneven ground or in a curve, the tire must not only absorb the forces exerted by the vehicle in order to retain its adhesion under these specific conditions (to which the force of the wind is added), but also the forces resulting from the engine torque and braking system.
The suspension is one of the most important elements for vehicle safety and road behavior. Today, several types of suspensions (front and rear) are used according to the vehicle performance criteria that the car manufacturers define (cost, behavior, comfort.). On the basis of these criteria, engineers can choose the type of suspension that will be the most suitable (McPherson, wishbone superposed, multi-arm, etc,) and then start working on the suspension settings.
If independent multi-link suspensions have the reputation of being the most advanced, this statement cannot be engraved in marble: Peugeot and Porsche are the perfect examples.
Adaptive suspensions bring added value to their driver by proposing different modes of operation. However, the effects are limited because they are restricted to modifying the damping characteristics. It would be hazardous for a manufacturer to propose two radically different modes (sport and comfort for example), at the risk of significantly modifying the characteristics of adhesion, for example causing a loss of adhesion. In an emergency situation, the vehicle's response must be predictable so as not to surprise the driver.
Finally, active suspensions represent the best possible behavior in terms of road behavior: they adapt to the roughness of the road to transport the occupants of a vehicle in the greatest comfort, especially when the suspension knows how to anticipate the road profile. Active suspensions are less suitable for sports vehicles because of their intrinsic weight.
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