Nose up, tail down attitude of vehicle without self-levelling suspension
Many vehicle systems on a conventional vehicle are negatively affected by the change in attitude coming from changes in load - specifically a heavy load in the rear seat or luggage compartment.[1] This change in attitude affects aerodynamic properties, headlight aim, braking, bumpers, shock absorption from the suspension and the vehicle's performance in a collision.[2]
Most of the braking power is on the front wheels of a vehicle, which means you will have more effective braking when more weight is over the front wheels. When the rear end has a heavy load, the braking is not as effective. The weight is concentrated on the rear end of the vehicle, and the rear brakes need to do all of the work. When braking quickly in this situation, the front brakes will be easier to lock up because of the lack of weight transfer to the front of the vehicle. Self-levelling suspension lifts the rear end of the vehicle up to spread out the weight more evenly. This puts the weight back onto the front end of the vehicle, which lets the brakes do their job more effectively.[3]
This is nonsense as this effect may only be achieved by changing load/cargo distribution. Raising or lowering a suspension and hence the load/cargo does not significantly move the suspension/wheel loading, unless the vehicle has a very unusual extreme suspension geometry and the centre of mass of the load is very much higher than the pitch centre, then some dynamic effects could be seen.
There is an inherent conflict in suspension design - if the springs are soft, the car will be comfortable but dramatically affected by load.[4] If the springs are hard, the car will be uncomfortable, but less affected by load.[5]
Numerous manufacturers realize this conflict and have pursued different avenues to achieve both comfort and load capacity simultaneously.
History
In France in 1954, Citroën introduced the first self-levelling rear suspension on a production car, and then in 1955 pioneered self-levelling of all four wheels, using its hydropneumatic system. These cars maintain an exact height over the road when the engine is on - height control valves attached to the roll bars via linkages would open to add or drain fluid from the suspension, and when the desired height was reached the valve would automatically close due to its design.[6] Later models would use electronic height sensors and motors so adjustment could be achieved with the engine off.
This system allowed the suspension to achieve an unusually soft ride quality.[7]
Since then, millions of fairly inexpensive Citroën cars have been equipped with self-levelling as an unobtrusive, but integral design feature. The Citroën's dashboard (later console or fascia mounted controls) includes a position lever which allows the driver to select whether the car would sit with the body in high, intermediate, normal or low positions, the extremes used for maintenance such as changing wheels or hydraulic system work. Up until 1995 when they added "antisink" to the range when the engine was turned off, the suspension slowly lost pressure until the car rested on the bump stops. When the engine was restarted it rose back to its pre-selected height. The addition of anti-sink added 2 non-return valves and an extra accumulator so that when hydraulic pressure was lost the valves would close and keep the remaining fluid in the system, and leaving the car resting at a normal height when parked.
In the United States, William D. Allison developed Torsion-Level Suspension used on 1955-1956 model Packards. This was an interconnected suspension, with torsion bars that ran along each side of the frame, connecting the front wheel to the rear (on the same side). Because this system is so affected by load, a supplemental, electronic leveling system was added, which uses a level sensor and a single motor to load/unload a pair of auxiliary bars to adjust vehicle attitude, but not overall ride height.[8][9] This early attempt was an important step on the road to self-leveling, even if a full load would cause the whole car to lower evenly, rather than maintain height.
In 1966, Rolls-Royce licensed Citroën's hydropneumatic system to fit to the rear axle of the Silver Shadow.[11] At first, both the front and rear of the car were controlled by the levelling system; the front levelling was removed in 1969 as it had been determined that the rear levelling did almost all the work. Rolls-Royce achieved a high degree of ride quality with this arrangement.[12]
Land Rover developed a different system for the Range Rover in the early 1970s. The Range Rover was intended to be as comfortable on the road as a normal saloon car yet as capable of heavy off-road use as a traditional Land Rover. This highlighted the contradiction in suspension design as the Range Rover used all-round long-travel coil springs. If these were kept soft to maintain comfort they would compress too much under a heavy payload, restricting axle travel when off-road and compromising handling. Land Rover developed a self-levelling rear suspension using the "Boge Hydromat" self-energising hydraulic strut.[13]
Of similar construction to a hydraulic shock absorber the strut used the motion of the suspension travelling over bumps to pump itself back up to a pre-set height. It was sufficiently powerful to regain up to 85% of normal ride height with a full load over the rear axle, and had the advantage of requiring no external power source or a dedicated hydraulic system in the vehicle. The same system was applied to the Land Rover in the 1980s when these vehicles adopted the Range Rover's coil spring suspension. In the 1990s Land Rover, in pursuit of the same blend of on- and off-road ability, developed an air suspension system that was both self-levelling and height adjustable. Originally this was in conjunction with live axles but it is now used with fully independent suspension using wishbones.
A shock absorber or damper is a mechanical or hydraulic device designed to absorb and damp shock impulses. It does this by converting the kinetic energy of the shock into another form of energy which is then dissipated. Most shock absorbers are a form of dashpot.
The Citroën DS is a front mid-engined, front-wheel drive executive car manufactured and marketed by Citroën from 1955 to 1975, in fastback/sedan, wagon/estate, and convertible body configurations, across three series of one generation.
A torsion bar suspension, also known as a torsion spring suspension, is any vehicle suspension that uses a torsion bar as its main weight-bearing spring. One end of a long metal bar is attached firmly to the vehicle chassis; the opposite end terminates in a lever, the torsion key, mounted perpendicular to the bar, that is attached to a suspension arm, a spindle, or the axle. Vertical motion of the wheel causes the bar to twist around its axis and is resisted by the bar's torsion resistance. The effective spring rate of the bar is determined by its length, cross section, shape, material, and manufacturing process.
Suspension is the system of tires, tire air, springs, shock absorbers and linkages that connects a vehicle to its wheels and allows relative motion between the two. Suspension systems must support both road holding/handling and ride quality, which are at odds with each other. The tuning of suspensions involves finding the right compromise. It is important for the suspension to keep the road wheel in contact with the road surface as much as possible, because all the road or ground forces acting on the vehicle do so through the contact patches of the tires. The suspension also protects the vehicle itself and any cargo or luggage from damage and wear. The design of front and rear suspension of a car may be different.
The Citroën GS is a front-engine, front-drive, four or five door, five passenger family car manufactured and marketed by Citroën in two series: for model years 1970–1979 in fastback saloon and estate bodystyles and subsequently as the GSA for model years 1980–1986 in hatchback and estate body styles – the latter after a facelift. Combined production reached approximately 2.5 million.
Hydropneumatic suspension is a type of motor vehicle suspension system, designed by Paul Magès, invented by Citroën, and fitted to Citroën cars, as well as being used under licence by other car manufacturers. Similar systems are also widely used on modern tanks and other large military vehicles. The suspension was referred to as Suspension oléopneumatique in early literature, pointing to oil and air as its main components.
The Citroën C5 is a large family car produced by the French manufacturer Citroën between 2000 and 2018 in France, and between 2008 and 2022 in China, over two generations. It replaced the Citroën Xantia, in the large family car class, and is the first modern Citroën with "Cx" naming nomenclature, previously used by its ancestors, the C4 and C6 from 1930. A crossover, unrelated to the previous generations, was released in 2021, with crossover styling and marketed as the Citroën C5 X.
The Mercedes-Benz 450SEL 6.9 is the high-performance version of the W116 S-Class saloon. It was based on the long-wheelbase version of W116 chassis introduced in 1972 and equipped with Mercedes-Benz M100 V8 engine and full hydropneumatic suspension system. The 450SEL 6.9 was presented to the public at the Geneva Auto Show in 1974 as the successor to the original high-performance saloon from Mercedes-Benz, the 300SEL 6.3 (1968–1972). Like the 300SEL 6.3 before it, the special displacement number (6.9) was affixed to the trunk lid to differentiate itself from the 450SEL. The 450SEL 6.9 was produced in very limited numbers from 1975 to 1980.
The Citroën XM is a front-engine, front-drive, five-passenger, five-door hatchback noted for its hydropneumatic suspension. Manufactured and marketed by Citroën from 1989 to 2000, with a minor facelift in 1994, XM production reached 333,405 over the course of 11 years.
The Citroën Xantia, pronounced "Zan–ti–a" is a large family car (D) produced by the French automaker Citroën, and designed by Bertone. Presented to the press in December 1992, the car was produced between 1992 and 2001 in France, with a facelift in the end of 1997.
Automobile handling and vehicle handling are descriptions of the way a wheeled vehicle responds and reacts to the inputs of a driver, as well as how it moves along a track or road. It is commonly judged by how a vehicle performs particularly during cornering, acceleration, and braking as well as on the vehicle's directional stability when moving in steady state condition.
In both road and rail vehicles, the wheelbase is the horizontal distance between the centers of the front and rear wheels. For road vehicles with more than two axles, the wheelbase is the distance between the steering (front) axle and the centerpoint of the driving axle group. In the case of a tri-axle truck, the wheelbase would be the distance between the steering axle and a point midway between the two rear axles.
An anti-roll bar is an automobile suspension part that helps reduce the body roll of a vehicle during fast cornering or over road irregularities. It links opposite front or rear wheels to a torsion spring using short lever arms for anchors. This increases the suspension's roll stiffness—its resistance to roll in turns.
Active Body Control, or ABC, is the Mercedes-Benz brand name used to describe electronically controlled hydropneumatic suspension.
Air suspension is a type of vehicle suspension powered by an electric or engine-driven air pump or compressor. This compressor pumps the air into a flexible bellows, usually made from textile-reinforced rubber. Unlike hydropneumatic suspension, which offers many similar features, air suspension does not use pressurized liquid, but pressurized air. The air pressure inflates the bellows, and raises the chassis from the axle.
A suspension lift is a modification to a vehicle to raise the ride height. It is done for the purpose of improving the off road performance of SUVs or trucks and other off-road vehicles, or for cosmetic purposes. Suspension lifts can enable steeper approach, departure, and breakover angles, higher ground clearance, and helps accommodate larger wheels and tires. Due to the raised center of gravity, maximum safe operating angles can be reduced and roadholding is often significantly impaired. Sensors, transmitters and cameras may need to be recalibrated, physically relocated or modified to maintain normal functioning of features like AEB, ACC or FSD. Suspension lifts are also found on a number of high performance sports cars, in which a very low ground clearance is used to improve handling, using an aerodynamic effect known as downforce; examples include the Ferrari 488, Lamborghini Huracan, McLaren 720S, and the second generation Ford GT. Such vehicles activate the suspension lift while traversing road bumps and ramps to avoid damaging the front underbody of the vehicle when driving on public roads; the suspension lift is deactivated on race tracks and level roads.
Hydrolastic is a type of space-efficient automotive suspension system used in many cars produced by British Motor Corporation (BMC) and its successor companies.
Height adjustable suspension is a feature of certain automobile suspension systems that allow the motorist to vary the ride height or ground clearance. This can be done for various reasons including giving better ground clearance over rough terrain, a lower ground clearance to improve performance and fuel economy at high speed, or for stylistic reasons. Such a feature requires fairly sophisticated engineering.
An active suspension is a type of automotive suspension that uses an onboard control system to control the vertical movement of the vehicle's wheels and axles relative to the chassis or vehicle frame, rather than the conventional passive suspension that relies solely on large springs to maintain static support and dampen the vertical wheel movements caused by the road surface. Active suspensions are divided into two classes: true active suspensions, and adaptive or semi-active suspensions. While adaptive suspensions only vary shock absorber firmness to match changing road or dynamic conditions, active suspensions use some type of actuator to raise and lower the chassis independently at each wheel.
The following outline is provided as an overview of and topical guide to automobiles:
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