Self-levelling suspension

Self-levelling refers to an automobile suspension system that maintains a constant ride height of the vehicle above the road, regardless of load.

Purpose

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]

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 hydropneumaticsystem. 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 1957, Cadillac, introduced the Eldorado Brougham, a Rolls-Royce Silver Cloud competitor, featuring a new Air suspension with a self leveling feature.[10]

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. The original coil-springs-with-hydraulic-strut system is still used on certain models of the current Land Rover Defender.

BMW, Ford, GMC, Jaguar, Mercedes-Benz, Scania AB, Subaru and Volvo have each pursued numerous avenues to address this issue, including air suspension and rear axle mechanical devices.

Notes

  1. http://www.equipmentworld.com/understanding-your-pickups-max-payload-capacity-and-how-helper-springs-make-a-smooth-ride-of-a-heavy-load/
  2. http://www.bmw.com/com/en/insights/technology/technology_guide/articles/self_levelling.html?source=categories&article=self_levelling
  3. https://racingrebel.com/5-reasons-why-air-lift-1000-air-bags-are-amazing/
  4. http://papers.sae.org/2004-01-1078/
  5. “The ride comfort vs. handling compromise for off-road vehicles”. Journal of Terramechanics44: 303–317. doi:10.1016/j.jterra.2007.05.001.
  6. “Citroën’s hydropneumatique”. citroenet.org.uk. Retrieved 2 November 2016.
  7. http://www.autoevolution.com/news/citroen-hydropneumatic-suspension-explained-49954.html
  8. http://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?post_id=58253
  9. http://justacarguy.blogspot.com/2014/05/packards-torsion-level-suspension.html
  10. https://history.gmheritagecenter.com/wiki/index.php/1957_%26_58_Cadillac_Eldorado_Brougham
  11. “Archived copy”. Archived from the original on 2015-01-20. Retrieved 2015-01-20.
  12. http://www.rrsilvershadow.com/Emenu/hydr.htm
  13. http://www.uniquecarsandparts.com.au/specifications/land_rover/1972_range_rover.htm
Dual ball joint suspension A dual ball joint suspension uses a pair of arms, one in tension, one in compression, to replace a wishbone, in a MacPherson or SLA suspension. The outer end of each arm terminates in a ball joint, hence the name. General description The two arms, the spindle, and the body, form a four-bar link. Use of the linkage at the lower suspension connection of either a MacPherson strut or a short long arms suspensionreadily gives an effective virtual ball joint outboard of the spindle, which is very useful for a suspension designer, allowing negative scrub radius whilst allowing the ball joints to move in and thus out of the way of the brakes. Some manufacturers use a double ball joint arm to replace both wishbones on a short long arms suspension. This provides further opportunity for optimising the geometry. Examples It is used on large cars such as the Lexus LS 460, BMW X5, Ford Territory, and General Motors' Zeta-derived models. Disadvantages The extra ball joint adds weight and cost. ...
Hydrolastic Hydrolastic is a type of space-efficient automotive suspension system used in many cars produced by British Motor Corporation (BMC) and its successor companies. Invented by famous British rubber engineer Alex Moulton, and first used on the 1962 BMC project ADO16 under designer Alec Issigonis (of Mini fame), later to be launched as the Morris 1100. Description The system replaces the separate springs and dampers of a conventional suspension system with integrated, space efficient, fluid filled displacer units which are interconnected between the front and rear wheels on each side of the vehicle. Each displacer unit contains a rubber spring, and damping is achieved by the displaced fluid passing through rubber valves. The displaced fluid passes to the displacer of the paired wheel, thus providing a dynamic interaction between front and rear wheels. When a front wheel encounters a bump fluid is transferred to the corresponding rear displacer then lowers the rear wheel, hence lifti...
Beam axle A beam axle, rigid axle or solid axle is a dependent suspension design, in which a set of wheels is connected laterally by a single beam or shaft. Beam axles were once commonly used at the rear wheels of a vehicle, but historically they have also been used as front axles in rear-wheel-drive vehicles. In most automobiles, beam axles have been replaced by front and rear independent suspensions. Beam axle and Panhard rod on a 2002 Mazda MPV Implementation Solid axle suspension characteristics: Camber change on bumps, none on rebound, large unsprung weight With a beam axle the camber angle between the wheels is the same no matter where it is in the travel of the suspension. A beam axle's fore & aft location is constrained by either: trailing arms, semi-trailing arms, radius rods, or leaf springs. The lateral location is constrained by either: a Panhard rod, a Scott Russell linkage or a Watt's linkage. While shock absorbers and either leaf springs, c...
Hydropneumatic suspension 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, notably Rolls-Royce (Silver Shadow), Maserati (Quattroporte II) and Peugeot. It was also used on Berliet trucks and has more recently been used on Mercedes-Benz cars, where it is known as Active Body Control. Similar systems are also widely used on modern tanks and other large military vehicles. The suspension was referred to as oléopneumatique in early literature, pointing to oil and air as its main components. The purpose of this system is to provide a sensitive, dynamic and high-capacity suspension that offers superior ride quality on a variety of surfaces. A hydropneumatic system combines the advantages of two technological principles: Hydraulic systems use torque multiplication in an easy way, independent of the distance between the input and output, without the ne...
Anti-roll bar An anti-roll bar (roll bar, anti-sway bar, sway bar, stabilizer bar) is a part of many automobile suspensions that helps reduce the body rollof a vehicle during fast cornering or over road irregularities. It connects opposite (left/right) wheels together through short lever arms linked by a torsion spring. A sway bar increases the suspension's roll stiffness—its resistance to roll in turns, independent of its spring rate in the vertical direction. The first stabilizer bar patent was awarded to Canadian inventor Stephen Coleman of Fredericton, New Brunswick on April 22, 1919. Anti-roll bars were unusual on pre-war cars due to the generally much stiffer suspension and acceptance of body roll. From the 1950s on, however, production cars were more commonly fitted with anti-roll bars, especially those vehicles with softer coil spring suspension.   An anti-roll bar (in black) on the rear of a Porsche, which traverses the underside of the car. Flexible bushings attach it to ...