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
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...
De Dion tube A de Dion tube is an automobile suspension technology. It is a sophisticated form of non-independent suspension and is a considerable improvement over the swing axle, Hotchkiss drive, or live axle. Because it plays no part in transmitting power to the drive wheels, it is sometimes called a "dead axle". De Dion suspension uses universal joints at both the wheel hubs and differential, and uses a solid tubular beam to hold the opposite wheels in parallel. Unlike an anti-roll bar, a de Dion tube is not directly connected to the chassis nor is it intended to flex. In suspension geometry it is a beam axle suspension. de Dion suspension characteristics: Camber change on one sided bumps, none on rebound. de Dion tube is shown in blue. The differential (yellow) is connected directly to the chassis (orange). De Dion rear axle History The de Dion tube was named after Comte Jules-Albert de Dion, founder of French automobile manufacturer De Dion-Bouton. The tu...
Electromagnetic suspension Electromagnetic suspension (EMS) is the magnetic levitation of an object achieved by constantly altering the strength of a magnetic field produced by electromagnets using a feedback loop. In most cases the levitation effect is mostly due to permanent magnets as they don't have any power dissipation, with electromagnets only used to stabilize the effect. According to Earnshaw's Theorem a paramagnetically magnetised body cannot rest in stable equilibrium when placed in any combination of gravitational and magnetostatic fields. In these kinds of fields an unstable equilibrium condition exists. Although static fields cannot give stability, EMS works by continually altering the current sent to electromagnets to change the strength of the magnetic field and allows a stable levitation to occur. In EMS a feedback loop which continuously adjusts one or more electromagnets to correct the object's motion is used to cancel the instability. Many systems use magnetic attraction pulling upwards...
Leaf spring A leaf spring is a simple form of spring commonly used for the suspension in wheeled vehicles. Originally called a laminated or carriage spring, and sometimes referred to as a semi-elliptical spring or cart spring, it is one of the oldest forms of springing, appearing on carriages in England after 1750 and from there migrating to France and Germany.   A traditional semi-elliptical Hotchkiss leaf spring arrangement. On the left, the spring is connected to the frame through a shackle. Leaf springs front independent suspension, front-wheel-drive Alvis1928 Independent front suspension by transverse leaf spring Humber 1935 Independent front suspension by semi-elliptical springs Mercedes Benz 230 W153 1938 Leaf spring on a German locomotive built by Orenstein-Koppel and Lübecker Maschinenbau A leaf spring takes the form of a slender arc-shaped length of spring steel of rectangular cross-section. I...
Corvette leaf spring Corvette leaf spring commonly refers to a type of independent suspension that utilizes a fiber-reinforced plastic (FRP) mono-leaf spring instead of more conventional coil springs. It is named after the Chevrolet Corvette, the American sports car for which it was originally developed and first utilized. A notable characteristic of this suspension configuration is the mounting of the mono-leaf spring such that it can serve as both ride spring and anti-roll spring. In contrast to many applications of leaf springs in automotive suspension designs, this type does not use the spring as a locating link. While this suspension type is most notably associated with several generations of the Chevrolet Corvette the design has been used in other production General Motors cars, as well as vehicles from Volvo Cars and Mercedes-Benz. Fiat produced cars with a similar configuration, using a multi-leaf steel spring in place of the FRP mono-leaf spring. Design The C5 Corvette's rear suspension ...