{{short description|Automobile suspension systems}} '''Height adjustable suspension''' is a feature of certain automobile [[suspension (vehicle)|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,<ref>{{cite book|title=Be Your Own Auto Repair Technician |first=Joseph |last=Miller |publisher =Global Media|year= 2007 |isbn =978-81-904575-1-4 }}</ref> or for stylistic reasons. Such a feature requires fairly sophisticated [[engineering]].
[[File:HighCX.JPG|right|thumb|200px| [[Citroën CX]] in high position]]
Height adjustment is most often achieved by [[air suspension|air]] or oil compression used for the "springs" of the vehicle{{snd}} when the pressure is varied, the vehicle body rises or lowers.
==Factory systems for cars== [[File:1954 Citroen 15CVH.JPG|right|thumb|150px| Height adjustable suspension from 1954{{snd}} high position]] [[File:Audi A8 Adaptive Air Suspension Control.JPG|left|thumb|150px|An [[Audi A8]] [[Multi Media Interface]] control screen for its Adaptive Air Suspension, which gives the vehicle clearance a range from 95 mm to 145 mm]] [[File:Dublin bus kneeling bus with ramp.jpg|right|thumb|150px|Kneeling bus in [[Dublin]] ]] The first instance of a production vehicle with adjustable (rear only) suspension was on the [[Citroën Traction Avant|1954 Citroën 15CVH]]. This vehicle featured a [[self-leveling suspension|self-leveling]], height adjustable [[hydropneumatic suspension]]. Since that time, these systems have appeared continuously on [[Citroën]] models, including the [[Citroën DS|DS]] and [[Citroën CX|CX]].<ref>{{Cite journal| url=https://www.autoevolution.com/news/citroen-hydropneumatic-suspension-explained-49954.html| author = Mihnea Radu| title = Citroen Hydropneumatic Suspension Explained| journal = Autoevolution| date = September 30, 2012}}</ref>
Height adjustable suspension was banned in the [[United States]] from 1974 to 1981, due to the stringent interpretation of passenger vehicle [[Bumper (automobile)|bumper]] height regulations by the U.S. government agency [[National Highway Traffic Safety Administration|NHTSA]]. <ref>{{Cite journal| url=https://reason.com/1978/03/01/billion-dollar-bumpers/| author = Jack Solomon | title = Billion Dollar Bumpers| journal = Reason| date = March 1978}}</ref>
Subaru was one of a few manufacturers who offered the feature after the ban was lifted on the [[Subaru XT]], the [[Subaru Leone]] wagon and the [[Subaru Legacy]] for a short time.
Many modern [[SUV]]s use height adjustability as part of [[active suspension]] systems to improve the vehicle's versatility on and off-road. <ref name="AutoBlog.com">{{cite web|url=https://www.autoblog.com/2015/11/17/how-does-adjustable-suspension-work/|first=|last=|website=www.autoblog.com|title=How Does Adjustable Suspension Work?|date=17 Nov 2015|publisher=Verizon Media|accessdate=7 Aug 2020}}</ref> The [[Range Rover]] offered this feature from 1993. New models of the [[Ford Expedition]] have a computer-controlled system designed for convenience, which lowers automatically when the doors are unlocked by remote, returns to normal height when the vehicle is started, and (on 4-wheel-drive models), raises when the 4×4 system is engaged.<ref name="AutoBlog.com"/>
Some cars use these systems to improve the vehicle's handling by lowering the vehicle's height during higher speeds{{snd}} a current example being the [[Mercedes-Benz]] [[Active Body Control]] system. <ref name="AutoBlog.com"/> Another example is the [[Audi A8]], which when driven at speeds of more than 120 km/h for more than 30 seconds reduces its clearance from 120 mm to 95 mm.<ref>{{cite book|title=Audi A8 Betriebsanleitung|page=151|chapter=adaptive air suspension|publisher= [[Audi AG]]|year=2008|trans-title=Audi A8 Operating Instructions|language=German}}</ref>
The 2011 [[Jeep Grand Cherokee]] includes adjustable suspension in the 4×4 model. It automatically lowers when the Park selection is engaged, and also at high speeds to improve aerodynamics. It also allows drivers to manually raise the suspension for off-road situations.
Starting in 2012, the [[Tesla Model S]] and [[Tesla Model X]] offer their own patented height adjustable suspension as an option. Two goals are accomplished{{snd}} the long, low slung car can be raised to avoid road obstacles{{snd}} it also lowers at speed on the highway to improve aerodynamics.<ref name="AutoBlog.com"/><ref name="Inside EV">{{Cite journal| url=https://insideevs.com/news/337504/lets-look-at-teslas-patented-self-adjusting-air-suspension/| author = Domenick Yoney| title = Let's Look At Tesla's Patented Self-Adjusting Air Suspension: This patent has its ups and downs| journal = Inside EVs| date = 10 April 2018}}</ref>
==Factory systems for buses== Height adjustable [[air suspension]]s are also equipped on some buses, mainly [[Low-floor bus|"low-floor"]] [[city bus]]es or "kneeling buses", however in recent years some high floor charter/intercity buses have been equipped with this feature as well. This allows the floor to be [[barrier-free|lowered]] when the bus is stopped, to allow people with large or bulky luggage, passengers with disabilities, or simply when stopping at stops with unusual curb arrangements to enter or exit the vehicle easily. It can also raise the bus more than normal, which is useful when navigating roads with large amounts of speed humps, crossing railway lines or in other unusual, but not always uncommon situations.
[[Assistive technology]] for persons with disabilities includes vehicles modified with height adjustable suspension, for example to allow wheelchair entry to the vehicle.
==Aftermarket systems== [[File:Lowrider lifting mechanism.jpg|thumb|200px|Rear axle suspension of a lowrider]] In 1959, auto customizer Ron Aguirre scavenged the Pesco pumps and valves from a [[B-52 Bomber]] and adapted them to the front suspension of his X-Sonic bubble-topped [[custom car]] [[Chevrolet Corvette|Corvette]], allowing him to change the height of the car with a switch on the dashboard. <ref name=''GS''>{{cite web |url=https://goldenstate.is/going-low-slow-east-los-angeles/|publisher=The Golden State Company|title=Going "Low and Slow" in East Los Angeles|accessdate=24 August 2020}}</ref> This was done to evade California law regarding ground clearance. <ref>{{cite web |url=https://msu.edu/~torresm2/ch3.html|publisher=Michigan State University|title=Cruising into History under the Law's Nose|accessdate=24 August 2020}}</ref> Aguirre is commonly accepted as the first person to create a [[Lowrider]] car with hydraulically adjustable suspension.<ref>{{cite web |url=https://www.roadandtrack.com/car-culture/news/a29982/the-grey-lady-discovers-a-world-of-candy-flake-color-in-odessa-texass-biggest-lowrider-show/|publisher=Road & Track Magazine|title="Ghetto Famous" Lowriders Reach For The Sky In Odessa, Texas|author=Rong, Blake|work=Road & Track |date=16 July 2016|accessdate=24 August 2020}}</ref>
[[File:Reactor by Gene Winfield.jpg|thumb|200px|[[The Reactor (show rod)|The Reactor]] by [[Gene Winfield]]]] In 1964, another customizer, [[Gene Winfield]], created [[The Reactor (show rod)|The Reactor]], with [[Citroën]] height adjustable [[Hydropneumatic suspension]], a car novel enough to guest star in several television programs. <ref name="automag28jun2017">{{cite web |url=https://www.automobilemag.com/news/two-one-off-socal-customs-set-invade-pebble-beach-concours/ |title=These Two One-Off SoCal Customs Are Set to Invade Pebble Beach |last=Jurnecka |first=Rory |date=28 Jun 2017 |website=www.automobilemag.com }}</ref>
[[File:Chevrolet Impala 1964 Lowrider.JPG|thumb|200px|Aftermarket height adjustable suspension installed on a 1964 [[Chevrolet Impala]] [[lowrider]] ]] Adjustable suspensions have become intrinsically associated with [[lowrider]] vehicles. The popular image of these vehicles is of one "hopping" on its suspension, or sitting with one wheel completely off the ground. These systems were initially adapted from the [[hydraulic]] pistons, valves and pumps used to adjust the flaps on [[aircraft]]. Today however, many aftermarket companies produce parts and equipment specifically designed for [[lowrider]]s.
In recent years "air bag" systems (not to be confused with the [[air bag]] safety device) have gained popularity among car customizers. These [[air suspension]] systems use heavy-duty rubber "bags" to replace the stock shocks and springs, with either a compressor or tank of compressed gas used to raise and lower the vehicle at will.<ref name="AutoBlog.com"/>
==User control== The first height adjustable suspension systems were controlled by the driver manually. <ref>{{cite web|title = Suspension - V Height Corrector 3. Manual Height Control|publisher=citroen-ds-id.com|url=http://www.citroen-ds-id.com/index.html?hc/chapter4.html}}</ref>
With the development of computer controls, research is ongoing into electronic control of the height adjustment process. <ref>{{cite journal|url=https://www.tandfonline.com/doi/full/10.1080/00423114.2012.660167?src=recsys|title=A new pneumatic suspension system with independent stiffness and ride height tuning capabilities|author1=Zhihong Yin| author2 =Amir Khajepour|author3=Dongpu Cao|author4=Babak Ebrahimi|author5=Konghui Guo|journal=Vehicle System Dynamics |volume=50|issue=12|pages= 1735–1746 | date = 24 Feb 2012|doi=10.1080/00423114.2012.660167|bibcode=2012VSD....50.1735Y|s2cid=110106627|url-access=subscription}}</ref><ref>{{cite journal|title=Height and Leveling Control of Automotive Air Suspension System Using Sliding Mode Approach|author1=Hyunsup Kim|author2= Hyeongcheol Lee|journal=IEEE Transactions on Vehicular Technology|volume=60|issue=5| publisher=Institute of Electrical and Electronics Engineers|date = Jun 2011|pages=2027–2041|doi=10.1109/TVT.2011.2138730|bibcode=2011ITVT...60.2027K |s2cid=22122852}}</ref> Certain modern layouts allow electronics alone to make this decision without the driver's control, notably the [[Tesla Model S]] and [[Tesla Model X]]<ref name="Inside EV"/>
==References== {{reflist}}
[[Category:Automotive suspension technologies]]