Self Leveling Suspension

[ilsit]

How does it work exactly? Ive seen two different methods using pneumatics and hydraulics so, is it possible to keep something perfectly level for a period of time locking it in place so it doesnt adjust constantly? Does the Self leveling suspension on vehicles work like this?

[D-Type]

Self levelling suspension is a misnomer.

The purpose of suspension on a car is to attempt to smooth out the irregularities in the road. This is achieved by putting springs between the wheels and the body or chassis. But when a wheel hits a bump it will stiil go up and the sprung body will also go up so it will never be level. Once you put a spring anywhere and compress and release it you will find it oscillates so you have to install dampers or shcock absorbers to constrain the string to absorb the shock of hitting a bump but not to perpetuate the shock as a series of bounces or oscillations. The next step to consider is whether to link the different wheels and spring them jointly - transverse leaf springs vs individual springs, whether to have an axle linking the wheels or to allow them to move independently. Let's assume that you opt for all wheels being sprung in a manner that allows them to work (bounce) independent of each other. This generally gives a reasonable ride provided the spring rates and damper rates have beeen selected appropriately.
So far so good. But it is possible to get a better ride by linking the suspension units either with a compressible fluid (pneumatics) or an incompressible one (hydraulics). This is often termed 'self levelling suspension'. The idea is that as a front wheel goes over a bump, despite the spring, the front of the car will rise. Now, if you can make the rear of the car rise before it hits the bump then it will rise less when it hits the bump and the ride will be smoother. Overall the front and rear of the car will have remained relatively more level but both will still have risen. The two main manufacturers to sell cars with 'self levelling suspension' systems were Citroen with their pneumatic system on their DS, ID and BX ranges and British Leyland with their 'Hydrolastic' hydraulic system. Citroen also developed a degree of 'self levelling' mechanically on the 2CV by linking the front and rear springs.
Why don't all manufacturers adopt 'self levelling' suspension systems? One reason is that the suspension system does not only have to cater for the dynamics of proceeding in a straight line over irregularities, the system also has to cater for the different dynamics of cornering where a basic 'self levelling' set up can make the car less stable. The other reason is simply cost. The benefit of a 'self levelling' suspension system is marginal compared to the benefits from a well-developed conventional system.
To answer your other question "is it possible to keep something perfectly level locking it in place so it doesn't adjust constantly?" It is possible but only if it is not moving over an irregular surface. Even trains have sprung suspension, but it is simpler than cars as the level of the rails doesn't vary longitudinally in the random way that a road can but a train still needs suspension to cater for the different loads as it goes around corners.
On a race track, there has been a tendecy towards making surfaces smoother and smoother which allows the attitude of the car relative to the road to remain constant, this in turn allows stiffer springing which keeps the clearance to the ground constant and the aerodynamic down force to be maximised. This was even more necessary twenty or thirty years ago at the height of the "ground effects era" eith skirted cars and shaped undersides. But thingts have now reached a stage where amodern F1 or sports prototype car would not be able to race at some of the classic tracks - the original Nurburgring, the original Spa and road circuits like Rouen, Montjuich and Pescara. Tracks that have been used for the last 50 years like Monza, Silverstone, and the current Spa are now far smoother than they were.
I know I've rambled around a bit, but I hope I've helped make things clearer.

[nigelred5]

Our cadillac had self levelling suspension. load the trunk with baggage, it sinks to the ground. turn on the ignition and up it goes. Put my 6'6", 300lb brother in law in the back seat and one corner drops, then the car self levels itself. Driving down a very bumpy road, the bumps and dips are almost imperceptable. Follow the car down the road and the body barely moves. the Caddie uses a combination of pneumatic over hydraulic springs and magnetically damped shocks. cadillacs system is called CVRSS- continuously variable road sensing suspension and ELC-electronic level control.
This is a ten year + old systemexplanation.

http://www.cadillacforums.com/cadillac-suspension.pdf

[Rollo]

The W116 450SEL 6.9 Mercedes-Benz that I had, had no springs in it at all. The suspension was as M-B called it the "oleo-pneumatic" system, which was basically the same gas spheres arrangement from the W100 600 Grosser but with a liquid under pressure.

You can "lock" the suspension in a W100, W116 and W126 by inserting the rubber blocks which came supplied so that you can put the car on a hoist. The problem is that to drive around with them still in (as I needed to when moving the car between workshops on the odd occasion), the ride is incredibly bumpy.

[Zico]

We have a Peugeot Expert dissabilty vehicle with self levelling pneumatic rear suspension, One of the airbags developed a leak 2 weeks ago and because we had to wait 2 weeks to get the part from Peugeot I decided to put some tireweld in it chancing my arm that it might do the job... it did! :D
Its still up now and no leak, great, cheap temporary fix. ;)

[ilsit]

Thanks everyone for your replys