Air Suspension Explained | All you need to know

As i said, I'm not a suspension expert. All air suspensions still use a hydraulic or a mechanical spring to maintain the structural integrity. Its not possible to just float a 20-30 ton bus on just air springs. These bellows are basically made of rubber and due to the uneven weight distribution in vehicles, they front and rear bellows would be distorted differently. I pulled these images from the net for air bus suspension and as you can see they all have either a leaf spring or a hydraulic (closed loop) shocks for attaching to the chassis. These springs/shocks take the main load while the air spring only damps the vibrations.

Such air bellows are also used in military vehicles for Radar vibration damping too. Normal suspension would damage the radar so while the vehicle in moving the bellows are inflated and when vehicle is stationary and when the radar is working, these bellows are deflated. Even in these vehicles the bellow only inflates a guided platform on which the radars are placed. I've also attached the data sheet of Parker bellows. From page 7 you can see the limits of radial and axial mismatch depending on how many bulges there are in the bellows.

If the vehicle is only attached to air bellows, a simple dynamic braking would make the bellow go beyond its maximum misalignment limits.






Air Bellows.pdf

^ They are just the usual suspension dampers , they cannot carry any weight. The links are usual 3 link or 4 link set up with Anti toll bars, which is adequate for that task to keep the suspension located within the requirements for the bellows to function.

After riding in Volvo 8400 for many years , when I got into a normal high floor bus elsewhere I felt the springiness , continuous movement in those buses especially when nearly empty, they had leaf springs of course.

So clearly air suspension has a superior ride but why in cars do they have both metal springs and air ride? Does it have something to do with high speed handling ?

My understanding of air suspension is that its main purpose is to behave like an adjustable rate spring so that the ride height is maintained at a set level irrespective of the load.

So when there are more passengers, the vehicle height reduces due to the additional weight. This is sensed by the controller and commands the air valve to add air to increase the pressure in the suspension to increase ride height.

When there are fewer passengers, the load reduces and increases the ride height beyond the set limit. Then controller then open the air valve to exhaust air and reduce the pressure. The vehicle then lowers the ride height.

Mechanical springs generally aren't adjustable for preload (in cars, buses etc.). So they are sized to maybe 70% to 90% of max payload. This means that at part load, the springs are too stiff for a good ride. Like all things in engineering, everything is a compromise between different requirements, each pulling in different directions.

In the case of Jeroen's Jaguar, I think (I'm not sure) the air suspension may be acting as a supplement to the mechanical suspension to control ride height. The adaptive dampers complement this by adjusting for a perfect ride in all conditions. Jaguars are known to favour great ride quality over hard sports car suspension.

Motorcycles have had preload adjustable suspension for a long time. But they aren't meant to be frequently adjusted - it's impractical to change preload every time a passenger gets on or off the bike. But BMW now has optional electronically adjustable suspension on their big bikes.

https://www.bmw-motorrad.co.uk/en/en...sa-esatwo.html

Incidentally, some bikes (motorcycles and bicycles) come with air suspension.

BMW HP2
https://www.press.bmwgroup.com/globa...sion-system-04

Aftermarket
https://www.arnottcycles.com/

Bicycle
https://airbike.uk/collections/mtb-f...ack-gold-120mm

On premium cars where you will have these air suspension systems, the main purpose is to provide a more comfortable ride. The fact that is able to maintain a certain ride hide, or adjust it based on speed, is more about performance and comfort too.

Jeroen

I agree. Comfort is greatly affected by the spring stiffness, although damping also a major role. Generally a softer spring provides more comfort but it also allows the suspension to sag more, which affects handling.

Great thread, and a very interesting topic. I am a fan of air suspensions for the comfort and refinement they offer, and in spite of the probably of failure at some point, I wouldn't think twice about buying a vehicle with air suspension.

I think though this may be more Jaguar focused in some aspects. The air suspension systems in the BMWs, Mercs, Audis etc don't have any mechanical spring inside. They are just the damper with an air spring fitted on top. Of course the rest of the system is pretty much similar.

By and large they have become a lot more robust and reliable, than what they were even 15 years ago. Few Mercs like the W164 and W220 and even some W221s gave air suspensions a bad name here. In the newer ones the failure rate is a lot less, and will almost never leave one stranded with the vehicle dropped low.

The only suspensions which I feel are even better than air suspension, are the hydraulic set ups, what Mercedes called ABC, and later on Magic Body control, and Audi launched something similar with the A8 a few years ago (as an option), and even the new Panamera revealed yesterday has this.

There’s also Hydraulic suspension that we find on Toyota Land Cruisers/ LX. Pretty reliable, just an oil change at 100K mark.

Also, some of the newer cars like i7 have independent 4 wheel suspension that appears to be controlled by air and electricity(electronic dampers?). Though all articles talk about the same being Air suspension, it’s definitely different than before as the time it takes to rise up is fraction of what it used to be earlier.

Those are air suspensions too. Some other cars like Porsche, Land Rover etc use a larger resevoir with the systems running at a higher pressure. Its pretty much a closed loop system, with the compressor just being an aid. Hence the raise and drop is a lot faster. The open loop systems like what Merc uses take much longer to raise and lower.

You know what they say:

Having said these days Landcruisers pack a lot of very complicated systems too. Likely still far superior to Landrover of course.

I am not sure if this is the latest, but I came across this Toyota Land Cruiser suspension article.

https://mag.toyota.co.uk/how-does-ki...n-system-work/


Could be, all the electronic dampers I am familiar with, work on the same principle. The dampening characters are electronically controlled. Either by metallic fillings that when in a magnetic field alter viscosity in the damper and or throttling the restriction in the damper through which the fluid needs to pass.

On the Jaguar X350 series the air and electronic damper work together. However as far I can tell the air suspension is basically a more sophisticated version of the springs, or adjusts the springs to be felt more comfortable.

One interesting thing I did not mention. According to Raymond on the X350, the dampers do affect vehicle height a little bit. On most suspension systems it will be the springs (and bellows of course, when fitted) that determine the vehicle height.

I have fitted dampers that had to be tensioned somewhat in order to be installed, so it's likely they have some effect. But I always presumed it was marginal at best. But apparently, not always. Live and learn as they say.

Jeroen