Go Home! DIY Self Leveling Air Suspension See All Photos

I began this project in July 2003 (It's now September 2004) and it is only just about finished! It came about after I bought 35" Simex Extreme Trecker tyres for my Land Rover Defender - and it rolled around like a blamange on the road!

Initially I wanted to build an active anti roll system which would try to level the vehicle on bends, if not make lean into bends like a motorbike!

I have seen the odd mechanical solution to this, but I figured, air was the way to go.

I had recently acquired a number of BIG pneumatic rams. They are 100mm bore by 300mm stroke. If I could feed these with 100psi, I could apply a force of 1200Lbs per ram to make the vehicle lean.

I built an experimental setup with the rams attached to the rear axle in exactly the same way as shock absorbers (dampers) and used them in conjunction with the existing springs.

    The rams were fed with air from a 2 x 2 port valve which could feed air into either end of the ram allowing me to force the ram to extend as well as contract, thereby doubling the force I could apply.

The valve was driven by a simple mechanical pendulum which when off center, closed the contacts on one of two micro-switches.

If the vehicle leans to the left, the pendulum moves to the left which opens the valve which tries to make the vehicle lean to the right - until it reaches equilibrium.
    Surprisingly, this worked very well! It would lean appropriately as I cornered and would maintain the vehicle more or less level all the time.

The main problem I encountered was that occasionally, the vehicle and suspension would start to oscillate! Usually when driving fast! It was quite frightening!! The only way to stop it was to kill the power.

This proved that some kind of pendulum based self leveling system could be made to work with appropriate damping.
    Air Bags (Springs)  
    Although 2400Lbs of force leaning the vehicle sounds a lot, it was only enough to level fairly gentle corners. I decided to grab the bull by the horns and go for a full air suspension setup using 'Air Bags' similar to those used by Range Rover.

The trouble with Range Rover bags is the lack of travel - only about 9" and I already had more travel than that in my raised springs.

I considered using a pair of air bags mounted end to end in some kind of telescopic guide - and still think this has potential, but simply couldn't find any used air bags in good enough condition. If I was going to buy new, I might as well get something better to start with.
    After a lot of Digging, I found a company called AutomInt who supply air springs for trucks. I had a chat with them (very friendly & helpful) and they suggested springs for a DAF Truck - Part number 1T26D-7. I promptly bought a set of four.

I enquired about dampers also - as, as you can probably see, the spring rate changes with the air pressure and in order for the suspension to remain critically damped, the damping coefficient should change with the height. They said that the a standard damper is fine and has enough breadth of damping that it will work over the range of heights. In practice, it only need be correct at whatever ride height you prefer to use on the road.

Rancho adjustable dampers might be worth a look - but probably not worth it!

The next problem was the valves. The air bags are single acting, in other words, you can inflate them and they will expand, but you rely on gravity or another external force to deflate them. The valves I had already were capable of inflating the bag, but as soon as you stop inflating, the bag was connected to the exhaust and it deflated - they were not designed to hold a position, just to move to a position.

    I considered a number of alternatives for valves. Firestone and a number of other companies make complete, assembled valve blocks designed to drive 2 or 4 air bags and if I had my time over - I would have bought one of these but thought they were too expensive at the time at about £700 in all.
I settled on buying 8 x 3/8" 2 port valves from a company called Air Technics in Horsham. They were only about £25 each - so £200 in all, but I needed to make the manifolds and buy all the air fittings separately to connect them up which added at least another £100 to the bill
I spent the winter modifying the Land Rover, making new spring mountings etc. to replace the original coil springs with the air bags and by February, it was all complete.
    Fitting the rear springs was pretty easy. I found that the 'top hat' or spring cup already had holes in it which matched the studs which stuck out the top of the air spring. These were turned upside down and bolted to the air spring. The bottom of the spring is mounted by a single bolt (1/2" UNC) which screws into the middle of the spring. I made a plate which bolts to the top of the axle with a long 1/2" bolt welded to it. The spring then simply screws to the top of the axle. The top of the spring, with the original spring cup attached can then just sit in the existing chassis spring hanger. I made a small spacer to raise the height a little, but this is not necessary.

The springs have about 13 inches of travel - far more than a coil spring. I decided not to include a dislocation mechanism as it already has more articulation than most coil setups with the dislocate. The limiting factor on articulation is now the 'A' frame ball joint, so until this is modified, there was little point in making them dislocate.

    The front was more complex, but only by virtue of having to move the damper which normally lives inside the spring. There are plenty of companies selling turrets to let you mount a second damper next to the spring - so if I were you, I'd buy one of these and fit the spring itself as per the back. I decided to make my own since I had the facilities.

    The whole thing is plumbed in 10mm OD nylon tube with push fit connectors - very easy!

    Air Supply  
    There are several options for this. The system could easily run from an electric compressor such as that used in Range Rovers (pretty cheap on eBay and from junk yards) with a decent size reservoir.

    I already had an air system fitted with enough capacity to drive air tools etc. It is based on a standard Land Rover air conditioning compressor. Normally they are filled with oil, which leaks out into the air supply (we don't want that). I drained the oil and filled it with grease instead! It has run happily for a couple of years now so I would suggest this works. If you enlarge the image, you can see the grease nipple fitted to the oil filler plug on top of the compressor.
    The compressor is connected via a dryer to the reservoir. The dryer is important to remove moisture from the air which otherwise might condense out in the air springs - and fill them up eventually!

    For a reservoir, I use my roll cage which was built with this in mind. Most of the tubes are connected together giving me a volume of about 15 litres. I have recently added a second reservoir made from one of the bottles air conditioning gas is supplied in. It is suspended between the chassis rails at the rear of the vehicle. That adds another 15 litres or so.

The reservoir is important because the suspension (and air tools) can use air far faster than the compressor can generate it. Because neither is running all the time, reservoir supplies the excess and the compressor can catch up in-between

    Initially I just built a box covered in toggle switches which could switch combinations of the valves on and off. I have for example one switch which will inflate all four bags and another which will make the vehicle lean to one side. It was very simple and worked well for showing off! It was not what I'd built it for though, I wanted an active anti roll and self leveling solution.

    To control the system, I decided to use a computer! The one I selected is called a 'Basic Atom', supplied by Milford Instruments. It is a self contained unit, only really requiring power which runs a version of the BASIC language. It has 16 digital I/O lines plus another four Analogue lines capable of measuring a voltage from roughly 0 to 5v in 1024 steps. Here is my final version of the control program - in case you are interested!
    The pendulum to control the lean is all very well, but now we have replaced the coil springs with air bags, we need to control the ride height as well. Range Rovers achieve this by using axle position sensors (essentially a potentiometer which is turned as the axle moves up & down spitting out a voltage proportional to position).
    I was a bit worried about these getting ripped off or damaged and decided to use another solution. I figured that the pressure in each air bag is proportional to the weight on each wheel. Therefore, if the pressures are all the same, the weight on each wheel is the same, and in an off-road situation this hopefully means that all the wheels are in contact with the ground. To achieve this, I fitted four pressure transducers supplied by NRS Refrigeration supplies in Crawley. They are intended to measure the pressure of gas in air conditioning systems and have a range of 0 to 300psi - ideal! The voltages output by the transducers are read by the analogue inputs on the Basic Atom - nice & easy.

    Instead of a mechanical pendulum, I bought a thing which is intended to be used in radio controlled submarines to make them sit level in the water by automatically adjusting the bow planes. It came from Pandan Models and has the part number: APC-4.

It spits out a signal intended to drive a RC servo where the servo position is proportional to the degree of lean. Fortunately, this signal is easy to read with a computer - you just measure the length of the pulses it spits out - also nice & easy.

    The computer now knows what pressure is in each air bag and how far the vehicle is leaning to one side or the other.

If you are cornering, you naturally lean to the outside of the bend. This increases the pressure on the outside two wheels. If you rely on equalising the pressure between the air bags, it will lower the pressure in these to match, making you lean over even further - until you fall over! The pendulum therefore needs to have priority over the pressure equalisation.
    Initially, I decided to control the system with a single button and two variable resistors on the dash. The button was to control whether the vehicle was on or off road, one pot was for ride height and the other for lean - which overrode the inclinometer used to self level.

    I found however that although this worked, it did not allow me to exploit the full potential of the system and very often I switched back to the box with toggle switches to give me individual control of the air bags.
    I designed a new interface based upon a 4 line by 20 character LCD display. Milford conveniently supply these as well as the computers. The one I bought is described as a data entry terminal as it supports 16 push buttons and sends the button pressed back to the controlling computer. This made it easy to build a control box only needing four wires connecting it to the computer (Ground, +12v, RS232 In and Out).

    The display has four buttons on either side corresponding to the four display lines plus another two on top. The two on top scroll between pages and the four on either side act as 'soft' buttons (like on your mobile phone) where the label for the button is displayed on the LCD and it's function changes with the screen.

    The LCD is capable of displaying simple graphics - and I have a plan to write a 'Defender' style game to run on the computer with vehicle shaking effects!!! - but for clarity & speed, the screens are mostly textual.
    There are three screens:  
    1. Full Manual control.
This shows the pressure in each of the air bags. The buttons allow you to increase & decrease (UP & DN on the display) the pressure in each bag. There are also four 'blocks' in the middle. The top two move from side to side as the vehicle leans and the bottom two depending on the differential pressure front to rear. If all four blocks are in the middle, the vehicle is level.

    2. Off Road control.
This is a set of functions which you might want to use off road such as leaning from side to side, front to rear and forced articulation. This is intended for say a ditch crossing or perhaps to climb up a step with one wheel. It lowers the pressure on two diagonally opposite wheels and increases the pressure in the other two.

    3. Full Auto control.
This is the clever bit! It takes in readings from the inclinometer and the pressure sensors and tries to do the equivalent of moving the four blocks into the middle of the screen (as in screen 1). It has taken a lot of work to make the whole thing stable (critically damped) whilst still reacting fast enough to be useful, but not so fast that it uses air faster than it can be pumped.

    It has buttons to let you trim the lean left to right as well as front to rear as well as set the ride height.

It has an on and off road setting (top button on either side of the screen) which changes the level of damping, gain & hysteresis to make it react more quickly and dramatically off road. On the road, it makes occasional small adjustments to self level. The adjustments need to be small as they feel as though they are amplified when traveling at speed and it feels like some kind of 'bucking bronco'! On a Land Rover, if the vehicle changes height, it has a small, but noticeable effect on the steering. This I think is the cause of the 'bucking bronco' feeling.

After a lot of playing, I think it's just about right now!

    Click Here to see it in action!

    Punctures! - How to repair a punctured air bag (air spring)
Although they are pretty tough and well protected, it is possible to puncture an air bag!

Traditionally, you would have to chuck it away and buy a new one, but I wondered if it would be possible to repair the puncture? - Then forgot all about it assuming it wouldn't happen!

At Slindon Safari Winch Challenge this year - I did manage to puncture a bag on a splintered bit of wood which I kind of drove into. Tony Cordell tried valiantly to fix it with his motorcycle puncture repair kit, but to no avail. The bag contorts so much as it moves, and the surface is covered in small ribs that there was no way it would stick.
    I did a bit of hunting and found this thing called 'Safety Seal' from the US. It is intended to fix punctures in tyres without taking the wheel off the car and only takes seconds to do. I chatted up the European Importer and soon I had a kit!
    It has a special needle which you use to inject a plug into the hole. It's illegal to use on tyres in the UK - but there's nothing stopping you from using it on an air bag! They work pretty well on tyres too - you know, that set you only use off road?!
    For bigger holes, I recommend sticking two seals into the hole (thread two through the needle). Over a certain size, one is likely to be forced out by the pressure! In practice that leaves you no worse off than before and you can then fix it as before (with two seals) and carry on!
    After getting my puncture, this has given me much more peace of mind - particularly on long journeys or abroad where recovery might be difficult. I know that I can fix it in a couple of minutes and don't even have to remove the bag.
    Click Here to see all the photos of the build process.