| |
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.
|
| |
AutoMint
|
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
|
|
| |
|
Control |
|
| |
|
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. |
|