Let’s start by getting some of the terminology right – there is no such thing on a car as a shocker (sometime called shock absorber) – the spring is the shock absorber, and then the spring resonance is damped by the damper. What many call the shock or shock absorber is in fact the damper.
Cars therefore have a spring which is normally metal and wound in a coil, and a damper. On the front of most modern cars the spring sits around the damper, but is sometimes separate from it on rwd cars.
Another thing to note is that on most cars there is no such thing as a coil over conversion – most cars have coil over damper setups anyway (specifically most fwd cars).
So what is the air all about then?
Well in essence (and a great over-simplification) we replace the standard coil spring that all modern cars have with an air bag. An airbag is often also called a bellows, or air spring.
Of course there is much more to it than that, but that’s the basics, so lets for a minute forget everything else.
With a normal coil spring the rate is fixed. Its measured for the most part in pounds per inch – i.e. how many pounds of load are required to compress the spring by 1 inch. That figure would normally be in the order of 100-1000.
A 200 lb/inch (yes I know the metric Police are probably on their way as we speak, but since almost all air kits have their foundations in the USA they are almost all imperial) spring will compress by 1 inch when a load of 200lbs in placed upon it, and by 2 inches when a load of 400 lb is placed upon it.
Again it’s a simplification, but for the most part the spring rate is fixed at manufacture. Because the weight of the car is constant and the spring rate is constant, so the ride height of the vehicle is constant. This remains the case until you move the car and go over a bump when the load on the spring then varies, and so the ride height varies.
When you go over a bump the spring tries to return the car to its normal ride height, and would spring up and down about 20 times before it came to rest (or you went over another bump) if it were not for the action of the damper to damp out this resonance and make the spring stop after about 1.5-2 cycles.
When you fit air you change the metal spring for an air spring. The basically carries out the duty of the spring, and still needs a damper to operate correctly.
The advantage to the air spring is that it is a variable rate spring – the more air you pump in then the higher its spring rate. Thus by inflating the bag we can take the spring from 0lb/inch (no air) to probably 600lb/inch with 200psi in it.
This variable rate has two effects, firstly the higher the spring rate then the more load is required to deflect the spring, but the second effect is that the ride height of the vehicle varies in proportion to the air pressure exerted inside the bag.
Technically it’s not a linear scale but again for simplicity we will assume that it is. Therefore the height of the front bumper from the ground might be 4 inches at 0 psi, and rise to 9 inches at 200psi. Thus we can control the height of the vehicle simply by the addition or subtraction of air from the bag.
One thing to note about air systems is that contrary to what you might be used to the car will actually corner better when it is higher (and the spring rate is higher) as it will tend to roll less into the corner.
Air Bags and Fitting
The air bags come in various sizes, and shapes, but for the most part on European Modified cars (fwd not rwd or 4wd) we tend to use the double convoluted bags in bead seal and crimped end varieties. these have a range from fully up to fully down of about 5 inches which is good for what we want, although it should be noted that the vehicle is normally driven at a height somewhere towards the middle to upper 2/3 of the range.
The bags must be fitted by brackets as the bolt onto a flat surface, and the originals did not. For the front of fwd a different approach is needed as the airbag must surround the damper for the suspension to function. this is a complex item, and it is not surprising that the cheap import kits do not tend to offer reliable solutions to the problems experienced in mounting the bags in this manner.
Well with all this talk about air, the question is where does it come from? That’s a good question, with the very very basic van kits (some people sell these for modded cars) you need to go to a workshop or petrol station and using a tyre inflator have the system filled with air – that’s great for a van or motorhome needing a fill once a month or so, but for a car, we need an on board inflation system.
The slightly less basic van systems use a very small compressor with no tank, and the compressor pumps up the bags directly. One competitor does in fact supply these for modded car, but since they take around 5 minutes to inflate the car are once again best left to vans, and motorhomes.
We use a compressor which fills a tank with air at a pressure of between 150 and 200psi. The compressor is powered from the battery (we often fit a second battery for this purpose so you can never end up with a flat battery that cannot start your car).
The tanks can be from between 1 gallon and 6 gallons, although experience shows that above 3-4 gallons a second compressor is probably a good idea, to prevent premature failure of the compressor.
A slow system uses a small compressor, and small tank and will lift the car 1-1.5 times before the tank is empty. The compressor would take around 5 minutes to refill the tank, and the car would take around 15 second to lift the average car from fully down to fully up.
A fast system (our standard) uses a far larger compressor (see picture next to coke bottle). The compressor is larger but actually runs far slower – only about 1500 rpm, for durability, and fills a tank of around 3 gallons. Using 3/8 valves and lines this would lift the car in about 2-3 seconds, would lift the car 2.5 times on a tank of air, and take around 3 minutes to refill the tank.
An ultra fast system would use probably two compressors, a larger tank and would also use ½ inch valves instead of the standard 3/8 valves. This would lift in well under 2 seconds, and take less time to refill the tank, although not half the time.
A mega Fast system would use even higher spec products.
Each system is custom built to the customer’s specifications depending upon what the objective is.
The control system is an entire area on its own. The most basic system used on slow, and ultra slow systems is a basic mechanical paddle which basically opens and closes a tap to let air in and out. This is very unsophisticated, and very slow but there is almost nothing to go wrong and it is the system of choice on vans for load levelling and load support. These systems typically take between 20 seconds and 5 minutes to lift the vehicle.
The next step if for solenoid valves. These are basically electrical taps that open and close electrically. Cheap solenoids (most imported US kits) are nothing but trouble, and continuously go wrong. High quality valves can cost up to ten times the cost of cheap and unreliable Chinese made (US supplied) ones.
Solenoid valves open up a number of control possibilities. The most basic is a simple electric window switch – up is up/ down is down and you can stop the car anywhere you want in between the two.
The next is a ten switch box which gives the ability to lift and lower the entire car in 1 hit, and is often fitted on a long lead so it can be operated from outside the car.
There are modified play station controllers, and (illegal in the UK) 24 function (get one to work if you can) remotes imported from the USA, although a great idea we would not advise use in the UK as the frequency they use is a military one!!! For the most part you are safe as out of the ten we have none work!!! Be very careful import US good, they never have a guarantee.
We have started using a computerised controller which has built in gauges to read the pressure in each bag. This has the added benefit of UK legal remote control which allows the entire vehicle to be adjusted in one hit to three preset positions. These are normally set to fully down, drive and fully up, but can be set to anything you want. I highly recommend this item for simplicity, reliability and cool factor.
The most basic systems have no way of knowing how high the vehicle is without getting out and looking.
Mid level systems use standard 52mm gauges which have two needles in each gauge and read the pressure (mechanically).
The computer listed above uses sensors in each circuit to read the pressure for the computer, and displays it on an LCD display.
On the whole the best components in each area will provide the fastest operating system. This can also (but not always) be the most reliable systems. It is a peculiarity of the market that mid level systems tend to be the least reliable (although note the word least, they are not unreliable), and top level computerised system is the fastest and very reliable in use.
Reliability should be considered a very important factor, as any fault on the system weather mechanical, electrical, or pneumatic will tend to render the vehicle un-useable, and difficult to recover due to the low stance. In the event of a failure the car will almost certainly end up at the lowest setting possible.
Thats a lot of waffle – I’ll add some pictures as soona s I can to illustrate it better.