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Technical - How to build an Air Suspension System |
Why do I want an Air Suspension system?
At Air Suspension Systems we have found that our customers are divided over whether a better ride quality or a lower stance is most desirable. Our street rod customers want a Cadillac ride and our sport truck customers want to drag the ground. The following answers explain how you can achieve a lower stance without sacrificing ride quality using our suspension systems.
What is ride quality?
Ride quality means different things to different people. Someone who is accustomed to driving a new Lexus or Mercedes will have an entirely different idea of ride quality from the guy who drives a 10 year old pick-up. We define good ride quality as the ability to minimize the effects of road irregularities to the vehicles passengers.
When the vehicle encounters a pothole or bump in the road it should traverse the obstacle with as little body motion as possible. Basically we want to achieve a smooth ride. It seems like a simple explanation, but is more complicated to actually perform!
How do I get a good ride quality?
To understand how to achieve a good ride quality, it helps to understand exactly what happens within the suspension. When the wheel hits a bump, for example, it must ride up over that bump. Ideally the suspension would absorb this impact with no transfer of motion to the body.
In the real world at least some of this motion will be transferred through the spring to the body. How much transfer takes place is affected by several components of the suspension.
The spring - be it coil, leaf, torsion, or air - is what holds the vehicle up and also controls the rate of compliance of the suspension. Springs are typically rated by two specifications, load capacity and spring rate. Load capacity represents the amount of weight that a spring will support at a given height. Spring rate represents how much weight change it takes to change the spring's height by 1 inch.
For example, if a spring had a spring rate of 200 pounds per square inch (psi), it will take a 200 pound weight change to make the spring gain or lose 1 inch. The higher the spring rate, the more weight change it takes to change spring height.
Most traditional spring manufacturers list the spring rate of their springs, not the load capacity. That is why you will find references for 200lb, 450lb, 500lb springs, etc.
The load capacity of a traditional spring can't change at a given height unless the diameter or wire thickness is physically altered. Airsprings, however, are rated by load capacity at the industry standard of 100 psi because changes in air pressure greatly influence both load capacity and spring rate.
The shock absorbers also control the rate of compliance of the suspension. A proper shock absorber must be matched to the vehicle weight, the suspension and the spring used in that suspension. A leafspring, coilspring and an airspring all have very different spring rate patterns and would require different shock valving. A shock absorber that is too soft would cause the suspension to bottom out on hard bumps. One that is too stiff will result in poor a ride over small bumps. Since there are a wide range of vehicle weights, spring types, and customer preferences, shock absorbers that are adjustable go a long way towards optimizing ride quality for your car.
The tyres and suspension bushings also influence ride quality. Most original equipment manufactured (OEM) suspensions use large soft rubber bushings and tall tyres to enhance ride quality and noise transmission. Some street rodders like to use polyurethane bushings and short sidewall tires. It is especially tough to get a decent ride quality with this combination.
No, really, how do I get a good ride quality?
Now that you have enough technical information to bore you for awhile, we will talk about real world combinations. The typical OEM vehicle will use a tall sidewall tire, large rubber bushings, a soft spring rate spring and a progressively valved shock absorber. Some of the higher end vehicles use electronically adjustable shocks to optimize the ride and handling over a wide range of road conditions. Hundreds of hours and millions of dollars are spent by manufacturers to optimize each OEM suspension application. When that OEM combination is changed, all of that research becomes void.
When it comes to building your street rod, your first priority is to use components that will fit or are affordable. While the general guidelines concerning bushing and tyres can be followed on any car, not everyone will spend the time and money necessary to repeatedly change springs and shocks to get the best ride quality. This is exactly where the air spring suspension shines! An air spring suspension system can be tuned to the parameters of the vehicle and the owner by changing air pressure while on the road. Likewise, an adjustable shock can be tuned to individual taste.
In theory it is possible to make a vehicle ride as good with traditional springs as with an air suspension, but this would require the aforementioned spring and shock testing. All of this tuning would need to be repeated when there are significant changes made such as fuel load, passenger load or road surface changes. With an air suspension and an adjustable shock, these changes can be made in a fraction of the time.
As with anything adjustable, it is possible to make the wrong changes. Common mistakes are making the air pressure too low or the shock or shock adjustment too soft. This will allow the airspring to collapse rapidly over a bump. Symptoms of low air pressure are a bouncy ride on rolling bumps and bottoming out on potholes. Many times adding air or using a firmer shock will actually result in a better ride quality.
The best way to start tuning an air suspension is to inflate the airsprings to the height at which they are designed to work best. Any competent air suspension manufacturer will be able to tell you this information. Your preferred ride quality should occur within half an inch of that height, regardless of the air pressure. After that is accomplished, start playing with the shock adjustment.
What air pressure should I run?
Different vehicles will require different amounts of air pressure. Run the amount of air pressure it takes to get the airspring to its design height. Keep in mind that the weight of the vehicle is only one factor in determining how much air pressure is required.
A much more important influence is the geometry of the suspension. The relationship of the load point vs. the pivot point vs. the position of the airspring will be a major factor in determining the selection of the airspring as well as what air pressure is required. If you are building your Air suspension from scratch, these are just some of the factors that have to be considered. While you are at it, you must also account for ground clearance, driveline angles, and balljoint travel limits. If you have purchased a pre-designed system from a manufacturer, these issues should have been addressed.
I want to build a custom suspension from scratch.
No Problem! Building a successful Air suspension is no different than building any traditional suspension. It is actually easier. In a leaf, coil, or torsion suspension you have to some educated guesswork as to where the vehicle ride height will end up and how the ride quality will be. Everyone has their favorite combination for using coil springs out of a Malibu with 1 coil cut off, or leafs from an S10 with a leaf removed. This will all work to a certain extent, but let's look at why they work and what would make them NOT work well.
Lets say your buddy builds a 48 Ford with a Camaro front subframe and S10 leafsprings in the rear. It rides GREAT! You want to build one the same way. Only yours will be a convertible with a big block. It now sits too low and rides like hell. Now you can start the research all over to find a set of coils and leafs that will be appropriate for your car.
With an air suspension it is simply a matter of adjusting the air pressure to accommodate the extra weight in the front and the lesser weight in the rear. The added benefit is that when you add 3 friends in the rear of that convertible for cruising, all it takes is an extra squirt of air to re-level the car and restore the ride quality.
I want some real guidelines.
There are a few things to be aware of when building any suspension, including an air suspension:
- Ride height -
The vehicle needs to be mocked up at your intended highway ride height. This highway ride height needs to reflect proper ground clearance, which should be at least 4.5 inches. An adequate suspension travel has at least 3inches during compression and 2 inches in rebound). After that, start looking for a place to install the airsprings at their intended design height. This is VERY important! If you have an airspring that is intended to ride at 9 inches tall and you try to make it run at 7 inches tall, the best ride quality will make the car ride 2 inches taller than you want. If you deflate that airspring to that 7 inch height to make the car sit right, the ride quality will suffer. Any competent airspring supplier will be able to give you the compressed, ride height and extended dimensions of their airsprings.
- Ground Clearance -
A lot of customers want to lay the car on the ground. DON'T DO IT! You must have at least an inch or so of ground clearance when the air system is fully deflated. You also need at least 4.5 inches of ground clearance at ride height to be able to negotiate speed bumps and other road obstacles safely.
- Driveshaft and balljoint travel -
Balljoints will only travel so far before they bind. The driveshaft will go only so far into the transmission tunnel before it hits. It is your job to find these limits and take them into account.
- Airspring clearance -
The only rule here is that the airspring must never rub on anything at any time. Ever.
It requires considerable thought and planning to properly satisfy all of these details. The nice thing about an air suspension is that you can inflate and deflate the vehicle through its full range of travel to check out all of these parameters. A conventional suspension makes it a much tougher accomplishment.
What about the compressor system?
After you have properly designed and installed your Air suspension, you will need a source of compressed air and a way to control it. In theory it is possible to use an inflation valve or even a bottle of compressed air to inflate your suspension. However, the most efficient method is an on-board compressor with a reservoir tank and a control valve.
With the compressor system you can upgrade, customize, economize or overkill without drastic compromise. Ride quality tuning is done in very small air pressure increments. It is very hard to inflate or deflate in small enough increments to achieve a great ride quality as the airsprings are quite small. In addition, when you add to the load, in the form of fuel, people or luggage, you have to go looking for an air hose!
You can quickly see how convenient an on-board system can be. Any stand alone air suspension really needs an on-board compressor and control system.
When it comes to compressor systems there are no real limits. The main difficulties here are rise time and convenience. The faster you want the vehicle to come up the more compressors, pressure and reservoir tanks you will need.
A typical street rodder who is not concerned with rise time will probably use a single compressor with a 2 gallon tank and a 2 way controller. If you want that car up in 2 seconds it will take a pair of 150 psi compressors, a pair of 3 gallon tanks and a 4 way control system. Obviously this extra equipment will be more expensive.
Why would I want to have 4 way control?
There are several reasons to control the airsprings independently. Probably the biggest is to eliminate air transfer when cornering. In a 2 way control configuration the airsprings are tied together from side to side. When you negotiate a turn to the left, for example, the vehicle will tend to lean to the right and place more load on the right hand suspension. The loaded right hand airspring will then start to transfer air to the unloaded left hand airspring, thereby worsening the problem.
Another issue is the leaning vehicle.
If everything is equal, meaning weight, frame level, and bushing resistance, then the car will level. In the real world driver weight, gas tank weight, and uneven suspension bushing resistance may combine to create an unlevel car. Then you have to wedge the car level with a bit more air pressure on one side or one corner. These are common problems, especially for top-heavy cars with polyurethane bushings and no swaybars.
The fifteen seconds it takes to rise may not seem like a long time but if you are sitting there with your finger on the switch it can seem like forever!
A 4 way control system will offer a faster rise time because it will move more air. The RidePro solenoid system also offers a simpler plug-in installation and a digital gauge option.
As with any serious suspension project, a proper air suspension system requires serious planning and forethought. DO NOT be afraid to ask questions! Ask several different sources. You will soon get a consensus of reliable information. When you find the right enthusiast that will give you the information that you need reward him with your business - you never know when you may need more information!
Summary
Advantages of an air suspension
- You can optimize the load capacity and spring rate for nearly any weight vehicle.
- You can RE-OPTIMIZE your suspension when the load changes, such as when adding extra fuel, people, luggage or a trailer.
- You do not have to know what the finished weight of the vehicle will be to select the correct components. You can tune the suspension to the exact finished vehicle weight.
- An air suspension is tunable for different driving styles and ride quality characteristics
- Your vehicle can be lowered dramatically for parking and fairgrounds cruising, and then restored to highway ride height for a safe, comfortable trip home
- You can compensate for various loads to maintain a consistent ride height, an air suspension can actually offer BETTER tyre wear and handling.
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