How To: Air Suspensions 101

So you want to slam your rat rod, huh? Cool. Before you rush out to the garage and rip out your suspension, there are some steps you need to take to ensure you understand what you are getting into. As with all my other writings, this tech article is meant to serve only as a guide. Many of the technical aspects of certain types of suspensions are only briefly touched upon in order to keep things as clear and concise as possible. As it is, I am going to have to split this article into four consecutive posts due to its length. So let’s get on with it:

Part I:


Understanding The Air Suspension Components:

Simply stated, air bags are the devices that are responsible for raising, lowering, and adjusting the ride height of a vehicle with an air suspension. For the vast majority of two-wheel drive vehicles, air bags replace the front suspension springs between the frame and the lower A-arms. Rear air bag setups can vary, depending on the type of rear suspension the vehicle has. Most commonly, air bags are either installed in such a manner that they either ***ist in the load bearing capacity of the rear leaf springs (air over leaf), or they completely replace the rear leaf springs. We will discuss these two different rear air bag setups later in the article. It is important to note that air bag is a generic term. Where p***enger vehicles are concerned, there are two primary types of air bags that you should be aware of:

Air Springs: Air springs consist of a rubber bellow(s), sealed by a top and bottom plate. When inflated with compressed air, the rubber bellow(s) will expand, causing the distance between the top and bottom plates to increase. When the rubber bellow(s) is deflated, the distance between the top and bottom plates will decrease. As such, inflating and deflating the air springs will either raise or lower vehicle with an air suspension. Air springs are available in several types, lengths, and load capacities. Air springs offer several advantages, including low cost, multiple size/shape/load capacities, and ease of installation. The primary draw back to air springs is that they are load supporting only, and offer no suspension dampening. Separate shock absorbers must be run with air springs to dampen and control the suspension bounce produced by the air springs. As a terminology note, air bags that have two bellows are typically referred to as “double convoluted” (see Figure 1), three bellows are “triple convoluted”, and so on.

Figure 1: Double Convoluted air spring (click on thumbnails to view larger image)


Air Shocks: Air shocks share the same principle as air springs, with one key difference. The bellow(s) of an air shock fit around a shock absorber, rather than simply being sealed with a top and bottom plate. The primary benefit of this design is a single unit that will both offer load support and suspension dampening (see Figure 2). Advantages of air shocks include compact, all-in-one design, ease of installation, and fewer parts due to the elimination of an external shock. Disadvantages include more limited size/shape/load capacities, and cost.

Figure 2: Two pair of Double Convoluted air shocks with integrated 10-way adjustable shock absorbers. Notice that the front suspension air shocks have upper stud/lower eye mounts, where the rear suspension air shocks have both upper/lower eye mounts. We will see why this is important later in the article.



Air Compressor: As its name implies, the air compressor is the device that compresses air to provide positive air pressure to inflate the air bags. Compressors can be either electrical or mechanical. Electric air compressors offer several advantages, including flexibility of mounting locations, size, and ease of installation. Disadvantages include cost, slower inflation times, and noise (see Figure 3). Mechanical, or engine driven, air compressors function much the same way a vehicle’s air conditioning compressor does. Mechanical air compressors are mounted by brackets connected to the engine block, and are typically driven by a belt connected to the crank pulley. Mechanical air compressors offer the advantages of low cost, and fast inflation times. Disadvantages include difficult installation, and engine drag (see Figure 4).

Figure 3: Generic electric air compressor


Figure 4: Generic mechanical air compressor


Air Tank: Air tanks are simply sealed holding cells that store compressed air supplied by the air compressor. Tank sizes can vary widely. However, 2-5 gallon tanks are the most common sizes for p***enger vehicles (see Figure 5). Though air tanks are not absolutely necessary for an air bag system to function, they serve two critical purposes. Air tanks store enough compressed air to keep the air compressor from constantly running. Therefore, it is only necessary for the compressor to maintain the air pressure in the tank, resulting shorter duty cycles and longer compressor life. Secondly, air tanks offer enough air volume to inflate all the air bags quickly and simultaneously, promoting even inflation and ride height. Given these critical functions, air tanks are very important, and should not be overlooked when piecing together and air suspension.

Figure 5: Typical 5 gallon aluminum air tank. Notice the tapped bung provisions for separate lines to each of the four air bags, as well as an inlet line from the compressor.



Air Distribution Block: This is arguably the least understood part of an air bag suspension. Much like a hydraulic brake system, an air bag suspension requires a distribution block to provide pressurized air to each of the four air bags. The distribution block is the actual hardware component that governs air pressure to the air bags, by either inflating or deflating the air bags through two-way valves controlled by an electric solenoid. Distribution blocks typically have separate valve provisions for an air tank and each of the four air bags (see Figure 6). Distribution blocks can be electronically connected to separate control units, that can perform a mul***ude of advanced functions including air pressure monitoring, automatic leveling, preset ride heights, active suspension adjustments to counteract body roll, etc. These control units can be mounted inside the vehicle’s cabin, providing both control over, and information about, the air suspension.

Figure 6: High quality air distribution block with integrated electric solenoids. This distribution block can be connected to multiple control units, thus giving it great flexibility.


Air Lines And Fittings: Air lines and Fittings come in all sorts of sizes and configurations, depending on your particular application, and can range from 1/8” ID to 3/8” ID. Air lines can be made of rubber, braided stainless steel, or other materials. Only DOT approved, pressure rated pneumatic line should be used for air suspensions. Never use non-pneumatic tubing, as it will certainly fail under the high psi demands of the air suspension. When purchasing air lines and fittings, it is important to keep in mind that larger inner-diameter lines and fittings will allow for quicker inflation times.

 

Part II:

Determining Your Air Suspension Components: Now that we have covered the basic components of the air suspension, you will need to determine whether to use air bags, air shocks, or a combination of the two. There are a mul***ude of air suspension manufacturers, each of who offer numerous kits. To determine which air suspension parts or kit is right for you, there are a few things you need to consider:

Determining Measurements: In practical terms, air suspensions have only two primary states: Ride Height and Parked Height. Ride height is the vehicle’s height when the air suspension is inflated. Parked height is the vehicle’s height when the air suspension is deflated. Ride Height is typically 60-80% of an air suspension’s maximum extended length, where Parked Height is typically 100% of an air suspension’s maximum compressed length. The difference between the maximum compressed and extended lengths determines the air suspension’s stroke.

Measuring: Using a tape measure, measure from the bottom of the frame immediately behind the front wheel, to the ground. Then measure from the bottom of the frame immediately in front of the rear wheel to the ground. Make note of both measurements. If the body is channeled over the frame, the same measurements can be made from the bottom of the rocker panels. Next, envision the height at which you would like your vehicle to be at Parked Height. Subtract the envisioned Parked Height from the current height. That difference will determine the amount of suspension drop required. Second, you need to consider the vehicle’s desired Ride Height. You want to make sure you can raise your vehicle enough have reasonable approach and break-over angles, as well as be able to navigate obstacles such as speed bumps.

Considerations: There are practical limits to how much you can lower a vehicle. The front upper and lower ball joints have a finite range of motion. Usually, front suspension bump stops are installed to keep the ball joints from being over extended. Front bump stops typically represent 95% of a ball joint’s range of motion. For a car that is to be driven regularly, the front suspension should never be lowered beyond the suspension bump stops (see Figure 7). Lowering a vehicle beyond the working range of the front ball joints places great stress on the joints, and can result in premature failure. This can be extremely dangerous since the ball joints are what hold the front spindles on the upper and lower A-arms. Also, lowering the front suspension beyond the bump stops can result in high degrees of negative camber, which can often be very difficult to properly correct.

Figure 7: Note the Energy suspension polyurethane front bumper and frame stop directly behind the sway bar connection.


A second consideration to keep in mind is the interference of the frame with the rear axle. For many p***enger vehicles, there comes a point during the lowering process when the frame will come into physical contact the rear axle. Unfortunately, this often creates a disparity between the front and rear heights. The solution is to “C-notch” the frame. C-notching the frame provides the necessary additional clearance for the axle when the frame is lowered (see Figure 8).

Figure 8: Wonderful view of a C-notched frame. Photo courtesy of ubolt.




Determine A Budget: Air suspensions can run a wide range of prices. Based on the amount you would like to lower your vehicle, and the specific measurements you have made, the exact parts available for your particular application will be vary. This is where it is important to research different manufacturers to determine what parts, suspension kits, electronics, etc. are available. The cost of a complete air suspension can be anywhere from $500-$3000+. Therefore it is very important to know what parts are necessary to lower your vehicle, and the cost of said parts, before you tear out the existing suspension. Being surprised by the cost of air suspension parts when your vehicle is on blocks is something to be avoided.

 

Part III:

Air Suspension Design: Despite all the different applications, p***enger vehicle air suspensions are typically designed one of three ways. The primary differences from one air suspension to another concern the different types of components used. With the exception of very early model solid front axle vehicles, front air suspensions are for the most part generic. However, rear air suspensions can be setup several different ways. Let’s take a look at the various configurations.


Front Suspension: Almost all early to mid model two wheel drive p***enger vehicles use an A-arm configuration for the front suspension. Typically, a coil spring or strut sits in a spring pocket, which is part of the vehicle’s frame (see Figure 9). The bottom of the spring is then connected to the lower A-arm. This spring is load bearing, and supports the sprung weight of the vehicle (see Figure 10). When installing an air suspension, an air spring or an air shock replaces the coil spring or strut ***embly.

Figure 9: Front spring pocket viewed while the frame is upside down for painting


Figure 10: Original metal coil spring in the frame’s spring pocket and connected to the lower A-arm


Air Springs: Since air springs are typically short, they often requite a metal extension from the air spring’s top plate to reach the underside top of the spring cup. This extension is often has a threaded stud to allow it to be bolted to the top of the spring cup As we discussed earlier, air springs require separate shock absorbers to control and dampen the suspension bounce caused by the elasticity of the air bellow(s) (see Figure 11-12). No matter what the specific vehicle application is, the basic components for this type of front air spring design remain the same. As such, there are a mul***ude of front air suspension kits that come in this type configuration, for a wide range of vehicles.

Figure 11: Example air spring front suspension, complete with metal extension, new tubular lower A-arm, shock absorber, and bracket.


Figure 12: Air spring front suspension with metal extension, shock absorber, and bracket installed, using stock lower A-arm.



Air Shocks: As we discussed earlier, air shocks serve as both the air spring and shock absorber, in one single unit. If your vehicle has a strut ***embly, the air shock will replace the strut. If your vehicle has coil springs and separate shock absorbers, the air shock can replace both the springs and shocks, thus creating a very clean appearance (see Figures 12-13). Unlike air springs, air shocks are long enough to reach the underside top of the spring cup. Sometimes it is necessary to add a spacer(s) to the top of the air spring to account for spring cup taper (see Figure 14).

Figures 12-13: Air shock installed in spring pocket




Figure 14: Air shock installed in spring pocket. Notice washers were used to account for the space between the top underside of the spring pocket and the top of the air shock. The washers provide just enough spacing to prevent the frame’s weight from resting on the edges of the top “hat” of the air shock, due to the tapered sides of the spring cup. Relief had to be ground into the frame to allow adequate room to install the air lines and fittings. Braided stainless steel air lines were used here since the air lines run extremely close to the exhaust headers.


Connecting the lower portion of the air shock to the A-arm is straightforward. A very useful component to accomplish this is a bar pin eliminator (see Figure 15-16). The bar pin eliminator is easy to install in the lower eye bushing, and allows for the proper bushing pre-load.

Figure 15: Exploded view of a bar pin eliminator ***embly


Figure 16: Bar pin eliminator installed in lower eye busing, and connected to lower A-arm.


That covers the design of a front air suspension for vehicles with an A-arm configuration. As you can see, there is really only one design method, which can be achieved using either an air spring ***embly or air shocks.

 

Part IV:

Rear Suspension: Just as air bags replaced the load carrying capacity of the coil springs or struts of the front suspension, air bags serve the same function for the rear suspension. However, there is one critical difference between the front and rear suspension, and that is location of the rear axle. There are three common methods of locating the rear axle with an air suspension: Leaf spring, Parallel 4-Link, and Triangulated 4-Link. Although there are several other types of rear suspension setups, this article will only deal with these three designs. Since this article deals with the design of air suspensions, many of the technical aspects of planning and installing parallel and triangulated 4-link suspensions will not be covered.

Air Over Leaf: This is the easiest type of rear air suspension design to implement, requiring the least amount of fabrication and the fewest components. The leaf springs of a stock suspension perform three critical functions. They support the sprung weight of the vehicle, locate the rear axle back-to-forward and side-to-side, and determine the pinion angle. When lowering a vehicle using the air over leaf design, the leaf spring packs will need to be dis***embled, and most of the leaves will need to be removed (the number ot leaves removed will vary depending on your application). The spring packs are then reinstalled, thus properly relocating the axle. With the majority of the leaves removed, the springs’ load bearing capacity has been virtually diminished. As such, and air spring is subsequently installed on both the driver and p***enger side, between the either the top of the axle or the leaf spring pack, and the frame (see Figure 17). In this configuration, the air spring ***umes the load carrying capacity of the leaf spring. When inflated, the air spring will raise the vehicle, and will lower the vehicle when deflated.

It is important to note that the pinion angle may need to be adjusted to account for lowered suspension. Pinion angle adjustments can be achieved with spring perch shims.

Figure 17: Air over leaf diagram


Figure 18: Air over leaf setup installed on a 1961-1967 Buick p***enger vehicle


Triangulated 4-Link: This type of rear suspension requires quite a bit of planning and fabrication. However, the benefits of this design allow for a great deal of flexibility. A triangulated 4-link suspension replaces the entire rear leaf spring suspension ***embly with upper and lower control arms. The lower control arms typically run parallel to one another, as well as parallel to the ground, and serve to locate the rear axle forward-to-backward. The upper control arms however, are triangulated, which locates the axle side-to-side, and prevents any side-to-side movement of the axle (see Figure 19). The length of both the upper and lower control arms can be adjusted to alter the pinion angle.

Figure 19: Wonderful view of a Parallel 4-Link rear suspension. Notice the triangulated upper control arms mounted to the top of the Ford 9” rear axle, and the parallel lower control arms mounted to the bottom of the axle tubes. Photo courtesy of ubolt.


Once the upper and lower control arms of a the triangulated 4-link have been installed, it is necessary to install either air springs and shocks, or air shocks to support the vehicle’s weight. In many cases, a frame crossmember is located above the rear axle, and runs horizontally from the driver side frame rail to the p***enger side frame rail. This crossmember was a common place to mount the upper tabs for the rear shock absorbers, and makes for a wonderful attachment point for the upper air spring perches (see Figure 20).

Figure 20: Air springs installed between the rear axle and a fabricated crossmember. The air springs are bolted to spring perches welded to the rear axle tubes and crossmember. Photo courtesy of ubolt.



Parallel 4-Link: Like the triangulated 4-link, the parallel 4-link rear suspension requires quite a bit of planning and fabrication. The prarallel 4-link also allows for a great deal of flexibility. A parallel 4-link suspension replaces the entire rear leaf spring suspension ***embly with upper and lower control arms, and a panhard The upper and lower control arms run parallel to one another, as well as parallel to the ground, and serve to locate the rear axle forward-to-backward. The panhard locates the axle side-to-side, and prevents any side-to-side movement of the axle (see Figure 21). The length of both the upper and lower control arms can be adjusted to alter the pinion angle. Since the leaf springs have been removed, it is necessary to install air springs or air shocks to restore the load bearing capacity (see Figure 22).

Figure 21: Mock up of a parallel 4-link rear suspension. The leaf springs are only present to locate the axle prior to welding the parallel 4-link ***embly in place. Once welded, the leaf springs, perches and shackle hangers will be completely removed. Notice the panhard extending horizontally from the p***enger frame rail to the Ford 9” third member.


Figure 22: Mock up of a parallel 4-link rear suspension, using air shocks instead of air springs. The bottom eye of the air shocks will connect to the mounting flanges to be welded to the bottom of the axle tubes, and will then connect at an angle to tabs that will be welded to the crossmember. It is important to note that mounting air shocks at angle reduces their load capacity. The greater the mounting angle, the less the load capacity. This should be taken into account when designing the rear air shock mounts, and selecting the specific air shock units.


That concisely sums up the various types of rear air suspensions. As you can see, there are three primary rear suspension designs. Air over leaf requires the use of an air spring, while the triangulated and parallel 4-link designs can be achieved using either an air spring ***embly or air shocks.

Air Suspension Links And Resources:

http://www.airliftcompany.com/
http://www.suicidedoors.com/
http://www.ridetech.com/index.asp
http://www.airbagit.com/
http://www.airridepro.com/

There you have it. All you ever wanted to know about air suspensions. I'm tired of writing, so away I go. Cheers.

 

hahahahahhahaha

 

Who asked for that stuff?

 

Thanks.. i so wanna drop my rat now...

 

thanks for taking the time to post. im sure it's going to be useful for many wanting to learn the ins and outs of air bags...

-scott noteboom

 

i'm still trying to figure out how zman delete his reply?! vanished like this guy should

 

DAMN! That guy can type fast. Look at the time stamps on each of his posts!

probably an adverti*****t from the people that distribute to the linked websites... but useful info, none the less.

 

Who cares if its copy and paste? Useful information alot of us can use....Thanks for posting this.

 

Some of it is useful, but I'm having a hard time with the "mystery poster".

 

Thanks for the info
Bagging mine at the moment

 

This guy is on ol skool rodz and wrote all of these on there. He is copying and pasting to this board his tech posts.

Some good info in there.

 

killer post. cut/paste or othewise...its compiled and all in one place.

lord knows i need all the info i can get.

 

Bump for great tech!

 

You mean bump for the cut and paste...

 

I'm too lazy to read all that.... I'll stick to cut springs and lowering blocks.




(Good info)

 

I was just looking for it...