Ridin' Right

Every once in a while a new product crops up that significantly impacts the classic truck hobby. Certainly the availability of fiberglass bodies, reproduction bed assemblies, and reproduction frames fall into that category. Another that will doubtlessly make the list is air spring suspension.

Air springs are nothing new, they've been used on trucks, busses, and in industrial applications for quite some time. General Motors and Packard used them on production cars in the '50s and '60s, and they are found today under a variety of luxury cruisers and SUV's. And while it took some time for them to show up on the classic truck scene, you can bet that they're here to stay.

One of the pioneers in aftermarket air suspension business is Brett Voelkel of Air Ride Technologies. Brett has put air springs on everything imaginable and knows how to make them work like they should. Because of his experience and expertise on the subject, we asked him to respond to some of the more common questions truckers ask about air suspension systems, and throw in a few tips while he was at it.

CT: What do most of your customers expect from an air ride system?

BV: We have found there is about an even split between the desire for better ride quality and the desire for a lower stance. Our street rod customers want a Cadillac ride and our truck customers want to drag the ground. We can provide both, within reason.

CT: How do you assess ride quality?

BV: A good ride 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 ten-year old pickup. Technically, we define good ride quality as the ability to minimize the effects of road irregularities to the vehicle passengers. When the vehicle encounters a pothole or bump in the road, it should transverse the obstacle with as little body motion as possible. Simple to explain, but it's a good deal more complicated to accomplish.

CT: Okay, so how do you do it?

BV: To understand how to achieve a good ride quality, is to understand exactly what happens to the suspension when the wheel hits a bump. Ideally the suspension should absorb this bump 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 establishes the suspension's rate of compliance. Springs are rated by two specifications: Spring 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 height by 1 inch. For example, if a spring has a load capacity of 1,000 pounds and a spring rate of 200 pounds, it will take a 200-pound weight change to make the spring gain or lose 1 inch. The higher the spring-rate, the more load change it takes to change spring height. Most traditional spring manufacturers list the spring rate of their springs, not the load capacity. The load capacity of a traditional spring at a given height does not change. Air springs are rated by load capacity at 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 must be matched to the vehicle weight, the suspension geometry and the spring used in that suspension. A leaf spring, coil spring, and an air spring all have very different spring rate patterns and would require different shock valving to optimize ride quality. Too soft a shock may let the suspension bottom out on hard bumps. Too stiff will result in a poor ride over small bumps. Since there are wide ranges of vehicle weights, spring types, and customer preferences, adjustable shocks go a long way towards optimizing ride quality for your truck.

Tires and suspension bushing also greatly influence ride quality. Most OEM suspensions use large soft rubber bushings and tall tires to enhance ride quality and noise transmission. Some classic truckers like to use polyurethane bushings and low profile tires. It is especially tough to get a decent ride quality with such a combination.

CT: Other than being able to raise and lower your vehicle, how do air bags affect ride quality?

BV: This is exactly where the air spring suspension shines! Assuming that a feasible air spring is selected for the suspension, it can be tuned to the parameters of the vehicle and the customer 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. This would require the aforementioned spring and shock testing. All of this tuning would need to be repeated when there are significant changes made to the fuel load, passenger load, or road surface. With an air suspension and an adjustable shock, these changes can be made in a fraction of the time. It is analogous to tuning a carburetor or tuning EFI. The EFI is much more versatile. The analogy to EFI is relevant in other ways, too. As with anything adjustable, it is possible to adjust yourself into a terrible ride quality. The mistake most people make is too low of air pressure or too soft a shock [or shock adjustment]. This will allow the airspring to collapse too rapidly over a bump. Since an airspring is very progressive (spring rate rises in compression) it feels too firm. Many times adding air, or using a firmer shock will actually result in a better ride quality.

Symptoms of low air pressure are bouncy ride on rolling bumps and bottoming on potholes. The best way to start tuning an air suspension is to inflate the airsprings to their design height. Any competent air suspension manufacturer will be able to tell you this dimension. This is where the airspring is designed to work the best. The best ride quality should occur within a 1/2 inch of that dimension, regardless of the air pressure. After that is accomplished, start playing with the shock adjustment.

CT: What is the ideal air pressure for bags?

BV: That's a question with many answers! Run whatever air pressure it takes to get the air spring to its design height. A '32 Ford may take 45psi; a '57 Chevy may take 75-psi; an F-100 may take 60 psi. 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 versus the pivot point versus the position of the air spring will be a huge factor in determining the selection of the air spring as well as what air pressure is required. If you are building your air ride suspension from scratch, these are just some of the factors that have to be considered. While you are at it, you must account for ground clearance, driveline angles, and ball joint travel limits. If you have purchased a pre-designed system from a manufacturer, these issues have been addressed [or should have been].

CT: How difficult is it to build a custom suspension from scratch?

BV: No Problem! Building a successful air ride suspension is no different than any traditional suspension. It is actually easier. In a leaf, coil, or torsion suspension you have to have some educated guesswork as to where the vehicle ride height will end up and how the ride quality will be. Everyone has their favorite recipe for using coil springs out of a Malibu with one coil cut off, or leaves from an S-10 with one leaf removed. These will all work to a certain extent, but lets look at why they work and what would make them not work as well.

Let's say your buddy builds a '48 Ford with a Camaro front subframe and S-10 leaves in the rear. It rides great! You want to build one the same way, only yours will be powered by 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 leaves that will be appropriate for your vehicle.

With an air suspension it is a matter of adjusting the air pressure to accommodate the extra weight in the front and less weight in the rear. The added benefit is that when you add a bit of a load for that trip to the Supernationals, all it takes is an extra squirt of air to level the truck and restore the ride quality.

CT: What do you suggest for general air suspension guidelines?

BV: There are a few things to be aware of when building any suspension, especially an air suspension:

Ride Height
The vehicle needs to be mocked up at your intended ride height. This ride height needs to reflect proper ground clearance (at least 4 1/2 inches) and adequate suspension travel (at least 3-inches compression and 2 inches in rebound). After that, start looking for a place to install the air springs at their intended design height. This is very important! If you have an air spring 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 vehicle ride 2-inches taller than you want. If you deflate that air spring to that 7-inch height to make the truck sit right, the ride quality will suffer. Any competent air spring supplier will be able to give you the compressed, ride height and extended dimensions of their air springs.

Ground Clearance
A lot of customers want to "lay the truck 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. Remember that you need at least 4 1/2 inches of ground clearance at ride height to be able to negotiate speed bumps and other road obstacles safely. I don't care what anyone says, it is not cool to scrape!

Driveshaft and Ball Joint Travel
Ball joints 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.

Air spring clearance
The only rule here is that the air spring must never rub on anything at any time. Period!

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 much tougher to accomplish this.

CT: What about the compressor system?

BV: After you have properly designed and installed your air ride suspension, you will need a source of compressed air and a way to control it. While it is possible, at least in theory, to use an inflation valve or even a bottle of compressed air to inflate your suspension, the most efficient method is an on-board compressor with a reservoir tank and a control valve. It is with the compressor system that you can upgrade, customize, economize, or thoroughly overkill without drastic compromise. If you are doing a simple supplemental air spring over a leaf spring, then an inflation valve may be enough. Any stand-alone air suspension really needs an on-board compressor and control system.

Here is why: Ride quality tuning is done in very small air pressure increments. Because the air springs are quite small in volume, it is very hard to inflate or deflate in small enough increments to zero in on a great ride quality. In addition to that, when you add 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.

The sky is the limit on compressor systems. The main difference here is rise time and convenience. The faster you want your vehicle to come up the more compressors, the more pressure, and the more reservoir tanks you will need. A typical trucker who is not concerned with rise time will probably use a single compressor with a 2-gallon tank and a two-way controller. If you want that truck up in 2 seconds it will take a pair of 150-psi compressors, a pair of 3-gallon tanks and a four-way control system. Obviously this extra equipment will cost more money.

CT: Why would four-way control be an advantage?

BV: There are several reasons to control the air springs independently. Probably the biggest is to eliminate air transfer when cornering. In a two-way configuration the air springs are tied together from side to side. For example, when you negotiate a turn to the left, the vehicle will tend to lean to the right and place more load on the right hand suspension. The loaded right hand air spring will then start to transfer air to the unloaded left side air spring, thereby worsening the problem.Another issue is the leaning vehicle. If everything is equal, meaning weight, frame level, and bushing resistance, then the truck will level. In the real world, however, driver weight, gas tank weight, and uneven suspension bushing resistance may combine to make the vehicle lean. Then, just as with a coilover, you have to "wedge" it level with a bit more air pressure on one side or one corner. These are very common problems, especially for top-heavy vehicles with polyurethane bushings and no sway bars.

A four-way control system will also offer a faster rise time because it will flow more air. And while 12-15 seconds may not seem like a long time, if you are sitting there with your finger on the switch it may seem like forever! The RidePro solenoid system, for example, also offers a simpler plug-in installation and a digital gauge option. When new products are developed, such as electronic leveling systems and remote controls, they will require the use of a 12-volt solenoid air valve.