What is Ride Quality?
A good 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. 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. A simple explanation, but more complicated to actually perform!
How do I get a good ride quality with airsprings?
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 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 controls the rate of compliance of the suspension. Springs are typically rated by two specifications, Spring rate and load capacity. 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”. For example, if a spring has a load capacity of 1000lbs and a spring rate of 200 lbs/in, it will take a 200 lb weight change to make the spring gain or lose 1”. 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. 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 100psi 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 leafspring, coilspring and an airspring 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 poor a ride over small bumps. Since there are a wide range of vehicle weights, spring types, and customer preferences, adjustable shocks go a long way towards optimizing ride quality for your car.
The tires and suspension bushings also greatly influence ride quality. Most OEM suspensions use large soft rubber bushings and tall tires 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 such a combination.
No, really, how do I get a good ride quality with airsprings?
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. This is all done to optimize ride quality.
Some of the higher end vehicles use electronically adjustable shocks to optimize the ride and handling over a wide range of road conditions. Thousands of hours and millions of dollars are spent to optimize each OEM application. When that OEM combination is changed, all of that research is voided to some extent.
Now think about building your street rod. For many people the first priority is to use components that will fit or are affordable. While the general guidelines concerning bushings and tires can be followed on any or every 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!
Assuming that a feasible airspring 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 but it 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 adjustable shocks, 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 air pressure set too low or shock set too soft [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 designed 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. Your favorite ride quality should occur within a 1/2″ of that dimension, regardless of the air pressure. After that is accomplished, start playing with the shock adjustment.