Apparatus, Chassis Components

Understanding Air-Ride Suspensions

Issue 8 and Volume 22.

FAMA Forum   By Bob Albano

Air-ride suspensions have steadily increased in popularity in nearly all segments of the trucking industry. Approximately 75 percent of the trailers manufactured today and almost all highway tractors use air-ride suspensions.

Air has even proliferated into the specialty segments of the trucking industry. It is not uncommon to find air-ride suspensions on heavy-duty vehicles that operate in extremely rugged environments – dump trucks, refuse, logging vehicles, and even concrete mixers. Following this logical progression, the migration from leaf spring suspensions to air ride is gaining momentum on fire and rescue vehicles. Specifying the right suspension should be a high priority for decision makers in this industry. Suspension systems affect the performance and life of the vehicle, frequency of maintenance, and comfort and safety of the operator. Upgrading to an air-ride suspension typically results in less than a one percent increase in the total cost of an apparatus, a great value for benefits gained.

What Factors Control Ride Quality?

Suspensions with a lower natural frequency reduce the force transmitted to the vehicle, which improves ride and vehicle protection when properly engineered. Natural frequency includes the spring rate of the suspension and the amount of weight supported by the suspension. The spring rate of the air springs is a measure of the vertical stiffness of the air springs or how much the suspension deflects for a given load being carried. The natural frequency of a suspension will increase if the suspension spring rate is increased or decreased. Because of a softer spring rate, air suspensions provide greater equipment protection over traditional mechanical suspensions. This is because of the air springs’ ability to cushion the equipment and passenger load. Outstanding ride produces superior driver and passenger comfort while helping to protect critical, lifesaving equipment and costly electronic components from excessive vibration and road inputs.

Understanding Roll Stability

Roll stability is a measure of how well the suspension and chassis resist rolling or swaying, especially when encountering a turn. A vehicle’s roll stability can be measured in two ways: (1) determining the maximum cornering speed before reaching the rollover threshold (when tires lift off the ground), and (2) measuring the amount of roll that occurs for a given turn at a given speed (the roll angle). Many elements of a vehicle’s design play a factor in determining its roll stability. Roll stability for a fire and rescue vehicle is influenced by the height of the center of mass of the truck body and the axle track width. These factors can have the greatest impact on the roll stability of the vehicle.

While suspension roll stiffness contributes to roll stability, it represents only one component of the equation. The chassis’s flexibility and tire stiffness, for example, must also be considered when determining the truck’s overall roll stability. Air-ride suspensions have come a long way over the past few decades, and we’re now at the point where properly engineered air-ride suspensions can deliver roll stability near that of a steel leaf spring suspension. For fire and rescue vehicles, higher rollover thresholds allow improved vehicle performance during the challenging, evasive maneuvers that are experienced when rushing to the scene of an emergency.

So, how can the vehicle’s suspension affect roll stability? The level of roll stiffness provided by a suspension is the best way to directly affect the stability of the vehicle (more roll stiffness = more roll stability).

Another factor to consider is suspension vertical stiffness, which can be quite different – especially between air and steel leaf spring suspensions. The vertical stiffness of a leaf spring suspension can be greater than that of an air-ride suspension, resulting in a rougher ride. While a soft ride seems to be in conflict with roll stability, in reality it is not.

The answer lies in the fundamental difference between the suspension types. Since air suspensions provide a considerably softer ride than leaf spring suspensions, they must rely on other means to achieve roll stiffness. This other means, commonly referred to as auxiliary roll stiffness, comes from the horizontal cross brace used in certain air-ride suspensions. The auxiliary roll stiffness provided by the cross brace allows for roll stability characteristics similar to a leaf spring suspension while providing a vastly superior ride. This is the ideal combination of ride comfort and roll stability – in other words, the ultimate in equipment protection, safety, and driver and passenger comfort.

Not All Air-Ride Suspensions Are Equal

For suspensions that are going to be used in the fire and rescue market, roll stability, package space, maintenance, and durability should all be considered. Just because an apparatus has an air-ride suspension doesn’t mean it delivers a soft ride, nor does it mean it has inferior stability when compared to a spring ride suspension. The ride quality and vehicle control a suspension provides result from the design of an integrated suspension system.

There are air-ride suspensions designed to achieve optimum performance for standard fire and rescue applications. Some suspension manufacturers have optimized their configuration to minimize the amount of torque reactivity the suspension experiences. Suspension-induced driveline vibration is minimized, which reduces wear on components by keeping the axle at a nearly constant pinion angle throughout its travel. This provides optimum joint angle cancellation and significantly controls suspension windup and corresponding frame rise.

Fire and rescue apparatus typically exhibit a high center of gravity. A suspension with a high level of roll stiffness should be used to give the apparatus a feeling of stability when in operation. When considering different suspension options, it is also important to ask questions about what the suspension you’re considering was designed for. Most manufacturers will have a “vocational” rating that gives you an indication of what the intended use of that suspension was. Those ratings will range from zero to 100 percent vocational rated. Typically, the higher the vocational rating, the more roll stable and resistant to abuse the suspension will be.

ADVANTAGES AND TRADE-OFFS TO AIR-RIDE SUSPENSION

As the expectations for today’s apparatus grows, extending service life, adding valuable body mounted equipment, and protecting the equipment are becoming more and more important. Advantages to an air-ride suspension may include the following:

  • Reduced maintenance costs from less vibration and shock transmitted to the chassis and other body-mounted components.
  • Less “wear and tear” on the vehicle and equipment because of cushion and support from air springs.
  • Increased ride quality without sacrificing performance.
  • Ability to control body lean.
  • Enhanced driver comfort and safety.

It is possible that there are trade-offs as well. Fire Apparatus Manufacturers’ Association (FAMA) member companies can provide details on the types of suspensions offered and what trade-offs may be involved in specific situations.

FAMA is committed to the manufacture and sale of safe, efficient emergency response vehicles and equipment. FAMA urges fire departments to evaluate the full range of safety features offered by its member companies.

BOB ALBANO is the Fire & Rescue Business Unit sales manager for Hendrickson. He has been in the industry for 11 years and focuses specifically on working with fire apparatus manufacturers to ensure that Hendrickson products are designed to meet the demands of the fire and rescue market. He also works with dealers, federal, state, and local departments nationwide to assure that the correct suspensions are specified to meet the unique needs of each department.