From incremental improvements to innovative bounds, fire truck designs are in a constant state of evolution and suspension systems are undoubtedly a crucial area of focus. Suspension systems not only play a role in the overall performance and life of your vehicle but also the safety and comfort of its drivers. Additionally, superior suspension quality can protect the lifesaving equipment used by firefighters each day. However, choosing the type of suspension system that is best for an individual department depends a lot on the type of service that is planned for the vehicle. Determining the design should include a series of considerations including its planned use, the type of apparatus, a single or tandem axle, how much water it will carry, and more – are all important factors when selecting the suspension system for a fire truck.
Common Suspension Systems in Fire Apparatus:
Multi or Stacked Leaf: Can be used in any axle position and configuration and typically incorporates spring steel sections, or leafs, that are the same thickness and width and get shorter in length from the top down until the weight capacity is reached. The stacks can have eight or more leafs - all of which are bolted together with a common center bolt or pin, and clamps on the longer sections that make direct contact with each other.
Parabolic Leaf Spring: Similar to multi-leaf systems in regard to simplicity and maintenance with higher ride quality, and lower stability.
Walking Beam or Bogie: Only used in tandem drive axle configurations and in the heaviest applications. These systems are typically very stiff, but stable.
Air Ride: Ride height and ground clearance remains constant regardless of load weight, but may reduce road feel and stability. Suspension provides for a comfortable ride with minimal maintenance and axle versatility.
Independent: Allows each wheel on the same axle to move vertically and react independently of the others. Independent suspensions typically offer the highest quality ride, handling characteristics, and the ability for each individual wheel to respond to what it encounters on the road separately from the other wheels on the vehicle.
Independent Suspension Systems for Custom Fire Apparatus Chassis:
In the United States, approximately 60-70 percent of all major fire trucks are built with a custom chassis that is engineered, designed, and built to meet a fire department’s exact needs. Custom chassis most often ride, handle, stop, and set up at a scene better because they are developed with specific geographic elements and service applications in mind. In the mid-1990’s, Pierce Manufacturing’s parent company, Oshkosh Corporation, developed the TAK-4® Independent Suspension System used on military, severe-duty, and emergency vehicles.
In 2001, Pierce introduced the TAK-4® Independent Front Suspension (IFS) System on custom built Pierce chassis to provide a better road feel, control, and a smoother ride over any kind of surface.
Pierce now offers TAK-4 T3™ Independent Rear Suspension (IRS) which applies the same military proven performance to the rear ride performance. For the highest level of mobility, Pierce offers TAK-4 T3, which combines the advantages of TAK-4 IRS with a 100% mechanical rear steering system.
Ways Independent Suspension Improves Fire Truck Performance:
1. Ride Quality & Control
Compared to a standard leaf suspension, independent suspension delivers outstanding ride quality and vehicle control. Some systems offer up to 10” of suspension travel, a light spring rate, robust design, and independent wheel movement – keeping tires on track for improved overall handling and performance. Independent suspension systems will provide two, to two-and-a-half times wheel travel distance over the spring systems. Superior ride quality reduces stress not only on occupants but also gear, components and the vehicle.
Greater maneuverability, turning radius, diameter, and lane-to-lane turning are other benefits of independent suspension systems. Specifically, Pierce’s TAK-4 Independent Suspension System provides fire trucks with the highest cramp angle which can safely and effectively be maneuvered into strategic locations when responding to a fire. The maximum cramp angle is 45°, with a minimum of 40°. Just like the independent front suspension, mobility is maximized with the TAK-4 IRS - a 100% mechanical rear steering system free of electronics.
3. Steering & Braking
In rigid suspension systems, when one wheel bounces, the entire axle tilts. This will cause the opposing wheel to tilt inwards or outwards, preventing it from rolling in a straight line. The effect can cause an imbalance and decrease in steering control. Additionally, rigid axles are less responsive, especially in heavy vehicles, when turning corners or braking.
With the TAK-4 Independent Suspension System, the vehicle allows for larger front brakes which transfers weight quicker to the front wheels. The lower spring rates made possible by a torsion bar smooths out the road better than any straight axle rig because the wheels are allowed to respond independently to the road surface. This not only maximizes performance, but minimizes wear and tear on equipment, reduces maintenance, and saves on brake repairs. In fact, the system shortens stopping distance by 23 percent, has a 45-degree cramp angle, and improves ride quality by 340 percent (as measured by accelerometer testing).
4. Load Carrying
Integrating a higher rated front axle allows for greater load carrying capacity which enables firefighters to add more features and equipment to fire apparatus. This may include front suction or air condition that won’t exceed gross vehicle weight restrictions. With Pierce’s TAK-4 Independent Suspension System, a maximum front axle weight rating of 24,000 lb. is possible with 425/65R22.5 tires.
When selecting fire apparatus suspension systems, a focus on optimizing safety, stability, maneuverability, ride quality, load capacity, simplified maintenance and more are all important considerations. The advancements in independent suspension systems have provided those in fire service with many ad control of the vehicle, are all unparalleled qualities that continue to evolve with design and technology advancements.vantages over other suspension systems. The superior ride quality, enhanced operating features, and the added confidence that the driver is in control of the vehicle, are all unparalleled qualities that continue to evolve with design and technology advancements.
Do you want to learn more about fire apparatus suspension systems that offer a high-performance, dramatically smoother ride with outstanding steer-ability? Experience a better ride for you, your crew, and your equipment by contacting an experienced Pierce Manufacturing representative.