Fire Apparatus Supplement 2025: Axles, Brakes, Suspensions & Tires

Axles, brakes, suspensions and tires and all of their related components are part of an apparatus’ foundation, despite the fact that they won’t be used on every call or training session.

Understanding GAWR

Gross axle weight rating (GAWR) applies to the front-axle (steer) and to single-rear-axle and tandem-rear-axle (drive) assemblies. It’s calculated from the weight rating of the various components that make up the axle assembly, which include an axle with wheel ends, suspension, brakes, wheels and tires. The actual weight rating of a given axle assembly is based on the component that has the lowest rating.

Wheel and tire decisions

Wheel and tire sizing and tire load range depend on a rig’s steer and drive axles’ GAWR.

Tire decisions depend on whether wheels are painted steel or polished aluminum.

Aluminum wheels are lighter than steel wheels and dissipate the heat that’s generated by the vehicle brakes much better than steel wheels do.

Steel wheels require more maintenance than aluminum wheels do, because the paint that’s on steel wheels must be maintained to prevent rust and deterioration.

Some manufacturers put steel wheels on the inside wheel location and aluminum wheels on the outside wheel location of rear axles. For departments that are in an area where corrosive chemicals are used on the roads, I recommend a plastic spacer between rear dual wheels, particularly if they are aluminum.

Weight and balance study

Any time that an apparatus manufacturer is solicited for a vehicle specification, it will do a weight and balance study, which affects every component that’s discussed in this article and is critical to a successful build. The study either verifies the GAWR and gross vehicle weight rating (GVWR) in a vehicle specification or indicates that changes must be made. Section 9.1 of NFPA 1900: Standard for Aircraft Rescue and Firefighting Vehicles, Automotive Fire Apparatus, Wildland Fire Apparatus, and Automotive Ambulances covers this process and the equipment allowance that’s required for the manufacturer’s design.

When a department’s equipment cache weighs more than the NFPA allowance, it’s the department’s responsibility to inform the manufacturer. If the information that a department provides the manufacturer isn’t accurate and a new rig is overweight, the liabilities fall to the department.

Brakes

There are two choices for brake types for an apparatus committee to specify on a piece of fire apparatus: disc and drum.

Disc brakes are far superior to drum brakes. First of all, they react faster during anti-lock brake system (ABS) events than drum brakes do. Secondly, disc brakes aren’t subject to brake fade when they get hot, which makes the performance far safer than drum brakes. Whenever possible, disc brakes should be the brake of choice.

Currently, the maximum single axle weight rating for 17-inch disc brakes is 27,000 lbs. (54,000 lbs. for a tandem assembly). According to industry representatives, the limitation is dictated by the available wheel ends, not by the disc brake mechanism. In 2024, Meritor announced that it soon will market a new drive axle wheel end that allows disc brakes to have a weight rating of 33,500 lbs.

All that said, drum brakes still are widely used. The most common are 16½ x 7 inches. There are 16½ x 8⅝-inch drum brakes. These are more expensive but also more robust. Depending on GAWR, the terrain of its response district and the type of vehicle that’s being purchased, a more robust brake might be a safer choice.

Per Federal Motor Vehicle Safety Standards (FMVSS) No. 121, air brake systems require ABS. FMVSS No. 136 specifies electronic stability control (ECS) for heavy vehicles, which includes a function that’s called automatic traction control (ATC).

The specifications for these systems require several sensors and modulators. Each wheel end has a sensor, so a vehicle that has a single rear axle always has four wheel end sensors, and a vehicle that has tandem rear axles will have six. Single-rear-axle vehicles also have four modulators (the part that automates brake pulsing), so those vehicles are noted as a 4S4M system. On a tandem-axle vehicle, the rear wheels can be modulated as a pair (noted as 6S4M) or individually (6S6M). Both meet FMVSS.

ABS senses the wheels’ rotating motion. When the system detects that a particular wheel(s) isn’t spinning (thus, skidding), the system pulses the brake application to that wheel(s). This keeps the wheel(s) from locking up and aids in the driver’s ability to maintain vehicle control.

ECS monitors individual wheel speed (via input from ABS), steering angle, lateral acceleration and yaw (a vehicle’s shift from its center of gravity to the left or right). When ECS detects that the vehicle is approaching a point of instability, it cuts off the accelerator pedal and applies specific wheel brakes (one or more) to slow down the vehicle. Except for the temporary loss of accelerator function, this process is seamless to the driver.

ATC cuts back on the accelerator when the rear wheels spin. In a low-traction situation, such as snow or mud, the ATC’s shut-off switch allows the driver to spin the wheels to get the rig out of the situation.

Front axle suspensions

Front axles of fire apparatus are over-whelmingly of the beam type, whether they’re Dana’s or Meritor’s forged I-beam model or Hendrickson’s fabricated beam. They may be fitted with leaf spring suspensions, a traditional spring pack or, more recently, a parabolic spring assembly.

Independent front suspensions (IFS) also may be used, such as the ones that are offered by Reyco Granning and Pierce (TAK-4 IFS). An IFS unit replaces the axle with control arms that contain the wheel hubs, brakes, steering and suspension in a single unit. Although I believe that there’s a place in the fire service for IFS-equipped vehicles, justified uses are rare, in my opinion. An IFS is an expensive upgrade, and it requires more maintenance than a beam axle does. It also can complicate other routine maintenance procedures, such as oil changes, which adds to the operational cost over the life of the rig.

Cramp angle

Front wheel cramp angle plays a significant role in the turning radius of a vehicle. Generally, the larger that the cramp angle is, the tighter the turning radius.

The cramp angle often is different from left to right, so departments should look at both numbers. Furthermore, the width of the front tires, the front suction, the front bumper discharge, the location of steering components that are on the chassis, and the make and model of the front axle can affect cramp angle significantly.

Rear axles and suspensions

Rear axles are a vehicle’s driving force and should be specified based on the department’s response district.

Are rear axles with a driver-controlled differential lock ordered? If a tandem-rear-axle vehicle is ordered, is an interaxle differential lock specified? These add additional drive and traction to a rig’s rear axles. If a department is located in an area that has four seasons, I would include these in specifications.

An apparatus committee also should review rear axle gear ratio with the engineers from the apparatus manufacturer’s chassis division. A department that’s located in a flat area and has a large response district will need a different gear ratio than a department that’s in a hilly or mountainous area. Rear axle gear rations can be changed, but Allison Transmission gear ratios can’t.

For a single-rear-axle vehicle, the suspension choices are spring or air ride. Tandem-rear-axle suspensions can be spring, air ride or rubber block.

Spring suspensions usually are the least expensive option, and their ride quality is OK. In single-rear-axle vehicles, the most popular spring suspension seems to be Reyco Granning’s 79KB. This suspension has a very wide weight rating range, and the springs are hung without shackles, which eliminates a lot of time-consuming maintenance and costly repairs.

Air ride suspensions for single- and tandem-axle setups are the most expensive but offer the best ride quality. They are the most responsive and transmit the least amount of road vibration through to the chassis, which gives the crew a better ride and bounces the vehicle body and equipment around much less than spring suspensions do.

Air ride suspensions also self-regulate as the load on them decreases and increases and the air pressure in them decreases. This isn’t so significant in a pumper that has a 500-gallon tank, but in a 2,500-gallon tanker, in which the load swing is 10 tons or more, air ride suspensions can improve drivability, maneuverability and safety substantially.

Rubber block suspensions can be a useful middle ground between the previous two types. They’re more expensive than spring suspensions but not as costly as air ride suspensions. Rubber block suspensions have a better ride quality than spring suspensions but not as good as air ride suspensions.

There’s good action in rubber block suspensions: They absorb a fair amount of road vibrations instead of transmitting them to the chassis and body. As a result, maintenance is reasonable.