The Apparatus Architect: Part 26 - Designing Rescue Squad Apparatus

Michael Wilbur and Tom Shand discuss the type of chassis that will carry the body, personnel and equipment.


The November 2005 installment of The Apparatus Architect covered some of the considerations when determining the type and size of generator system that should be designed into new rescue apparatus. Once your apparatus committee has determined the preliminary tool and equipment list and the number of...


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The November 2005 installment of The Apparatus Architect covered some of the considerations when determining the type and size of generator system that should be designed into new rescue apparatus. Once your apparatus committee has determined the preliminary tool and equipment list and the number of personnel who will staff the rig, the next step is to identify the major fixed components that will require space within the rescue body. Component items such as generators, light towers, air cascade systems, ground ladders and fire pump systems will all have an impact on the overall design of the apparatus.

As the truck committee ponders the many choices in these areas, one of the most important decisions to make is the type of chassis that will carry the body, personnel and equipment. A wide range of commercial and custom-built chassis can be readily adapted for rescue squad applications. There are a number of considerations that should be reviewed by the committee before making a decision on which style of chassis may be best suited for the department.

Commercial Chassis Options

From an overall perspective, commercial chassis will generally cost less than a similarly equipped custom chassis; however, this should not be the most important factor when choosing a chassis. Commercial chassis will generally offer a smaller range of engine and transmission choices and are well suited for chassis applications up to 40,000 pounds gross vehicle weight rating (GVWR) where engines in the 300- to 350-hp range will meet the department’s needs. Commercial chassis are capable of carrying two people in the front of the cab and up to five in four-door-cab configurations. As these cabs are designed for inner-city delivery units, the space available for equipment such as self-contained breathing apparatus (SCBA) and radios are limited and may cause you to place some of this gear in other areas. Make sure that you see and review a chassis that is similar to what your department is considering before planning the interior layout of the cab.

Heavy-duty Class 8 chassis provide a wider range of diesel engine and cab options with heavier GVWR capabilities. These types of chassis are well suited for walk-in-body rescue units where high-horsepower engines are required and different types of auxiliary braking devices such as engine brakes and transmission retarders are available. Class 8 chassis also provide more options with respect to integral frame extensions, which can be useful for front-winch applications. Where a department has to design a unit to meet a specific overall length requirement, the heavy-duty chassis generally have an unlimited number of wheelbase offerings to match the needed cab to axle dimension for the body.

Both types of commercial chassis will require upgrades to the electrical system to meet the total connected load demands of the apparatus. As most chassis will come standard with only two or three batteries and a low-output alternator, both of these components will have to be replaced by the body builder. Depending on the number and type of body lights and warning lights that will be used, a minimum of four group batteries, together with a minimum of a 320-amp alternator and a load-management system, will need to be provided and installed. Consideration should be given to locating the batteries to a slide-out drawer in the cab-step area to allow access for maintenance.

Other considerations for commercial chassis should include the position of cab-mounted fuel tanks and exhaust systems and how these components will interface with body-mounting and other equipment. Step surfaces located under the cab doors may have to be covered with non-slip surfaces if not available from the chassis manufacturer, or you may have to specify that all step surfaces be covered with aluminum tread plate with step areas that meet the requirements of the National Fire Protection Association (NFPA) 1901 standard. If your truck committee is considering mounting equipment such as winches, reels and warning devices on the front bumper extension, make sure that these components are not affected by the tilting hood assembly or a stationary grill may be needed to provide clearance for this equipment.

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