How to spec a 911 rapid response vehicle

Calculate chassis load, determine fire suppression capability and do your math to build your 911 rapid response vehicle from the ground up


Let’s do some initial spec-writing for a new 911 rapid response vehicle for your fire department. Rapid response vehicles used by fire departments are going to increase exponentially over the next few years – that’s my prediction based on several key indicators:

  • A continuing decline in the number of volunteer firefighters across the country.
  • Decreases in staffing for career departments.
  • An increasing number of entry-level firefighters (both career and volunteer) with no experience driving or operating large trucks.
  • Increasing fire potential in the wildland urban interface (WUI) as development continues to encroach on the WUI.
  • Increasing costs of new fire apparatus.

Additionally, we are seeing – and will continue to see – innovative ideas and technologies that enable 911 rapid response vehicles to better fill a fire department’s needs.

More powerful commercial truck chassis, along with continued improvements in fire suppression technologies, are making the 911 rapid response vehicle a more effective and efficient fire apparatus. (Photo/Elmhurst Fire Department)
More powerful commercial truck chassis, along with continued improvements in fire suppression technologies, are making the 911 rapid response vehicle a more effective and efficient fire apparatus. (Photo/Elmhurst Fire Department)

Calculating 911 rapid response vehicle load

We’re going to start with chassis selection because it’s the foundation upon which everything else depends. We want to ensure the chassis for our rapid response vehicle doesn’t become overloaded before it ever hits the road. This is the selection criteria we’ll use to ensure that our chassis selection has enough payload:

  • Weights for equipment. We’re going to need a good scale, because every piece of equipment we consider for placement must be weighed, including full fuel tanks and any other fluids.
  • Weight for water. Remember in Firefighter I class when we learned that water weighs 8.35 pounds per gallon? That means if we have a 200-gallon water tank on our rapid response vehicle, we’ll be putting 1,670 pounds on the chassis.
  • Weight of all occupant positions. Manufacturers for medium- and heavy-duty trucks across the board use a figure of 150 pounds per seating position to calculate a truck’s payload. Our rapid response vehicle is going to have a crew cab with seating for four, with the fifth seat used for internal storage. We’re also going to increase that weight-per-person figure up to 200 pounds, so four seating positions adds 800 pounds to the payload.
  • Chassis curb weight. The truck manufacturing industry defines this as the shipping weight of the cab and chassis – without body and aftermarket upfits – and includes all standard equipment, fluids and a full fuel tank.

Totaling these components helps to accurately assess what gross vehicle weight rating (GVWR) emergency vehicle best suits your needs. GVWR is the maximum allowable weight of chassis, occupants and payload as determined by the manufacturer for the vehicle to safely start and stop.

Before we’ve even put anything else on our future 911 rapid response vehicle chassis, we’ve already accumulated 2,270 pounds of payload: 1,670 pounds of water and 800 pounds of firefighter.

Fire suppression capability of a 911 rapid response vehicle

From here, we’ll have to look for equipment that’s capable of doing the job, but remains lightweight. Fortunately, that’s one area where pump manufacturers have made great strides. In the current marketplace, we have several options for getting fire suppression capacity in the apparatus without adding too much weight.

But, before we go there, we need to make a fundamental decision: What fire suppression capability will our 911 rapid response vehicle have? There are several options:

The omission of plain water from a traditional low-pressure pump is intentional on my part. We’re building a fire suppression unit for the 21st century, not the 19th. Researchers with Canada’s National Research Council (NRC) conducted live-fire tests that compared effectiveness and efficiency for a variety of suppression agents and pump technologies: water from a low-pressure pump, water with a foam solution, CAF and UHP. Both the CAF and UHP outperformed the other two options, but UHP got the better of CAF in “time to fire knock down” and “amount of water needed for knockdown.”

Comparing CAF to ultra-high pressure knock-down times and water requirements. Source: Kim, A. and Crampton, G. Fire Suppression Performances of Manually Applied CAF, MPW/HPW and High Expansion Foam Systems. National Research Council Canada.

The good news is that the UHP pump can also be used with Class A, Class B and AFFF foams. So, we can get some of the best of both CAF and UHP, which will give our rapid response vehicle good fire suppression capability, while keeping the weight down.

  • Option No. 1: A UHF skid load package with a 150-gallon tank weighs in at 2,000 pounds, including the water weight. Net addition to the payload would be 2,000 pounds.
  • Option No. 2: An integrated UHP that’s powered by the vehicle’s PTO drive would weigh 425 pounds. Add a 150-gallon polypropylene water tank and the total weight for water, and the tank is 1,355 pounds. Net addition to the payload would be 1,780 pounds.

We’ll go with Option No. 2 and save 220 pounds of payload capacity for equipment.

Choosing the right chassis for a 911 rapid response vehicle

We’ve chosen the Dodge RAM 4500-series truck for comparison purposes only, along with the pertinent specification information for that model truck.

Let’s add up the numbers for the GVWR for our RRV with Option #2 for its fire suppression package:

Thus, we have 5,303 pounds of payload available for the truck body and equipment we want to have on board, roughly two and a half tons.

More powerful commercial truck chassis, along with continued improvements in fire suppression technologies, are making the 911 rapid response vehicle a more effective and efficient fire apparatus option for many fire departments. The key is to do your math from the chassis up.

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