By Bill Adams
“Apparatus Purchasing: Single or Dual Rear Axle? Part 1” (December 2017) described axle ratings and the governmental regulations and advisory standards apparatus manufacturers follow.
Included was commentary from fire apparatus manufacturers that build their own cabs and chassis. Part 2 continues the discussion with apparatus manufacturers that do not build their own. The intent is to help purchasers determine “How much weight can I put on this axle?” and “When should I go to a tandem rear axle?” All apparatus manufacturers purchase commercially made cabs and chassis and modify them for fire service use. Small to midsize apparatus manufacturers (OEMs) also purchase custom cabs and chassis from the limited number of OEMs that build and resell their own. Their input into this discussion as “users” is equally as important as the chassis builders.
OEMs not manufacturing a proprietary chassis can be caught between a rock and a hard place when discussing axles and axle ratings, especially if there can be multiple interpretations of standards, regulations, and industry norms. OEMs may have established in-house axle loading criteria that may not necessarily be in sync with those of the commercial and custom chassis manufacturers. It is tactless to claim one manufacturer’s interpretation of a standard is “better” than another’s.
Commenting are Mike Marquis, vice president, rescue sales, at Rescue 1; Mark Perkins, lead designer at Midwest Fire Equipment & Repair Company; Joe Messmer, president and owner of Summit Fire Apparatus; and Mike Watts, national sales manager for Toyne, Incorporated.
Do you recommend a maximum dead load weight for axles when a rig is “loaded” sitting in the barn?
Watts: “[National Fire Protection Association (NFPA) 1901, Standard for Automotive Fire Apparatus], in general, sets the guidelines for acceptable weight distribution for an apparatus. Toyne strives to stay within those guidelines and utilizes a large volume of historical data and engineering input when evaluating each situation. Axle loads have to be looked at differently with a fire apparatus vs. any general service vehicle because the fire apparatus is fully loaded 24/7/365. As such, the chassis has to be properly set up for this condition. However, there is no one-size-fits-all scenario, as some tend to do.”
Messmer: “Summit utilizes the tried and true methods long ago established of 30 percent on the front and 70 percent on the rear. We do like to keep away from the total [gross vehicle weight rating (GVWR)] by about 10 percent.”
Perkins: “On trucks with 2,000-gallon booster tanks and smaller, Midwest calculates the maximum dead weight to be approximately 15 percent under the front axle rating. On 3,000-gallon or larger tanks, we want to be 16 to 17 percent under. We stay below the rear axle rating by a minimum of 1,000 pounds. The rear axle carries most of the weight, and a larger excess is advisable. The weight of the equipment listed in the NFPA standards must be included, and we always expect that even more equipment may be carried.”
Marquis: “The front axle capacity (gross axle weight rating-front or GAWR-FR) is always one of Rescue 1’s most important concerns. The combinations of the engines, front bumper extensions with hydraulic reels and tools, cab seating for members, and the general rescue design all can add a tremendous amount of weight onto the front axle. With front axle ratings capped at 23,000 pounds on a beam and 24,000 pounds on an independent front suspension, we must monitor the front axle weight starting with our first preliminary proposal. The cab to axle/wheelbase dimensions of the chassis may need to be changed to offload the front axle weights. The final weight of the laden front end should be near the capacity rating with some margin for growth. Matching the rear suspension capacity (GAWR-RR) to the actual laden load on chassis is critical. Overrating the suspension doesn’t necessarily make the vehicle better. Overrated suspensions can cause damage—not only to the crew and equipment carried onboard, but it could create structural issues to the chassis and body. The suspension is designed to absorb the energy as the vehicle is driven down the road. With overrated suspensions, the road shock is transferred directly to the body and not absorbed by the suspension. At the same time, rescue units are usually kept in service for 20 years or so, and you must plan for equipment growth.”
Do your recommendations differ for pumpers, ladders, rescues, and tankers?
Perkins: “The same rules apply for all. Pumpers and rescues usually carry more equipment than tankers and tanker-pumpers.”
Messmer: “Nope, there really is no difference. And, they do not differ between custom and commercial chassis.”
Is there such a happening as a dynamic weight loading on a front axle when a fully laden rig is sailing down the road and the binders are slammed on?
Perkins: “When the brakes are rapidly applied, momentum shifts forward and the water will shift forward, causing the front to dive down. We like to build the trucks with excess left over, providing a more controlled ride.”
Do you have any recommendations on when to go to a tandem rear axle?
Watts: “Any time you approach the limits of a single axle weight rating, the department should consider moving up to the next available rating. This could be a larger single axle or a tandem axle. If you are at the high end (33,000 pounds) on the singles, it makes sense to move to the 40,000-pound tandem.”
Messmer: “That is simple: It’s when they begin to exceed the ratings for the rear! When the weights get real close, you have to sit back and ask exactly what you are doing. Will this be a safe unit? Am I leaning out too much for the sake of trying to get it on one axle? Some things are not meant to be, and when you start weakening other parts of the build, you are only asking for trouble.”
Perkins: “Tandem-axle trucks have a larger turning radius. The extra set of wheels can take up compartment space, and there are more upfront costs. But if the customer needs a lot of equipment or a lot of water, it is the only option. Having a larger axle rating may create a stiffer ride, but emergency equipment can see rough use, and a stable ride will be more controllable.”
Any words of wisdom for purchasers when considering single vs. dual rear axles?
Watts: “If you significantly oversize the axle/spring combination, it actually creates a poor-handling rig and will induce a lot of stress into the body through excess vibration. There has been a trend to pack 10 pounds into a five-pound bag. Finances and staffing, in many cases, create a need to utilize the “rescue-pumper” concept similar to the Toyne PRV where the rig carries full pumper equipment and almost a full complement of rescue equipment. This requires the manufacturer to have more detail for both the equipment and the desired equipment layout early in the process to verify proposed weight distribution and proper axle selection.
“A maxed-out single will probably have spring fatigue issues at some point in its lifetime. With the tandem, you obviously have more spring (or air suspension) and most importantly brakes. Maxing out a single axle can lead to more and quicker brake fade because of heat buildup as well as more wear on the brakes themselves. Auxiliary braking systems (e.g., compression brake, transmission retarder, or Telma retarder) can help out but have their limitations because of environmental conditions (rain, snow, etc.). Another benefit to the tandems is that in many cases the wheelbase is actually shorter, which helps in turning radius. You can lose some compartment space and will have more tire maintenance with the tandem, but these can be good tradeoffs for the safety of the crew and the longevity of the rig. The same process of qualification is done regardless of whether a custom style or commercial style cab and chassis are used.”
Messmer: “Summit likes to keep away from the total GVWR by about 10 percent. That keeps the fire department out of trouble later in life when it just has to have a lot more stuff on the truck. Most of the time you can gauge how much weight they can add by how many compartments we’ve built into the truck. There are a lot of budget builders that do not pay any attention to the weights and center of gravity. Unfortunately, those are the trucks we all read about.
“For years, [some were] saying, for instance, that you cannot put more than 2,000 gallons of water on a single axle. Then the axle manufacturers started building larger axles, and the tire companies started making the tires capable of carrying the additional weight. The thing to always remember is to not exceed the weight ratings of the entire axle system—axle, suspension, frame, and brakes.
Perkins: “The curb weight of a custom chassis is very heavy on the front axle when compared with a commercial chassis, and this must be calculated in. A single axle will have better turning radius and usually is a shorter truck. Not as much weight can be carried. Single-axle trucks will have larger compartments.”
Marquis: “The actual cubic feet of usable storage area doesn’t necessarily increase when upgrading from a single rear axle to a tandem axle. Normally for rescue bodies greater than 22 feet in length, we will consider utilizing a tandem-axle chassis. This is not always because of the axle rating capacities but more for the stabilization of completed apparatus. There are exceptions depending on the design and layouts. The additional axle location takes up prime storage space. Tandem-axle chassis are more stable and have better braking and improved traction. There is also the additional cost for tandem-axle chassis.”
Advantages of Both
Mike Connely, national sales manager, Spartan ER, provides a brief synopsis of the advantages for both. “For the tandems, the obvious is the ability to carry more water and equipment; they appear to hold their resale value more so than a single-axle, and tandems have increased braking capabilities while driving as well as using on an incline during aerial operations,” he says. “There is also increased traction while driving on inclines and during poor weather conditions; there is more counterbalance for aerial operations; and there is less stress on chassis components such as brakes, shocks, and axles when you are using the appropriate axle because of weights. This will lower the cost of ownership over the life of the vehicle. This makes choosing the correct axle configuration even more critical.”
For single-axle configurations, Connely states, “Certain states require drivers to have licenses depending on the weight of the vehicle, so keeping to a single axle may be a way for some departments to have more access to drivers for the vehicles. An overall length requirement of a piece of apparatus will push you toward a single axle. Shorter wheelbases mean a shorter turning radius and the overall size of the vehicle, which allows access into more size-restricted areas. Weight is also a factor; how much water and equipment a unit can carry based on local laws regarding weight restrictions on bridges will come into consideration when looking at weights. Whether it’s an aerial, pumper, or tanker, a single-axle chassis is less expensive. This means you have less room to configure the rest of the apparatus, therefore not spending as much money as for a tandem.”
If a state police accident reconstruction team shows up with portable certified weight scales after a motor vehicle accident, drivers and owners of large commercial vehicles will cringe—especially if they knowingly pushed their rigs’ GVWRs to their limits. Ascertaining accountability and liability has kept many people gainfully employed in the legal profession. After delivery and acceptance of a new rig, fire departments seldom concern themselves with the weight of their apparatus unless some catastrophic event results in litigation. Purchasers are sadly mistaken if they believe it’s always the OEM’s fault if an in-service rig is overloaded or noncompliant. The importance of a rig’s “estimated in-service” weight has been addressed in depth by many commenters, myself included (http://www.rigspot.com/articles/2016/09/nfpa-1901-estimated-in-service-weight/). In essence, the purchaser tells the OEM what’s going to be carried on a rig, the OEM builds it accordingly, and the OEM informs the purchaser what can be carried on it weight-wise.
The annexes in NFPA 1901 contain explanatory material for informational purposes. A few interesting and applicable statements follow:
- “When the apparatus is ready for delivery and acceptance, the purchaser has a responsibility to check the completed apparatus carefully against the specifications, the contract, and the requirements of this document to ensure that all that was required is being delivered. This includes witnessing any required acceptance tests and verifying that the gross vehicle weight and the axle weight distribution are within the chassis and axle ratings.”
- “It is the responsibility of the purchaser to properly load the fire apparatus and place equipment to comply with the GVWR, the front-to-rear weight distribution, and the right-to-left load balance requirements of this standard.”
- “All vehicles are designed for a GVWR, which should not be exceeded by the apparatus manufacturer or by the purchaser after the vehicle has been placed in service.”
BILL ADAMS is a member of the Fire Apparatus & Emergency Equipment editorial advisory board, a former fire apparatus salesman, and a past chief of the East Rochester (NY) Fire Department. He has 50 years of experience in the volunteer fire service.