By Bill Adams
There appears to be an industry trend to maximize loading on single rear axles, especially on tankers (tenders) and some of the recently introduced aerial devices.
The trend isn’t new, nor is it restricted to rear axles. Manufacturers have been challenged with axle weights for decades. Getting a comprehensible answer to the question, “How much weight can I put on this axle?” is as difficult as asking what the weight distribution should be or has to be between the front and rear axles. Asking, “When should I go to a tandem rear axle?” can be equally frustrating. Purchasers must rely on the advice, expertise, and integrity of fire apparatus manufacturers (OEMs) to find the correct answers. That is problematic when inconsistent or ambiguous answers are given. Purchasers shouldn’t be overly critical of OEMs who give conflicting answers about axle ratings. They must negotiate a labyrinth of governmental regulations at various levels, recommendations by recognized fire service and industry-affiliated organizations, as well as the requirements of National Fire Protection Association (NFPA) 1901, Standard for Automotive Fire Apparatus. Each answer might be correct in its own context.
To understand how ambiguities can occur when discussing axle ratings, it is best to define the ratings. Fire trucks are commercial vehicles. NFPA 1901’s Annex B states: “This annex is not part of the requirements of this NFPA document but is included for informational purposes only.” Included in section B.2.6 is: “Since the passage of Public Law 89-563, the National Traffic and Motor Vehicle Safety Act of 1966, the federal government has adopted certain motor vehicle safety standards applicable to all manufacturers of trucks, including fire apparatus. It is unlawful for a manufacturer to deliver a truck not in compliance with these federal standards. These federal safety standards are frequently changed, and their provisions make the incorporation of certain features and devices mandatory. Apparatus manufacturers cannot build apparatus to specifications that would require them to delete required items or to include any that are illegal, and they face substantial penalties for infraction of these rules.” The italics are mine for emphasis. I don’t know of such a thing as the NFPA police. I don’t know who or how one determines illegality or liability or what the penalties could be. But, I’m reasonably sure the OEMs don’t want to find out.
The point of origin for the rules and regulations for commercial vehicles including fire apparatus lies within the bowels of the United States government. It could be the United States Department of Transportation’s National Highway Traffic Safety Administration (NHTSA) or some other agency that generates rules or regulations that are published in the Code of Federal Regulations (CFR). They become administrative law and are, in turn, published in the Federal Register. It is immaterial and too time consuming to establish which agency established what, when, and why. From the point of origin forward, the “understandability” of axle ratings appears to go downhill.
States might have their own criteria for axle ratings that might be more stringent than, but never less than, federal requirements. However, the NHTSA allows states to give fire apparatus exemptions to certain federal requirements. In 2011, the Fire Apparatus Manufacturers’ Association (FAMA) and the International Association of Fire Chiefs (IAFC) published a paper titled “Emergency Vehicle Size and Weight Regulation Guideline,” which can be accessed online. It’s worth reading.
It states: “Size and weight regulations applicable to emergency vehicles currently vary widely from one state to the next. This situation can lead to confusion, delays, and inefficiency. In an attempt to begin addressing this situation, the IAFC and FAMA offer this document as a guide to U.S. federal, state, and local officials responsible for creating and enforcing vehicle size and weight laws and regulations.” In the section regarding weights, the document points out that 25 states have general exceptions to federal requirements, four have their own specific weight requirements, one allows special permits, and 21 supplied no data on the topic. Purchasers rely on the OEMs to engineer roadworthy and compliant apparatus. If the apparatus is intended to traverse local roads and bridges with weight restrictions, purchasing specifications should reflect them.
Dave Scharphorn, from Rosenbauer Motors, says the Dana Model S35-590, rated at 35,000 pounds, is the largest single rear axle used in the fire service. He says the Reyco Granning IFS, rated at 24,000 pounds, and the Hendrickson SteerTek are available for front axles. Meritor, Hendrickson, and Marmon Harrington also supply axles for various fire service applications.
Ambiguity and NFPA 1901
Fire apparatus purchasing specifications should be clear-cut and detailed. Utopia is when specification requirements are not subject to individual interpretation by prospective bidders. Requirements in regulatory and advisory standards should be equally explicit. It doesn’t always happen. Some of the axle requirements in NFPA 1901, while well intended, can be ambiguous, causing confusion and even animosity between buyers and sellers. This is not an accusation of vagueness in the NFPA 1901 document. It just might be a matter of individual interpretation.
There are fewer than 10 domestic manufacturers of custom fire chassis that also build complete apparatus. Several sell the bare cab and chassis to OEMs that don’t build their own. There’s probably an equal number of domestic manufacturers of commercial cabs and chassis that apparatus OEMs use to manufacture complete rigs. None manufacture their own axles. Chassis manufacturers and apparatus OEMs may have divergent opinions and interpretations of axle requirements. Several statements follow from the NFPA 1901 document.
Sentence 3.3.81 GAWR (gross axle weight rating): “The final stage manufacturer’s specified maximum load-carrying capacity of an axle system, as measured at the fireground interfaces.” The key words in the NFPA’s definition are “final stage,” which means the manufacturer that builds the rig on a chassis is responsible to specify the maximum load-carrying capacity. The final stage manufacturer may not be the chassis manufacturer and definitely is not the axle manufacturer. Apparatus OEMs can have their own in-house determinations of what percentage of the GAWR they want to max out at. None should exceed the GAWR.
Appendix A, sentence A.3.3.81 has one portion stating, “The axle system includes, but is not limited to, the axles, tires, suspension, wheels, frame, brakes, and applied engine torque.” This sentence defines basic axle system components. An axle’s GAWR cannot exceed the highest rating of any component part. Ask your local apparatus vendor how the frame, brakes, and applied engine torque figure into the axle equation.
Sentence 188.8.131.52: “The front axle loads shall not be less than the minimum axle loads specified by the chassis manufacturer under full load and all other loading conditions.” I interpret this to mean that OEMs must build the rig so there’s always a minimum weight on the front axle. That weight is determined by the chassis manufacturer and is commonly referred to as a percentage of the GAWR. Is it possible for an OEM to have a front axle loading on a chassis it builds that is different than a like rig that’s built on a commercial chassis? Does a full load mean when the rig is sitting in the barn ready to go or when it’s fully loaded responding with a full crew?
Sentence 184.108.40.206: “The manufacturer shall engineer the fire apparatus to comply with the gross axle weight ratings (GAWRs), the overall gross vehicle weight rating (GVWR), and the chassis manufacturer’s load balance guidelines.” I believe most people understand complying with the GAWR and GVWR as they interpret them. Has a purchaser ever asked what constitutes a chassis manufacturer’s “load balance guidelines”? Ever see them in writing?
While not directly related to single or tandem axles, the preceding illustrates the perplexity in debating axles and why OEMs may have different answers to the same question. Several apparatus OEMs, axle manufacturers, and every custom chassis manufacturer were asked to comment on axles in general and, in particular, when purchasers should consider using a tandem rear axle. Part 1 has comments from OEMs that build complete apparatus—including the cabs and chassis. Commenting are Mike Connely, national sales manager for Spartan ER; Dave Scharphorn, director of sales for Rosenbauer Motors; John Schultz, director of pumper and custom chassis products at Pierce Manufacturing; Joe Hedges, aerials and custom chassis product manager at E-ONE; and Grady North, pumpers and commercial chassis product manager at E-ONE.
When a rig is fully loaded sitting in the barn, is there a recommended maximum dead load weight for each axle?
Scharphorn: “All Rosenbauer apparatus are designed to be loaded to 90 to 95 percent of capacity all of the time. This is [the] ‘sweet spot’ where the suspension and brakes are most efficient.”
Schultz: “Pierce Manufacturing adheres to all axle manufacturers’ installation guidelines. Pierce Manufacturing does not make axle recommendations that are outside of axle manufacturers’ guidelines.”
Connely: “All Spartan vehicles are designed for the vehicle to be operated at full GAWR, regardless of whether it is a pumper, ladder, rescue, or tanker. The percentage of GAWR will vary from vehicle to vehicle, depending on how the truck was configured and loaded minus the equipment and personnel that are not in the vehicle during this time.”
Hedges: “Our standard procedure is to set the GAWR with a minimum of three percent excess capacity in addition to NFPA-required loads or customer-specified requirements, whichever is greater. We also follow the National Truck Equipment Association (NTEA) guidelines concerning front-to-rear weight bias.”
[Author’s note: The NTEA Handbook lists recommended front-to-rear-axle weight distributions for various types of commercial chassis such as conventional, tilt-cab, tandem, and so forth. It is unknown whether it is specifically applicable to fire apparatus or if it has been “officially” adopted by any governmental agency or regulatory standard. Ask your local vendor. Also, see “Apparatus Load Balance Guidelines” in the February 2012 issue of Fire Engineering at .]
Do GAWR recommendations differ for pumpers, ladders, rescues, and tankers?
North: “By our definition, pumpers typically carry 1,250 gallons of water or less. They are almost always single-axle custom or commercial chassis. E-ONE considers tankers and pumper-tankers anything 1,500 gallons or more. We allow up to 2,000 gallons of water on a single axle. Single-axle tankers will generally have a shorter wheelbase but a higher center of gravity. For this application, we require electronic stability control. Tanks greater than 2,000 gallons will generally require tandem axles because of the axle loading. Single or tandem axles for rescue units are usually dictated by the GVWR as specified by the NFPA or customer requirements.”
Scharphorn: “Since fire apparatus are typically loaded while sitting in ‘the barn,’ Rosenbauer designs apparatus to accommodate this. It seems to be an industry norm that the only time a fire truck is unloaded is after a call on the way back to the barn.”
Schultz: “Pierce Manufacturing follows axle manufacturers’ installation guidelines; these guidelines are not product-specific.”
Do GVWR recommendations differ for commercial cabs and chassis?
North: “The difference would be in available axle ratings. For example, custom chassis single axles can be rated up to 35,000 pounds GAWR while 30,000 to 31,000 pounds are generally the maximum GAWR for commercial chassis.”
Scharphorn: “There is not a difference between apparatus types.”
Schultz: “Pierce Manufacturing follows axle manufacturers’ installation guidelines; these guidelines are not product-specific.”
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?
Hedges: “There is, and I would refer you to articles written by vehicle dynamics experts on this topic.”
Schultz: “Pierce Manufacturing tests the stopping distance of its vehicles to ensure compliance with Federal Motor Vehicle Safety Standards (FMVSS). During heavy braking, weight is transferred from the rear axle to the front axle. We can feel this effect on our bodies as we are pressed against the seat belts. Because a deceleration force acts at the center of gravity of a vehicle, and because the center of gravity of the truck is located somewhere above the ground, weight will transfer from the rear axle to the front axle in direct proportion to the rate of deceleration. In so many words, this is the effect of weight transfer under braking—and can be as high as 70 percent or more. This is why most vehicles have larger front brakes.”
Connely: “Yes. When the vehicle is under braking conditions, the loading is shifted forward on the vehicle and is why it is essential to take into consideration the unloading of the rear axle when qualifying the brake system.”
Scharphorn: “Any vehicle traveling down the road will have weight shifted to the front when the brakes are applied. The brakes are selected based on the GVWR of the apparatus with this in mind.”
[Author’s note: FMVSS are developed by the NHSTA. They consist of dozens of individual standards addressing specific items. As an example, FMVSS 121 addresses air brake systems.]
Using a quint with the same pump, aerial, tank, and cab components as an example, what would the difference in wheelbase be going from a single- to a dual-rear-axle configuration?
Scharphorn: “There are many factors involved in this such as weight and balance, aerial mounting location, and outrigger location. There is a possibility that no change in the wheelbase will be required to go from a single-axle to a tandem-axle configuration.”
Schultz: “There is about a 27-inch difference in wheelbase length as a function of the axle alone. However, very rarely does the body dimension from the front of the body to the centerline of the first axle match between a tandem- and single-axle vehicle. For example, when looking at our single-axle Ascendant product vs. a tandem-axle Ascendant product, the wheelbase difference is 15 inches.”
Hedges: “An E-ONE tandem 100-foot ladder can have a wheelbase ranging from 230 to 245 inches depending on the model, where our single-axle 100-foot quint has a 235-inch wheelbase.”
What would be the difference in GVWR between single and dual axles?
Hedges: “A tandem E-ONE 100-foot ladder will have a GVWR anywhere from 67,000 pounds to 78,000 pounds depending on options. Our single-axle Metro 100 Quint has a GVWR of 59,000 pounds.”
Scharphorn: “There is a minimum of 5,000 pounds difference.”
Schultz: “The difference in GVWR would start at about 10,500 pounds.”
Is there a difference in stopping distances between single and dual axles?
Hedges: “E-ONE chassis are designed, built, and tested according to FMVSS 121 brake testing requirements.”
Schultz: “Both vehicles would be tested to comply with FMVSS standards. Comparative braking studies demonstrate that a single-rear-axle aerial stops shorter than a tandem-axle aerial. The heavier tandem-axle vehicle will take longer to stop, and it will exhibit higher brake and tire wear, increasing its cost to the owner over time.”
Connely: “There are too many variables that are taken into account to make this statement. Potentially yes, if the two vehicles were identical, except for the rear axle and wheels. Otherwise, the stopping distance is more relative to how the braking system is designed for each application, resulting in variable results.”
Scharphorn: “The braking distance will be very similar from one to the other. A couple of factors that do come into play are with a single rear axle—the brakes are always operating at their maximum, causing additional heat, brake fade, and wear. With a tandem axle, there is more braking surface area, which allows the brakes to remain cooler and the ABS to work more effectively, giving you a more controlled and stable stop, as well as increased brake life.”
What would be the difference in turning radius?
Hedges: “On paper, turning radius reports show a single and tandem will have an equivalent turning radius assuming the same wheelbase and front-axle cramp angle. Some, however, may argue that, because of the greater amount of tire scrubbing found on the rear-axle tires of a tandem, it may actually pivot a little forward of center. Another thing to consider is that tandems often have longer rear overhangs that can result in increased tail swing.”
Schultz: “The turning radius difference of the Ascendant examples above with a 15-inch-shorter wheelbase would be about 18 inches measured from curb to curb.”
Scharphorn: “The difference in turning radius can vary greatly between manufacturers based on the wheel cramp angle or how far a wheel can turn. As you can see from the chart (Figure 1), an apparatus with a wheelbase of 245 inches with a 45-degree cramp angle has the same turning radius as an apparatus with a 220-inch wheelbase and a 40-degree cramp angle. So, wheelbase length alone is not the only factor to consider.”
Is there any difference in braking capability?
Schultz: “Both vehicles would be tested to comply with FMVSS standards.”
Scharphorn: “See previous comment under ‘Stopping Distances.’ ”
Hedges: “Brakes provided by the axle manufacturers are rated for the GAWR. I would refer you to articles written by brake experts that cover brake performance including drum vs. disc comparisons.”
What is the approximate difference in cost going from a single to a dual rear axle?
Scharphorn: “The price difference can range anywhere from a few thousand dollars up, depending on what capacity you are moving from and what capacity you are moving to. You need to remember that you are going from four tires and rims to eight; however, the brake maintenance long term can help offset the initial costs if a tandem will be the best fit for the apparatus.”
Hedges: “When comparing single- to tandem-axle rigs, the price increase for a tandem includes the additional cost for not only the axles but also wheels, tires, brake system components, and the suspension. In addition, tandem products typically have longer frames and larger bodies (including, of course, a second wheel well). Factoring all this in, the price difference for a tandem-axle unit typically runs $35,000 to $45,000 over a single.”
Do you have any recommendations for purchasers when considering a dual rear axle on a custom chassis?
Schultz: “Pierce believes the customer’s performance needs and expectations of the apparatus should determine the axle configuration. The department’s requirements for items like total vehicle water, hose, equipment, and ground ladder capacities should drive the GVWR requirements. This makes the decision on a single or tandem axle easier to make.”
Hedges: “First and foremost, the configuration of the unit, including tank size; storage space requirements for equipment; and, if an aerial, the specific model will determine if the unit needs a tandem. Beyond that there are customers who require tandem-axle configurations because of the large amount of equipment they intend to carry. On the other hand, some want to have a single-axle product because of their lower cost, compact size, and lower cost of ownership (less tire wear is a key factor).”
Scharphorn: “Rosenbauer axle recommendations are based on the needs of the customer and the anticipated in-service weight of the completed apparatus, which is why communication with your dealer and OEM is such an important aspect of the fire apparatus. Everyone needs to understand the needs and requirements of the apparatus so that it can be built properly.”
Do you have any recommendations for purchasers when considering a dual rear axle when on a commercial chassis?
Scharphorn: “Same recommendations as our custom chassis.”
Schultz: “Our belief is that the customer’s performance needs and expectations of the apparatus should determine the axle configuration. The department’s requirements for items like total vehicle water, hose, equipment, and ground ladder capacities should drive the GVWR requirements. This makes the decision on a single or tandem axle easier to make.”
North: “The recommendations are almost always based on tank, body, and equipment loading that dictate minimum requirements for GAWR. When rear-axle requirements exceed 30,000 pounds GAWR, E-ONE recommends a tandem rear axle.”
Are there any long-term maintenance issues when going to a dual rear axle?
Connely: “With the latest technologies being used on the trucks today, the maintenance issues are not much more than that of a single axle. Where the concern comes into play is the increased cost and additional maintenance duration for the additional axle.”
North: “Obviously a tandem axle will have twice the number of tires, spring components, axle lube, etc. over a single rear axle, so maintenance on these items will be double. Tire wear can be significant for a tandem-axle vehicle that makes a lot of sharp turns as the rear-most axle will be subject to sliding during turns and could sustain more tire wear in this area. Providing too much axle capacity can also cause maintenance issues. If a 30,000-pound single-axle system is deemed adequate for the vehicle design, specifying a 44,000-pound tandem-axle system could provide a harsh ride and unnecessary pounding of equipment and body components because of the stiffer suspension system.”
Sharphorn: “There are definitely more moving parts with a tandem axle, which will cost more; however, the possibility of less abuse and overuse of the components can help prevent premature failure, and the reduced brake replacement will contribute to cost savings in the overall life of the apparatus as well.”
Schultz: “We would not label them as ‘issues.’ It is important to understand—from a total cost of ownership (TCO) perspective—that a second rear axle brings additional tires, brakes, and suspensions that will need to be maintained. Because of the heavier weight of a tandem-axle vehicle, it will take longer to stop and will exhibit higher brake and tire wear. These additional TCO expenses should be balanced against whether there is a single-axle GAWR that meets the department’s requirements.”
There appears to be an industry trend to maximize loading on single rear axles, especially on some of the newer aerial devices. Do you have any comments you would like to make on the subject?
Scharphorn: “Make sure that you know and understand what you are specing and buying. There can be limitations to aerials on single axles regarding functionality, including water flow, reach, and tip load in different operating conditions as well as the amount of equipment and personnel that the apparatus is capable of carrying.”
Schultz: “As a purchaser, it is important to configure a vehicle that has application approval of all components involved—including axles, tires, brakes, etc. Are these approvals being provided as an intermittent rating or continuous rating? How do these approvals fit the profile of your plans to use the apparatus? Has the proper testing to FMVSS standards been conducted? There is a lot to take into consideration.”
Hedges: “Single-axle aerials have, for years, been a popular market segment because of their versatility. While historically these aerials are in the 75-foot range, the recent introduction of 100-foot single-axle quint products has led to increased axle capacity requirements. The increased weight (GVWR) of these newer products is a necessity to support the higher overturning moments 100-foot aerials create when extended fully at zero degrees to the side with their full rated tip load. The single vs. tandem aerial decision should take into consideration budget, vehicle size, maneuverability requirements, and what is to be carried on the unit.”
Connely: “Manufacturers and customers are being asked to do more with less on fire apparatus designs. That includes putting longer aerial ladders on single-rear-axle trucks, larger water tank capacities, more hose load, larger generators, and more equipment in general on apparatus. Not that long ago, a 31,000-pound single-rear-axle GVWR was the highest rating you could get on a fire truck. As the industry evolved into quints, it was difficult to go beyond a 75-foot aerial ladder with 500 gallons of water. Top speed and distance were also limited by tire manufacturers. Today we are specifying larger cabs for more seating capacity, heavier engine/emissions requirements, and stronger body designs with increased compartment storage. Trucks are getting heavier, thus requiring a higher rear GVWR. The aerial industry is asking for more (greater than 100-foot aerial ladders) and pushing the envelope to increase single-rear-axle GVWRs. Most manufacturers are offering or working on 35,000-pound GVWRs on single rear axles to accomplish this. As we generally use the same axle, suspension, and tire suppliers, it’s a matter of ensuring all components are designed, tested, and certified to ensure all components are compatible and the vehicle is safe. Some larger fleet specs still have a requirement for the apparatus to have some margin between GVWR vs. actual GWV to allow for a little breathing room for adding equipment over the life of the truck, extending brake life, etc. If the truck has higher run rates (fire, rescue, emergency medical services), it is pushed to its limits. If it’s used as second due basically for truck company operations, there is less negative impact. In many cases, it’s a matter of limited budgets and operating leaner.”
Any words of wisdom for purchasers when considering single vs. dual rear axles?
Connely: “Dual rear axles should be considered when GAWRs exceed 33,000 pounds or where stability concerns come into play, such as on a tanker, rescue, or aerial.”
North: “Single vs. tandem rear axles is usually an outcome based on many factors, not a starting point for specifications. Some of these factors are as follows:
- GVWR and GAWR requirements based on tank, body, equipment, etc.
- Maneuverability such as turning radius and overall length.
- Traction—eight tires vs. four tires on the drive axles.
- Available suspension systems.
- State or local regulations regarding tire footprint load ratings.
- Maintenance and tire wear.
Scharphorn: “The most important thing is to get an apparatus that is going to safely perform all of the functions the department needs in all operating conditions. This is done by always making sure that you are clearly communicating your needs, such as equipment load, operating conditions, personnel requirements, and any additional expectations that your department has to your sales representative and the manufacturer to ensure that the apparatus you are getting has the correct product configuration for your department’s needs—whether that is single- or tandem-axle apparatus. The determination to go from a single to a tandem rear axle is based on the estimated in-service weight requirements.”
Part 2 will have comments by independent apparatus manufacturers that do not build their own cabs and chassis.
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.