Weight Distribution Affects Apparatus Handling And Stopping
By Gary Handwerk
There are many factors influencing fire apparatus safe handling and stopping, and we are fortunate to have carefully-engineered truck chassis, designed to handle specific loads over more miles than we will see in the fire service.
On the other hand, there are too many apparatus in service around this country that do not handle as well as they should, or worse, are just plain dangerous.
The reason why is incorrect weight distribution. The causes for improper weight distribution are many: inappropriate original apparatus design; poor specifications which didn’t indicate the full loading of the finished apparatus; adding features to apparatus during refurbishing or just plain old-fashioned overloading.
Weight distribution problems are commonly caused by overloading an axle, under loading an axle or having too much weight on one side of the axle or the other.
Before getting too deep into the problem, we need to learn a few definitions.
Some Definitions First
First, gross axle weight rating (GAWR) is the maximum certifiable weight which a given axle may support. The certifiable weight is determined by the lowest rated one of the following components: tires, suspension, hubs, wheels, rims, steering, bearings, brakes, chassis frame and the basic axle assembly itself.
For example, a truck with 14,600-pound rated axles, tires, wheels and brakes with a 12,000 pound set of springs will have a certified weight rating of 12,000 pounds.
Next, there’s gross vehicle weight rating (GVWR) which is the total maximum certifiable weight of the vehicle, including payload. That means GVWR equals GAWR front and rear, less the limitations caused by chassis frame, steering or drive train capabilities.
Individual GAWRs
I know what you are thinking. The National Fire Protection Association (NFPA) 1901 does state you cannot exceed the GAWR front or rear. Yes that’s true. However, the caveat is the loading shall be within the range set by the chassis manufacturer.
Even with that cautionary standard, it doesn’t stop departments from buying a chassis with a much larger axle than needed. Heavier than necessary axles negatively affect apparatus handling, braking and component life. The standard also does not stop departments from overloading apparatus, which seems to occur over time.
Don’t fall into the trap of assuming apparatus is OK just because it doesn’t exceed its GVWR. The unit’s individual GAWRs need to be examined, as well as the loads side to side on each axle.
Let’s look at three different apparatus. The first – call it Apparatus A – has 9,500 pounds on the front axle and 30,500 pounds on the rear axle. Apparatus B has 15,000 pounds on the front axle and 25,000 pounds on the rear axle and Apparatus C has 14,000 pounds on the front and 26,000 pounds on the rear.
While none exceed the 40,000 GVWR, apparatus A and B have problems. Apparatus A has an under loaded front axle and a much overloaded rear axle. With this configuration it’s likely to steer into corners and stop poorly. Additionally, the ride will be harsh and, eventually, there may be problems of components cracking or falling off due to the shock caused by under loading of the front axle.
As a general rule, never exceed an axle certified weight rating and, when fully loaded, at least 75 percent of the axle rating should be used.
Taking a look at Apparatus B, it’s easy to see it has an overloaded front axle. Granted, it will drive better than Apparatus A, but it’s not safe, and excessive wear should be expected with the extra weight on the front axle.
One may wonder how apparatus that are not built for the job and have improper axle loading come into existence.
A Recipe For Trouble
The first way is from the fire department buying its own chassis, which is a guaranteed recipe for trouble. Local chassis sales representatives, in most cases, know nothing about fire apparatus and will not make the optional selections needed to build a good apparatus.
Too often, fire departments make decisions based for two reasons – “bigger is better” or “how can we save money.”
Departments that feel compelled to buy chassis on their own really need to get exact specifications from the apparatus builder. That way, the departments can make sure they get what they need.
The second way trucks with poorly distributed weight end up on the road is by departments not specifying, or taking into account, all the equipment and weight the apparatus will be carrying.
According to NFPA 1901, a pumper has an equipment allowance of 2,000 pounds. However, many departments are really running a pumper/tanker, a rescue pumper or a rescue pumper/tanker and end up carrying 3,000 to 4,000 pounds of equipment.
To make matters worse, much of that extra weight is behind the rear axle, adding more to the rear axle while taking away from the front axle. It’s always best to list every piece of equipment the apparatus will be expected to carry as part of the specifications and let the apparatus builders’ engineers find an appropriate home for it and account for the weight.
The final way apparatus with inappropriate weight distribution configurations end up on the road is through fire departments picking builders that do not have the engineering capabilities to get the job done. The good news is there are very few of this kind of builder left. It’s best for departments to know who they are dealing with and what kind of capabilities the builders have before awarding the bid.
Getting Overweight
Finally, department should, under no circumstances, accept apparatus until it is fully loaded and weighed. The apparatus should be weighed front to back, and side to side and then total overall.
Another factor contributing to overweight apparatus is the tendency of departments to add new equipment as money becomes available or is donated or technology evolves. Over time, equipment piles up, and the apparatus becomes overweight.
Departments often replace steel water tanks with plastic tanks, and it’s difficult to resist the urge to add water to apparatus when a new tank is ordered. While plastic is lighter than steel, more substructure will be needed to support the new plastic tank and that will add weight.
It’s important to remember that older apparatus were never designed for plastic tanks, so all the possible weight savings may not be realized. Tank conversion really ought not to be done until the original apparatus has been weighed and analyzed.
It’s important to remember that when weight is added to apparatus, some will go on the front axle and some will go on the rear axle. How much depends on where the load is placed.
If the load is placed between the front and rear axle the following formula and definition applies: K = distance in inches from the center of the load to the center of the rear axle; T = distance in inches from the center of the load to the center of the front axle; WB = wheelbase in inches; L = Load in pounds; F/X = front axle; and R/X = rear axle.
The load on the front axle equals K times L divided by WB, and the load on the rear axle equals T times L divided by WB.
For example, to determine how much load a 1,000-gallon tank will add to the front axle load, the factors are: K= 2 inches; WB= 170 inches; and L= 10,000 pounds, which is 1,000 gallons of water, the weight of the tank and the supporting structure.
The load on the front axle will be 2 inches times 10,000 pounds divided by 170 inches, which equals 117.65 pounds.
If the tank is moved forward on the chassis by an additional 6 inches, the new load on the front axle will be 8 inches times 10,000 pounds divided by 170 inches, which equals 470.59 pounds.
If the load is placed behind the rear axle center line, the following definition and formula applies: Z= distance in inches from the center of the load to the center of the rear axle; WB = wheelbase in inches; F/X = front axle; R/X = rear axle.
The load on R/X equals Z divided by WB times L + L. The load removed from the F/X equals Z divided by WB times L.
For example if a 1,000-pound diesel generator is added in a rear compartment on a pumper, using example measurements (Z = 47 inches, WB = 170 inches, and L = 1,000 pounds), the formula will look something like this: Load added to rear axle = 47 inches divided by 170 inches times 1,000 pounds + 1,000 pounds = 1,276.5 pounds.
The load removed from the front axle = 47 inches divided by 170 inches times 1,000 pounds = 276.5 pounds.
In non-engineering, non-technical terms, this simply means if a load on an apparatus, like a water tank, is placed closer to the rear axle, more of its weight will be on that axle. If you move the tank forward by 6 inches, some of the weight will be moved to the front axle and some will be removed from the rear axle.
Additionally, if a load is added behind the rear axle, some of that load weight will be removed from the front axle and that weight along with the original load will be added to the rear axle.
It should be obvious that placement of loads on apparatus is critical. It’s not just the total weight, but where the weight is located that also counts.
Axle Side-To-Side Loading
It is also important to consider side to side axle loading. The NFPA 1901 standard allows for a 7 percent side to side load variation. What is critical when looking at top-side weight is not to exceed the individual single side tire(s) loading. For example, if you have a 12,000 pound GAWR and the tires are rated at 6,000 pounds each, or 12,000 pounds for a pair, you cannot exceed a load on one side more than the tire rating of 6,000 pounds. (See accompanying chart)
As noted in the outset, there are too many overweight apparatus in service today, and there are things that can be done to remedy the situation.
The first and perhaps easiest solution is to remove or move equipment. This includes hose in the bed and personnel in the cab. If this doesn’t fix the problem, the water load may need to be reduced. Reducing water load can be done by using inflated truck tire inner tubes in the tank, but be careful not to affect air or water movement inside the tank.
In a few extreme situations, an apparatus may need to be taken out of service.
The new edition of NFPA 1911 soon to be released will call for weighing every apparatus each year, but do not wait for a new rule book to do what is right today.
I want to challenge every fire department reading this article to weigh each apparatus in their department, including reserve pieces. Go to the rock quarry, the grain depot, scrap metal yard, truck stop or wherever and check the front, rear, left side, right side and total weight of each apparatus.
If you are over or under weight on any axle, find out what must be moved or removed to fix the situation.
The life you save may be your own.
Editor’s Note: Gary Handwerk has been involved with the fire service industry for 35 years working for various fire apparatus or fire pump manufacturers. He is currently global pump product manager for Hale Products and has been a member of the National Fire Protection Association (NFPA) Fire Apparatus Standards Committee for 15 years.