In an open letter to the fire service that appears as an advertisement in several industry trade publications, Freightliner Corporation (parent company of America LaFrance) President and CEO Jim Hebe states, “The cab and chassis are the foundation on which everything else depends … the chassis gets the hardest workout and is subject to the most wear and tear … the cab and chassis have been willfully ignored as the key components of a fire truck.”
Hebe’s words couldn’t be truer. Unfortunately, it is not uncommon for fire apparatus of all types to experience a chassis problem or failure of some type at some point over the vehicle’s service life. With some basic knowledge, firefighters could have avoided many of these problems back when they were speccing the vehicle. It is also important to know many other problems and failures that occur to other parts of the vehicle can frequently be the result of too weak a chassis system. A weak chassis can also be partially responsible for poor ride quality.
Section modulus, minimum yield strength, and resistance to bending moment (RBM) are three factors that determine the size and strength rating of the chassis frame rails, which are the structural backbone of the entire vehicle. They are extremely important in determining the success or failure of the vehicle’s overall service life.
Section modulus refers to the physical size of the frame rail itself, expressed in cubic inches. It includes the rail’s height or depth, the flange’s width, the inside radius at the flange, and the material’s thickness. These numbers are combined in a mathematical equation that determines the section modulus rating.
There are actually two different section modulus ratings. The nominal or design rating is for a given set of numbers. The rating assumes that the frame rail manufacturer will make every single part to these exact measurements.
However, in the real world, frame rail manufacturers do not produce perfect parts-they will vary somewhat in size in the manufacturing process. Frame rails can typically vary up to as much as nine percent in either direction from the nominal or design rating dimensions established by the chassis engineer. For example, a frame rail may be calculated with a nominal (design) section modulus rating of 16 cubic inches, but the real rating can typically range from between 14.55 and 17.45 cubic inches. When writing a frame rail bid specification, plainly and specifically state that the size indicated is the nominal type rating.
When frame rail reinforcements such as “C” channel or in-verted “L” types are used, the reinforcement section modulus is added to the main frame rail section modulus rating to obtain the final section modulus rating-for example, a main frame rail with a 14-cubic-inch nominal section modulus rating combined with a reinforcement section with an eight-cubic-inch nominal section modulus rating results in a total 22-cubic-inch rating. However, the increased rating is only in the reinforced area; the section modulus will still be 14 cubic inches in the unreinforced area. Think of section modulus as the amount of mass in the frame rail system that determines chassis stiffness.
Frames are reinforced several ways. The most common method is to add reinforcement from the front suspension mounting brackets all the way through to the end of the frame rails. This method is often used for three-axle chassis, such as those of aerial trucks.
Another common method is to add reinforcement from the front suspension mounting brackets all the way through to the end of the rear suspension mounting brackets, which is generally used on two-axle chassis. Both methods cover key stress areas in the center of the chassis and should help prevent frame failure.
The actual amount of section modulus required is determined by gross vehicle weight rating (GVWR), frame rail length, number of axles, type of suspension, and vehicle application. As the GVWR increases, so does the section modulus requirement. As the frame rails increase substantially in longitudinal length, the section modulus requirement also increases. Since a three-axle truck chassis is almost always longer and heavier than a two-axle chassis, the section modulus requirement also increases. An apparatus that will be used in an off-road application typically requires an increased section modulus rating compared with a chassis that will be used strictly in an on-highway application. Finally, air suspension systems create a shock force in the immediate area of the suspension system, and it is a common practice to increase the section modulus rating in the area of the suspension system with a frame rail reinforcement of some type. This helps to prevent cracked frame rails over time.
Chassis engineers have determined over the years that the section modulus rating has a direct correlation to ride quality, which is frequently improved as frame rail stiffness (section modulus) is increased.
The steel used in frame rails is heat-treated in some manner in the manufacturing process. This is a factor in the minimum yield strength rating of the material, expressed in pounds per square inch (psi). Many medium-duty trucks in the 25,000- to 33,000-pound GVWR range use frame rails in the 50,000- to 80,000-psi minimum yield strength rating, depending on chassis manufacturer. Most Class 8 trucks with a GVWR of more than 33,000 pounds use a high-tensile, heat-treated steel frame with a 110,000-psi rating.
Notice that up to now we have not discussed strength or comparative strength. In theory, section modulus and minimum yield strength, each considered alone, do not determine strength. However, when the two ratings are multiplied, they do determine comparative strength or RBM. To obtain the frame rail RBM rating, multiply the section modulus rating (in cubic inches) by the minimum yield strength rating (in psi) for an RBM result in inch-pounds-for example, a 16-cubic-inch section modulus rating multiplied by a 110,000-psi minimum yield strength rating results in a frame rail with a 1,760,000-inch-pound RBM rating.
Using this method, we can now do comparative strength ratings. If the PSI rating of the material is the same, then the frame rail with the highest nominal section modulus rating will be the stronger rail. However, if the section modulus rating or the psi yield strength rating varies, then multiply to find the RBM inch-pound rating to determine which frame rail is stronger.
Finally, one big no-no is to allow anyone in the manufacturing process of the vehicle to flame- or torch-cut the frame rails in key structural areas. This specifically includes the entire center section of the chassis from the start of the front suspension system to the end of the rear suspension system. Flame-cutting destroys the heat-treating process in the area where the rail is cut. This requirement should definitely be included in your frame rail bid specification.
All information and data presented in this article are for general minimal guideline purposes only. The ratings shown include nominal type section modulus ratings in the RBM calculation. The vendor you select for purchase may have different minimal frame rail RBM recommendations and requirements. To reduce the chance of a frame rail failure over the service life of the apparatus, thoroughly discuss minimal frame rail RBM requirements with the vendor when speccing and purchasing apparatus.
JIM WILKINS is a California-based fire apparatus specification and cost control consultant with a background in chassis and power train engineering. He has a bachelor of science degree from California Polytechnic State University in San Luis Obispo.
