Tips on truck frames
Published in: Trailer/Body Builders
Date: 6/1/2005
By: Rick Weber
RICHARD TONER has spent entire days giving a truck-frame seminar for the National Truck Equipment Association. He knows that much about truck frames.
Toner, president of Toner & Associates (Pentwater, Michigan) and the NTEA’s first staff engineer, said the frame is “the backbone of the truck” because “it carries the loads we put on trucks.”
He said most trucks use ladder-type frames with side rails and crossmembers, and the frames are subjected to three types of loads: vertical, torsional, and side.
Side rails support vertical and side loads such as engine, transmission, fuel tanks, battery boxes, suspensions, bodies, work equipment, and cargo. The crossmembers provide torsional rigidity and support components such as the engine, transmission, and radiator. In addition, he said the crossmembers prevent the side rails from twisting with side loads such as the fuel tank and battery box.
“A frame section supported at each end loaded in the middle is in compression at the top and in tension at the bottom,” he said. “There is a neutral axis where there is no stress in the frame. Holes and welding at the neutral axis will not significantly affect frame strength. Holes and welding near the flanges can cause frame failure. Chassis manufacturers restrict the size and location of holes, and most also restrict welding on the frames.”
He said moments are force, or weight, multiplied by distance. One pound times one foot is 1 ft-lb. One pound times 12″ is 12 in-lb. They are both the same moment. He said moments on a truck frame can be measured from the front axle, the front end of the frame, or any other point as long as all forces are included. Up forces are positive and down forces are negative.
“The high-moment area is right behind the cab,” he said. “And where do we like to put mounts? Right behind the cab. They’re OK if you stay away from the flanges.
“Now you see tractors converted into dump trucks. A tractor frame is not a straight frame. They don’t have the right crossmembers to be dump trucks. We seem to pay attention to side rails. We don’t pay enough attention to crossmembers. With the same load, the maximum tractor moment is more than twice the straight truck.”
He elaborated on some truck frame nomenclature, saying: a web is the vertical section of a frame rail; a flange is the horizontal section of a frame rail, located at the top and/or bottom of the rail; a centroid is the center of the material in a cross-section of the frame rail; and shear center is the point that takes a vertical load without collapsing the frame rail.
He said yield strength defines the material and is the maximum stress that the material will sustain without permanent distortion. It is related to ultimate strength.
“If it changes shape, you’ve exceeded the yield strength,” he said.
He said typical values are: 35,000 psi for mild steel, 110,000 psi for alloy steel, and over 110,000 psi for heat-treated steel. When mixing materials of different yield strengths, the lowest value must be used for calculations.
“If I have a frame that is 50,000 psi and reinforcement that is 50,000 psi, use 50,000 yield strength,” he said.
He said the section modulus defines the shape of the frame material in inches to the third power. It is related to the moment of inertia and stiffness. He said published literature can be as much as 30% too high.
“Information is getting better – I will say that,” he said. “But don’t trust them. It’s not the engineers who are providing the bad information – it’s the marketing guys. They can’t sell a frame that is 30% less strong than it was last year.”
Toner said ultimate/tensile strength is the maximum stress before failure by separating; dynamic loading is loads put into a frame from acceleration, braking, road irregularities, etc; and fatigue is the behavior of a material when subjected to cyclically applied stress, which can result in a crack and failure.
“Fatigue is what kills truck frames – not tensile strength or yield strength,” he said. “Fatigue is when you bend something until it breaks. Every material has an elastic zone. If I bend the material and don’t exceed the yield strength, it will come back. If I get past the yield strength, I’ve strained it. It doesn’t go back to the original shape.”
Toner said resisting bending moment (RBM) describes the ability of the frame to resist the moments from the load and truck components; section modulus describes the shape of the material; and yield strength describes the material strength.
Toner said the RBM is the product of the section modulus (SM) and the yield strength (YS) in inch-pounds and is a measure of the capability of the frame rail. He said some manufacturers of equipment such as cranes specify either a section modulus or RBM for the chassis frame. Chassis manufacturers supply frame information with RBM, section modulus, and yield strength in truck data books and body builders books.
So, RBM = SM x YS.
“Why is that important?” he asked. “It describes the maximum load the manufacturer says we can put on the frame in moments.”
Reinforcements popular
He said reinforcements are popular because they’re easy to put on. But “I’m not big on bolted reinforcements. If you weld it in, it’ll be strong.”
Moment of inertia is a mathematical representation of the shape of a frame rail, and is represented by “I.” SM = I/d. And deflection = W x l3/k x E x I.
He said as wheelbases get longer, deflection becomes important. The rule of thumb is that with wheelbases under 160″, design for strength; and with wheelbases over 160″, design for deflection. For the same load, a 10% increase in wheelbase requires a 10% increase in RBM for strength and a 33% increase in moment of inertia for the same deflection.
“If you’re going to lengthen the wheelbase on a chassis, never go beyond the wheelbase the chassis manufacturer recommends without taking a look at it and seeing what you’re going to do,” he said.
He said endurance limit is the maximum stress that a material can tolerate indefinitely without failure; fatigue strength is the stress level corresponding to a definite life; and stress concentration is a hole, weld, crack, notch, or other discontinuity that concentrates stress. An open hole concentrates stress from 2.5 to 3 times. A Grade 8 bolt reduces the concentration to 1.5 to 1.9.
“If I’m below, say, 45,000 psi in fully reversed bending stress, I can do it forever and it won’t fail,” he said. “If I’m above 60,000, then I have a finite life. You want to stay below the endurance limit.”
Stress concentration is a sixth-power effect when the stress level is above the endurance limit. Doubling the stress decreases the life by a factor of 64, so a 300,000-mile truck becomes a 5,000-mile truck. Increasing the stress by 12% decreases the life by one-half. Decreasing the stress by 12% doubles the life.
“Truck frames are usually modified to change the chassis wheelbase for proper weight distribution,” he said. “Frame modifications are more common on medium- and heavy-duty trucks, although some light-duty chassis are extended for applications such as car carriers. When possible, it is better to move the position of the rear axle than to cut and modify a frame.”
He said fish plates are large flat plates bolted to the web of the frame rail, and are usually 3/8″ to 1/2″ thick and increase the section modulus considerably. The height of the fish plate often exceeds that of the frame rail. Fish plates are often installed between the rear of the cab and the end of the frame to handle the stresses imposed by a crane mounted directly behind the cab. The ends of the fish plates should be tapered to reduce stress concentration in this area. Fish plates should be bolted using the match-drilled technique so the fish plate and frame act as one. Rivets, brackets, and other components in the area where a fish plate is to be installed will have to be removed and reinstalled with the fish plate in place.
Frame splices
On frame splices, he said the general rule is that reinforcements should taper a minimum of two times the frame height. Volvo recommends a taper of 20-30 degrees; 20 degrees is 2.7 times the frame height, and 30 degrees is 1.7 times the frame height.
He said if the increased wheelbase is less than or equal to the maximum wheelbase manufactured by the chassis manufacturer with the frame on the vehicle, then follow these recommendations:
Single-member rail – the minimum reinforcement should equal the RBM of the original frame rail.
Multiple-member rail – the minimum RBM of the reinforcements should be equal to the RBM of the strongest single member. Splices should be staggered at least twice the height of the frame rail.
Toner’s Frame-Splicing Commandments:
Don’t go from stiff to flexible. Taper and stagger reinforcements.
Stay out of high moment areas or extend reinforcing to a lower moment area.
Match steels for yield strength as closely as possible.
Extend reinforcements a minimum of twice the frame height past the splice before beginning the taper.
The strength of a frame rail is in the flanges.
More steel is stronger than less steel.
Use a straight cut at the splice.
Toner said the purpose of body mounts is to attach the body to the truck frame without doing more harm than good and retain the body in all horizontal directions.
“Harm can be done by concentrating stress and not allowing the frame to flex,” he said. “Rear mounts could be shear plates or some other rigid configuration that prevents movement. Front mounts should be flexible or placed to avoid stress concentrations and high moments.”
He said mounts can be combined to take advantage of the best features of more than one type. Spacer strips used between the body and truck frame perform multiple functions: cushioning member, sacrificial wear member, and stress-spreading member. Rigid mounts should be used at the rear and flexible mounts at the front. The front mount should not be at the front of the body.
Toner said there are three basic types of body mounts: flexible, rigid, and combination. Rigid mounts should be used at the rear, and flexible mounts at the front.
He said that even though U-bolt body mountings are popular, they are among the least effective mounting systems.
“Are they bad by themselves? No,” he said. “But they don’t tend to stay tight.”
He said if they are used, proper frame spacers must be used. Some of the problems are loosening, not preventing forward movement of the body, and frame damage. He said frame flanges should never be notched for a body mount.
“The body should not be rigid at the front,” he said. “It is better to be mounted solidly at the rear and float at the front. Avoid high moment areas for the mounting brackets. Use existing frame holes when possible.”
References
The Procedure Handbook of Arc Welding from The James F Lincoln Arc Welding Foundation. 216-383-2211. www.jflf.org.
Steel design software and manual from the Auto/Steel Partnership Program in Southfield, Michigan. 248-945-4777.
Government Printing Office for Safety Standards. www.gpo.gov. Code of Federal Regulations, Title 49 (pick year), Parts 500-599, Part 571 of Federal Motor Vehicle Safety Standards.
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