Tips on truck frames
Jun 1, 2005 12:00 PM, By Rick Weber
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 × l3/k × E × 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:
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Single-member rail — the minimum reinforcement should equal the RBM of the original frame rail.
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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.
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Toner's Frame-Splicing Commandments:
Don't go from stiff to flexible. Taper and stagger reinforcements.
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Stay out of high moment areas or extend reinforcing to a lower moment area.
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Match steels for yield strength as closely as possible.
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Extend reinforcements a minimum of twice the frame height past the splice before beginning the taper.
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The strength of a frame rail is in the flanges.
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More steel is stronger than less steel.
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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.
