AT one point during his “Truck Frame Overview For Body Mounting” session, Dick Toner asked for a show of hands. Which frame splice is the strongest: Straight butt joint? Staggered butt joint? Or tapered butt joint?
When the votes were in, he let everyone in on a secret: There is no correct answer. They're all the same.
He said if the frame is not properly reinforced, it doesn't matter which type of splice is used because the joint on the bottom of the frame will fail from fatigue.
“The bottom rail is in tension,” said Toner, who gives one-day seminars on frames for the NTEA. “It's being pulled apart. The weld we put on the bottom rail is going to fail first. That's why it doesn't matter what kind of butt joint you put in.”
He said the general rule is that reinforcements should taper a minimum of two times the frame height — which fits into Volvo's recommendation of 20-30 degrees, 20 being 2.7 times the frame height and 30 being 1.7 times the frame height. So for a 9" frame, you go 18" each way and taper from there.
“Stiff to flexible is bad,” he said. “You want to spread the stress out as much as you can. Any kind of tapering you can put on is good. Sometimes you have to take off crossmembers and shorten them. Sometimes you get away without it.”
The Seven Commandments
His frame-splicing commandments:
Don't go from stiff to flexible. Taper and stagger reinforcements. “If you're in a situation where you're putting an inside reinforcement on and an outside reinforcement on, stop them in different places. Taper, taper, taper. Stagger, stagger, stagger.”
Stay out of high-moment areas or extend reinforcement to a lower-moment area. “That's when you're working on a straight truck and you know you're going to have to do something behind the cab. Get as far up under the cab as you can. It's amazing how frame moments drop off when you get under the cab.”
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. “I can't repeat that enough times.”
More steel is stronger than less steel. “If there's a question, increase the gauge. Increase something. When you're putting a reinforcement on a truck, the difference between, say, 1/4" steel and 5/16" steel weightwise is next to nothing. If you're putting a 6' piece on, the weight change is nothing. I've found through the years that if you add weight to a customer's used truck being repaired, they love to see steel on there. They don't care about the extra pounds. It's when the fleet goes to buy the chassis at the dealer that they care about a few extra pounds.”
Use a straight cut at the splice.
Toner said the purpose of body mounts is to attach the body to the truck frame and prevent the body from moving on the truck frame in a horizontal direction — forward, rearward, and laterally. 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 in high-moment areas.
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.
He said if U-bolt body mountings work, use them. But he added a strong caution.
“I've seen more damage done to truck frames with U-bolts than any other single type of mount,” he said. “A mechanic gets under there and says, ‘That looks loose.’ He puts the air gun on it and torques it until he collapses the frame rail.”
He said that even though U-bolt body mountings are popular, they are among the least effective mounting systems. If they are used, proper frame spacers must be used. Some of the problems: loosening, not preventing forward movement of the body, and frame damage.
“Sometimes they don't have the right size U-bolt, so they notch the frame flanges,” he said. “As you can imagine, there aren't many ways you can concentrate stress better than that. If the truck lives its life driving around the city at 30mph never fully loaded, that will probably live.”
On stress concentration, he said the 6th Power Effect holds that doubling the stress decreases the truck's life by a factor of 64. Therefore, a 300,000-mile truck becomes a 5,000-mile truck. Reducing the stress by 10% doubles the life, and increasing the stress by 10% halves the life.
“That 10% change is not a lot,” he said. “It really isn't.”
He said most trucks use ladder-type frames with side rails and crossmembers. 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, the crossmembers prevent the side rails from twisting with side loads such as the fuel tank and battery boxes.
He said frame strength is described through yield strength, section modulus, and Resisting Bending Moment (RBM). Yield strength defines the material and is the maximum stress, in pounds per square inch, that the material will sustain without permanent distortion. Typical values are 35,000 psi for mild steel, 80,000 for alloy steel, and over 100,000 psi for heat-treated steel. Section modulus defines the shape of the frame material in inches cubed and is related to frame stiffness and flexing. Chassis manufacturer-published section module literature can be as much as 30% too high. RBM is the product of the section modulus and yield strength in inch-pounds, which describes the ability of the frame to carry loads.