BOXING A FRAME Most new cars are of unitized construction; they basically have no frame. However, old style hot rods have either U-channel or hat-section shaped frame rails. The U-channel (sometimes called C-channel) side rails are found on Ford and Mopar models from their very first model years up through the 1940s. Some early GM vehicles use the 4-sided hat-section type rails. |
Front section of this early Mustang shows how the frame/body are combined into a unitized construction. Rear of the body (not shown) completes the structure of this modern type vehicle. |
Rear of this Model-A frame shows how thin and flimsy the frame was on these early Fords. |
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This new pickup frame still uses U-channel for the frame with a boxing plate welded on at the crossmember attachment point. |
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Both types use a series of crossmembers to space the rails apart and serve as locaters for other components. The early configurations resembled a ladder and were not very rigid. They were prone to twisting as the vehicle crossed irregular terrain. The addition of X-members added rigidity but they still twisted under loads applied at the corners. Early Fords were not affected much by this because the buggy-style cross spring was attached along the centerline of the vehicle and the axles located by radius rods (wishbone) that also attached along the centerline. As the wheels climbed up and down over irregular terrain the axles pivoted around the centerline without transferring any of the twist to the frame rails. Body roll was addressed in later models with anti-roll bars. GM and Mopar vehicles used semi-elliptical springs mounted parallel to the frame rails at the corners of the vehicle. This configuration stabilized body roll better but introduced more twist to the frame rails and allowed less suspension travel. GM added rigidity by making 4-sided hollow rails (hat-section) but they were not very strong because they used thinner metal construction. Mopar frame rails were of thicker materials and cross-section that made them more rigid but still exhibited some twist. |
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To solve this problem of frame twist hot rod builders added a fourth side to the U-channel, usually referred to as “boxing”. This, along with X-members and additional crossmembers, added the necessary rigidity to the frame when more modern suspensions that attach at the corners were utilized in today’s modern hot rods. |
This cross-section of a U-channel shaped rail illustrates where the edges of the rail and boxing plate should be beveled prior to welding. Weld should penetrate all the way through so that there is still sufficient strength after grinding the edge. A nut or block of steel that can be threaded should be welded inside at attachment points if access holes are not going to be cut in the boxing plates. |
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Cutting off the rivets with a hammer and chisel also works if you don’t have access with power tools. |
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Summary: Frames on many early model vehicles were made of lighter materials and lacked rigidity. They were adequate for the slow speeds and small powertrains that these vehicles were equipped with. However, when adding bigger engines and more modern suspension systems it is necessary to reinforce these frames to withstand the additional loads. This is usually accomplished by adding a fourth side to the C-shaped frame rails (boxing) commonly used on these early models. |