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Boxing a Frame: Article by Jim Clark

Story & Photos By Jim Clark (The Hot Rod MD)

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.

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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.

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Rear of this Model-A frame shows how thin and flimsy the frame was on these early Fords.

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Rails were not any bigger throughout the length of this Model-A frame and served more as body and sheet metal attaching points than as structural members.

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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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Most modern vehicles that have separate frames are of boxed tube construction; made in two halves and joined together by welding.

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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When viewed from the end the hat-section, frame resembled a top hat.  Light steel plate was stamped into the inverted “T” shape and the flat strip added onto the open bottom side.

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Because the early hat-section style frame was made from lighter material than the U-channel frames, adding a reinforcing plate to the frame is necessary to prevent the attachment from ripping off under load.

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.

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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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Most vehicles built using early style frames utilize just the side rails so removing the stock crossmembers is often necessary.

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Some of the original rivets can be drilled out or the rivet heads cut off with a grinder.

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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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After beveling the edges the plates are clamped in place and welded at small intervals.  The x-member made it necessary to install separate plates in small sections.

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Frames with kickups require accurate cutting and fitting of the boxing plates.

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Frame flanges should be heated and straightened before the plates get welded at intervals like this.

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Welding continues until all of the gaps in the welding have been joined.  One continuous weld would cause too much distortion to the frame so it has to be done in segments.

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After all of the welding is done the edges can be ground to finish the edge.  It’s very important to get good weld penetration or the joint will be weak.

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This is a finished ’32 Ford frame utilizing reproduction frame rails and crossmembers and fully boxed on both rails.

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Frame diagrams like this are available from sources like Speedway and Wescott.  Having the correct dimensions is critical on these early style vehicles because the components must lineup for proper assembly.

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Boxing plate, 1932 Ford front half—19404
Speedway Plate
, boxing, 1932 Ford front half.  10-gauge mild steel, designed to box from ahead of the front crossmember to about the middle of the frame.  Plate is 60-1/2" long.  Order two of these boxing plates to box the front half of a pair of rails.

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Shorter sections of boxing plates are also available from companies like Welder Series, precut to fit between crossmembers on various popular models.

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X-members kits like this one for a Model-A frame are available to provide more rigidity to early ladder-style frames.

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This frame rail has been C-d for clearance and added rear axle travel.  Boxing plate has been inset creating additional rigidity to the C-ed section.

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Original early frames often have fatigue cracks after years of hard use.  Those cracks can be welded but because of the age of the metal it may just crack somewhere else.  Inspect them very carefully.

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Crossmember kit, front, flat, 24" wide
Welder Series'
 Crossmember kit, front, flat, for early Ford-style front suspension (transverse leaf).  This crossmember helps get the frame lower by about 1-1/2 to 2'' compared to a stock Model 'A' crossmember.  The front face of the crossmember is already formed.  Simply fold, by hand, the rear faces along the laser-cut slits and tack in the center plate.  Then weld all the edges.  This crossmember is 24'' wide and can be trimmed to fit your frame.  A hole for the spring center bolt is centered in the crossmember.  Use a 1-3/4'' wide spring.  
Also available 28'' wide.  A 28" wide crossmember can serve double duty as the front “C” notches, too.

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U-bolt Eliminator kit
The Welder Series' U-bolt eliminator kit is an ideal companion part to use with this crossmember (#WS15710).  Stock radiators will need modifications to the lower tank.  The ''butterfly'' shape of this crossmember gives more strength to the frame and looks way better than a simple channel. WS21864.

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.


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