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1955 Chevy Bel Air Project - Shoebox Structure
We Follow The Build Of A Fatman Fabrications Tri-Five Chassis For Installation In Our '55 Project.
Sep 1, 2010
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1955 Chevy Bel Air Project - Shoebox Structure
The first step is to assemble the pieces that make up the outer rails on the chassis jig. The rails are made of box steel tubing and offer superior rigidity over the stock rails.
Each piece has alignment tabs like these welded in so the rail pieces align properly for welding once they're set up in the jig.
Using basic measuring tools and the alignment line down the center, the individual framerail pieces are lined up, then secured in the jig for welding. After they're secured, the pieces will be measured one more time to verify alignment before being welded. All it takes is for one little piece to be off, and nothing else will line up correctly during the frame's construction.
With everything set, the rail pieces are tack welded together. In the initial construction stages, all parts are only tack welded, so if anything needs to be moved around to keep the frame true and measurements correct, only a small weld must be ground off. Only once everything is aligned, tack welded, then double-checked again for proper alignment and position will the chassis have the finish welding done.
Before the crossbraces can be set in place, the outer rails must be positioned correctly apart from each other, and verified that they are parallel. The builders at Fatman are fanatical about taking measurements and making sure everything is correct, so every frame built will be as perfect as possible, and have no fitment or suspension tuning issues.
With everything set, the mounting point for the first rear crossmember is located, measured, and scribe marked. The crossmember will be set in place, checked for alignment, then tack welded in place.
At the front of the frame, the crossmember that mounts the rack-and-pinion and engine block mounts is set in place. Placement of this piece is critical, because if it's off, it will throw everything else out of alignment.
Next up is the radiator core support crossmember. It's alignment and placement is checked and set based off the engine/rack-and-pinion crossmember so when the car is assembled, the sheetmetal will line up properly on the new chassis.
The outer crossmembers are set in stages, front-rear-front-rear. As another crossmember is laid in, it's clamped in place, measured and checked for placement, then tack welded in position.
With all the outer crossmembers in place, the inner crossmember/X-member is built in pieces and tack welded in place. Fatman builds this piece with routing holes for the exhaust system so no matter how low to the ground the car is set, the exhaust won't be scraping pavement. The forward exhaust holes are oblong to allow for movement of the exhaust system for installation and removal. Placement of this structure is critical because the transmission mount bolts to the front, so there's no such thing as measuring too many times before tack welding everything.
Tying the two sides of the X-member together is this top loop and a bottom plate. This part of the chassis was specially designed to offer maximum rigidity for the frame, and the driveshaft loop in the center not only adds to that, but protects the passenger compartment from unexpected visits by the driveshaft in case anything fails in the drivetrain. The top loop also clears the factory tunnel on the Tri-Five body, so no body modifications are necessary.
With all the crossmembers in place and tack welded, the whole chassis is remeasured to make sure everything is aligned and located properly. Once it passes inspection, all the crossmember pieces will be fully welded to the framerails and the adjoining components.
Taking a break from the frame, one of the next steps is assembling the rear end housing. Fatman can use pretty much any rear end housing in its Tri-Five chassis as long as it meets the required width measurements. In our case, the requirement is a 59 1/2-inch hub-to-hub axle assembly. This will allow us to mount up to a 10-inch wide wheel/tire combo with 6 1/4-inch backspacing and 5 3/4-inch frontspacing in the rear without having to cut the wheelwells. The mounting brackets for the four-link bars are set in place on a jig, then the housing is dropped in. Once measurements and alignment are set, the brackets will be tack welded in place. For our chassis, we'll be using a Moser-supplied 12-bolt rear.
Moving back to the frame, the mounting points for the upper control arms and shocks are set in place, aligned, then tack welded.
Back on the rear end, the housing is rotated and aligned in the brackets so the pinion angle and alignment will be correct when mounted in the chassis. If the alignments are off here, then the car will never handle right and be an absolute bear when it comes to tuning the suspension.
Here, the mounting tabs for the rack-and-pinion unit are aligned and welded in place. This lower crossmember also serves as the mounting point for the lower control arms.
Next up, the transmission crossmember is clamped in so it and the engine mounts can be set up and secured to the chassis. When built, a Fatman chassis is set up for the transmission/engine combo specified by the customer. If you decide to change transmissions later on, the chassis has holes drilled for multiple crossmember mounting points. All you'll need is a new, correct transmission crossmember from Fatman, and you'll be good to go.
A dummy engine block and trans similar to what the customer specifies are set in place so the mounts can be properly positioned. Once everything's in place for the engine mounts and the measurements and alignments check out, they're tack welded on.
With the engine mounts set, the trans crossmember is located and then the mounting tabs are tacked in for drilling later.
These reinforcement tabs support the body mount, and serve as a mounting point for the frame-mounted, four-link suspension brackets.
Moving to the outside of the frame, the Fatman crew starts welding in the body mounting tabs. Fatman has all the measurements for the body mount locations on every Tri-Five model, so once you specify what body style you're using, the correct mounts are set in place. The Fatman mounts are much more substantial than the factory units, cutting down on body flex and adding more rigidity to the car's overall structure.
This is one of the rear sway bar endlink/body mounts. Because the location on the frame for these never changes, they can be mounted using a template and tacked in place.
With the sway bar mounts attached to the rear end housing, it's ready for the shock mounts to be tacked in, and the placement and attachment of the frame mounts for the four-link also attached.
The measurements checked out, so the four-link frame mounts are tacked in place. The mounts feature multiple locations so the suspension can be tuned as necessary.
With the bars set, measurement and placement for the crosslink mount is addressed. The crosslink gives the four-link design extra lateral strength for stability in cornering and other maneuvering.
With the rear and four-link set, the coilover mounting tabs are located and welded on.
Back at the front, these support tabs are tacked on. They bolster the engine mounting bolt tube so it can handle anything mounted to it. The special radius design of the mount was done so that the weight of the engine will cancel out the inward pressure exerted on the frame by other parts of the vehicle and chassis, improving handling, balance, and adding to the chassis’ already robust strength.
With all of the crossmembers, body, and suspension mounts set, the next step is to finish weld all those pieces, then hose the entire assembly down with paint before finally assembly of the suspension, brake, and steering components.
Here's what the typical front suspension will look like on a Fatman Tri-Five chassis. Customers can either specify QA1 adjustable coilovers, or RideTech components. The control arms are Fatman's own in-house design that feature strength and construction equivalent to the chassis they're mounted on. The standard spindles used are a Mustang II design that determines the brake application. If customers want additional drop over standard, other spindle options are available. For brakes, Fatman recommends Wilwood Engineering, but anything that fits the Mustang II spindle can be used.
The rack-and-pinion for the Fatman chassis is an improved Mustang II design that offers a quick ratio and superior steering. This is a "stock" unit, so if you ever have a rack failure, you can easily obtain a replacement at your local parts store without issue.
We mentioned before the fully adjustable four-link suspension with Z-bar and coilover shocks. The only modifications required to the body will be a small revision of the left rear trunk floor for proper clearance of the upper coilover/ShockWave mount, and an aftermarket fuel tank in order to provide proper clearance for full suspension travel. Those two small mods allow a 5-inch stroke coilover to fit for extra suspension travel over the more commonly used 4-inch units.
Here's what the finished chassis looks like, ready to have a classic Tri-Five body bolted to it.
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