The New Chasis
1. We bought 80 feet of 2x4x1/8-inch rectangular steel tubing from our local supplier. Sixty feet would have done the job, but we bought extra just in case. We used an 11-inch-diameter chop saw to cut the tubing to length. Notice that the end of each piece has been beveled, so that when we butt two pieces together, the weld will fill in the gap for a stronger joint. | 
2. Construction began by clamping the Scott's front IFS crossmember 1/4-inch off our frame table. This would keep the crossmember and the engine oil pan off the ground when the suspension was cycled upward. We knew going into this project that we'd be running a 30-inch-tall tire and that the front suspension offered 8 inches of travel. We'll need to make some modifications to the upper control arms to gain enough suspension travel for that diameter tire, but we'll get to that later on. | 
3. We purposely ordered the crossmember narrower than Scott's normally builds them because that would make the control arms longer, which would translate into more suspension travel. The downside of that is it means we'd have to make compound cuts in the tubing to adapt the crossmember to the wider frame dimension. We could have narrowed the frame to match the crossmember, but then we'd run into clearance problems around the transmission, and it would mean making extra wide body mounts. Once again, the chop saw proved invaluable in this effort, and O.C. Auto Shop's Mike Kim made quick work of the complex cuts, while the author MIG-welded the parts together. |
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4a. Like the front of the frame, the rear of the frame had to be stepped inward to accommodate the independent rear suspension | 
4b. At this point, the diff was secured to the framerails with scrap steel to establish the wheelbase of the chassis, but we'll make adjustments later to set the pinion angle once the engine and tranny are in place. For now, this gives us a point of reference to design the rear suspension |
5. Later on in the buildup, the middle section of the frame will receive not only gussets at the joints but additional crossmembers to prevent the 'rails from twisting and bending. This is a key step before the chassis hits the ground. You can't have long sections of chassis like this without some reinforcement. | 
6. Unfortunately, we didn't have the wheels for our project at this time. We were in a hurry, though, so we guesstimated how wide we could make the rear track width by measuring the inside of our bed and subtracting the width and backspacing of the wheels we ordered. Then, we wimped out and subtracted an additional inch from each side, just to be sure when we got our wheels that they'd actually tuck inside the bed. The guys from O.C. Auto Shop laser-cut a steel jig to hold the Ford Expedition rear hubs in place at our desired ride height. Just outside of the framerail, you'll see the FK Rod Ends spherical bearings we'll be using as the pivot points for the suspension. | 
7a. Construction continues with the fabrication of the rear control arms |
7b. We used 1.5x0.120-inch wall chrome-moly tubing for maximum strength. The advantage of using spherical bearings at the pivots is their extreme strength. God forbid we have some road debris hit one of the wheels or smack into a curb at high speed; these pivots will not break. All of the mounting points will be reinforced with gussets, as well | 
8. After Mike had the basic design of the control arms mapped out, he set about aligning the rear diff to the jigs and chassis. It was time to design the rear cradle that would hold the rearend securely in place. | 
9a. If you recall from the Feb. '07 issue of Sport Truck, we ran an article on how laser-cutting works. Well, here's the finished product of that story, sheetmetal bulkheads that we'll use to mount the differential to. The bulkheads were cut from 1/8-inch-thick chrome-moly steel with a ton of holes that we punched with dimple dies. Dimpling the holes made the flat steel stock incredibly strong. Once Mike Kim was through TIG-welding the structure together, it was nearly bomb-proof. |
9b. The gusset plates were TIG-welded to the chassis | 
10a. While Mike K. was welding, the author was busy cutting out gussets for the chassis joints. | 
10b. The gusset plates were TIG-welded to the chassis |