11a. Once the front half of the chassis was reinforced, we cut more tubing to create the rear frame section (A). We used this slick aluminum fuel tank from Brothers as the template for the shape of the frame. It's sold as a replacement tank for '67-'72 Chevy and GMC trucks, but its low profile also made it ideal for our hammered chassis | 
11b. You might have noticed that there is no kick-up at the rear of the chassis that is often found on lowered trucks. The reason for this is that since the rear suspension moves independently of the rear differential, we were able to hard-mount the differential and run the framerails below the control arms. This design has its good points. First off, it keeps all of the components low, which means the bed floor won't have to be raised substantially. But, since the framerails don't kick up, it also means they will be visible from behind the truck, unless we bolt a back bumper in place to hide them. We'll cross that bridge when we get to it. | 
12. UpCycling the IRS without the axles in place revealed 17 inches of travel, which is way more than we need. Once we shorten the Expedition's factory axles and install them in our new setup, we'll end up limiting travel at 12 inches, which is plenty to lay the truck on the ground and then get up and roll down the highway. |
Down | 
13. Another move that substantially boosts the strength of the chassis is adding triangulated crossmembers. Using 1-1/2x0.120-inch wall chrome-moly tubing, we welded X-shaped braces throughout the frame, from front to back. When these X-shaped crossmembers are welded into the perimeter of the frame, the frame ends up becoming a box with a bunch of triangles in it, and that's a good thing. Triangles are the strongest geometric shape you can design. | 
14a. Next, we mocked up the engine and transmission, so we could create mounting points. Our LS engine came with vertical aluminum factory Corvette mounts that wouldn't work in our chassis because the steering shaft would be in the way. So, we made cardboard templates for new mounts and transferred those to 1/4-inch-thick cold-rolled steel plate. The mounts were MIG-welded to the chassis, and we used Delrin plastic bushings at the mounting points |
14b. The rear transmission crossmember was made from 1/8-inch-thick sheetmetal and designed so that not only would the tranny be easily removed for service, but so that we could route the exhaust system through the chassis. | 
See More!
Check out how these bomb-proof bulkheads are made in the February '06 issue of Sport Truck. | |
The Final Word
Here's an overview of the chassis thus far, minus the front and rear airbag suspensions. We've made great progress, working at night and on weekends, and believe it or not, we got this far in a little more than a month. Next month, we'll wrap up the suspension and start working on getting the drivetrain in shape. Stay tuned!