Our F-350 has always had a...
Our F-350 has always had a tail-high stance, sitting almost 7 inches higher in the rear. We wanted to drop it to near level and maintain all its load capacity.
When we started on our Hot Rod Hauler, a 1973 Ford F-350, some of our goals were to lower the rear slightly, improve the ride when empty, and still maintain all of its load carrying capacity. We concluded the only practical way to get everything we wanted was with auxiliary air springs, so we turned to Joe Morrow and the crew at Chassis Tech.
Chassis Tech offers two kits for installing airbags for towing applications, the Basic Kit uses 6-inch ’bags that mount to the springs and the Heavy Duty kit that uses either 7.25-inch or 8.5-inch ’bags that mount inside the frame above the axle. For this application we choose the Heavy Duty kit with the smaller ’bags, but because of the tight confines between the ’rails, we mounted them to the outside of the frame directly above the axle. Space was the primary consideration, but we also felt that placing the ’bags as far apart as possible would help stability. We should point out that in many cases these towing kits simply bolt on, but for some applications, particularly on older trucks, some fabrication may be necessary.
While it was a tight squeeze to get the ’bags between the inside of the tires and the bedsides, they do have adequate clearance throughout the suspension’s range in travel. The ’bags we used expand very little under pressure, and in most cases, ½-inch clearance all around is adequate, though we allowed for 1-inch in all directions.
As they would no longer be needed, we removed the helper springs from each side, which also lowered the truck by ½-inch. Since the helpers didn’t come into play unless the truck was loaded we decided to remove two additional leaves from the spring pack to try and soften the ride when empty.
To accomplish our altitude...
To accomplish our altitude and load goals, we turned to Joe Morrow’s Chassis Tech/airbagit.com and ordered a heavy-duty towing kit.
Disassembling the spring pack was easy enough; ironically it was something that had nothing to do with the suspension that took most of our time. Evidently the original owner had installed a trailer hitch that was subsequently removed before we got the truck. Unfortunately, remnants of the hitch were still welded to the frame right where the airbag brackets needed to be. Held in place by extremely ugly “bubble gum” welds we figured the brackets would pop right off—we were wrong. Those welds were ugly but strong and it took much longer to remove the old brackets than it did to install the new pair.
While air springs were first used in the ’20s, today most commercial trucks with gross vehicle weights of 80,000 pounds or more are equipped with them, yet there are still those who are concerned about their reliability. The fact is that unless the ’bag is allowed to rub against something, failures are virtually unheard of. Add to that the ability to adjust ride height and load carrying capacity and it’s no wonder that many high-end cars are equipped with them. Certainly they’re perfect for our purposes.
The stock spring pack had...
The stock spring pack had not one but two overload springs, however, the basic spring was so stiff that the truck was extremely rough riding even when empty.
When selecting an air spring, Morrow advises that the diameter of an air spring is directly related to the pressure required to attain ride height. A small diameter ’bag will take more air pressure to support the same amount as a larger ’bag. The best bet is to always use the largest ’bag you can fit into the available space. If you do not have enough room, you should do whatever it takes to make room, even if it means redesigning the control arms or trimming the frame.
Check out the following photos on the installation of our rear air-bags and stick around, as next time around we’ll deal with the front suspension and the air management system. CCT
With the frame supported and...
With the frame supported and the shocks disconnected, the U-bolts were removed and the rear axle was dropped far enough to remove the center bolt from the spring pack.
We removed the upper and lower...
We removed the upper and lower overloads as well as the two shortest spring leaves.
With the four leaves removed,...
With the four leaves removed, the center bolt was replaced in the remaining six.
Airbags come in a variety...
Airbags come in a variety of configurations. A pair of these tapered sleeve ’bags will support approximately 2,500 pounds and are rated at 150 psi maximum.
Our towing kit came with DeNominator...
Our towing kit came with DeNominator II air springs—7.25 inches in diameter, 2.8 inches collapsed, 12 inches extended, with a capacity of 3,049 pounds per ’bag and are rated at 600 psi maximum.
Our application called for...
Our application called for some fabricated frame brackets, these were made from ½-inch plate that was folded on an ironworker.
As we wouldn’t be able to...
As we wouldn’t be able to get to the top of the brackets once they were welded in place, they were given a coat of paint from a rattle can.
Normally these ’bags are mounted...
Normally these ’bags are mounted inboard of the framerails. Due to the offset of the differential housing, the sway bar brackets and shock mounts, we positioned them outboard. It’s a tight fit, but everything clears, however, this wouldn’t work on a dualie.
For a lower ’bag mount we...
For a lower ’bag mount we started with the original spring plate. Note the original center bolt has yet to be shortened.
A 4½-inch piece of ¼-inch...
A 4½-inch piece of ¼-inch wall, 2-inch square tubing was welded to each spring plate. The square-tube spacers allow enough room to install and tighten the nuts on the U-bolts.
Bases for the ’bags were made...
Bases for the ’bags were made from ¼-inch plate. Holes were drilled to attach the ’bag to the plate, the attachment bolts are installed and tightened from inside the tube.
A test fit revealed two things:...
A test fit revealed two things: there wasn’t room for the air lines or for access for the attachment bolts. Installing the ’bags upside down solved the air line problem; as the bottom of the ’bag will be moving, the supply line will be braided stainless steel from a bulkhead fitting in the frame.
To provide access to the top...
To provide access to the top retainer bolts, the flange on the bottom of the bed was trimmed and the brackets were spaced down far enough to get the bolt in place and tighten. Our next chore is some frame detailing and plumbing the air system.