Chassis: Front Suspension

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Chapter 3: Chassis - Front Suspension

Twin I-Beams - an Independent Alternative for Your Hot Rod


An introduction to the Ford Twin I-Beam

There are many types of hot rod suspensions, and they each require their own particular fabrication methods and techniques. This chapter will show the fabrication of a Ford Twin I-Beam independent front suspension on the ladder frame we have just completed.

The Ford Twin I-Beam is in the "swing axle" category of independent front suspensions. You can picture it as simply splitting a traditional straight axle in two and then hinging it at each of the cut ends. Ford engineers quickly saw that this design would create camber issues if such short axles were simply hinged at the center of the car. To alleviate the problem, Ford lengthened each axle so that each could be hinged near the opposite wheel. The longer the axle, the less the impact on camber. However, this design change requires that the axles overlap, with one axle in front of the other (Photo 3-1).


Photo 3-1 The Ford Twin I-Beam - illustration courtesy of Ford Motor Company.
"The idea behind the engineering was simple: replace the traditional one-piece solid-beam axle with two separate suspension beams designed to move independently. This new twin I-beam suspension would allow both tires to be isolated from one another and theoretically allow them to stay in better contact with the road. The design sought to maximize durability and simplicity over a conventional A-arm suspension by using long beams that would pivot from the opposite sides of the truck. The long beams meant the tires would see less camber change as the suspension cycled through its range of travel. To locate the beams front-to-rear, radius arms were mounted in parallel with the truck's frame, and coil springs were used to carry the load." Source


The Twin I-Beam suspension is unique to Ford among mass-produced automobiles. It was introduced in their F-Series pickup trucks beginning in 1965 and remains standard on F-250's and F-350's to the present. It was also included during limited years on the Ranger, Bronco II, E-Series Van and even early Explorers. It was used on the highly popular F-150 from 1965 until 1997.

It should be noted, however, that not all Twin I-Beams are the same. From 1965 until 1979, F-100 and F-150 I-beams were equal length and were forged. The axles for the scratch-built rod shown in Photo 3-2 are from a 1978 donor, and, as you can see, at first glance they look just like a traditional straight axle you might find under any typical hot rod. It is only on closer inspection that you can identify them as independent twin beams.


In 1980 and 1981, the axles were still forged, but Ford redesigned the geometry, the axles were shortened a bit, and one was made shorter than the other. 1981 twin beams were used in my roadster build (Photos 3-3 and 3-4) and as you can see, the beams have about a 12" difference in length. These '80 and '81 beams are usable, but I don't feel they are nearly as attractive as the pre-1980 beams.


Photo 3-2 1978 twin I-beams with air bags on this rod. Photo attribution
Photo 3-3 1981 twin beams are shorter and are not equal length. Photo attribution
Photo 3-4 Another view of 1981 beams adapted to a hot rod chassis. Photo attribution


After 1981, Ford began using a stamped steel process for making their twin beams. This took a great deal away from the traditional look of earlier beams, and, unless the suspension is going under a full-fendered rod where they will not be readily seen, I would not recommend their use, based purely on aesthetic reasons. These newer beams can be made to work, they just won't be as good-looking or give the traditional appearance provided by the earlier axles.

Another very important difference between twin beams is brakes. From 1965 to 1972, Ford mounted drum brakes on all of their axles. In 1973, they initiated disc brakes for all their twin beams, greatly improving their desirability for hot rod use. Fortunately, should you already have a set of older beams with drum brakes, all of the disc brake parts from 1973 to 1979 beams can easily be swapped onto the early axles. A guide for doing this conversion can be seen here.

The most desirable twin beams remain those taken from the 1973-1979 F-100 or F-150. Obviously they must come from 2WD trucks. In my area of the country, where there is lots of snow plowing and lots of stump pulling, 2WD trucks are considered less desirable than their 4WD counterparts; they're usually less expensive but a bit more difficult to find.

Special considerations when using Twin I-Beams

Photo 3-6 Turn the L on its side and the long leg becomes the axle. Photo attribution
Because of their configuration, twin I-beams have some unique features that the fabricator must take into account during design and construction. If you think of the axle and wheel as the letter "L" turned 90 degrees counterclockwise, with the short leg of the L representing the wheel and the long leg of the L representing the axle with a pivot hole in the end (Photo 3-6), you can see how the I-beam configuration affects camber. Photo 3-7 shows the effect of going over an exaggerated bump or having the axle installed at an incorrect angle. As the wheel rises, the pivot point remains fixed. As a result, the wheel is no longer vertical to the road, as represented by the vertical line to the right of the wheel. If driven this way over a protracted period of time, the inner edge of the tire would wear significantly faster than the outer edge. If installed this way, it can produce a dangerous handling situation.
Photo 3-7 When the axle rotates, camber issues may result. Photo attribution
Photo 3-8 Properly configured twin beam at rest. Photo attribution
The builder must also consider the axle geometry in relation to the frame, the mounting bracket pivot point, and the spring rate and spring location. Photo 3-8 illustrates a right side I-beam at rest and in the correct position to achieve proper camber and safe handling. But note what happens in Photo 3-9 if the pivot point of the axle is mounted at the wrong height in relation to the frame. While this illustration is quite exaggerated, any error in the height of the pivot point will result in incorrect camber, poor tire wear and potential handling problems.
Photo 3-9 Poorly located axle pivot can result in tire wear and poor handling. Photo attribution
Photo 3-10 Incorrect spring position or spring rate can also create camber issues. Photo attribution
Also note what happens in Photo 3-10 if the axle is not held at the proper height near the wheel end. This height is determined by the spring rate and the spring mounting location. This illustration can also help visualize how camber can be adjusted, within limits, once the suspension is completed. By increasing or decreasing the spring rate, the frame is raised or lowered. This, in turn, raises and lowers the axle pivot mounting bracket and thus the axle pivot point. And as the axle pivot point moves up or down, it will change the camber of the wheel/tire. In real life, you can see how the geometry of the Twin I-Beam setup looks in these shots of my '32 pickup during fabrication. This is another of my cars that was scratch-built. In Photo 3-11 you can see the spring (air bag) location and how the axle pivot mount is under the frame rail on each side.
Photo 3-11 Example of Twin I-Beam mounting in this rod. Photo attribution
Photo 3-12 Location of pivot mounting bracket (arrow). Photo attribution
In Photo 3-12, you can see the pivot mounting bracket (arrow) on the underside of the frame. Photos 3-13 shows the same suspension later in the construction process. The Twin I-Beam geometry thus requires that the pivot location for each axle be correct in relation to the frame height, the track width of the front end and the scrub line for your particular car. On most I-Beams from 1973-1979, the bottom of the axle should run approximately parallel with the ground in order to achieve zero camber while the car is stationary. This also means that the bottom of the axle should run parallel with the front crossmember of the frame. The top of the axle, on the other hand, is tapered, being taller at the king pin than it is at the pivot point. So always use the bottom side of the axle as your reference point when determining the position and height of your mounting brackets.
Photo 3-13 Another view of the Twin I-Beam mounts. Photo attribution

Fabricating the axle mounting brackets

Photo 3-14 Sketch front suspension and frame to scale to determine location of the axle pivot holes. Photo attribution
To properly size our mounting brackets and determine the correct location of the pivot hole center, we can either draw the axle and frame to scale on graph paper, or we can mock up the axle in correct position in relation to the frame, and get our pivot center location from there. For this project, the sketch shown in 3-14 was used to determine the height of the axle mounting brackets and the location of the pivot hole to be drilled in the bracket in relation to the frame. Note that this design is for an underslung frame, so care must be taken to ensure there will be room for adequate axle travel without hitting any frame components, while maintaining a proper scrub line so nothing on the car will contact the pavement in the event of a tire blowout.
Photo 3-15 Mark the frame for the axle mounting boxes. Photo attribution
Photo 3-16 Cut axle mounting boxes from 2x3 tubing. Photo attribution
Using dimensions taken from the donor truck and your sketches, mark each side of the frame to locate the axle mounting brackets (Photo 3-15). The brackets are made using two "boxes" cut from 1/8" 2x3 rectangular tubing (Photo 3-16). Using your prior calculations and drawings, mark and drill holes in the box for the axle mounting bolts (Photo 3-17).
Photo 3-17 Drill the boxes for axle mounting bolts. Photo attribution
Photo 3-18 Plug the top of the box. Photo attribution
Next, weld a "plug" to seal the top of the box (Photo 3-18). Using a cutting disk or jigsaw, cut a hole in the side of the box large enough to install the axle end and to allow axle travel once the mount is finished (Photo 3-19).
Photo 3-19 Cut access holes for the axles. Photo attribution
Photo 3-20 Mounting box support pieces. Photo attribution
Our mounting box is 3" wide and it will sit perpendicular to our 2" frame, so that there will be 1" of the box that overhangs the outside of the frame rail. This overhang is supported and tied into the frame using support pieces cut from 1/8" flat stock (Photo 3-20). Position and weld the box and support pieces to the frame as shown in Photo 3-21.
Photo 3-21 Mounting boxes and support pieces welded to frame. Photo attribution
Photo 3-22 You can now test-fit your beams. Photo attribution
You can then test-fit your axles into the boxes as shown in Photos 3-22 through 3-24.


Front springs and spring perch

The spring system for this project is a fairly unique design that incorporates quarter elliptical leaf springs. However, instead of connecting the axle and leaf with spring eyes and shackles, the leaves will ride on rollers or glides, to allow the leaf spring to expand and contract during road travel. The rollers, in turn, will be set in an adjustable bracket, which will allow the car to be raised and lowered to create the correct frame height and axle camber. An easier and more direct way to accomplish this end would be to use a readily available coil over shock arrangement. But buying stuff off the shelf is often not the hot rod style. So, this project will modify the semi-elliptical rear leaf springs from the donor truck, and convert them into quarter elliptical springs for use on the sedan delivery chassis.

To stay with a more traditional look, the front springs will be mounted in the traditional "cross spring" position, that is, running parallel with the front crossmember between the two front wheels. Unfortunately, the rear leaf springs from the donor truck can not be used in their stock configuration as a cross spring.

Rear leaf springs for most cars and trucks have a non-uniform arch, meaning the front of the spring is arched differently than the rear of the spring. This is done by design, to assist with rear end axle torque and loading on the spring. Front "cross springs", on the other hand, are uniformly arched, that is, the arch to the left of the spring's center is exactly the same as the arch to the right of the spring's center.

In order to use the leaf springs from the donor truck, they need to be converted to quarter elliptical springs, by cutting each spring stack in half. All the left side halves, which now have the same arch, will be used to make two quarter elliptical spring stacks for the new front suspension. All of the right side halves, which now have the same arch, will be used to make two quarter elliptical spring stacks for the new rear suspension. Photo 3-25 shows one of the resulting quarter elliptical spring stacks after going through the cutting process.

To mount the quarter elliptical spring sets, a perch must be fabricated at the center of the front crossmember. The perch consists of a stout bracket with a "spring box" mounted at the top to hold the ends of each spring stack. The center bracket is made from a 10" length of 1/8" wall, 1 1/2" diameter steel tubing and two 1/4" steel gussets (Photo 3-26).

Photo 3-25 Leaf springs cut in half to make quarter ellipticals. Photo attribution
Photo 3-26 Pieces for center spring mount. Photo attribution
Photo 3-27 Center spring mount tack welded to frame. Photo attribution
Photo 3-28 Another view of the center spring mount. Photo attribution
The bracket pieces are positioned and tack welded in place as shown in Photos 3-27 and 3-28. At this juncture, a hole is also drilled through the top of the front crossmember on each side of the spring perch bracket (Photo 3-29). These two holes will be directly above the two large holes we drilled in the underside of the front crossmember during frame construction (See Chapter 1). The holes will be used to bolt auxiliary end supports to the spring box. The large holes on the underside of the crossmember will allow us to get a socket up through the frame to tighten the bolts holding the auxiliary support bars in place.
Photo 3-29 Bolt holes are drilled for the mount's side braces. Photo attribution
Photo 3-30 Spring box - top view. Photo attribution
Quarter-inch flat stock is used to build the "spring box", and holes are drilled to mount the box to the center bracket and to bolt the spring stacks inside the box. Photo 3-30 is a view of the top of the box and Photo 3-31 shows the underside of the box. Photo 3-32 shows the box temporarily mounted on the center post. Note the 1 1/4" bolt in the center of the box. The nut for this bolt was welded to a circular piece of 1/4" steel plate after a properly-sized hole had been drilled through the plate. The steel plate was then welded to the top of the center post, nut side down, so that it was hidden inside the post.
Photo 3-31 Spring box - bottom view. Photo attribution
Photo 3-32 Spring box bolted to center mount. Photo attribution
This center bolt is not enough to withstand the extreme torque that will be passed through the spring stacks to the mount. So, auxiliary supports are placed at each end of the box. These supports are made using 1" steel pipe with a nut welded to the end as shown in Photo 3-33. The supports will be bolted to the front crossmember using the holes we drilled on each side of the center post. To mount the spring stack, holes must be drilled in each leaf to match the mounting holes already drilled in the box (Photo 3-34). Be aware that spring steel is very difficult to drill. So, take your time and use lots of lubricant. Photo 3-35 shows a bottom side view of the spring stacks bolted in the mounting box and the auxiliary support bars welded in place. Photo 3-36 shows the spring perch and springs being test mounted on the frame.
Photo 3-33 Spring box side support braces. Photo attribution
Photo 3-34 Leaves drilled for mounting in spring box. Photo attribution
Photo 3-35 Leaves bolted into spring box and side braces welded on. Photo attribution
Photo 3-36 Quarter elliptical springs bolted in place. Photo attribution

Four-bar fabrication

Photo 3-37 Pieces for four-bar axle bracket. Photo attribution
You can purchase pre-made four-bar kits, but it is unlikely you will find anything that fits the unique Twin I-Beams. To make your own four-bar system, you first need a means of attaching the bars to the axles. The quarter elliptical springs also need to maintain contact, in some way, with the front axles in order to control up-down movement. A single bracket can satisfy both of those needs.

Fortunately, I-beam axles provide a great attachment point for your hand-fabricated four-bar system: the radius rod attachment hole. This hole runs vertically through the axle, not far from the king pin, and can be used as the starting point for creating the axle brackets.

We will begin by constructing the portion of the axle bracket that will provide an adjustable mounting point for the leaf rollers or glides. This portion of the bracket is comprised of 1/4" flat stock pieces as shown in Photo 3-37. The bracket needs to be removable, so it is made in two sections, a front and a back, which will slip over the hole in the axle and then be bolted in place with a single grade-8 bolt.
Photo 3-38 Making an adjusting slot. Photo attribution
Photo 3-39 The finished slot. Photo attribution
To allow my leaf spring rollers to be adjusted for frame height, slots must be created in the uprights of the bracket. To make these slots, first drill a 1/2" hole at each end of what will eventually be your slot. These slot holes are spaced 3 1/2" apart (Photo 3-38). Then, using a 4 1/2" cutting disk, remove the material from between the two holes. You will end up with a slot (Photo 3-39) that allows the spring roller to be adjusted upward and downward. The pieces for the front and back of the bracket are welded and then slipped onto the axle, where they are temporarily held in place with a bolt (Photo 3-40). If you look closely, you will note that the bracket is actually in two parts, with the large mounting bolt going through two plates on the top and two plates on the bottom. This allows the front half of the bracket to be removed, and then the back half. If the bracket was welded together in one piece, you couldn't remove it from the axle without entirely removing the axle from the car and slipping the bracket over the pivot end of the beam.
Photo 3-40 four-bar axle bracket installed. Photo attribution
Photo 3-41 Weld tabs to the bracket to attach the four-bar rod ends. Photo attribution
With the main section of the axle bracket in place, mounting tabs for the four-bar rod ends can be cut, positioned and welded in place. Photo 3-41 shows the lower left four-bar tabs being set up for welding. Photo 3-42 is an overhead shot of the lower right tabs welded to the bracket. A bracket is also needed on each frame rail to attach the other end of the four-bars. This bracket is made from pieces of 1/4" flat stock (Photo 3-43), which are drilled and then welded together to form the bracket (Photo 3-44). Position the bracket on the frame using your four-bars as a guide and clamp the mount in place for welding (Photo 3-45). Photo 3-46 shows the four-bar system temporarily installed to check for fitment.
Photo 3-42 Top view of the tabs. Photo attribution
Photo 3-43 Pieces for four-bar frame bracket. Photo attribution
Photo 3-44 Bracket pieces welded. Photo attribution
Photo 3-45 Bracket clamped to frame for welding. Photo attribution
Photo 3-46 The finished front four-bar system. Photo attribution

Leaf rollers

Photo 3-47 The leaf springs will glide on rollers. Photo attribution
One of the more unique features of this suspension is that it is shackle-less. Instead of attaching the leaf springs to the axle with spring eyes and shackles, the eyes are removed from each spring stack, and the longest leaf of the stack rests on a roller, allowing the leaf to glide in and out as the spring stack is compressed or decompressed during normal road travel. Without allowing for this movement, the spring would bind immediately. The first set of leaf rollers were made from the components shown in Photo 3-47. A grade-8 bolt, a brass bearing sleeve and an outer plastic tube (PEX tubing) were incorporated to reduce squeaking and friction. The components were then assembled in the adjustable axle bracket as shown in Photo 3-48.
Photo 3-48 The assembled rollers shown in adjustable mounting brackets. Photo attribution
Photo 3-49 The original rollers were later upgraded to these 900 lb. 7/8" roller bearings. Photo attribution
However, this design proved to be less than ideal. Over time, after the engine, transmission and body were installed and the suspension sat for long periods during completion of the car, the "bounce" of the suspension became more and more stiff. This was due to the plastic outer tubes "flattening" slightly due to the weight of the car, and causing more binding than they were designed to eliminate. To remedy the problem, manufactured roller bearings were used to replace the homemade glides. The components of the new rollers are shown in Photo 3-49, and the assembled roller for one leaf spring is shown in Photo 3-50. Photos 3-51 and 3-52 show the rollers installed on the axle. These bearings are 7/16" wide and each is rated at 900 lbs. They ride on a 1/2" grade-8 bolt, and they are kept in position on the bolt with small sections of steel sleeve to prevent lateral movement.
Photo 3-50 The bearing assembly. Photo attribution
Photo 3-51 The bearing assembly installed. Photo attribution
If you look closely under the rollers, you will see a nut supporting the center of the roller bolt. This was used temporarily, as a safety device to keep the rollers in place should the roller bolt loosen and slip downward under the weight of the spring. This little safety measure made so much sense that it was incorporated into the final fabrication of the mounts. Once the final ride height was determined, and the position of the adjustable rollers was set, steel blocks were fabricated to fit under each roller bolt. This ensures that if a roller bolt ever became loose while on the road, the rollers themselves can not drop down, thereby causing the frame to drop on that side of the car.
Photo 3-52 Another view of the bearing assembly installed. Photo attribution

Testing the suspension

Photo 3-53 The engine and transmission were set on the chassis to test front suspension geometry. Photo attribution
Thus far, the front suspension components have been fabricated using only theory and best guesses as to how they will perform. So at this juncture, it is time for the first big test of all that theory. By clamping a couple of 2x3 steel tubes across the bottom of the frame and setting the rear of the frame on jack stands, the engine and transmission can be set down in the chassis to give a rough idea of how much the springs will compress under the weight, and whether the axles will end up in the correct position. Photos 3-53 through 3-56 show the assembled front suspension sitting on its own front legs for the first time. And given that there is a certain amount of adjustment available to raise and lower the frame and adjust camber, it appears we are in the ballpark for creating the correct geometry for the suspension to operate properly.
Photo 3-54 The completed front suspension (less shocks). Photo attribution



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