Chassis: Rear Suspension
From The Scratch-Built Hot Rod
Chapter 5: Chassis - Rear Suspension
Preparing the basic rear suspension components
|Building the rear suspension begins by placing the rear of the frame on jack stands or blocks at your anticipated final ride height. Then, with the wheels bolted on, roll the rear end unit into position as closely as you can to its final ride position under the frame. Make sure the axle is maintained at a right angle to the frame rails. Block the wheels so that they can't creep forward or backward. Next, lift the pinion and place it on a floor jack. Then, jack the pinion up or down until the pinion angle is between 2-3 degrees up (higher toward the front of the car). This angle may vary with different applications, but it should always match the downward angle of your transmission shaft. You can measure the angle across the face of the yoke as shown by the arrows in Photo 5-1.|
Fabricating the rear four-bar system
Our rear end radius rod system will be the same four-bar style as we used on the front suspension, including pre-threaded sleeves and heim-type rod ends. The sleeves should be threaded with left and right threads on either end, and the rod ends should be left- and right-threaded as well. This makes it a snap to adjust the rear end and front end with a simple twist of the sleeves.
To attach the bars, we need to fabricate mounting brackets for the rear axle, and mounting brackets for the frame rails. We will begin with the axle mounting brackets.
The design of this particular rear axle bracket is somewhat unique, and may not be the prettiest, because it was designed to limit any welding directly on the rear axle housing. These housings are quite easy to warp if you do not have the proper equipment to hold them solidly during the welding process and do not use the proper welding techniques. These brackets are therefore designed to attach primarily to the leaf spring mount and shock mount, which have already been solidly welded to the housing. This may look a little odd, but it greatly reduces the amount of heat going into the axle housing itself, and allows the hobby welder to do the job, rather than taking the rear housing to a pro to do the mounts.
|The axle mounting bracket is made from 1/8" wall, 2x3 tubing. Measure and cut the tubing to length based on the estimated distance between your upper bar and your lower bar. Bars can be set up in a number of different configurations, so your bracket length may be different than what is shown here, but the same general procedure can be applied. Next, drill a hole near the top of the bracket for mounting the upper radius rod. Then cut out a section on the front (forward facing) side of the tube, large enough for the radius rod to be installed and to achieve sufficient travel during road conditions (Photo 5-2). You will also note in this photo that the top and bottom of the bracket have been cut at an angle. As you will see in photographs to follow, the bracket for this particular car is mounted on the axle at an angle that matches the angle of the frame kick-up section. This is to allow the upper and lower radius bars to be the same length. So, the angle used to cut the top and bottom of the mounting bracket will match the angle between the lower frame rail and the frame kick-up section.|
|The bracket is then positioned by using a mocked-up set of radius bars, and is tack welded as shown in Photo 5-3. While most of the welding is done on the existing shock bracket rather than the axle housing, there is a weld joint directly between the axle housing and new bracket as indicated by the arrow in the photo. Photo 5-4 is an overhead view of the bracket being tack welded in place. Note that the leaf springs have been temporarily clamped in place to help ensure that the location of the axle is correct and that the four-bar location will not interfere with the springs.
The bottom radius rod connects to the axle using the original stock shock mount, which has been drilled out for a 5/8" bolt (Photo 5-5).With the radius rods connected to the axle, you next move to the frame to fabricate mounts for the other end of the radius rods. And for that, you need to determine the angle of your four-bars. Don't simply assume that "parallel with the ground" is going to suit your particular chassis. Each chassis is different and each application is different, particularly if racing is going to be involved. To determine the angle for your bars, this source will provide an excellent description of the process, as well as links to an online calculator that can determine this angle for you. For this particular project, an angle of 5 degrees will be used for the bars.
Using a magnetic angle gauge attached to the bottom radius rod, move the bar up and down until the gauge reads 5 degrees. Then, mark the frame at the exact center of the rod end and drill a 5/8" hole through both walls of the frame. Your rod end should be extended approximately 1/3 its thread length out of the sleeve (at both ends) when determining this mark. (Note: when mocking up bar lengths, remember that the total amount of rod end adjustment is NOT the total length of the threads on each rod end. You need approximately 1/3 of the length firmly threaded into the sleeve to provide proper support. So, the actual adjustment length will be 2/3rds of the total thread length.)
|Although it is not absolutely necessary, our design calls for the radius rods to run parallel with the frame, rather than angling outward or inward. To do this, clamp the bar in position parallel to the frame using a wood spacer between the bar and the frame. Then, measure the distance between the frame and the inside surface of the rod end, and deduct the width of any washers that you might be using. Cut a spacer from 1/8" wall 1" steel tubing which has been reamed out to accommodate a 5/8" bolt. This spacer is then clamped to the frame with the mounting bolt and tack welded in place as shown in Photo 5-6. Photo 5-7 shows a close-up of the spacer ready for welding.|
|Once you have the spacer attached and the lower bar bolted in place, move to the top bar, lining it up to run parallel with the lower bar. Drill the mount hole and make a spacer as you did for the lower bar. Photo 5-8 shows the upper and lower spacers welded to the frame. Photos 5-9 and 5-10 show the completed driver's side rear four-bars. With these in place, move to the passenger side of the frame and construct those mounts in the same way. Photo 5-11 shows both sides of the four-bar system completed.|
The rear springs
|With the rear end now held in place by the radius rods, you can move on to the fabrication of your spring system. For this car, we are using quarter elliptical springs similar to those used on the front suspension. The springs are cut from the "back halves" of stock F-100 semi-elliptical springs in the same way the front springs were made from the front halves of the semi-elliptical stock springs.
The mounting plate for the spring stacks consists of a 12" long perch and four "legs" to attach the perch to a frame crossmember and drop it down 2" from the crossmember (Photo 5-12). All pieces for this mount are 1/4" flat stock steel. Four holes are drilled in the mounting plate for attaching the spring stacks.To weld the four legs square to the perch plate, a piece of 2x3 square tubing is used as a guide, and the bracket parts are clamped in place around the tubing (Photo 5-13). The plate and legs are then centered on a 2x3 crossmember which was cut to length and shown earlier in Chapter 2.
|At this point, the exact height and position of the spring perch on the crossmember is a bit of guesswork. But, based on the front end spring compression, we can make an educated guess as to where it will work. Photo 5-14 shows the legs of the spring mount welded to the crossmember and the spring stacks bolted in. The crossmember and spring pack are installed as a single unit between the frame rails, located just behind the apex of the frame kick-up. Photo 5-15 shows the crossmember and spring pack being positioned and clamped to the frame rails for welding.|
Just like the front suspension, the leaf springs in the rear will glide on rollers held within adjustable mounting brackets. The brackets are shown in Photo 5-16. The glide components are shown in Photo 5-17 and the mounted glides are shown in Photo 5-18. As was done with the front suspension, these glides were later upgraded with roller bearings.
The glide brackets are then tack welded to the stock leaf spring pads, thereby minimizing any welding directly on the axle housing (Photo 5-19). The chassis can now be fully suspended on its own springs, both front and rear (Photo 5-20).
Front and rear shock mounting
|Pete and Jake's shorty shocks will be used, both front and rear. Photo 5-21 shows the parts used for mounting each rear shock. The two "ears" at the bottom of the photo will be welded to the frame and become the upper shock mount. The grade-8 bolt with the head cut off will be welded to the axle's original leaf spring pad and become the lower shock mount.
Since there is no body on the car, we have to do some guesswork to determine the upper shock mounting position. The lower mount is fixed by its location on the axle. So this mount (the bolt shown in Photo 5-21) is positioned and welded so that it lines up with the existing leaf spring pad.The upper mount must be set so that, hopefully, the shock rod will be approximately halfway extended when the car is completed and sitting at rest. There may be some formula to calculate this positioning with mathematics, so if you can find the formula, use it. Otherwise, you may have to cut and re-position your mounts later if your best guess is a bit off.
The method used for this car was to set the leaf spring glides to their lowest position, thus dropping the frame to its lowest position. The shock was then installed onto the lower mounting bolt and the shock rod fully extended. A mounting "ear" was bolted to each side of the upper shock end and the ears positioned against the frame so that the shock was lined up square. The ears were then welded in place (Photo 5-22).
If at all possible, design as much adjustability into your shock mounts as you can. Or, as an alternative, leave this step until much later in the project when the full weight of the car is resting on the chassis. We lucked out with the method used on this project, but the chassis does have some adjustment built into the leaf spring rollers, and that saved having to cut and re-mount the shocks later.
|For the front suspension, the shocks had to be mounted so that they operate opposite of how shocks typically operate. This is because the car has an underslung frame. If you look closely at Photo 5-23, you will detect that when the wheel hits a bump and the axle travels upward, the shock will extend. When the wheel hits a pothole and the axle travels downward, the shock will compress. On typical frames, this would be just the opposite, with the shock compressing with upward axle travel and extending on downward axle travel.
But as long as you have 50/50 shocks, this makes no difference whatsoever to your shock; it couldn't care less what direction it is going. It will provide the same degree of dampening going up as coming down. Just to be certain, Pete and Jake's was contacted about mounting the shock this way and confirmed the mounting theory was correct and that the shocks would work properly.Photo 5-24 is a close-up of the front shock mount. As you can see, two "ears" were welded to the frame on the lower end of the shock and the upper end of the shock was bolted to the axle mounting bracket which was fabricated in Chapter 3.
|To complete our basic chassis, we must have a way to keep the rear axle from moving from side-to-side. There are a number of ways to accomplish this, but we will be using a simple Panhard bar for this project.
The basic physics of a Panhard bar is illustrated in Photo 5-25. Source The Panhard bar, incidentally, is named after Rene Panhard, who was the founder of the Panhard motor company in France. Source SourceThe Panhard bar runs parallel with the rear axle, one end securely attached to the frame and the other end securely attached to the axle or an axle component. 1x2 rectangular tubing is used to fabricate the Panhard frame mount (Photo 5-26).
|The bar itself is a long sleeve with left-hand threads in one end and right-hand threads in the other. Heim-type rod ends are inserted in each end of the sleeve. The bar is mocked up in the frame mount and then the components are adjusted until the Panhard bar is parallel with the axle, parallel with the ground, and will not come into contact with the axle center section of any other part of the rear end. The frame mount is then welded in place (Photo 5-27). The free end of the Panhard bar will be attached to the spring roller mounting bracket near the opposite wheel of the car. A small extension with a hole drilled in it is welded to this bracket and then a spacer is cut so that the Panhard bar will clear the center section of the axle (Photo 5-28).|
|The spacer and Panhard bar are then bolted in place (Photo 5-29). The assembled Panhard bar is shown in Photo 5-30.
The basic rolling chassis is now complete (Photos 5-31 to 5-36).
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