REAR SUSPENSION
There are many factors to consider when designing a three or four link rear suspension.
I have been experimenting with all of these variables using a Computer Aided Design software and have learned some interesting things that I would like to share with you. The
suspension I am using has two lower trailing arms that will extend from the frame to the lower outer ends of the axle. The upper arms will extend from the frame rails to the center of the differential, primarily providing side load capability as well as resisting differential rotation.
Ideally, your pinion angle of the rear differential is parallel to the transmission shaft at all times. This keeps your u-joints from wearing out prematurely. All of the above variables affect differential rotation during the travel. Ideally, the upper arm is equal in length and parallel to the lower arm, resulting in zero differential rotation throughout the travel. As your lower arm gets longer relative to the upper arm, then rotation gets worse. If you move the upper arm pivot vertically it increases rotation at full compression while decreasing rotation at full extension.
Upper Arm Length - Since the upper arms are the only means of restraining the axle from side loads, they should be as short as possible. If they went straight from the differential to the frame they would provide excellent side load capability, but would not allow any suspension travel. A compromise is needed between side load and suspension travel. The frame pivot point I have chosen is just to the rear of the rear cab body mount. This area is extra beefy and easily accessible. If you go too much forward of this, your side load capability will be severely degraded and you will also have interference problems with the cab during full compression.
Location of shock/spring on lower arm - I will be using 16" travel coilover shocks and would like to get as much travel as possible. If the shocks mount in the center of the arm, then 32" of travel might be possible. The disadvantages of this are that the spring rates would need to be very high or the truck would easily roll and bottom out. Moving the attach point closer to the axle gives more control but less travel.
Location of Lower Arm Attachment to Axle - The primary load on the axle is upward from the tires. This load goes into the lower arm and to the springs into the frame. Should the attachment of the lower arm be in front of the axle, below the axle, or somewhere else? If the attachement of the lower arm is in front of the axle (at the 3 o'clock position), then the axle will want to rotate when a vertical load is applied (always). This rotation is reacted by the upper arms, but why should the have added loads? Putting the attach point directly below the centerline of the axle prevents this rotation and is the best from a load standpoint. However, under normal cruising conditions, the upper arms will not be loaded and if there is any clearance in the heim joints, they will tend to rattle and make noise. By positioning the attach point slightly forward of the centerline (I chose 30 degrees), a slight preload is maintained on the upper struts at all times.
Lower Arm Length - Lower Arm length equal to the top arm would provide no differential rotation, but would cause a bigger problem, it would cause excessive yoke movement at the transmission (pushing in as the suspension extends). If the lower arm is twice as long as the upper arm, you would have the same problem in reverse (pulling out as the suspension extends). I played with many length of lower arms to find one that keeps both the yoke movement and differential angle within a tolerable range.
Vertical Location of Upper Arm relative to Lower Arm - This distance affects differential rotation and yoke displacement. In one position, you can equalize the rotation between full compression and full extension, but the yoke displacement is higher. I have tried to minimize yoke displacement (less than 1.5" for almost 26.5" of travel) and have sacrificed a bit of differential rotation at compression (4.3 degrees) I feel this is a very good compromise.
Here is the
layout of a 4' x 8' 3/16 steel plate we are having lasercut.
The blue parts are the radius arm, the white parts are the crossover steering,
and the maroon parts are rear suspension
We removed the spring brackets and all cross members except the one by the rear cab mounts. The brackets are tacked onto the rear end slightly forward of being straight down. If these brackets are straight down, all of the load from an impact is put into the heim joint without wanting to twist the axle. I have seen some people mount these connections directly forward of the axle, causing a moment on the axle that the upper arms need to react. We were originally going to mount the trailing arms on the outside of the frame in the hole that is already there. This spot worked when we were going to use the longer 5R55E transmission, but when we decided to use the C4 that is 10" shorter, we had to move the rear suspension mount points to keep the driveshaft length change to an acceptable level. Directly below the frame and about 1" aft of the hole is the new location. We realize this lowers the trailing arm and reduces ground clearance a bit, but it was necessary.
We did a fit check of the 4 link rear and all worked well. We then proceeded to weld up the trailing arms. The flat plates have small tabs (.1 high x .2 wide)every 6-8 inches which help to hold the parts together when clamped. After tacking the entire arm it is welded all around.
Full droop
Full compressed and full lean
Ride Height
Here is how we are attaching the 4 link to the frame (March 02)
