[stallion112]

I have spent quite a few years working with automotive OEM’s designing and validating steering systems and driveshaft’s so universal joints, or cardan joints, are a familiar topic.

My Kellison Stallion was built in the mid ‘70’s so the original steering intermediate shaft utilized Pinto / Bobcat components. This included a thick one inch cable that rotated at a slight angle in place of one of the cardan joints (in addition to the one cardan joint that remained). It was used by the OEM primarily to pass relatively new crash requirements since it would easily fold with a high enough impact to the vehicle and minimize the energy transferred to the upper column. Because of the cable the steering in the Stallion was smooth throughout its full rotation but, since the original cardan joints were worn out from age I decided to design a new system from the steering wheel through to the pinion shaft on the rack & pinion.

A rule of thumb for a slow speed, low torque steering application is for the cardan joint to not exceed 30 degrees of operating angle. Even then, the entire system has to be properly phased. Phasing refers to the relationship of one set of yoke “ears” (the section of the yokes that support the bearings) with respect to the opposing set of yoke ears.

At first glance, a series of shafts that are connected by cardan joints appear to all be rotating at the same velocity. However, a closer look reveals that they are not. Imaging two shafts that are joined together in the middle by a cardan joint. Now hold it up to eye level and in the horizontal position. The set of yoke ears connected to the shaft in the right hand are “Plane A”. The set of yoke ears connected to the shaft in the left hand are “Plane B”. Now rotate the shaft assembly. The instantaneous velocity of the set from “Plane A” is the same as the instantaneous velocity of the set from “Plane B”. Now keep the shaft in your right hand horizontal and tilt the shaft in your left hand at an angle. When you rotate the shaft assembly now you can see that the set in “Plane A” or your right hand still rotates in the vertical plane, but the set in “Plane B” or your left hand rotates in a sinusoidal motion relative to “Plane A”. For every one full rotation the speed and velocity of the shaft in the right hand equals that of the shaft in the left hand. However, during each rotation the instantaneous velocity of “Plane B” both accelerates and decelerates relative to “Plane A”. In addition, due to the sinusoidal motion there is an axial load being generated through the system. The higher the operating angle the higher the axial load that is generated.

I know this sounds complicated but the bottom line is that each cardan joint needs to be cancelled out or balanced (phased) with another cardan joint operating in an equal and opposite motion. This is especially important with high speed, high torque applications like a drive shaft.

One way to help balance out the system is by using a double-cardan joint. This is simple a back-to-back cardan joint assembly. Input equals output at all points throughout rotation. My Stallion uses a three joint system, two cardan joints at each end with a double-cardan in the middle.

I would recommend using Heim type joints on one side of the cardan joints to help support the shaft and keep it rotating about its intended axis. Otherwise they can move around when the system is rotated and make the steering feel “lumpy”. This is especially true when using a double-cardan joint. Make sure that the bearings in the column are properly supporting the shaft as well since they perform the same function as the Heim joints.

Another thing I suggest doing is setting up the system by using wooden dowels or broom sticks along with the cardan joints to determine the length, angle, and phasing of the steering intermediate shaft between the lower column and rack & pinion. It is much easier than cutting DD or splined steel shafts.

Lastly, some type of a rubber isolator should be used to dampen vibration from the chassis. I prefer the older style donut shaped isolators but the ones that are incorporated into tube shaped yokes provide for a slightly stiffer feel. The isolator should be located next to the pinion shaft from the rack & pinion to minimize vibration.