Tripod Clocking

» Posted by on Oct 6, 2012 in Current projects and Tech Notes | Comments Off on Tripod Clocking

Tripod Clocking

I’m a huge fan of tripod joints for high power applications. Tripods are one of the two common joint designs used on the ends of the halfshafts that connect the transmission to the drive wheels of a car with independent suspension. The other common joint type is a CV, or constant velocity joint. The reasons that I like tripods over CVs are that they are more efficient than CVs up to a joint angle of 8°, their service life in high-power applications is vastly greater, and they don’t need an exotic and annoying grease like Krytox in high power applications. Krytox is an amazing grease, but the only effective solvent for it is freon. Freon is not exactly a practical solvent. At Hall Racing, we went from having to do CV maintenance every day at the superspeedways to half-season inspection intervals with tripod joints. That made a giant difference in maintenance workload on the team.

When you are assembling tripod joints onto a halfshaft, you are faced with a choice. Since the coupling between the tripod and halfshaft is a spline, you can clock each tripod joint to any angle that the splines will line up. Should you align the two tripod joints on each halfshaft with each other, stagger them 60° apart, or just plug them on at a random angle?

Halfshaft assembly with aligned tripod clocking

 

Halfshaft assembly with 60° staggered tripod clocking

As it turns out, it does matter how the joints are clocked relative to each other, and that clocking depends on the driveline geometry and suspension alignment of your car. So, there is no one-size-fits-all answer to this question.

Many years ago, I performed a kinematic analysis of a tripod joint to find out what makes them tick, and if there were any oddities in the way they work. I learned some interesting things from that investigation:

  • A tripod joint produces an exact 1:1 drive ratio all the way around, regardless of joint angle, so it is a true constant velocity joint.
  • When the joint angle is not zero, the center of the tripod joint is offset from the housing axis. The joint center displacement increases with the joint angle.
  • As the car moves down the track, the tripod center precesses in a circle around the housing axis at an average of 3 times the shaft speed. I call that an average speed because it accelerates slightly for 30°, then decelerates slightly through the next 30°, and so on.

The fact that the joint center is offset when there is a non-zero joint angle is a source of vibration. That vibration can be minimized by clocking the two joints on each halfshaft optimally. The optimal clocking will offset the two ends of the halfshaft in opposite directions, so that they will precess out of phase with each other, and the center of gravity of the halfshaft assembly won’t move much.

If the tripod clocking results in the two joint ends being offset in the same direction, it can produce a vibration that is severe enough to halt a test day at a high speed track. This happened to us at Hall Racing in 1994 at the Phoenix International Raceway mile oval. That was our first oval track test day with the new Reynard 94I, which was our first Indycar with tripod joints on the halfshafts. That’s the day that we discovered that on oval tracks, the tripod clocking had to be different on the left and right sides of the car, because the right side had negative camber and the left side had positive camber.

Here is the way to clock your tripod joints optimally:

If the two tripod joint angles on a halfshaft are in the same direction, then the tripod clocking should be staggered 60° in order to offset the two joint centers out of phase with each other. An example of driveline geometry that produces both joint angles in the same direction is a car with the diff center slightly below the axle center, negative camber, and a small or zero fore-aft offset between the axle center and the diff center. Here is a rear view illustration of that driveline geometry:

Driveline geometry with both halfshaft angles in the same direction

If the two tripod joint angles on a halfshaft are opposite to each other, then the tripod joint clocking should be aligned. That will offset the two joint centers out of phase with each other in order to minimize the movement of the CG of the assembly for minimum vibration. An example of driveline geometry that produces opposite joint angles is a car with the diff center above the axle center and negative camber. Another example is a car with a large fore-aft offset between the diff and the axle center. Still another example is a car with the diff center a long way below the axle center. Here is a rear view illustration of that:

Driveline geometry with opposite tripod joint angle directions

So, now you know how to clock your tripod joints when assembling them.  I’ll bet that you didn’t think you would learn that today, did you?