CV or UJ in place of layrub?

[Forumadmin]

So layrubs are over £100 a piece and a Jav needs 4 so is it worth replacing them with a Universal Joint UJ or Constant Velocity Joint CV?

The output of a universal joint does not turn at a constant velocity when run at an angle. You need a second UJ running at an equal (or equal and opposite) angle to the first in order to cancel the cyclic speed variation that the first one introduces. With a CV at one end, there is nothing to cancel out this speed variation so it's passed straight into the driveline.

The fact that someone found fitting a CV at one end improved the level of vibration tends to suggests there was a problem with their original 2 UJ configuration; it's certainly far from an optimal solution unless the UJ is never operated at an angle (and why would you need one it if wasn't?).

I have seen driveshafts assembled where the phasing of the UJ's is 90 degrees out, which instead of cancelling the speed variation makes it much worse giving horrific vibration. Even if the phasing is correct, having the rear axle nose tilted up or down also means the angles at each end may not cancel, again increasing vibration. Fitting a CV at one end might improve things in these situations, but it's not fixing the underlying problem.

I received so many conflicting opinions, I telephoned the head of Technical Engineering at Dana/Spicer........manufacturers of U-Joints and CV Joints. He said that while J-Joints are designed to absorb "twisting" forces, they probably would handle "thrust" forces as well. However, he said that they aren't designed to handle consistent angles greater than 4 degrees. Greater than 4 degrees would create excessive vibration and lead to premature failure. In my case I would need about 7 degrees constant angle, so I have ruled out U-Joints for my solution.

We then spoke about CV Joints. He said they could handle the angle, but are not capable of handling much over 2,000 RPM, or they will overheat and burn up. Not an option for me as I would be turning in excess of 4,000 RPM.

In the real world it is possible to use a CV joint at 4000 rpm and 10 deg so I might think about this for by prop from overdrive to rear axle.

Perhaps this

Note that UJs must not run in line and need a couple of degrees of deflection otherwise they wear rapidly.

[Srenner]

You might recall that I replaced the two rear Layrub on the Jav with u-joints before the Big Alaskan Adventure, leaving the front one between the gearbox and the front prop shaft. The rear shaft was balanced (only having u-joints), but no shop was interested in working with me on trying to balance the front shaft. I played with the jubilee clip and chased that without much success. Still had that nasty shudder at the wrong (right) road speed. I'm all in favor of sorting a replacement. I have driven Jups with all u-joint prop shafts and really can't feel a difference. IIRC, Stars & Stripes is such a vehicle and we did 3500 miles in less than a month without issue. Car us unchanged since then, bit I doubt it has covered near the same mileage in the last 17 years.

We use CV joints to replace the rubber donuts in the rear of Lotus Elans, but these are after the dif reduction, so turning slightly above 2000 rpm at full tilt.