Topic: By design?

[neale_rs]

Looking at the Rubicon Express 2.5 inch lift kit for a YJ I noticed they include shackles that provide 0.5 inch of lift and also transfer case lowering hardware.  It would seem the shackles would point the pinion up a bit and the transfer case lowering would point the TC output down a bit.  The overall effect should  be to keep the yokes closer to parallel.  So maybe lift shackles are good to use in the back as long as you have a MML or TC drop.  Any comments on this strategy?

[Jeffy]

Seeing as some YJ's had bad U-joint angles coming from the factory they are playing it safe.  the other issie is not only the angle but the length of the driveshaft.  As you add lift, you're increasing the distance between the end of the transfer case and the pinion yoke.  The shackles will point the pinion out of phase with the transfer case but keep the pinion angle happy.  Lowering the transfer case brings the driveshaft back into phase.  Phasing has to do with the alignment of the driveshaft.  For a single-cardan joint, both U-joints must be in alignment with each other.  This means the angle of the drivetrain must be parallel with the angle of the pinion shaft for it to run properly.  When they are out of phase, it can cause vibrations and increased wear.

[neale_rs]

Ok, so it sounds like the phasing is most important in avoiding damage due to vibrations so long as the operating angles of the u-joints are not exceeded.  Thanks.

[neale_rs]

I took some measurements yesterday and calculated that the angle on the stock DS on my '95 YJ with old stock springs, 2.5L is at least 13.9 degrees.  It is probably even worse because the DS does not go straight back but rather angles to the side a bit, so this would add a bit more to the angle.  The Tom Woods web page says 15° is the most that is acceptable.  A strict application of that rule of thumb means a SYE is needed for even the slightest lift on a YJ.  That being said, 15° is probably very conservative since lots of YJs seem to be running fine with greater angles.

[Bounty Hunter]

That sounds like a lot of angle, is that the angle of each ujoint calculated between the pinion and driveshaft, and tc output and driveshaft?

The max operating angle of a 1310 ujoint is 30º, and safe continuous operation is generally half of that, hence the 15º.  The life of a ujoint is dramatically decreased at 50% of max operating angle, but life gets longer as the angle decreases.

[neale_rs]

The 13.9 is angle up from the floor baseline.  Last night I used an angle finder and the more carefull measurements and found that the pinion is up 2 degrees from the floor and the DS is up 12 degrees from the floor so the real operating angle of the DS is 10 degrees.  Sounds much more reasonable for a stock vehicle. Based on some trig it looks like you can get a 13.9 degree operating angle with a single cardan DS, super short SYE and 2.5 inch lift.  A bit more than the 10 degrees my mechanic recommended but still within the 15 degrees.  Looks like even for a 2.5 inch lift a CV style driveshaft is really the best way to go for max reliability.