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SYE Conversions and the Rear Driveshaft Angles

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When installing a CV or SYE rear driveline, the rear axle pinion angle is important.  A Jeep XJ Cherokee owner asked me about this angle and axle shimming (changing the pinion shaft angle).  Here is our exchange:

XJ Cherokee Owner's Question:  "Mr. Ludel...I am wondering if you have an SYE on your XJ?  I had one put on with a Tom Woods driveshaft.  [The Jeep] rides smoother with its 2 1/2” lift, but I did not install shims. Are  shims a must with an SYE?...Lynn W."

My reply:   Hi, Lynn…An SYE driveshaft has a double-Cardan or CV front joint.  A double-Cardan joint has self-cancelling angles.  The angle at the pinion/axle end of the rear driveshaft is important, however.  That angle with an SYE/CV driveshaft should be 1.5 to 2-degrees.  This is measured with the vehicle on the ground or the axles weighted.  If the rear joint is straight (0-degrees), there will be inadequate rotation of the needle bearings inside the U-joint.  The joint will fail prematurely…Too much rear U-joint angle is not good either. 

So, to answer your question, you do want to shim/rotate the rear axle and pinion shaft to achieve a 1.5 to 2-degree rear U-joint angle.  I use hard steel (not aluminum) shims...Moses

Here are Lynn's photos...This shaft needs a rear U-joint angle adjustment:


Lynn's SYE and CV driveshaft has a Double-Cardan front joint...Here, the double U-joints cancel each other's angles...


Lynn's rear axle pinion joint angle needs to be corrected.  This single Cardan joint should run at 1.5 to 2 degrees, set with the vehicle's weight on the axles...To correct the pinion angle, wedge shaped shims fit between the rear axle's spring perches and the leaf springs.  Pinion/U-joint angle is corrected with these shims.

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Lynn shared this photo.  Here, he has only measured the driveshaft slope...Below the photo is my explanation of how to measure the driveshaft-to-pinion shaft (U-joint tilt) angle:


Lynn...I looked closer at your photo and saw the string measurement.  This must be your driveshaft (tube) slope with the vehicle level.  The measurement you want is the angle between the driveshaft (tube) and the axle pinion shaft.  If you draw a line straight out from the pinion shaft centerline and measure its relationship to the driveline slope, you have the angle you want.  Put simpler, you can place your string gauge on the flat outer flange of the pinion yoke with the pinion/U-joint yoke facing straight up and down.

Note:  For pinion angle reference, the axle’s pinion shaft centerline is parallel to the pinion yoke’s outer flanges.  The magnetic angle gauges attach to the U-joint yoke’s outer flange(s).  It is important that the U-joint yoke’s flanges face straight up and down when measuring this angle.

The 1.5 to 2 degrees is the angle or tilt of the rear U-joint.  If your driveline slope with a string line gauge is 20 degrees, the axle’s pinion centerline angle should be 18.5 to 18 degrees.  (In geometry or trigonometry, the angle between the driveline tube and pinion shaft centerline is two intersecting slope lines with a deviation of 1.5 to 2 degrees.)  With the CV driveline, the axle’s pinion yoke joint angle approaches a straight line with the driveshaft tube, deviating only 1.5 to 2 degrees from straight.  That tilt has the axle pinion shaft pointed slightly downward.

The usual tools for measuring this angle would be a protractor gauge or a magnetic spirit/bubble level gauge.  Both measure in degrees like your string gauge and plumb bob.  Your approach can work if you use the gauge and plumb bob/string properly.  When the axle housing is rotated correctly, the driveshaft tube will appear to run nearly straight into the pinion, the tilt being only the slight 1.5 to 2 degrees.

Below is a factory/Mopar Inclinometer (7663) tool for measuring U-joint flange angles and, in this instance, the front axle caster angle.  This gauge has a spirit/bubble and a tilt scale in degrees.  The top of the gauge has a magnet that attaches to a U-joint yoke flange:

Adjusting camber of front axle while setting lengths of link arms.

Here is a time-honored Spicer Anglemaster gauge, the latest "Anglemaster II" version.  Note that this gauge attaches on the flatly machined flange end of the U-joint yoke.  This angle can be quickly compared to the slope angle of the driveshaft.  The difference between the two angles is the U-joint tilt or joint angle.  These angles are always measured with the vehicle at normal/curb ride height and vehicle weight on the axle(s).  Do not take driveline and joint angle measurements with the axle hanging off the floor.  The vehicle should be standing on level ground or with the axle(s) supported safely on jack stands or tripod stands.  Driveline angles for trucks or SUVs that carry a hefty load should be measured with the full payload on the axles:


For more information on an SYE conversion and the use of a CV rear driveshaft, visit:  4WD Mechanix Magazine article on an NP/NV231 transfer case SYE kit installation, click here.  For details on our XJ Cherokee's 6-inch long arm lift that required an SYE and CV rear driveline, click here for the magazine article at 4WD Mechanix.


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Additional exchange with Lynn:

 Question from Lynn:   "I am hoping this is the last time I’m bothering you about pinion angles. Thanks again for the in depth explanations...I downloaded an app to measure angles a little more accurately.  I came up with 13.5° on driveshaft and 8.5° on pinion yoke. So does that work out to be a 3° shim I need?  Also I believe when shims are installed on CV driveshaft the beefy end will be facing the rear of Jeep?"

Lynn...Your measurements must be with the vehicle on level ground and the body's weight fully on the axles…If you measured the pinion yoke at the flange flat (with the yoke flanges pointed straight up and down), then the pinion shaft is on an 8.5-degree slope or angle.  If the pinion shaft is on an 8.5 degree slope, and the driveshaft is on a 13.5-degree slope angle, the difference between the two angles is 5 degrees.  You want to reduce this difference to 1.5 to 2 degrees.  Rotating the axle housing and pinion shaft upward with a three degree shim set should work.  The end result will be 2-degrees of U-joint tilt or angle.

Most shims, by design, allow minor adjustment in either direction, but 2 degrees is fine.  I run 2-degrees on my XJ's rear U-joint angle.  When you are through installing the shims, with the spring U-bolts torqued evenly to specification and the Jeep on level ground with body weight on the axles, you should have a 2-degree or less rear U-joint angle or tilt.  The driveshaft slope angle may decrease slightly when the pinion rotates upward, dropping the difference figure closer to 1.5 degrees.  Let us know your final readings.

The "shims" are wedge-shaped metal plates that fit between the axle perches and the leaf springs on your Jeep XJ Cherokee.  (The TJ and newer Wranglers have link-and-coil spring suspension, and the shim fit is different.)  You will be raising the vehicle safely, supporting the body/frame, allowing the springs to sag fully, and then loosening the spring U-bolts evenly.  When there is enough gap between the spring centering bolt and the axle spring perch to allow inserting the wedge shim, the shim's narrow end will face forward.  When both shims are in place, the tapered shims rotate the axle housing and pinion shaft upward to reduce the U-joint angle...Shims are not installed on the CV driveshaft itself; the tapered shims fit between the axle spring perches and leaf springs to rotate the axle housing and pinion shaft.  This rotation changes the pinion angle.

I use steel (not aluminum) shims that match the width of the leaf springs.  Shims need to support the leaf springs properly.  Torque the spring U-bolt nuts in cross and evenly to factory torque specification. I like to re-check the nut torque after driving the vehicle and cycling the suspension.






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