First, purchase (or Borrow) an angle finder gage. Measure your angles, then make a change, only if needed.
The goal is to decrease the u-joint operating angles, but only if the u-joint operating angles can remain equal at both ends of the drive shaft. The stress of unequal u-joint operating angles far exceeds the stress of equal, but steep, operating angles.
I lifted my 88 to a true four inch lift gain, for a year or so with a combination of add-a-leafs, with no pinion angle change. Later, sag and shackle changes compounded to alter the sweet balance of angles. I was lucky, for a while, but the typical XJ is not so forgiving (although the 87-90 models are more forgiving).
The pinion angle relative to the t-case almost always changes when the vehicle is lifted. The spring arch is changed (or unchanged on the leading half), the shackle length is changed, or the leverage on the spring pack is altered (by blocks) to change the pinion angle when under power.
Each of these changes tend to point the pinion up, while traveling down the road, decreasing the u-joint operating angle between the drive shaft and the pinion.
The lift also increases the u-joint operating angle between the transfer case and the drive shaft.
The result of the two changes is a lower than stock operating angle at the pinion, and a higher than stock operating angle at the transfer case. The imbalance between the two angles with respect to the drive shaft (the common link) sets up the vibration.
The u-joint with a greater operating angle requires more travel and centerline offset (more movement of the mass center) per drive shaft revolution, and the greater movement simply shakes that end of the drive shaft more. You feel the weight of the drive shaft pushing on the u-joint bearings.
When the operating angles at the two ends of the drive shaft are near equal the shaking at both ends will cancel each other out.
The stock XJ t-case output shaft is typically pointed down ~5 degrees, and is matched with a pinion angle pointing up ~4 degrees. The one degree difference allows spring wrap to point the pinion up the extra degree, for equal 5 degree input and output shaft angles, at highway speed.
These measurement are with respect to a level ground reference. A conventional drive shaft will allow up to ~3 degrees of u-joint operating angle imbalance before it becomes a noticeable problem (if the operating angles are relatively small). Some lifts keep the pinion angle within the three degree error range (and some do not).
When we lift the XJ equally, front and rear, the t-case angle will not change and we only worry about the pinion angle change. A three inch lift usually results in ~1-3 degrees of upward pinion angle change (a net 6-7 degree up angle).
A 2.5-3 degree shim fitted fat end forward will drop the pinion back down and equal out the input and output shaft angles (a net 3-4 degree op angle). The result might be off a degree or two but within the error range of the conventional drive shaft on the safe side where spring wrap will not cause critical imbalance.
If it's still a little off (say, t-case ~5 degrees & pinion ~6 degrees) we can drop the t-case about one-inch to make the balance ~6 degrees & ~6 degrees (within the error allowable when under load).
Fitting the shim fat end back points the pinion up, and may take a very slight problem (a one degree rise in pinion angle) and make it worse (a three+ degree rise result). The result is still within the error of the conventional drive shaft, until you load the springs with a torque load at speed. This is the buzz felt when on the gas (when you feel nothing during normal town driving).
The other problem with this exercise is few lifts raise the front and rear equally. The compound impact of a tall rear lift (common because the stock XJ squats) results in the t-case angle changing to 4 degrees or less. This now adds to the shim requirement for equal u-joint operating angles. Also take care to watch the motor mount condition. Failing motor mounts tilt the engine forward, and result in a shallower t-case angle. Again, an angle finder gage is needed to measure the actual conditions).
Since it is near impossible to guess the resulting true height gain on both ends of a lift (not to mention the later sag) a check with an angle finder is always a good exercise to follow.
All XJ's lift a little different. The 87-90 models seem to be the most forgiving. The low pinion height of the 8.25 (and the long unsupported tail shaft of the 97+ t-case) seem to combine to amplify any u-joint angle imbalance. Unfortunately these two late model features (or tall lift heights) also increase the operating angle of both u-joints, to a point where the u-joint life is reduced. You have a fairly heavy drive shaft pounding away on the bearing surface of the u-joints with every revolution.
This is where a double cardan style constant velocity joint (CV) shaft helps, by reducing the mass that is pounding on the u-joints. The u-joint operating angles in & out of the CV joint are the same as at the ends of the drive shaft, but the drive shaft mass is much less (it's only the little CV joint bridge). The twin u-joints are also so close together that they naturally seek an equal operating angle. We end up with equal u-joint operating angles at the CV joint, with equal mass centerline velocities, and very little potentially imbalanced mass to shake the drive shaft. This allows drive shaft to operate at a much steeper angle (a taller lift height) before the mass and imbalance forces become great enough to feel.
The pinion end of the CV shaft however now needs to have a near zero operating angle at the solo u-joint. We also still have the spring wrap issue to deal with. The result is a decision to point the pinion at the CV joint (or the t-case output) and then adjust the pinion one more degree down for spring wrap. We end up raising the pinion angle up with the CV style shaft (fat end back with a shim) to try and neutralize the solo u-joint angle.
The difference in need to shim the pinion down (conventional drive shaft) or up (CV style drive shaft) gets confusing at times. Throw in an unbalanced lift height, questionable spring wear, questionable motor/trans mounts, and the variations in how the pinion angle will change, and it's way to easy to make an incorrect armchair diagnosis (not to mention get confused). Sometimes the drive shaft angle is just too steep to help a vibration problem that is exaggerated by a dented or bent drive shaft (and the problem keeps returning after new u-joints are installed).
I hope this helps, and explains why a check up with an angle finder is best to determine the actual shim need (if any).
Happy Trails!
By Ed Stevens
Originated from a post on the
Modifications Tech Forum
, and the author of this tech article disclaim any and all liability associated with any "do it yourself" vehicle modifications and/or repairs.
Content © 1999- 2002 North American XJ Association
The goal is to decrease the u-joint operating angles, but only if the u-joint operating angles can remain equal at both ends of the drive shaft. The stress of unequal u-joint operating angles far exceeds the stress of equal, but steep, operating angles.
I lifted my 88 to a true four inch lift gain, for a year or so with a combination of add-a-leafs, with no pinion angle change. Later, sag and shackle changes compounded to alter the sweet balance of angles. I was lucky, for a while, but the typical XJ is not so forgiving (although the 87-90 models are more forgiving).
The pinion angle relative to the t-case almost always changes when the vehicle is lifted. The spring arch is changed (or unchanged on the leading half), the shackle length is changed, or the leverage on the spring pack is altered (by blocks) to change the pinion angle when under power.
Each of these changes tend to point the pinion up, while traveling down the road, decreasing the u-joint operating angle between the drive shaft and the pinion.
The lift also increases the u-joint operating angle between the transfer case and the drive shaft.
The result of the two changes is a lower than stock operating angle at the pinion, and a higher than stock operating angle at the transfer case. The imbalance between the two angles with respect to the drive shaft (the common link) sets up the vibration.
The u-joint with a greater operating angle requires more travel and centerline offset (more movement of the mass center) per drive shaft revolution, and the greater movement simply shakes that end of the drive shaft more. You feel the weight of the drive shaft pushing on the u-joint bearings.
When the operating angles at the two ends of the drive shaft are near equal the shaking at both ends will cancel each other out.
The stock XJ t-case output shaft is typically pointed down ~5 degrees, and is matched with a pinion angle pointing up ~4 degrees. The one degree difference allows spring wrap to point the pinion up the extra degree, for equal 5 degree input and output shaft angles, at highway speed.
These measurement are with respect to a level ground reference. A conventional drive shaft will allow up to ~3 degrees of u-joint operating angle imbalance before it becomes a noticeable problem (if the operating angles are relatively small). Some lifts keep the pinion angle within the three degree error range (and some do not).
When we lift the XJ equally, front and rear, the t-case angle will not change and we only worry about the pinion angle change. A three inch lift usually results in ~1-3 degrees of upward pinion angle change (a net 6-7 degree up angle).
A 2.5-3 degree shim fitted fat end forward will drop the pinion back down and equal out the input and output shaft angles (a net 3-4 degree op angle). The result might be off a degree or two but within the error range of the conventional drive shaft on the safe side where spring wrap will not cause critical imbalance.
If it's still a little off (say, t-case ~5 degrees & pinion ~6 degrees) we can drop the t-case about one-inch to make the balance ~6 degrees & ~6 degrees (within the error allowable when under load).
Fitting the shim fat end back points the pinion up, and may take a very slight problem (a one degree rise in pinion angle) and make it worse (a three+ degree rise result). The result is still within the error of the conventional drive shaft, until you load the springs with a torque load at speed. This is the buzz felt when on the gas (when you feel nothing during normal town driving).
The other problem with this exercise is few lifts raise the front and rear equally. The compound impact of a tall rear lift (common because the stock XJ squats) results in the t-case angle changing to 4 degrees or less. This now adds to the shim requirement for equal u-joint operating angles. Also take care to watch the motor mount condition. Failing motor mounts tilt the engine forward, and result in a shallower t-case angle. Again, an angle finder gage is needed to measure the actual conditions).
Since it is near impossible to guess the resulting true height gain on both ends of a lift (not to mention the later sag) a check with an angle finder is always a good exercise to follow.
All XJ's lift a little different. The 87-90 models seem to be the most forgiving. The low pinion height of the 8.25 (and the long unsupported tail shaft of the 97+ t-case) seem to combine to amplify any u-joint angle imbalance. Unfortunately these two late model features (or tall lift heights) also increase the operating angle of both u-joints, to a point where the u-joint life is reduced. You have a fairly heavy drive shaft pounding away on the bearing surface of the u-joints with every revolution.
This is where a double cardan style constant velocity joint (CV) shaft helps, by reducing the mass that is pounding on the u-joints. The u-joint operating angles in & out of the CV joint are the same as at the ends of the drive shaft, but the drive shaft mass is much less (it's only the little CV joint bridge). The twin u-joints are also so close together that they naturally seek an equal operating angle. We end up with equal u-joint operating angles at the CV joint, with equal mass centerline velocities, and very little potentially imbalanced mass to shake the drive shaft. This allows drive shaft to operate at a much steeper angle (a taller lift height) before the mass and imbalance forces become great enough to feel.
The pinion end of the CV shaft however now needs to have a near zero operating angle at the solo u-joint. We also still have the spring wrap issue to deal with. The result is a decision to point the pinion at the CV joint (or the t-case output) and then adjust the pinion one more degree down for spring wrap. We end up raising the pinion angle up with the CV style shaft (fat end back with a shim) to try and neutralize the solo u-joint angle.
The difference in need to shim the pinion down (conventional drive shaft) or up (CV style drive shaft) gets confusing at times. Throw in an unbalanced lift height, questionable spring wear, questionable motor/trans mounts, and the variations in how the pinion angle will change, and it's way to easy to make an incorrect armchair diagnosis (not to mention get confused). Sometimes the drive shaft angle is just too steep to help a vibration problem that is exaggerated by a dented or bent drive shaft (and the problem keeps returning after new u-joints are installed).
I hope this helps, and explains why a check up with an angle finder is best to determine the actual shim need (if any).
Happy Trails!
By Ed Stevens
Originated from a post on the
Modifications Tech Forum, and the author of this tech article disclaim any and all liability associated with any "do it yourself" vehicle modifications and/or repairs. Content © 1999- 2002 North American XJ Association
