That is a good suggestion. I will try when I have my Jeep back and the problem still persists.
Harmonic vibrations in Jeeps: A new theory (please read!)
- Thread starter psrivats
- Start date
That is a good suggestion. I will try when I have my Jeep back and the problem still persists.
Pay close attention to my wording. You CANNOT drive on the unit bearings without stub shaft properly bolted in place. The two halves will separate, your front tire(s) will leave the vehicle and you will die a horrible death.
A good reason for him to get some spare outer stubs and unit bearings to hang on for trail spares.
In all of the trails I've run with all of the repairs I've been involved in, not once did I wish I had a spare unitbearing.
I have not read all 33 pages of post but has the OP tried pulling the rear driveshaft and running on the front only?
Thanks Blaine, you did a better job of explaining the lateral movement caused by the trackbar than I could do.
Thanks Blaine, additionally my point is that not only longitudinal tilt of the pinion needs to be considered but also lateral alignment of the pinion needs to be centered on the transfer case output shaft and be square to the t-case.
I don't disagree on the face of the theory. Too many folks are running offset pinions on 8.8's for me to give that as much value as it appears to deserve.
OP here. I've done everything that's been discussed on the forums on this topic, and then some. Not an easy problem to resolve
Are those folks running a driveshaft standard u-joints or a DC with a standard u-joint driveshaft? For purpose of this discussion I was assuming the latter was the case.
I see your point. What are the common failures you've experienced with the front axle? Is it the stub shaft ears, or inner stub, thus wiping out the ujoint? I'm just thinking in terms of an easier trail repair to get the rig off the trail. I agree, the chances of a unit bearing being damaged is slim, but in an effort to minimize down time I figured a ready to go assembly would save time?
Your logic is sound but it requires carrying around a pain in the ass to protect part that is heavy. Most common failure is broken yokes from a spit cap which then breaks the joint.
In closing, a driveline system alignment is not black magic, it is just simple geometry. A driveline system must be aligned in all three axis, X.Y and Z, according to the dictates of the system design. If the driveline system is out of alignment on any of its axis then it will have a vibration, again simple geometry dictates that result. If the driveline system is properly aligned in all axis then one needs to look elsewhere for the vibration problem solution.
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Roadhawg011
Member
Wow that went sideways a bit.
For my 2c, my vibes seemed to get worse after going up from a 2inch BDS lift to a 3.5inch RE lift and adding a trackbar bracket in the rear. If anything the axle is more centered now than it was on the 2inch lift with no bracket.
(I am RHD though, so the rear trackbar is quite short and attaches to the axle just past the diff.)
Questions,
- The theory is the new style Skid plate causing/not helping with some of this vibration....Does the old style skid fit on the newer Jeeps?
- I have the Teraflex super short SYE setup. Am I going to be able to get rid of these high speed vibrations ever or am I going to be stuck to sub 100km/h forever
?
For my 2c, my vibes seemed to get worse after going up from a 2inch BDS lift to a 3.5inch RE lift and adding a trackbar bracket in the rear. If anything the axle is more centered now than it was on the 2inch lift with no bracket.
(I am RHD though, so the rear trackbar is quite short and attaches to the axle just past the diff.)
Questions,
- The theory is the new style Skid plate causing/not helping with some of this vibration....Does the old style skid fit on the newer Jeeps?
- I have the Teraflex super short SYE setup. Am I going to be able to get rid of these high speed vibrations ever or am I going to be stuck to sub 100km/h forever
?
Would the construction of the double cardan joint itself have any impact on vibrations?
Considering that we know that a single u-joint at an angle does not result in a 1:1 ratio, but two u-joints cancel each other out over the span of the joints, that would mean that the double cardan intermediate carrier must accelerate and decelerate twice with every rotation of the driveshaft.
A larger intermediate carrier, such as a 1350 carrier, is going to have significantly more rotational inertia than a smaller carrier like a 1310. Not only does the 1350 have additional mass, it has a larger overall diameter, both of which increase its rotational inertia. But it must angularly accelerate and decelerate the same amount as the smaller carrier. So in theory, a 1350 DC shaft should have significantly more vibration than a 1310 DC.
This vibration would present itself as torsional vibrations, so balancing of the driveshaft would do nothing. A properly tuned torsional mass-damper could negate it, but I doubt the factory harmonic balancer is correctly weighted and damped to effectively do so. Since the rest of the shaft travels at a relatively constant speed, no mass can be added to the shaft to effectively counteract the vibration induced by the acceleration and deceleration of the DC joint.
There would only be a few ways to reduce or eliminate this effect.
The first, or most obvious, is to delete the driveshaft. This is not practical, so I will ignore it.
The second would be to use a double-double cardan driveshaft, or a driveshaft with DC joints at both ends. If the DC joints were installed 90 degrees from each other and run at the same angle, one intermediate carrier would be accelerating at the same time as the other one would be decelerating. With a sufficiently stiff center tube, these accelerations would cancel out, and few to no torsional vibrations would leave the shaft and enter the transfer case or differential.
The third option which I briefly suggested earlier would be to ditch the double cardan joint for a true CV joint like a Rzeppa joint.
Unlike a double cardan joint, a true CV joint like a Rzeppa has no parts that must angularly accelerate or decelerate. Thus, it will not create torsional vibrations like a double cardan joint will, especially at extreme angles.
This effect is actually called out in the Wikipedia page where I got the images from. Scroll down to the section on double cardan joints.
https://en.m.wikipedia.org/wiki/Constant-velocity_joint
"Double Cardan joints require a centering element that will maintain equal angles between the driven and driving shafts for true constant velocity rotation. This centering device requires additional torque to accelerate the internals of the joint and does generate some additional vibration at higher speeds."
Considering that we know that a single u-joint at an angle does not result in a 1:1 ratio, but two u-joints cancel each other out over the span of the joints, that would mean that the double cardan intermediate carrier must accelerate and decelerate twice with every rotation of the driveshaft.
A larger intermediate carrier, such as a 1350 carrier, is going to have significantly more rotational inertia than a smaller carrier like a 1310. Not only does the 1350 have additional mass, it has a larger overall diameter, both of which increase its rotational inertia. But it must angularly accelerate and decelerate the same amount as the smaller carrier. So in theory, a 1350 DC shaft should have significantly more vibration than a 1310 DC.
This vibration would present itself as torsional vibrations, so balancing of the driveshaft would do nothing. A properly tuned torsional mass-damper could negate it, but I doubt the factory harmonic balancer is correctly weighted and damped to effectively do so. Since the rest of the shaft travels at a relatively constant speed, no mass can be added to the shaft to effectively counteract the vibration induced by the acceleration and deceleration of the DC joint.
There would only be a few ways to reduce or eliminate this effect.
The first, or most obvious, is to delete the driveshaft. This is not practical, so I will ignore it.
The second would be to use a double-double cardan driveshaft, or a driveshaft with DC joints at both ends. If the DC joints were installed 90 degrees from each other and run at the same angle, one intermediate carrier would be accelerating at the same time as the other one would be decelerating. With a sufficiently stiff center tube, these accelerations would cancel out, and few to no torsional vibrations would leave the shaft and enter the transfer case or differential.
The third option which I briefly suggested earlier would be to ditch the double cardan joint for a true CV joint like a Rzeppa joint.
Unlike a double cardan joint, a true CV joint like a Rzeppa has no parts that must angularly accelerate or decelerate. Thus, it will not create torsional vibrations like a double cardan joint will, especially at extreme angles.
This effect is actually called out in the Wikipedia page where I got the images from. Scroll down to the section on double cardan joints.
https://en.m.wikipedia.org/wiki/Constant-velocity_joint
"Double Cardan joints require a centering element that will maintain equal angles between the driven and driving shafts for true constant velocity rotation. This centering device requires additional torque to accelerate the internals of the joint and does generate some additional vibration at higher speeds."
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Roadhawg011
Member
Managed to fiddle a bit now.
My rear Pinion seems to be pointing 2 deg lower than the drive shaft. Zeroed my iphone on the shaft. Compared that to the flat section on the back of the axle, axle is pointing 2 degrees down.
I don't know if that's enough to cause problems. I dont really have decent flat ground to work on, so I'll get to my office warehouse on the weekend and properly set it to as close to 0 as possible. I need to actually set up all the control arms in the rear so that should be done first, and then the pinion angle I guess.
I've been putting it off because of what a pain in the ass it is, as well as one of the nuts on my metalcloak control arms being STUCK. Managed to visit a truck mechanic we know today and borrow a giant "Kind Dick" (lol) wrench and bust it free.
Other question:
Should the yokes on either end of the drive shaft not be 90 degrees apart? Or no?
My rear Pinion seems to be pointing 2 deg lower than the drive shaft. Zeroed my iphone on the shaft. Compared that to the flat section on the back of the axle, axle is pointing 2 degrees down.
I don't know if that's enough to cause problems. I dont really have decent flat ground to work on, so I'll get to my office warehouse on the weekend and properly set it to as close to 0 as possible. I need to actually set up all the control arms in the rear so that should be done first, and then the pinion angle I guess.
I've been putting it off because of what a pain in the ass it is, as well as one of the nuts on my metalcloak control arms being STUCK. Managed to visit a truck mechanic we know today and borrow a giant "Kind Dick" (lol) wrench and bust it free.
Other question:
Should the yokes on either end of the drive shaft not be 90 degrees apart? Or no?
Roadhawg011
Member
I setup my rear axle on the weekend and got the pinion angle as near as dammit 0degree different to the shaft.
Still vibrates. I wanted to pull the rear shaft and see if that hanges anything but the shop that installed my SYE clearly didn't give the yoke bolts to the shop that installed the CV driveshaft. So they just used regular head bolts and they're very difficult to undo...no clearance between the bolt yeah and the yoke. I'm going to have to try find the proper 12 point bolts somewhere.
Even though the high speed vibration is still there, Im super happy I did the rear setup. The rear suspension feels 100x better. I think the rear axle was fighting itself because I just jammed the new control arms in, making them the same lengths, rather than making them fit the axle. It rides so much softer now it's unbelievable. So my Sunday spent under the thing wasn't a waste.
Still vibrates. I wanted to pull the rear shaft and see if that hanges anything but the shop that installed my SYE clearly didn't give the yoke bolts to the shop that installed the CV driveshaft. So they just used regular head bolts and they're very difficult to undo...no clearance between the bolt yeah and the yoke. I'm going to have to try find the proper 12 point bolts somewhere.
Even though the high speed vibration is still there, Im super happy I did the rear setup. The rear suspension feels 100x better. I think the rear axle was fighting itself because I just jammed the new control arms in, making them the same lengths, rather than making them fit the axle. It rides so much softer now it's unbelievable. So my Sunday spent under the thing wasn't a waste.