| Author |
Message |
    
Randall Smith
| | Posted on Tuesday, November 12, 2002 - 11:11 pm: | 
|
Here's my latest theory on why some trucks get a vibrating front driveshaft much more easily than others with a bigger lift: Lateral misalignment of the front drivetrain.
I made a pointer which bolts onto the front diff. Twenty inches out the pointer is one inch off center of the driveshaft. This equates to a 3 degree(sideways)angle that my front u-joint is operating at on flat ground. So from the side view my drive shaft comes straight out of the diff, but from below view the DS is cocked 3 degrees.
My Tom Woods driveshaft was not intended to operate like this. Check out the "Tech" pages at www.4xshaft.com, specifically "Geometry". Somehow the engine/transmission assembly has be come misaligned with the front axle assembly.
I have also changed almost every part in the front end, some of them several times. "Rebalancing" the DC driveshaft has already been tried twice.
My original thought was to make an ajustable length panhard rod(which I may do anyway) to pull the front axle assembly over to the drivers side. This would straighten it out, but there are so many other things attached to the axle!
My current thought is to replace the engine and trans case rubber mounts. And I would pry the engine/trans assembly over to the passenger side before tightening bolts. I have never done anything like this and the shop manual doesn't say anything about drivetrain alighnment. Is this possible?? Help me out! I need an out on this vibration problem.
Randall |
Ken Dunnington (Ihwillys)
| | Posted on Wednesday, November 13, 2002 - 10:35 am: |
|
What you are getting at is the alignment of four things:
The centerlines of the front/rear output shafts of the transfer case
The centerlines of the pinions front/rear
The output shafts in the t-case are parallel. The best scenario(for a non-CV jointed shaft) is to have the pinion shafts parallel to the output shafts. This coupled with properly phased u-joints and individually balanced driveshafts will result in the least amount of vibration from this part of the drivetrain. The pinion can be offset laterally from the t-case output, as long as it is parallel to the output or u-joint phasing is used to account for an angle of incidence between the two.
My '98 DI did not come with the front driveshaft u-joints phased at 90. When I questioned this here, it was brought up that they are all like that, thus LR intended this. LR used u-joint phase to diminish/alleviate some source of vibration. I am not certain of the source but have assumed(possibly incorrectly) that the front pinion is not parallel to the t-case output and that the alternative phasing of the u-joints is dealing with this. The rear u-joints are phased 90 degrees, as is conventionally done. I assume the front/rear axle shafts are parallel to one another and are perpendicular to the pinion shafts. Thus the front/rear pinions are in parallel vertical planes to one another, so if the front pinion is not parallel(in a vertical plane) to the front t-case output, the rear pinion would not be parallel to the rear t-case output, assuming the t-case outputs are parallel to one another. But because the rear u-joints are phased 90 deg(with no noted vibration), the rear pinion is reasonably parallel to the t-case output. So, this leaves us with the front pinion not being parallel in the horizontal plane to the t-case output. If the pinion is pointed up(relative to t-case output) stock, the horizontal planes of the pinion and t-case output intersect behind the front axle and a lift will increase the angle of incidence. If the pinion is pointed down stock, the planes intersect ahead of the axle and a lift would lesson this angle, bringing them closer to parallel. But, remember the front driveshaft was phased from the factory to deal with this, so any change in the angle of incidence would result in increased variation in velocity of the pinion with respect to the output velocity, thus increased vibration. So, to deal with this, the driveshaft phasing might need to be experimented with. I'll bet your Tom Woods driveshaft has the u-joints phased at 90 deg. If the lift brought the pinion and t-case output closer to parallel, then this is good, otherwise...
Lateral misalignment in and of itself is not damning. I've seen a rear offset diff(like the Disco has) have a driveshaft run from a center output t-case with no vibration. The question is how out of parallel is each pinion to the t-case output. If they are not the same 'out-of-parallel' (as I suspect is the case from the factory) then aligning the t-case to one will leave the other still out. Diminish the affects using u-joint phasing like the factory did.
If you do attempt to *realign* the t-case, keep in mind that you are aligning to the front and the rear pinions simultaneously.
I'd get out a plumb bob, protractor, square, and some chalk and get an idea of what's what, before I started moving things around.
If your t-case is laterally out of parallel to the front, it likewise is to the rear or your vehicle has a more serious problem(front and rear pinions laterally(vertical plane) should be parallel).
Ken |
Paulschram
| | Posted on Wednesday, November 13, 2002 - 10:53 am: |
|
Randall:
I know we've discussed this at length, but have you confirmed that the front axle is located correctly? Any chance you might have a bent radius arm or something similar? Perhaps it might be a good exercise to measure the distance from center-line of front wheel to back on each side to see if they are indeed correct and equal.
Is there any chance that a PO might have been involved in a crash? I MIGHT be able to get you the dimensions to confirm or deny this idea.
Still suffering along with you and searching for an answer,
Paul |
Randall Smith
| | Posted on Wednesday, November 13, 2002 - 04:48 pm: |
|
Ken:
Thanks for the time that you took to write this. I will have to print it and read it several times later so that it will make complete sense. And also others may understand their vibe problems better after looking at our posts.
According to the Tom Woods site, 3 degrees is the maximum misalignment. Mine is right at 3 degrees. If this is not straight then the front u-joint will vibrate and there is not another u-joint on the shaft to canel the vibration out.
I do plan to measure everything before anything gets moved. Hence the adjustable length panhard rod. Will follow up later.
Paul:
The truck has been in several wrecks(I already have frame specs). Also I have got a custom made radius arm on the passenger side which may be part of the problem. I'm fairly certain that the axle location is not exactly right, moreso than the engine mounts.
I have spent thousands of dollars replacing parts, but my drive train is like new now. The only part that helps stop the vibe is a DC drive shaft. The only change that eliminates the vibration is to remove the front driveshaft.
If I can prove that misalignment between the diff and trans case is the real culprit, then this could help a lot of other people.
Randall |
Ken Dunnington (Ihwillys)
| | Posted on Wednesday, November 13, 2002 - 07:43 pm: |
|
Randall
I hope I'm helping.
I checked out the Tom Wood's tech/geometry site and it states:
"Doing the math for an automotive sized driveline , using a 4" swing diameter and assuming the transfer case out-put and pinion shafts are parallel, the actual cosine for an angle of 15 degrees. I calculate the result of the net difference in linear distance traveled through the arc of each of the "U" joints' path , to be roughly .0014" per occurrence . I believe there are enough clearances in the universal joint , the slip yoke & spline stub along with a torsional modulus of elasticity in the tubing and other components to accommodate this. "
So the shop that made your driveshaft says up to 15 degrees...provided the pinion and output are parallel.
The only reference I see to 3 degrees on that page is in a longevity example, not vibrational. Perhaps I missed what you are referring to.
Because you've got it isolated to the front driveline I wouldn't change anything anywhere else. Eg, leave the position of your transfer case alone, it's the pinion that needs to be moved. Get those angles figured, they'll illustrate what's wrong.
Ken |
Randall Smith
| | Posted on Wednesday, November 13, 2002 - 09:45 pm: |
|
Ken
You are helping.
If you click "Technical", then "What type of shaft", the second paragraph states that a double cardon driveshaft needs to be lined up within 3 degrees at the diff end. Mine is off a solid 3 deg sideways and probably 1 deg up and down. So it still vibrates with the DC DS, but not as bad because the DC takes care of the other vibration(inherent in full time 4WD) so well.
I just finished carefully examining the engine location within the frame, and everything looks correct. So I will be tweeking the axle assembly instead. A quarter inch here, a tenth inch there should prove this theory right.
Also while I was groveilng about under the truck, noticed something else wrong. There is a long steel rod connecting the engine to the transfer case(I think it is to help control engine torque). The nut which fastens this rod to the engine was visably loose. It seems that those characters who replaced my trans case forgot to tighten it. This could've been a factor too.
Randall |
Ken Dunnington (Ihwillys)
| | Posted on Wednesday, November 13, 2002 - 11:26 pm: |
|
Ah yes! I didn't realize you had a CV joint driveshaft, or double cardan. Yes, the u-joint at the pinion needs to have almost no angle because the two joints in the CV are providing the phasing. So your 3 degrees of lateral misalignment is not good. Sorry I didn't catch on earlier. Hmmm, I'd still try to determine the position of the axle relative to the rest of the vehicle to try to formulate how to get rid of that misalignment. That adjustable panhard bar just might do what you want, I don't know. I'm in over my head with this one, as my experience is with the dead simple leaf spring set-up.
Ken |
|
Closed: New threads not accepted on this page