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newbie alert: what causes the vibes?

Beej

Beej

NAXJA Forum User
Location
The Wet Coast of BC
I am a newbie, and I have searched for the answer to this already with no consistent explanations. I have a 97 with a 3" lift and 31s. It has a command trac and an 8.25 rear with a limited slip(?). It has minor vibrations at speed around the 55mph mark. What i want to know is, what is the problem with newer xjs that seems to result in vibes after lifting? I want to address the vibes and I think I will do the SYE thing by the holidays, but I am wondering what causes it? I know this is a seriously newbie question and i would not ask if i could find the answer. One thing i did find referred to was something about the tcase output. What is it that is different on my jeep as opposed to older ones? Hopefully someone can fill in my blanks without too much flaming? I already know how to fix it, I just want to know why it happens.
Thanks ever so much in advance,
Beej.
 
Tire size, tire type, allignment, warped wheel, bad tire. Take your pick. Most likely and alignment issue. Or you can do what I do, avoid 55 mph....... :)
 
In '96 the tailcone was changed to a longer one that has a built in seal so you could pull your rear d-shaft and not leak all the fluid out. This longer tail cone means a shorter d-shaft and steeper angles.
 
CW said:
In '96 the tailcone was changed to a longer one that has a built in seal so you could pull your rear d-shaft and not leak all the fluid out. This longer tail cone means a shorter d-shaft and steeper angles.
Click to expand...
Thank you very much, simple and to the point. Enjoy the reputation points!
Beej.
 
Beej said:
Thank you very much, simple and to the point. Enjoy the reputation points!
Beej.
Click to expand...

CW's comments were right on the mark. You might also consider checking the angle of your u-joints to see if you shim the rear axle you might be able to eliminate the vibration without the SYE. If shimming alone does not fix it then a transfer case drop and shimming would be your next step.

Best of luck,
Michael
 
jeepguy97 said:
If it only happens at 55 it is most definatly a tire or alignment issue.
Click to expand...
I don't agree. It could be a tire issue but I have never seen an alignment cause vibrations. Well I take that back it could be the caster in the front end making the front driveshaft out of alignment. However I know that driveshaft alignment is more often the cause. two or three degrees out is all it takes to make the shakes. My jeep is running about 9 inches of lift in the front and I cannot get the caster to come in and still keep a good angle on my front driveshaft without cutting and rotating the knuckles. It shakes like crazy at about 68-70 but goes away above that.
You could be totaly right but it most likley is a driveshaft angle issue. :peace:
 
jeepguy97 said:
If it only happens at 55 it is most definatly a tire or alignment issue.
Click to expand...
Negative - I have had vibrations at 50-55 on my 89 XJ when I had a bad u-joint in the rear driveshaft. I have vibes again with my SYE because the shims in my D44 are too steep at 4*. I'll swap those out for 2.5* shims tonight and that should take care of the vibes that I get under acceleration at around 45mph and again at 65mph+.

For the shaft angles, since you are using the stock rear driveshaft the angle of the driveshaft in relation to the t-case output and to the pinion should be the same. When you lift it that will change those angles and because it's a 96+ t-case they are very prone to vibes.

Tom Woods has some excellent illustrations and info about the angles on his website at http://4xshaft.com/driveline101.html
 
jeepguy97 said:
If it only happens at 55 it is most definatly a tire or alignment issue.
Click to expand...
why? I've had two sets of tires on it as well as the stockers and it vibed at the same speed each time. And of course its been aligned more than once. I've had the lift on for quite a while now. Also, I jacked it up and ran it up to past 55mph with no wheels on and it still had the vibes. I'm guessing its not a tire or alignment issue when it still does it while up on stands.
Beej.
 
Yucca-Man said:
For the shaft angles, since you are using the stock rear driveshaft the angle of the driveshaft in relation to the t-case output and to the pinion should be the same. When you lift it that will change those angles and because it's a 96+ t-case they are very prone to vibes.
Tom Woods has some excellent illustrations and info about the angles on his website at http://4xshaft.com/driveline101.html
Click to expand...
thanks for the link and info. My angles are the same, they did not change with the lift, although i don't know why.
I figured the vibes were a rear driveline problem.
Beej.
 
Funny thing is that you may want to have the rear pinion pointed a degree or so lower than the numbers indicate, because of spring wrap when you accelerate.
 
CW said:
In '96 the tailcone was changed to a longer one that has a built in seal so you could pull your rear d-shaft and not leak all the fluid out. This longer tail cone means a shorter d-shaft and steeper angles.
Click to expand...

Actually the tailcone got shorter but the output shaft remained the same length meaning more un-supported shaft hanging out!
 
Yucca-Man said:
Funny thing is that you may want to have the rear pinion pointed a degree or so lower than the numbers indicate, because of spring wrap when you accelerate.
Click to expand...
really? I'm not sure i follow this. I know what spring wrap is and how its caused, but what about for the majority of time when i am not accelerating? Wouldn't having the angles mismatched result in vibes at steady speeds? Just asking...cause i just read on that tomwoods site that they need to be the same.
 
Even at steady higher speeds you are pushing against the rear axle, which will point the pinion up farther, bringing your d-shaft angles into position so they don't vibe.
 
Yucca-Man said:
Even at steady higher speeds you are pushing against the rear axle, which will point the pinion up farther, bringing your d-shaft angles into position so they don't vibe.
Click to expand...
Got it. Thanks, I hadn't considered that. Maybe I will look at shimming a tad and see if that helps prior to the SYE. Also, I guess I'd better join NAXJA soon to pay for all the free advice! :D
Beej.
 
I didn't see this mentioned and if I am repeating then forgive me....

Sometimes when you have vibrations that you may associate with mis alignment or an unbalenced tire may be the result of a poorly balenced drive line... if your vibrations come at 55mph and stay consistant it could be either your tires or allignment. If, when you hit what ever speed, staying constistant, and you get the vibes but they seem to come and go, It could be a slightly bent driveline, bad U-joint, maybe a bearing in the t-case or diff. but not likely to be the latter.
 
I'm simply saying that usually vibes at one speed only are not the result of a lift. It is usually a tire balance issue or as ststed above a driveline balance issue.
 
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