zj front driveshaft vibration

[bobgoblin]

hey everyone. i have a problem that i need some help with.

background: i have a 98 4.0 zj with a 249, 3.5 re lift, 31X10.50 at's, and 4.10 gears.

ever since i had the gears installed i have a bad vibration from the front driveshaft. i had the rear balanced, but on the front i have the shaft with that stupid rzeppa joint at the differential and was told i cannot balance it. the vibration gets worse when i accelerate and its awful when i hit 70 or so (i havent taken it faster recently). my question is this: should i just replace the driveshaft with one that uses normal joints? if so, what parts exactly do i need?

or any other advice will be greatly appreciated.

thanks in advance for the help. this forum is great. i havent seen info this useful since ju back in '99.

jason

[mrblaine]

I can't help with the joint replacement, but it's pretty common to have a vibe that wasn't there before after regearing. You are now spinning the shaft much faster than before and the minor out of balance problem has now manifested itself as the rpm's have increased.

It was always out of balance, just not as noticeable.

[bobgoblin]

true true...... i just hope there is some way to remedy this situation. i have encountered so many problems on this jeep that i have fixed one by one; i hate to think that this one cant be solved.

thanks for the help

jason

[Allen]

Originally posted by mrblaine
I can't help with the joint replacement, but it's pretty common to have a vibe that wasn't there before after regearing. You are now spinning the shaft much faster than before and the minor out of balance problem has now manifested itself as the rpm's have increased.

It was always out of balance, just not as noticeable.

Interesting.....I never thought of that.

Allen

[John]

What about a hub conversion?

[Jerry Bransford]

Originally posted by John
What about a hub conversion?

That sounds like a real good idea.

[TJRON]

Well, some one once once told me RPM is RPM no matter what the gear ratio. If your Jeep didn't vibrate before the gear change when accelerating or at a certain RPM, there's no reason it should now.
Can you borrow, beg or steal a friend's drive shaft to test with?
Are all the u-joints and yokes in perfect shape?
I hope it's not the gears but you said it all started with a gear change......
Good Luck,
Ron

[bobgoblin]

i dont anyone with a zj, so i dont think i can borrow one...that is a good idea though...

also, the u-joints and yokes seem ok. do you think that something done incorrectly during the gear change could have made this happen? that would suck.

is there no way to rid myself of this rzeppa? didnt the earlier zj's come with a regular ujoint at the differential?

thanks for all the suggestions. keep 'em coming



jason

[Mark Hinkley]

Jason,

Are you trying to use a CV style (has a rubber boot) driveshaft on a lifted ZJ?

If so, you need to change the driveshaft to a regular style ujoint one.

mark
orgs mfg

[cbassett]

Originally posted by TJRON
Well, some one once once told me RPM is RPM no matter what the gear ratio. If your Jeep didn't vibrate before the gear change when accelerating or at a certain RPM, there's no reason it should now.

Your friend was partially right, but only in that "RPM is RPM".
Think of an out of balance tire: Around town the rig runs smooth, but shakes like an earthquake at freeway speed. At faster speeds the tire is making more Revolutions Per Minute, exposing the balance condition.

ZJ front vibes are fairly common when going >3", a good percentage of them being front driveline balance. Just as Blaine explained, the driveshafts are not spinning faster due to the R&P ratio.

I had a bear of a time ridding myself of vibes when I first lifted and regeared my ZJ. Vibes were aweful aboove 45mph. Having the front driveshaft balanced helped alot, but not entirely. I finally dialed them out entirely with pinion angle adjustments (upper and lower adjustable arms required) as well as moving the front axle forward a bit. When dialed, my pinion angle was at 0*; this made for very crisp steering, but nearly constant input was required on freeway driving.That was at 5.5". When I switched springs and went to 6.5" I never could dial out the vibes at 6.5", so I carried the front shaft in the cargo bay- and installed the shaft at the trailhead while everyone else aired down and bench-raced.


Oh yeah, if you're not already on a u-jointed (at the axle) front driveshaft, you need to convert. The CV can't take the angles and will die with little provocation.

[bobgoblin]

yep, that's what i was looking for. sorry if my questions were confusing.

so as far as replacing it, to whom do i turn? what all do i need?

cbassett - i have the 249, so i cannot drive without the front shaft (that is correct, right?). otherwise, i would be worrying about this later, as my wife of 1 week would rather me not be spending even more money on this jeep right now.

thanks so much everyone. it kinda bugs me to have to ask so many questions, but i guess you gotta ask to learn.

j

[TJRON]

Very good explanations except he said it started with a gear change, not a lift......

So when did it start, with a lift or the gear change?

[cbassett]

And the explanations cover R&P change-induced vibes. Just added the lift-related experiences for extra info.

you can drive a 249 with no front d/s, but it's not recommended. I know folks that had 249s that used the same "install the front d/s at the trailhead" strategy as me, and they never had issues.

[TJRON]

Originally posted by cbassett
And the explanations cover R&P change-induced vibes.

Naw, I don't believe it. Not when excellerating or going the same RPM.

[Stu Olson]

Ron,

If installing a R&P gear, say you are going from 3.73 to 4.56, you do agree that at the same engine RPM, the vehicle will be moving slower (hence, the reason to go to lower gearing and get a slower crawl speed).

So....I think you also belive that in order to go 60 MPH down the highway, we now have to have more engine RPMs (because of the lower gearing)....right?

So....the driveshaft will be spinning faster.....it has to....it is trying to get the vehicle up to 60 MPH and it has lower gears than it use to.

So....a slight imbalance that was not even noticed with 3.73 gears (doing 60 MPH) is now noticed after regearing to 4.56 gears because your shaft is spinning faster. Just like the tire comment.....it drives fine at 45 MPH around town but the out of balance tire jumps all over the road at 70 MPH.

[TJRON]

Originally posted by TJRON
If your Jeep didn't vibrate before the gear change when accelerating or at a certain RPM, there's no reason it should now.

I don't think I mentioned higher speed in my statement. Thanks for all the help though.....

[mrblaine]

Originally posted by TJRON
I don't think I mentioned higher speed in my statement. Thanks for all the help though.....

So which are you today? Thick or cantankerous?

The driveshaft is spinning at higher RPMs for the same highway speed.

You said- Well, some one once once told me RPM is RPM no matter what the gear ratio. If your Jeep didn't vibrate before the gear change when accelerating or at a certain RPM, there's no reason it should now.

If rpm was in fact rpm no matter the gear ratio, we'd never regear to reduce rpm's. A certain rpm will have the driveshaft spinning faster because it now takes more revolutions of the driveshaft to propel the vehicle the same distance all other things being equal.

[TJRON]

I guess I'm thick 'cause I'm happy with my new clutch and spare transfer case with no farm machinery howl in my Jeep.

What I mean by rpm is rpm is like this:
If the engine is running at 2500 rpm the driveshaft is going XX rpms.
Change gears and run the engine to 2500 rpm and the drivshaft is still going XX rpm.
I also doubt a small gear change would make the drive shaft vibrate more while accelerating normally.

the vibration gets worse when i accelerate and its awful when i hit 70

Now I may be wrong but it seems to me he is saying the vibration is there all the time and worse at higher speeds.
If it wasn't there at all before the gear change it shouldn't be there now at lower speeds.

[Stu Olson]

OK...that's enough.

This is one of those discussion that Ron, after having some 2 or 3 months go by, will find this old thead and say..."Oh, that is what you meant!"

Ron....Glad to hear you have removed the farm machinery from your rig! Life should be more quiet in lo range again.

[TJRON]

Stu,
I hope it's just temporary until I work something out with Tera. I'll by new planetary gears if that's what it takes. I hope they'll work with me though......

Being I used to be the Board Bulldog let me ask a question:
If Bobgoblin didn't have a drive shaft vibration at 70 mph before his gear change would he expect to have one now at 55 or 60 mph?
If he didn't have a drive shaft vibration while accelerating from 1800 rpm to 2500 rpm before the gear change should he have one now?
He seems to have dropped from the discussion but I wish he'd answer thes questions.

[cbassett]

Is your TJ a manual tranny Ron? If so, go for a spin and then report back the MPH you run in each get at 2500 RPM (with your left foot completely off the clutch ).

[TJRON]

Originally posted by cbassett
Is your TJ a manual tranny Ron? If so, go for a spin and then report back the MPH you run in each get at 2500 RPM (with your left foot completely off the clutch ).

Let's save time and how about you answering the above questions?

[Mark Hinkley]

Originally posted by TJRON
Stu,
I hope it's just temporary until I work something out with Tera. I'll by new planetary gears if that's what it takes. I hope they'll work with me though......

Being I used to be the Board Bulldog let me ask a question:
If Bobgoblin didn't have a drive shaft vibration at 70 mph before his gear change would he expect to have one now at 55 or 60 mph?
If he didn't have a drive shaft vibration while accelerating from 1800 rpm to 2500 rpm before the gear change should he have one now?
He seems to have dropped from the discussion but I wish he'd answer thes questions.

The funny thing about this whole thread is that Jason has figured out his problem but Ron hasn't realized is that Jason's problem has nothing to do with Ron's train of thought.

Jason's problem: vibs after re-gear.
Why because Jason is still running the CV boot style driveshaft. I would bet the guys who installed the gears did not reinstall the driveshaft correctly. With this driveshaft it is very critical adjust the length of it when installing it. Yes it is adjustable in length. Hence the vibs after the re-gear. There is also the situation where the cv boot joint doesn't like the new shaft rpm at that particular speed. This second one is less likely but possible. These cv boot style joints are a pain in the a$$. Jason's fix: replace the driveshaft.

Ron's train of thought is correct but is not looking at the whole picture. Driveshaft RPM is RPM at a certian engine RPM. However when you re-gear engine RPM changes at a particular MPH. With a higher gear ratio you will drive slower at a particular engine RPM ie. a lower MPH. At the same MPH with eash gear ration your engine RPM and driveshaft RPM will be different. Thus the reason why these guys are saying a vib can be more noticable or newly noticable with a gear change.

Now from a driveshaft builders point of veiw there is a thing called critical RPM and it is around 800 RPM on the driveshaft. This is where most driveshafts are balanced to. By theory if it doesn't vib at 800, critical RPM, then it shouldn't vib at any RPM. Now as 4wheelers we all know nothing works that way with every driveshaft/lift combo. That is why sometimes when one re-gears at a certain lift heigth one might have no vibs while the next guy might have vibs and needs the shaft to be balanced better.

Hopefully Jason's new wife has realized that playing with Jeeps is a lifestyle and she has learned to live that lifestyle and not fight the fact Jason will always be spending money on his second passion his, Jeep. Being his first passion is his wife.


mark
orgs mfg

[cbassett]

I'll answer your first question with an analogy: If a tree doesn't fall in the woods, does it make a sound?

For number two: Drive shaft vibrations that occur only during acceleration are typically a result of improper pinion angle.

Neither question addresses the original issue unfortunately, so how about that test drive?

[bobgoblin]

wow.

first and foremost i must say that i am somewhat proud my little old question has turned into such a conversation between people that i have been learning from for years. i feel honored.

ok. i know my problem is a front driveshaft problem. i have known for years that the cv-style will fail on me. ever since i installed my lift 2 years ago, the front shaft has made a more noticeable "whoom whoom whoom" sound as i drive. i knew it was less happy with me. when i regeared in june, i took it easy on my gears for a month or so and used my wife's car for my daily 25 mile highway commute. now i have started driving the zj and so i notice this. it makes sense that a slight imbalance would become a bigger inbalance at faster speeds, but i can feel this vibration starting around 50 mph. before, i could take the zj up to ~85 and never noticed this vibe. but maybe i just didnt know what i was looking for.

i do, however, think i just need to replace the shaft. now my question is this: what exactly do i need? i fear my knowledge of drivetrain is less than other parts of a vehicle, but i am learning. do i need a u-joint shaft from an earlier zj? or a tom woods type custom shaft? any help would be appreciated.

or possibly should i first have it aligned, to see if that corrects the possible pinion problem?

and, yes, ashley knew of my "habit" long ago. she thinks its cute. i knew i had made a wise choice when, on the way back from our wedding in memphis, we stopped to pick up my new scrambler half top. and she didnt mind a bit.

jason

[TJRON]

Thanks Mark.
I understand everything you said and I always did.
I really don't understand how the driveshaft on a Grand is configured but that shouldn't effect my reasoning which is and always has been, a gear change does not effect the balance of a drive shaft at a given RPM.

And YES I DO REALIZE IF THE SHAFT GOES FASTER, ANY VIBRATION WILL INCREASE!

When I try to analyze a problem I try to look at before and after. If I forget speed and keep the engine rpm where the drive shaft didn't vibrate before the gear change and duplicate this rpm after the gear change I would hope to get similar results. If I didn't, I would look to a problem with the gears or something wrong with the driveshaft besides an out of balance condition.
If I increase the rpm higher than what used to be ok and the shaft started vibrating I would look to an out of balance situation showing up with the gear change.

I'm glad Jason got it fixed, that's the important thing.

[TJRON]

Originally posted by bobgoblin


....it makes sense that a slight imbalance would become a bigger inbalance at faster speeds, but i can feel this vibration starting around 50 mph. before, i could take the zj up to ~85 and never noticed this vibe.......

[bobgoblin]

Originally posted by TJRON

oh goodness...what have i done?

[Darrell C]

Originally posted by bobgoblin
oh goodness...what have i done?

Don't worry bobgoblin, it happens quite often