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post #1 of (permalink) Old 03-02-2002, 09:53 PM Thread Starter
 
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On the track of vibrations (LEVE, please?)

OK, as many of you may remember I've been hunting down a vibration in my CJ-7 for a while now. First I thought it was driveline. Got that resolved, have EVERYTHING checked out, and am confident that it's NOT driveline related any more.

Setup: 83 CJ-7 with stock axle housings running Goodyear MTR 33X10.50's on AR767 steel rims.

Symptoms: Get a pretty severe shake between 60-70. Causes the shifter, steering wheel, and cage to shake. Above and below it's not as pronounced, or not there at all. No pulling to either side.

Done to date: Had wheels static balanced (one side of rim) and it got better, but didn't go completely away. Had the wheels dynamically balanced (both sides of rim) and problem changed from 50-60 to 60-70.

The guy at the tire place says it's probably bad steering stabilizer, swears tires/rims/alignment are good. The guys at the 4wd place say stabilizer is fine, but balance/tires/rims/alignment must be off.

I would greatly appreciate any sagely advice.

'83 CJ-7 nothing original but the tub and axle tubes
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post #2 of (permalink) Old 03-02-2002, 11:09 PM
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Re: On the track of vibrations, need advice

Remove the stabilizer and take it back to the front end shop. It sounds like tire balance if it just start vibrating on it's own. If it starts when you hit a bump, it's most likely front end alignment.

Check the bushings on the shock absorbers. They are not the problem, but if you find one worn much worse than the others, you have found which wheel is causing the problem.

I had a similar problem with my '55 T-bird, it just started shaking at 60 mph. I took it to a mechanic who was recommended by several people I worked with. They had all taken there cars to him and swore that he was great. Well, I take the T-bird up to him and the first thing he does is a front end alignment. When I get it back, I take it out on the interstate and no change.

The next day, I take it back and he does a wheel balance. When I pick up the car, he tells me that he did a static balance and then put them on the car and did a spin balance. He tells me that he had to add a half-ounce to the left front. Back out on the interstate and no change.

The next day is Saturday, so I decide to look into it myself. I notice that the left front shock absorber is loose and the rubber bushing is part worn away. I took the left front wheel off and found the problem right away. There were two pieces broken out of the lip on the brake drum. It looked like the previous owner had set a jack stand under the drum. There is no way that a static balance followed by a spin balance on the car would not have shown a problem.

Do a visual check yourself. Take the wheels off. Look at the rotors/drums. Check the tires for a small bulge indicating a possible slipped belt. I do 80 in my CJ all the time and I don't have a problem.




I've replaced so many parts that my old Jeep has become my new Jeep.
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post #3 of (permalink) Old 03-03-2002, 12:12 AM Thread Starter
 
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Re: On the track of vibrations, need advice

Good ideas Taz, will do those, but that brought several other items to mind:

1) New bearings within 5000 miles (they are installed properly and check out fine). Also replaced rotors and rebuilt front disks within 10K miles. Rears were rebuilt with new drums last weekend.

2) New ball joints, TRE's, swaybar bushings, and drag link within 5000 miles.

3) The stabilizer in question is no more than 5000K old, is visibly OK, with poly bushings that still look good.


'83 CJ-7 nothing original but the tub and axle tubes
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post #4 of (permalink) Old 03-03-2002, 12:25 PM
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Re: On the track of vibrations, need advice

I wasn't implying that the stabilizer is bad, even if it were, I can't see where it could be the problem. Just remove it so the front end shop can't blame it as the problem. The stabilizer is not really required for normal driving.

Did the problem start when you replaced the rotors? You may have gotten one that is out of balance. If you could find somebody that can balance check the spin balance the wheels on the Jeep, it might show where the problem is. Actually balancing that way is not a good idea because the brake drum/rotor is being balanced with the wheel and everything has to go back together in the same relationship to keep the balance.

I assumed that the front end shop would have found any problem with the tie rod ends, ball joints, etc., but I may be giving them too much credit.

How much weight was added to balance the wheels. More than 5 ounces on one wheel can be an indication of a problem.



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post #5 of (permalink) Old 03-05-2002, 02:52 PM Thread Starter
 
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Re: On the track of vibrations (LEVE, please?)

Leve, now that you're back in the saddle, I would really like to hear any sage words you can offer on diagnosing this thing... Any help would be GREATLY appreciated.

'83 CJ-7 nothing original but the tub and axle tubes
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post #6 of (permalink) Old 03-05-2002, 03:14 PM
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Re: On the track of vibrations (LEVE, please?)

You've replaced almost everything I can think of...so I'll keep this simple...I'm leaning towards a problem with the tires or rims because there is no pulling. Dynamically balancing a tire is great... if there's the correct weight distribution on the tire. Without the correct weight the balancing is rotational only without weight causing vector changes.

Have you noted ANY unusual tire wear?

Here's what I'd do first off...

1. Rotate the tires, front to back, left front to left back, etc.
2. Take the Jeep to up speed.
3. Did the symptoms change, or go away?

If so the problem's in the rims or tires. Now you need to narrow it down to one tire... so rotate one at a time back to the front... and see how the symptoms change. As for tires or rims... that can get touchy... and I'll have to think about this one a while...

If the vibration has not changed the problem's in the running gear and further isolation is needed.

Let me know what you find.

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post #7 of (permalink) Old 03-05-2002, 03:50 PM
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Re: On the track of vibrations (LEVE, please?)

When the alignment shop pronounced the alignment as good, did they tell you what the caster is? I suspect it might not be enough.

Have you checked the steering box, mounts and column for looseness?

Insufficient caster coupled with a little play in the steering can set up a cyclical shaking that can get pretty violent at the harmonic speed.

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post #8 of (permalink) Old 03-05-2002, 04:29 PM Thread Starter
 
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Re: On the track of vibrations (LEVE, please?)

Good points all Jim, to answer:

I believe the caster was out by 1-2 degrees (can't remember weather it was positive or negative). I may be wrong, but from what I have heard, caster affects general drivablity, and the thing drives like a dream, race-car tight except once the vibes start. But hey I'm more than willing to shim it if it'll help.

As for steering, I'm running the MORE HD bracket, with the brace, and a borgeson shaft, all replaced within the last 10,000 miles and tight. Is there something I should check in the steering column itself or from the firewall back (that's about the only thing that hasn't been replaced so far)?

'83 CJ-7 nothing original but the tub and axle tubes
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post #9 of (permalink) Old 03-05-2002, 05:16 PM
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Re: On the track of vibrations (LEVE, please?)

The '78 service manual calls for 3 degrees of caster. If yours is off 2 degrees, you could have one degree. The common recommendation on the board for Jeeps with oversize tires is 6 degrees. Caster provides a strong self-centering force to the wheels. Without enough, the wheels tend to wander. When the natural harmonic frequency matches up with the tire rotation frequency is when the cyclic shaking sets in.

In your steering, two more things to check are the joints in the steering column, one at the top of the box and one where it breaks out of the firewall. Also look for looseness in the box itself. Have a helper rock the steering wheel back and forth while you examine everything that carries the movement. When the steering wheel is rocked back and forth a half inch, you should see the road wheels moving slightly.

In 77K miles I have adjusted my steering gear twice, and it needs it again. To do it properly it must be removed and taken to the bench. Read up on the procedure before you try it. It's not difficult, but it must be done by the book. In addition to other tools, it requires an inch/pound torque wrench, which is not very common.

A little looseness allows the wheels to move slightly, and insufficient caster doesn't provide the self-centering to hold them in place. Then just a bump in the road or a small tire imbalance will set it off.

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post #10 of (permalink) Old 03-05-2002, 07:54 PM
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Re: On the track of vibrations (LEVE, please?)

My bet is on the tires. Either one or more of the tires is bad (way out of balance off the production line) or the guys at the tire shop don't know what they are doing. I have had this problem BAD on my 94 F350 Ford Crew. I would have Les Schwab balance the tires and the problem would come right back. The next day I would have to take the truck back to the tire shop. A different tire jocky would rebalance them and tell me just how out of balance they were. The second rebalancing would fix the problem for anywhere from a week to two months then it would be back to the shop for another balance. The last time I had to do this I had the manager of the store balance them personally and all has been good for about six months.

I would take them back to the shop you got them from and lean on them very hard. The shop should keep balancing them for free or at least get the tire factory rep to check it out. (Try yelling "Firestone" at the top of your lungs a few time. That should get someones attention.)

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