Death wobble continues..

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Phils67
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Re: Death wobble continues..

Postby Phils67 » Wed Jun 13, 2018 3:50 am

I can't find anything loose. The box is older, it came out of the XJ Cherokee donor when I swapped the 4.0 in. The steering is smooth feeling at the wheel. The steering shaft is a double U joint style and it has a tilt column, both of which aren't that old from Flaming River. I have to check caster angle like Stuka recommended and go from there. It makes sense the more i think about it being how the front tires flail around when it happens. It seems more wheel end related than suspension related.
1967 J2000/ 4.0MPI/T18/D20/35s

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tgreese
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Re: Death wobble continues..

Postby tgreese » Wed Jun 13, 2018 6:50 am

Where I first heard "death wobble" was not in ref to leaf-sprung Jeeps but instead to lifted and big-tired XJs. It's a resonant condition that is AFAIK set off by road conditions. In the XJs, this results in the wheels rapidly slamming back and forth, typically so frightening that the operator slams on the brakes and pulls over. With the introduction of the open knuckle axles, the Jeeps of the day could have wobble that was felt in the steering at boulevard speeds. This was an annoying but not frightening condition. Jeep's solution was to fit a steering damper on warranty. Eventually all the open knuckle Jeeps came with a damper to prevent this mild resonant condition.

Closed knuckle Jeeps could have wobble, but it was (again AFAIK) attributed to worn parts. A closed knuckle axle should have enough friction to damp out any inherent wobble. And I'm pretty sure the steering geometry changed a bit with the introduction of the open knuckle axle.

I know that the open 30 is a popular upgrade for early CJs that came with the closed 25 or 27. Both thoise and the lifted later CJs seem to function best with somewhere around 5 to 7 degrees of positive caster. This is a bit more than the factory spec of 3-4 degrees. The FSJs in '76 are supposed to be 4-5 degrees. Note positive caster is with the top of the pivot trailing, like a chopper bike (which has huge positive caster).

Also, I suspect enough negative caster could make a death wobble. Consider the shopping cart caster that flops back and forth - that is from quite a lot of negative caster.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 FCA Jeep Cherokee KL Trailhawk, ECO Green

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Phils67
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Re: Death wobble continues..

Postby Phils67 » Wed Jun 13, 2018 7:21 am

Ive owned countless XJs, all had death wobble and all were a bad track bar. In the case of my 67 J2000 there is no track bar due to it being leaf sprung. The favorable opinion of the majority seems to be castor related. My digital angle finder is in my car, tonight i will check the angle and report back. My driveway is almost perfectly flat. Should give me decent readings. Just looking by eye this morning before work i wouldnt be surprised if it has negative to zero castor, but time will tell. I am itching to see the reading, 5pm cant come fast enough
1967 J2000/ 4.0MPI/T18/D20/35s

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Re: Death wobble continues..

Postby SJTD » Wed Jun 13, 2018 7:53 am

If the ground isn't flat you just need to add/subtract its angle from the caster reading.

Oh, and I used to get the wobble in my '65 Valiant on the freeway once in a while. Or was it a Dart?
Sic friatur crustulum

'84 GW with Nissan SD33T, early Chev NV4500, 300, narrowed Ford reverse 44, narrowed Ford 60, SOA/reversed shackle in fornt, lowered mount/flipped shackle in rear.

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tgreese
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Re: Death wobble continues..

Postby tgreese » Wed Jun 13, 2018 8:17 am

Is there a track bar on the later GWs? Pretty sure there is a sway bar but no track bar. Later CJs have a sway bar too. Leaf spring suspension should hold the axle securely against lateral movement, unlike coils. I imagine that death wobble for coil sprung systems comes from the lateral compliance of the coils, plus some small imbalance wobble or bump wobble that builds. This ends up "steering" the tire back and forth (ie steering points straight, axle moves back and forth), which builds rapidly.

There's a post on IFSJA that claims you can't have a track bar on a passenger drop axle. http://www.ifsja.org/forums/vb/showthread.php?t=185419
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 FCA Jeep Cherokee KL Trailhawk, ECO Green

RamJetFSJ
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Re: Death wobble continues..

Postby RamJetFSJ » Wed Jun 13, 2018 9:12 am

Later GWs had a trackbar front and back, and a front sway bar.

A worn track bar bushing will definitely cause death wobble in a coil spring setup, as there is nothing keeping the axle from moving side to side. On leaf springs, the leaf springs keep the axle from moving side to side.
80 Wag in 73 attire, Ram Jet 350 power

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Phils67
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Re: Death wobble continues..

Postby Phils67 » Wed Jun 13, 2018 10:16 am

I just went home on my lunch break and checked that angle. With the ground completely level giving me a zero-degree reading directly in line with the center of the wheel it shows that my Caster angle is positive 1.75 degrees. With my lift and 35 inch tires I think I am going to have to shim the diff now to somewhere between 5 and 7 degrees and see how it responds. Guess now is a good time to do the U-bolt flip I have been wanting to do as well.
1967 J2000/ 4.0MPI/T18/D20/35s

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Stuka
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Re: Death wobble continues..

Postby Stuka » Wed Jun 13, 2018 10:31 am

Positive caster will definitely cause what you are seeing. Just to make sure, when you placed the degree finder on the axle, negative degrees were to the front, and positive to the rear? If you had it flipped, it means you had 1.75 degrees of negative. Which is better than 1.75 positive, but still bad.

The above was backwards as pointed out below. This is not the first time I have had it backwards in my head :)

Basically you want to the axle to be rolled back, which points the pinion down some. Bad for drive shaft angles (you should be fine), but good for drive-ability.

Lots of company sell shims. But you may need to find a spring pin with a longer centering pin. Some shims come with a new pin.
1981 J10 AMC 258 with HEI and Weber 32/36 DGEV Carb, T176, NP208, D44/M23 with 4.56 gears and Detroit Locker, 6" lift, 33x12.50 Cooper STT's, BJ's Offroad Rear Dozer Bumper.
2017 JKU Rubicon Lifted
Pevious Jeeps: 1975 Cherokee, 2008 JK, 2005 KJ, 1989 XJ

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tgreese
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Re: Death wobble continues..

Postby tgreese » Wed Jun 13, 2018 10:53 am

Pretty sure you have it backwards ...

Image

Positive caster is what you want.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 FCA Jeep Cherokee KL Trailhawk, ECO Green

weeegoneeer
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Re: Death wobble continues..

Postby weeegoneeer » Wed Jun 13, 2018 11:16 am

Caster makes a big difference - I had zero caster on my last jeep and it was a challenge to keep on the road. My caster angle changed after to PO put a mild lift on it. Think about the wheels on shopping carts - the reason they always point in one direction is that the center point of the wheel is offset.

A quick way to tell if your caster is really bad is if you let go of the wheel when in a turn, does the steering wheel want to return to center, or will your truck continue making a turn with hands off the wheel?

I could take exit ramps hands-free. After correcting the caster it was definitely better mannered.
1984 GW

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Stuka
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Re: Death wobble continues..

Postby Stuka » Wed Jun 13, 2018 2:20 pm

tgreese wrote:Pretty sure you have it backwards ...

Image

Positive caster is what you want.


Yes, you are right. Not the first time I have gotten it backwards. I have edited my above post.
1981 J10 AMC 258 with HEI and Weber 32/36 DGEV Carb, T176, NP208, D44/M23 with 4.56 gears and Detroit Locker, 6" lift, 33x12.50 Cooper STT's, BJ's Offroad Rear Dozer Bumper.
2017 JKU Rubicon Lifted
Pevious Jeeps: 1975 Cherokee, 2008 JK, 2005 KJ, 1989 XJ

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Phils67
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Re: Death wobble continues..

Postby Phils67 » Wed Jun 13, 2018 4:50 pm

So my solution, if everyone here will agree, is to buy some shims (6°?), cock the diff backwards (pinion toward the ground), drive this thing over bumpy roads like i just stole it, and report back? I am fine with that if you allthink it will be beneficial. I am leaning towards that, i am just unsure of what degree shim to get. 6 degrees seems like itll lay it back and definitely solve the issue if thats the cause.
1967 J2000/ 4.0MPI/T18/D20/35s

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Stuka
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Re: Death wobble continues..

Postby Stuka » Wed Jun 13, 2018 5:26 pm

I think 6 may be too much. That would give you almost 8 total. I think 4 degree shims would work fine.

Also, make sure they are steel shims. You do not want the aluminum ones, I have seen them break in half. And if your centering pin doesn't stick through enough to go into the axle, you will need to get a longer one.
1981 J10 AMC 258 with HEI and Weber 32/36 DGEV Carb, T176, NP208, D44/M23 with 4.56 gears and Detroit Locker, 6" lift, 33x12.50 Cooper STT's, BJ's Offroad Rear Dozer Bumper.
2017 JKU Rubicon Lifted
Pevious Jeeps: 1975 Cherokee, 2008 JK, 2005 KJ, 1989 XJ

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Phils67
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Re: Death wobble continues..

Postby Phils67 » Wed Jun 13, 2018 5:34 pm

Ill start with 4 then. Ordering them as soon as i finish posting this reply
1967 J2000/ 4.0MPI/T18/D20/35s

RamJetFSJ
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Re: Death wobble continues..

Postby RamJetFSJ » Thu Jun 14, 2018 7:01 am

An oversized tire doesn’t require as much caster as a stock size, so yes, I think the 4* is a good starting point. Be sure to get steel, and the kind that boys to the spring pack with a new center bolt. You will need to use some big c-clamps to hold the spring pack together when you remove the center pin.
80 Wag in 73 attire, Ram Jet 350 power

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tgreese
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Re: Death wobble continues..

Postby tgreese » Thu Jun 14, 2018 7:25 am

Spell check? 8-) "bolts to the spring pack"

You may need new spring center bolts to draw the pack together. Napa has them. Note you're also supposed to replace the u-bolts ... one time use only. Nominally.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 FCA Jeep Cherokee KL Trailhawk, ECO Green

RamJetFSJ
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Re: Death wobble continues..

Postby RamJetFSJ » Thu Jun 14, 2018 8:15 am

Sorry. I NOMINALLY use spell check...
80 Wag in 73 attire, Ram Jet 350 power

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tgreese
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Re: Death wobble continues..

Postby tgreese » Sat Jun 16, 2018 7:13 am

Amused, not criticizing. Figured the spell checker changed it for you from "bols" or such to an actual word.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 FCA Jeep Cherokee KL Trailhawk, ECO Green

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Phils67
Posts: 339
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Re: Death wobble continues..

Postby Phils67 » Sun Jun 17, 2018 6:39 am

Ordered the 4° shims for my current axle and welded the spring perches on my future axle swap since my brain is in castor mode. I zero'd out the knuckles and welded the perches 4.5° positive. The new axle is open knuckle so hopefully any variances can be made with the adjustment of the ball joint sleeves. Does that sound correct? I decided to keep my post mount springs up front when i do the swap and went SOA as well. I plan on using the 16.5 steel wheels with a 34x9.5 or 35x10.5 tire so hopefully i wont have turning issues with the wider suspension style on the open knuckle. I know i have questioned it alot but thats where i am at as of today. Anything here sound like itll be an unresolvable issue?
1967 J2000/ 4.0MPI/T18/D20/35s

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Stuka
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Re: Death wobble continues..

Postby Stuka » Sun Jun 17, 2018 10:31 am

That *should* be ok provided the shims correct the issue with your current axle. And I am pretty sure they will.
1981 J10 AMC 258 with HEI and Weber 32/36 DGEV Carb, T176, NP208, D44/M23 with 4.56 gears and Detroit Locker, 6" lift, 33x12.50 Cooper STT's, BJ's Offroad Rear Dozer Bumper.
2017 JKU Rubicon Lifted
Pevious Jeeps: 1975 Cherokee, 2008 JK, 2005 KJ, 1989 XJ


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