Death Wobble

[Brian]

Hi all,

My 38 has developed a 38-43 MPH 'Death Wobble' this year. I have rebuilt the steering box and all the steering parts a few years back with very few miles on the rebuild. The tire alignment was done per the repair manual specifications. There is very little play in the steering wheel and drag link through the pitman arm.

The jeep tracks straight and works great then at approximately 38 MPH- 43 MPH I get a bad wobble, that goes goes away as it accelerates into the 45 MPH + speeds.

I know these jeeps are designed for slower speeds but occasionally I need to stay in the 50+ MPH where I drive it and my wife and kids drive it on occasion so I want to be safe.

Any help would be appreciated.

Thanks- Brian
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Brian '51 M38 USMC

[idiocrates]

I would say if there are some "parts" of the steering system that you've recently reworked it would be wise to consider those other parts that got little to no attention....such as the wheel bearings, king pins, and king pin bearings....as being suspect. From what I read here safe maneuvering is the by-product of a totally maintained system....not random replacement of parts.
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Jim

***********************
'?? - M38A-1

[Brian]

The front wheel bearings/races are new along with all of the ball joints, but I did not replace the king pins/king pin bearings yet, when disassembled they were not pitted and the races were not worn so I regreased and put it back together the way it came apart. If I replace the bearings and pins the whole king pin shim thing scares me a bit as I am not sure how to properly measure for the shims.

The jeep appears to have had low miles on it when it was released from military service so I made an assumption that the king pins were relatively good- my mistake.

I guess that you can never make any assumptions with these vehicles....

Has anyone had trouble replacing the king pins/bearings?
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Brian '51 M38 USMC

[MODIFIED]

Aside from the above mentioned, Check the balance of both front tires.
Did you opt to re-use the old knuckle seals when you serviced the existing king pins?
Other aspects to consider:
U-bolt to axle loose
Worn leaf spring bushings
Loose lug nuts
Tire pressure
Tire wear
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Always Looking for M170 Parts and bits

[wesk]

Sometimes a professional front end inspection will quickly ID the source of the problem and save you a lot of shot gunning troubleshooting time and material loss.

I've been at this for 50 years and a quick check of a closed knuckle 4wd front end is very simple and takes very little time and no special equipment.

The list of causes for the death wobble is very short.

1-King pin bearing wear resulting in a looseness that becomes very apparent at certain speeds and when aggravated by a bump. Usually a hard shot on the brakes will stop it. Occasionally you have to come almost to a stop to stop it. To inspect their condition you simply raise the jeep and grasp the top and bottom of the tire and rock it. If it moves fix the king pin bearings.

2-Heavy steering component looseness. Usually the tie rods and bellcrank will show their looseness all the time. It is the drag link and pitman arm and steering gear box that can hide a loose issue and then let it suddenly appear at certain speeds or after a bump as a death wobble. Again raise the jeep and lock the steering wheel down and check all components for looseness. Fix the worn part.

3-You hear balance brought up a lot but balance does not cause death wobble. An out of balance condition will result in wheel hop (vertical motion) that is transmitted as a hopping sensation though the floor. It is tire tread and sidewall damage that can cause a wobble tracking of the tire at certain speeds being more pronounced then others. This can be tough to isolate and generally swapping in a pair of known good front tires is the best way to troubleshoot this problem.

Remember when the tires are on the ground you should only be able to rotate the rim of the steering wheel 1/2 to 3/4 " side to side with no movement of the tires.
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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php

[skyjeep50]

King pin bearing pre-load is an important element of eliminating wobble. You don't need to measure shim thickness when setting the pre-load, only the force required to move the knuckle. Refer to page 257 of TM9-8014 for a pic of how it is done ("Downloads" section of this website). That's the manual for a M38A1 but the M38 manual leaves out some of the details needed to set pre-load. However, if you refer to a civilian maintenance manual for the CJ3A from which the M38 was developed - it addresses pre-load in much the same manner as the M38A1 manual - I'm not sure why the difference. Pre-load is measured without the knuckle seal in place so with a new seal installed, the force required to move the steering knuckle will be even greater. It's really quite stiff when moved by hand. That resistance to movement is what helps dampen any tendencies for wobble.
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1951 M38

[artificer]

Most of this is pretty easy:
1. With wheels on the ground have someone rock the steering wheel back & forth while checking for steering box slop, tie rod ends, bell crank etc.
2. Jack each front wheel [in turn] off the ground, grasp tyre top & bottom. Try to rock in & out.
If movement....you can have play in wheel bearings or king pin bearings.
3. Have helper apply brakes & what happens to the in & out play?
3a. Elimated or reduced...look @ wheel bearings....so adjust
3b. No difference or after adjusting wheel bearings any remaining movement is king pin bearings.

Wheel balance is as Wes says but being out will exacerbate other problems associated with "wobble".
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John GIBBINS
ASE Master Medium/Heavy Truck & Auto Technician 2002 USA
Licensed Motor Mechanic NSW # MVIC 49593 Current
YOU CAN'T TROUBLESHOOT WHAT YOU DON'T UNDERSTAND

[Brian]

Thanks for the information. I will follow the advice and I really appreciate the instructive posts.

Thanks-
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Brian '51 M38 USMC