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Scary death wobble - caused by bump steer?

NATEandALLI

Jr. Member
Joined
Jun 25, 2016
Messages
112
Background - Been resto-modding my wife's 73 Bronco. It's a street cruiser so we converted it to 2wd. Sold the D44 and got a free D30 which we cut the center section out of. We tied the two axles halves together with some 3/8" wall tubing that slid inside the axle tubes then we capped it off with tubing that was the same od as the axle. Rebuilt the kingpins and got them in spec with the pull test.

Up front we put on an old K bar S lift. I think it's either 2.5 or 3 inch. Was a complete setup with matching caster bushings. When we bought the Bronco, it had a funky stink bug stance so I think it had HD rear springs so we didn't even need to lift the rear at all. With the front lift and stock rear springs, the Bronco sits very evenly.

Got it running and driving a few weeks ago. Took it for it's first drive around the neighborhood and noticed a bit of a shimmy in the steering when I hit a small bump. I dismissed it and figured that if that was as bad as it was going to be then it was fine. Well I hit a slightly bigger bump going a little faster and we got some mega death wobble. Pretty big let down to have DW after doing so much work but we're determined to do something about it.
 
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NATEandALLI

Jr. Member
Joined
Jun 25, 2016
Messages
112
More suspension info. We have an adjustable front track bar. I'm pretty sure it's something someone made. Looks like they just cut off the end of a stock bar and tapped the inside of it and threaded in a bushed end. I roughly eyeballed centering the axle a while back but we'll get out a tape measure and get it correctly centered soon. One other thing about the track bar is I just tightened the nut on the axle "good-n-tite" but didn't actually torque it - torque spec is something like 150 ftlbs?

We took the Bronco to local alignment shop. I thought it visually has too much positive camber but it's actually in spec. The only thing out of spec was toe but the toe adjuster was frozen - it's being replaced. The caster spec seems low to me but it's in spec. I would attach the alignment results but I guess I can't.
 
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Digger556

Sr. Member
Joined
Aug 8, 2013
Messages
793
Background - Been resto-modding my wife's 73 Bronco. It's a street cruiser so we converted it to 2wd. Sold the D44 and got a free D30 which we cut the center section out of. We tied the two axles halves together with some 3/8" wall tubing that slid inside the axle tubes then we capped it off with tubing that was the same od as the axle. Rebuilt the kingpins and got them in spec with the pull test.

Up front we put on an old K bar S lift. I think it's either 2.5 or 3 inch. Was a complete setup with matching caster bushings. When we bought the Bronco, it had a funky stink bug stance so I think it had HD rear springs so we didn't even need to lift the rear at all. With the front lift and stock rear springs, the Bronco sits very evenly.

Got it running and driving a few weeks ago. Took it for it's first drive around the neighborhood and noticed a bit of a shimmy in the steering when I hit a small bump. I dismissed it and figured that if that was as bad as it was going to be then it was fine. Well I hit a slightly bigger bump going a little faster and we got some mega death wobble. Pretty big let down to have DW after doing so much work but we're determined to do something about it.


First and foremost, you need a 4-wheel alignment to determine where you are at.

If you are getting shimmy at low speeds, you likely have a caster issue. With good suspension geometry and alignment, these trucks will cruise at 85mph and feel confident.
 

Timmy390

Bronco Guru
Joined
Jan 1, 2011
Messages
5,609
Loc.
Conway, AR
I'm confused as hell as to what you did to that D30 and why?

If I read your post correctly, you took out the center section and welded the axles and housing together using tubes? Why on earth?

I fought bump steer for years......fixed it with welding plate on the frame to mount the steering box, added solid in frame spaces at the steering box. Added 7 degree bushings, new steering box all now tie rods and ball joints.

70-75mph no issues.....

Tim
 
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NATEandALLI

Jr. Member
Joined
Jun 25, 2016
Messages
112
More info. Steering box is a rebuilt unit that's been converted to 4.25 turn. From best I can tell, everything seems fine with it. We are not using a steering damper. It seems the general consensus is that we shouldn't need one. Alignment guy suggested adding one but I don't like the idea of masking a steering/suspension problem with a damper.

We're using a stock pitman arm and the track bar mounting points are stock. Alignment of track bar to steering drag link doesn't seem that bad. I can get a pic of it this weekend. I'll also check that the upper track bar mount hole isn't blown out and check that the lower mount bolt isn't loose.

Wheels are US Mags Indy Slot Mags. 15x9, -12 offset, 4.5" backspace. Tires are brand new Falken 33x12.5x15.
 
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NATEandALLI

Jr. Member
Joined
Jun 25, 2016
Messages
112
I'm confused as hell as to what you did to that D30 and why?

If I read your post correctly, you took out the center section and welded the axles and housing together using tubes? Why on earth?

I fought bump steer for years......fixed it with welding plate on the frame to mount the steering box, added solid in frame spaces at the steering box. Added 7 degree bushings, new steering box all now tie rods and ball joints.

70-75mph no issues.....

Tim

Thanks for the info on being a contributor.

As for cutting up a D30 (which was blown up BTW) we converted the Bronco to 2wd - how else would we have done it?
 

Timmy390

Bronco Guru
Joined
Jan 1, 2011
Messages
5,609
Loc.
Conway, AR
More info. Steering box is a rebuilt unit that's been converted to 4.25 turn. From best I can tell, everything seems fine with it. We are not using a steering damper. It seems the general consensus is that we shouldn't need one. Alignment guy suggested adding one but I don't like the idea of masking a steering/suspension problem with a damper.

We're using a stock pitman arm and the track bar mounting points are stock. Alignment of track bar to steering drag link doesn't seem that bad. I can get a pic of it this weekend. I'll also check that the upper track bar mount hole isn't blown out and check that the lower mount bolt isn't loose.

Wheels are US Mags Indy Slot Mags. 15x9, -12 offset, 4.5" backspace. Tires are brand new Falken 33x12.5x15.

I don't use a steering damper. Just no need for it IMO on my rig.

I do run a drop T-bar bracket and drop P-arm.......

Tim
 
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NATEandALLI

Jr. Member
Joined
Jun 25, 2016
Messages
112
Timmy, thanks for the info. Besides ours being 2wd, your combo is very similar to ours. 2.5" SL, 1" BL, Mustang EFI 5.0, etc. We're running an AOD with 4.56's.

This is my wife's cruiser Bronco. It'll never see dirt. We decided to do a legit 2wd conversion. Her dad and I are hot rodder/racers so it's in our nature to do stuff like this. I don't expect a dyed in the wool 4x4 Bronco guy to get it but it's what we wanted to do.
 
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NATEandALLI

Jr. Member
Joined
Jun 25, 2016
Messages
112
It looks like you could use a couple more degrees of caster.

I feel the same way. I was honestly shocked to see that the caster spec is so low. In my experience, a typical caster value is something like 4° so seeing the spec being 2.5° was surprising. On early Ford hot rods (Model A's and stuff), it's common to run 7° of caster. I run 22° on my race car but that goes 220 mph so not all that applicable here.

Any other votes for more caster? My only concern with too much caster is that too much can also induce death wobble but "too much" in this case is usually a lot - like over 15°. What would be an advisable amount of caster to shoot for?
 

Timmy390

Bronco Guru
Joined
Jan 1, 2011
Messages
5,609
Loc.
Conway, AR
Timmy, thanks for the info. Besides ours being 2wd, your combo is very similar to ours. 2.5" SL, 1" BL, Mustang EFI 5.0, etc. We're running an AOD with 4.56's.

This is my wife's cruiser Bronco. It'll never see dirt. We decided to do a legit 2wd conversion. Her dad and I are hot rodder/racers so it's in our nature to do stuff like this. I don't expect a dyed in the wool 4x4 Bronco guy to get it but it's what we wanted to do.

Different stroke.......all good.....

So what kind of air pressure you running? If over 30psi, lower it to 28-30. Seems to the be "butter" zone for most.....

Tim
 

Timmy390

Bronco Guru
Joined
Jan 1, 2011
Messages
5,609
Loc.
Conway, AR
Interesting to say the least.

Those steering angles are STEEP. That's going to cause some issues......

Tim
 
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NATEandALLI

Jr. Member
Joined
Jun 25, 2016
Messages
112
They look especially steep in those pics because that was before the engine/trans went in. They've calmed down a bit. I'll measure the angle this weekend.

I see you've added an adjustable drag link - would that be a worth while upgrade for us?
 
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