Technical The search for a cure-Death Wobble SOLVED!

By the way @Roothawg , that's how you supply pics. Great job.

 

Hey Root! Thanks for the photos! [it helps]

It is "shackle wobble" Your photo [I attached ] shows the shackle angle as near vertical when the suspension is hanging
When it is on the ground and loaded, the arc of the spring lengthens and the shackles are even more vertical.


Either shorten the spring or add a panhard bar .

With a cross steer you need the shackles almost horizontal
This is because one side arcs downwards with suspension compression and the other side stays horizontal [stabilizing the axle laterally]

 

It is actually "Kerrynzl" here helping via using my wife Mimi's account.
It is about time Ryan "un-banned" me.

Mimi is the geometry / maths expert here

I was banned over posting some vintage race car photos that caused a shitload of drama with one member who claimed he had received threats , [which were not from me]

 

MIMILAN,is right,
Your biggest prob *is axle shift do to shackle angle{ lets wobble},the spring is too long,so=you must add a front axle to frame panhard bar*/or fine a shorter spring/so shackles fully loaded are very close to 45*< that harder to do and not needed if you add panhard bar..< that is the root of prob,the little bit of scrub can be OK,if it was bigger scrub then I see in pic. it too would then be a prob.

 

@Roothawg
Also the shackle shock mounts are making it worse [actually triggering shackle wobble]

In this photo the force and resistance is the blue arrows [the shock is creating a pivot through resistance]
The shackle pivots "or cantilevers" around this shock mount creating lateral movement [red arrows]


The shock needs to be mounted on either the upper or lower pivot [depending on available travel]

 

Thanks. I can’t believe a Chassis company would design something like this. Ugh. I’m a suspension novice but I trust that when I buy something from a reputable chassis company, you hope they have it figured out.

 

I have a shorter spring btw. It’s a CE front spring, not reversed. It usually makes it stand up like a Jeep though.

 

Would something like this work better?
https://www.speedwaymotors.com/Shackles-with-Shock-Stud-Mounts

 

undoubtedly..

 

They are only a component manufacturer!! And rely on you being the Chassis manufacturer.
That is how the "kick the can down the road" [liability wise]

They make them to fit [thats all] You purchased them as it was a solution to shock placement .

Yes. and preferable have the shock mount on the Axle / upper side if possible.

Keep your longer spring because your ride height is already set where you want it.
And add a Panhard bar which is preferable with cross steer

 

Roger that.

 

I have had bad death wobble in 2 previous cars that showed up when you hit a bump at a certain speed. both times the problem was cured by reducing caster. More than 7 degrees in a street car is asking for death wobble in my experience. I have front shocks mounted to shackles on my current driver with no issues.

 

^^^^^

 

I had those same shackle/shock mounts on my 40 and it drove great. Didn’t have a pan hard or a steering dampener on it either

 

I have wondered about it. I have it mounted higher than the CE plates that normally locate under the tranny pan. Not sure if I am measuring correctly, but I don’t see a lot of options. I have a magnetic digital angle finder. I may have to pie cut the bones to take some out of it.

 

Adding a panhard bar would be the best answer. Also getting some wheels with less offset would help too. I bet after the panhard is installed, the long spring and vertical shackles will not be a problem.

If you have enough room in between the "speed holes" that somebody drilled in that poor axle, you could easily install a bung to take one end of the panhard. Or attach a couple tabs to the inside of the wishbone. Then a simple hanging bracket down from the frame rail to mount the other end. Super easy to do with the truck up on the lift like that.

While you are under there, check something out for me. Back the jamb nut off the rod end on the wishbone, and see if you can wiggle the threaded end. It doesn't look like you have a lot of threads into the tube. Hopefully that spot isn't a loose point.

 

Will do. I was thinking of moving the rod end deeper in the bones and moving the bracket up and forward a bit.

 

My 39 has brackets on the frame and bones where a panhard bar once resided. It drives pretty good but I’m tempted to put one back just to see what happens. Curious to why someone would remove a panhard bar.

 

Root, you mentioned it happens at 35 mph, like clock work! This statement leads me to believe you have a balance problem in your rotating assembly! Check for wheel run out and total wheel/tire/brake disc balance. It ‘s a whole lot easier and cheaper than buying and swapping parts!






Bones

 

Well, to be honest, it started at 35 only, then it was 35 and 45….then 35,45 and 60. Getting progressively worse as time went on. The last long road trip I did, I thought I was gonna have to find a lift somewhere.

 

Don't let the illusion of suspension links fool you .
If you ran a string line from the centre of the balljoint to the centre of the spindle ,that would give you a better view of the arc the front suspension moves on.
So your bones can be lower at the front, but the dropped axle will cause the wheels to arc backwards.

This is a common problem/illusion with novices when fabricating a Panhard Bar! They bend it like a "J" bar thinking it is now horizontal [this is common on lifted trucks]
In fact some component manufacturers make bent Panhard bars ["J" bar] BUT it is the Chassis manufacturer that needs to use it correctly.

The purpose of a J-bar is to go around components [like a driveshaft] and still have a normal arc between the pivots .
It is not for keeping the Panhard bar horizontal on a Redneck lifted truck [as I've seen plenty of times]

So when it comes to making/installing a front Panhard bar , the first thing you should do is draw some crosses on the centres of the drag-link ball joints , Then tape a string line from centre to centre.
This will give you a true picture of how the drag-link arcs.
You want your Panhard bar to be close to parallel as possible to the drag-link, and if possible the same length [this eliminates mechanical bump steer]

Three things contribute to your death wobble .
1: Shackle angle being to vertical [so there is no lateral control]
2: Split bones so the whole "bones and axle combo" swivels laterally like a parallelogram
3: cross steer which changes lateral movement into steering angle.

A panhard bar will address all ^^^ of these in one go!

Note: WE use "blue-tack" and string to dummy up any suspension links. Even after doing drawings and calculations.

 

I'm guessing its getting worse because a geometry problem of some sort is wearing the tires improperly , the More they wear , the worse it gets , at least that's what happened on mine ...

 

I think things were getting looser as the wobble continued. The more it wobbles, the more things back off adding more movement that adds to the wobble. A vicious circle.

 

Do you have some wheels with less offset you can swap on for sh!ts and giggles? Just to see if yours are the main problem.

 

Are we talking about a TRUE low speed death wobble, or just a severe vibration at different speeds?
I had a true death wobble occasionally in my old Sedan Deliv. and when I would hit an un-even bump at about 20 mph, the steering wheel would shake so violently that you couldn't hold on to it. The only way to stop it was to slow down. That's a true low speed death wobble. Is that what's occurring?

 

There are many things that trigger suspension oscillations [wheel offset, scrub radius , caster, wheel balance, tyres out of round etc etc]

In normal situation the chassis will dissipate these oscillations.
Driving opposite lock on a dirt road is a good example!
The steering will "tram-line" through all the ruts and you certainly feel it in the steering wheel [but it will self correct]
My O/T race car has a lot of "roll understeer" which is good for side-bite on short tracks. It tram-lines terribly on bumpy straights but it also self corrects [I can take my hands off the wheel at 170 mph]

@Roothawg 's problem is his car isn't self correcting but manifesting itself into a worse situation. [the remedy can be rectified with engineering.]

Root, I am going to suggest a little bit of "applied research and development" [aka "let's try it and see]
You will need access to a small piece of angle steel [probably 2"] and a drill press.
You will need to sit you car on the ground at ride height [and measure]

What we need to do , is make a "Temporary Dead Perch" for the passenger side. Then go and try it.
You do this by locking these 2 bolts together [below] with a piece of angle steel [flange down? at the shackle bolt and horizontal for the bone bolt and drilled to capture both bolts]
Just measure carefully and drill 2 holes



You will end up with a little bit of "bump steer" because the R/H half of the leaf spring has a shorter Arc than the drag-link. But it should eliminate shackle wobble.

Try that before you dump $$$$ into a panhard bar.

 

Kind of hard to solve a problem without knowing exactly what the real issue is.

Just trying to determine whether the OP is dealing with a severe vibration/oscillation (at various speeds) or a true low speed (rip the steering wheel out of your hands) non-correcting death wobble oscillation.

 

Yes, trust me…..the Fly also has the death wobble after a burnout. It’s the same.

Once I hook up the steering, I will drive it without the stabilizer to see if it has changed anything. I do have a new steering box and new shocks, and tightened the shock mount brackets, and added a leaf to the spring pack.

At least that way, we will know if any of those fixed the issue. If not, we start one at a time until we cure it.

 

Excellent plan! You should always cure the problem before adding the stabilizer for insurance!






Bones

 

Ok, update on the death wobble.
I finished installing the steering and took it out for a test drive.

The results are a definite improvement. About 2/3 of the wobble is gone. It’s migrated from 35 to 40 mph but the other speeds are gone. It’s bearable now. I will continue to try and improve the issues.

One thing is for sure. The Bilstein shocks are worth the money! Drastic improvement in the ride! Never buying any other shocks going forward.

I will have to work on the scrub issues and Panhard bar. I think Kiwi Kev is right, it prolly has too much caster.