Technical The search for a cure-Death Wobble SOLVED!

What everyone is calling "death wobble" I always called caster shimmy. We ran 10 - 12 degrees positive caster on the sprint car once we went to power steering. While running slowly around the track, I could play with the steering wheel a little and get a side to side caster shimmy. Turning the wheel a bit to load the right front would stop it. It was a combination of a lot a caster and low power steering pump output. I never once felt a caster shimmy at speed.

 

Thanks for chiming in!
That is a good example of what I'm saying.
If you let go the steering wheel ,a sprint car should turn left [because of LR / RF weight jacking is loading the caster on the RF]
A well set up sprint car should be "carrying" both front wheels at speed so most shimmy goes away.

One thing a sprint car does have is a front panhard bar so they don't get "shackle wobble"

One question... Do you run a steering damper? Or is the power steering enough

 

Hi Root, Check how the tie rod seats in the steering arms. I don't know if it was just a cheap steering arm, but the taper in the steering arm wasn't cut right, I had to use a tapered reamer to clean up the steering arm so the tie rod taper seated well in the steering arm. The tie rod was nice and tight but if it still didn't seat right in the steering arm, and had enough deflection for death wobble.

 

Hey man, good to hear from you. Been a while. I was looking at the arm thinking exactly that the other day. It didn't have much taper at all. I thought I had installed it backwards, until I took a closer look. It drew up tight, but it did make me scratch my head.

 

You can also sneak up on the taper with a unibit.............yes it works

 

No steering damper, the power steering took care of any shimmy. An early car with power steering was the dirt champ car that Don Branson drove. I've been told that you could actually see the front wheels doing a dance at speed, but Don never felt it in the wheel.

My first sprint car was a spring front much like an old Ford, but we only ran a shackle on one end of the spring (right side) and no panhard bar. That car didn't have power steering so we ran in the vicinity of 5 degrees caster and never experienced any shimmy. One time after rebuilding the car from a front end wreck, I didn't have the level I normally used to set caster. I eyeballed the caster and put way too much in. With a Hallibrand 6:1 steering gear, it felt like I was lifting the whole car going into a corner. I backed a lot of caster out after the first heat.

 

Not well though. Full contact is better. If a fella is going to be building old Fords, he either ought to have his own 7 degree taper, or know someone who does.

 

Actually @SamIyam and I have both posted this in the past. You WILL get a full taper even tho a unibit has steps. When I did the last one I merely touched up the hole with a carbide bit

 

Those plates/tools for measuring toe in, I'm not a fan; if you watch that video that Moriarity posted you'll see that the guy does not make his measurement with the tape out at the furthest points of the tires. That will change the measurement. Marking the tires in the center an measuring at the 3 & 9 o'clock positions will give you the correct measurements, but as @Boneyard51 mentioned above, you usually cannot get a tape measure into position to make the measurement on the rear side and on some cars not even on the front.

As I recall @Blue One made a tool for checking toe in that looked like it would be not too difficult to fabricate, and should give excellent results. Blue one, was that you that made the tool, and can you post it up here? Thanks.

 

One way I found to check tie rod flex was with a length of 3/4” angle iron and 3 or 4 hose clamps to hold to the tie rod and see if that helps solve problem.

 

The caster angle is 8 degrees..

im not a straight axle expert, but we just encountered a problem similar to this here.. (death wobble)

the solution was to reduce caster to about 6 degrees.

in our situation the issue came to light when we swapped wheels and tires (more rake)
your mileage may vary.. lots of good, more experienced responders here. Id change whatever was cheapest and easiest . and in our case it was an adjustment to lessen caster.

 

I don’t have a lot of adjustment there. It’s actually sitting higher than the CE kit.

 

You should be able to run up to 10 degrees caster and not have an issue (I do). The problem I ran into (death wobble) was cured by running the front tires on my roadster at 18-20 lbs. Also make sure everything is balanced. all the way down to the discs you're using.
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I had this problem several years ago on my 34 Dodge and I eventually narrowed it down to a bad steering column. I spent alot of money on Flaming River parts but I no longer have the problem.

 

I'll chime in again!
With the posie's front spring , what is the angle of the front shackles?

They need to be slightly below horizontal with split bones.
If they are too "vertical" the whole front end tries to "parallelogram" itself [with the split bones]

Either use a slightly shorter spring [near on impossible to get]
or
Install a Panhard bar and leave the shackle angle as is.

This is a common problem with reverse eye springs
The shape of the arch makes it slightly wider between the eyes when they are above the main leaf , so the shackle angle changes.

 

Well, It has a couple of the CE triangular shackles. Kind of a RUBE GOLDBERG deal.

It rolls over center as the spring arcs....

 

Chris, you know the HAMB Golden Rule!!!
Pictures - pictures - pictures on the front suspension on tires

 

I know Glenn. It's been home renovation week. No shop time.

 

YEP I agree! Pictures of the whole setup would be a help

Pictures Of this installed would be a great help.

I assume those triangular shackle were installed to get the shocks to clear the Bat-wings
The shackles are a theoretical straight arc between pivot points. [That is the angle I was referring to]

This is my guess on what is happening [below] [pictures are needed]

By adding a 3rd pivot point in the centre of the shackles to mount the shocks will complicate things.
The shock creates a point of resistance and becomes a central pivot point [like a rocker equalizer on trailer suspension]
Any vertical forces [bumps] at the axle will be met with resistance at the shock and the spring . The pivoting nature of this "at the shock" will transfer forces into lateral movement [instead of absorbing forces into the spring]

Disconnect the front shocks and try it [it will bounce, but it might not "death wobble"]


One last thing.....Is your vehicle cross steer??

 

Yes, cross steering with a Borgeson 525 box.

 

Does it have a Panhard bar?

 

Negative.

 

Static or dynamic tire balance A tire that is heavy on one side heavy straight across on the other side won't show up on a static balance and will shimmy.

 

This was first mentioned on page 2 - post #34 ^^^.
It may not be the answer, but it is a real good place to start.....

 

I have a stabilizer but the wobble has now exceeded its capabilities.

 

Try and get a photo for us.
My take is you are getting "shackle wobble"

This is triggered by one side hitting a bump ,or drooping into a pothole and the shackle angle changes [as it should]
The shackle angle causes the axle to shift laterally until both sides meet a point of equilibrium. Unfortunately this causes the cross-steer to change steering angle.

I would recommend a panhard bar the same length and same angle as the drag link
[it can be higher or lower, further left or right, further forward or rearward Just keep it at the same angle and length]
The Panhard bar should arc parallel with the drag link.

And mount the shocks so they act direct of the axle not from the shackles [a couple of tabs welded on the bat-wings]

 

Actually, I don’t think it is bump derived. It happened like clockwork at 35 mph.
I’ll take some pics.

 

Here you go. Unloaded suspension on the lift. I do think the scrub may be an issue, from what little I know about it. Just some random photos.

 

Steering stabilizer is just a bandaid to mask the problem. Panhard bar is what you should be thinking about.

Well, that doesn't sound like 'death wobble'. If it manifests at a particular speed, regardless of bumps, it seems more like a balance problem. @Mimilan will see you right. He/she knows more about this stuff than most of us put together.

And yeah, going by the pics, your scrub it fairly severe.......

 

It isn’t a dribbling type of a balance issue. I can watch the steering wheel oscillate from side to side.
I agree, I like to surround myself with smarter people than me.