Technical Violent front end shaking

Well this carpenter needs to call on all of you experts again for my 34 Dodge. All of the front end components are new from Speedway motors and I probably have only 1500 miles or so on the build. It is a superbell with a transverse leaf spring and shocks. Just recently when I hit a bump the front end starts to violently shake, it seems like the passenger wheel and transfers through to the steering wheel. Now I know what seems to be is not always the way it is but that is what it feels like. I checked for any apparent broken items and there are none. I jacked it up and checked for any play and everything is tight. The brakes seemed to be hanging up so I bled them off and went for a ride and the shake continued. Checked the brakes again and they were ok. Not sure where to go. HELP!

 

Search "death wobble". You will find things to check and fixes for it. Common issue that comes up often.

 

Yes, death wobble.

you should know the drill by now.... post pics of the entire setup...top. side, and front views.

so we can argue. lol

 

One thing I forgot to mention I had been riding all weekend and before I went out on Sunday I had greased everything real well. Coincidence?

 

Oh,the dreaded "death wobble".
The caster needs to be set correctly, too much caster will try to toe-in that side after a bump
and too much wheel offset [scrub radius] will trigger death wobble by trying to toe-out when it hits a bump
Smaller diameter wheels increase the scrub radius ,so they need more backspacing [+offset] to correct this]

The combination of too much caster and too much scrub radius will try to correct each other by oscillating back and forth [death wobble]

also with a transverse leaf spring you need the shackles as close to horizontal as possible. If you can't do this ,a panhard bar is reccomended

 

Had a tie rod end that didn't seat correctly when I tightened them up. Later after driving it a while the tie rod loosened up. Retightened and it was fixed.

 

How much caster do you have? On street about 6 - 8 degrees or maybe as a shock dampener.

 

Death wobble can be caused by many factors. Excessive caster, worn steering components, worn steering box, incorrect toe setting,tires out of balance. A common fix but not always correct is to add a steering dampner. That cures it almost always. My roadster has an 8to1 Sprint car box. I put in 9 deg of caster to make it not so twitchy at high speeds but it would death wobble occasionally slowly going over r.r. tracks. I didn't want to add a dampner for appearance reasons so I changed to 5 deg caster and the wobble went away.

 

Speaking from experience start by making sure your front end is properly aligned. One sign of that would be aggressive or odd wear on your tires. Simple stuff first.

Posted using the Full Custom H.A.M.B. App!

 

Kerry is holding down the (wobble)fort in place of the late, great, Dick Spadaro ...and doing a great job

 

So you have ditched the knee action front and gone to dropped I beam.
How have you located it ?
What steering are you using?
What shocks are you using?
So you have done 1500 Mls and it's been semi Ok?
Have you done a wheel alignment?
Please post some pictures as often they tell all.

 

remember when the altereds came up to race?
the front end would bout shake apart, (looked like)

then they would "Just go like stink."
They had tremendous amount of caster..

 

my coupe started the wobble on Sunday, my tires are wore bad so i order new ones today and planned on getting it aligned after i install the new tires. hope that fixes mine? i have a steering dampner on mine and it still did it, but iv been driving it for 4 years now and it just started. i have 7 degs, of caster.

 

How long before the relative merits of a steering dampener starts up?

Lots of pics usually give rise to the eagle eyed respondents resolving the issue around here.

Chris

 

Merits got nothing to do with it.
They have their place, but not as an instant fall back to cure persistant death wobble.
Just gotta cover the mechanical aspects and setup first...because wobble has a cause.
Get that stuff correct and then if a dampener is desired for some reason its cool.

I have one that I plan on trying as a way to "tune" my steering feedback from our badly tire grooved roads.
Had some wobble two years ago...fixed it with a steering box replacement and some new tierods.
No wobble anymore.
NOW is the time for me to try the steering dampener, not last year. The wobble would have likely been gone with the addition of the dampener, but the box and rod ends still would have been bad.
In such a case the dampner would have been a bandaid, not a tuning aid.

 

Steering Damper instantly saved my HemiRod. Simple, low-cost, bolt-on strut.
You can see pictures of it on my Website. (Not Hamb-freindly, so I won't post a pic!).

 

Sometimes changing tire brands will help, some tires just don't like some setups.

 

The car has been fine to this point. My wife and I just go every weekend. I will take pics today.

 

This IS a "correct fix" for that particular build/components. It backs up what Kerry has alluded to.

Your death wobble was not caused by loose tierod ends or loose box. Those problems allowed death wobble to "show up"(if it really was death wobble), but there is a root cause of geometry that you have not corrected, if it did have true death wobble.

We need a comprehensive, step by step, explanation on all of the things you mentioned, but I doubt it will/could ever happen without a computer video simulation of what setting affects what, causing the start of a wobble, and how it turns into a chain reaction...including showing tie rod flex and steering arm flex. Spadaro gave us all the info, but unfortunately it was spread out in bits, here and there, over several years of different wobble threads. He even mentioned once, on why a I-beam 18 wheeler NEVER gets death wobble.(think!) His writings on "caster trail" vs "scrub radius" should have been described clearer/simpler, so that all could follow along.

 

Yeah, we had the wobble too. Standard roadster I beam set up with about 6 degrees caster and pan bar.
Added a dampener and while this may not have cured the initial cause, it seemed to prevent the symptoms.

I remember my old stock VW bug (1967?) had a dampener too.

 

Maybe rephrase the "cured" part. That is a band-aid, to mask a geometry faults, and in some cases a weak undersize component like tie rod tube.

Earlier VW bug never used a damper. They came in around 1960 or so, and bugs were around since WW2.

Think of the difference to "18 wheeler" and the super lightweight unsprung weights of "VW bug" front end components and tires.

The truck has massive amounts of weight/mass of all parts beyond the kingpin,(tires/wheel/spindles/brake), which "absorbs"/dampens any "pothole forces" of a tire hitting it. The VW uses a damper to reduce that force felt at the steering wheel, as it has very little mass of components.

My emphasis on "absorbs" above: A damper is no different than a shock absorber. A steering damper absorbs shocks in the steering system so that there is no violent movement felt at the wheel. Using a damper to "cure" true death wobble, which is really a chain reaction, is a myth. It is a band aid when installed for that reason, pure and simple.
.

 

This gets to the root cause of true death wobble. Kerry mentioned "try to correct each other".

This also helps explain why an 18 wheeler has such a massive tie rod tube. Not because the trailer carries so much weight in cargo, but it is stronger to be able to "not flex" as one wheel fights the other on a pothole, and it's "spindle/brake/wheel and tire" is so heavy, and the other side is acting like an unmovable object.

if we build a rod with excessive caster, we are making the opposite wheel more difficult to turn as we hit a hole. As the other side first hits a hole, it pushes the tie rod. The weaker cheapie thin tie rod will flex, before it can overcome the caster on the other side. Now you have two wheels steering "out of sync". This will get far worse with what he said about incorrect "scrub radius", (when the wheel is far outside of the imaginary king pin centerline that should be under the tire footprint). If the tire is outside of that line, hitting a hole will have much more leverage, and stress the tie rod even more.

we need to remember that death wobble is a chain reaction. Anything we do that increases the chances of both tires fighting each other is a bad thing.

 

Yes, the damper is a shock absorber just like the shock absorbers you use at each wheel. Are those covering up suspension problems? Of course they are because that's what they are designed to do. Many vehicles are designed to include a steering stabilized to dampen the harmonics.

In this case, the car apparently hasn't exhibited this behavior in the past. That suggests the geometry was right at some point. It seems to have started after recent lubrication. Maybe there was enough friction in the components before to dampen the harmonics like the old friction shocks did. If there are no loose components, the wheel bearings are good and adjusted properly and the wheel/tire combination doesn't have any runout wobble, it could well be that loss of friction. A steering damper may be useful.

 

My car is over 30 years old and had "some wobble"...not Death wobble.
Inspection showed the output bushing of the corvair box to be worn bad. (a common problem)
I sourced new bushings, but a low mileage, short shaft box kinda fell into my lap so I went with that to create some extra interior foot room by realigning my column.
As for the geometry, I use zero offset front (and rear) wheels and the caster is set at 5* on the late 50's Chevy spindles.

When discussing a car that has been set up correctly and working great for more than a decade (and actually longer!)...it seems to me the problems will be easy to find and address in a close hands on inspection to find where previously good controlling conditions have changed.
The original geometric setup of the car has already proven itself in the real world.

I've seen death wobble itself appear with worn parts on other vehicles however.
A friends 2nd gen Ford van for one.
True some root cause is creating the forces that develop into death wobble...but the unworn parts had no problem keeping them in check.
Once wear crept in the conditions changed, death wobble appeared and shorts were soiled!

 

I absolutely agree that friction can be a deterrent to wobble. But it is not a fix it's a problem. A damper adds friction in the system, so it can hide wobble but not cure the cause.

Dick Spadaro was the expert. His advice helped me finally cure the roadster in my avitar years ago.

In my case fine tuning everything was needed and than the final overlooked detail was to perfectly center the steering box and the front wheels with the drag link off. Than adjust drag link so box is perfectly dead center when wheels are dead center. This is an important step that is often overlooked. Just cause you put the drag link 90 degrees to the ground or straight down or straight front don't mean the box is centered and that's what needs to happen.

I run a reversed Corvair,

 

So often overlooked!

 

I'm curious as to what starts the 'death wobble' are you doing the bump like going onto a bridge or does it just start on its' own? happens a different speeds?

 

Is it OK to disagree? lol ...Shocks do not mask, or/"cover up" suspension problems. They are designed to control/absorb/calm, a rapid movement, just like steering dampers are designed to do.

Example: 90/10 drag shocks were invented to allow uncontrolled lifting of the front of car for maximum weight transfer....so the "10" shows that the shock is barely controlling anything in "extension".

A steering damper reduces shock to the steering wheel, that was it's main task. A suspension shock is designed to do the same; calming a violent movement of unsprung weight/and or, the sprung weight. The very early hydraulic lever shocks were one-way only. On those cars, they did not do anything as the car went into a dip and springs compressed, but as the chassis moved up right after compression, the shock dampened that movement. Driving those cars, you sure can feel it, compared to a two-way. It was not long after, that designers found a way to have double action with a lever shock.

Death wobble is a unique phenomenon, in that the "initial movement", turns into a convulsion/chain reaction. Using a steering damper to lessen that first shock, certainly can eliminate death wobble, but there is still another problem. Dampers were never designed to offset death wobble.

If you see that I questioned "if" you really had death wobble It sounded like it was not, to me.

 

Agree on Spadaro, but a person needs to look up Dick's many threads to find all the missing pieces now that we can't ask.

...back to the side steer, pitman hanging down "straight down". One thing that many people do not realize on side steer cars, is that due to Ackerman, and the steering spindle stops locations...a drag link actually moves MORE in one direction, compared to the other. (a mind hurting thought) LOL..

So if all planets aligned with a very limited movement from a particular box and pitman length vs spindle upper steering arm length... and you are struggling to get fully to both stops, the pitman will not be at 90 degree. Yes, you need to be at dead center high spot on the worm gear, but that fact with certain column angles, and limited throw, can have the pitman not at 90.

 

That's true F&J but the primary problem there is you have different turning radius on each side when you turn the wheel lock to lock.
Here we are looking at the possibilities of maybe:
The shocks aren't up to snuff or aren't installed to get the best dampening from them.
Tire damaged internally when it hit the pot hole.
Tie rod or drag link flexing when one wheel hit the pot hole and then oscillating causing the wobble. Back to the undersize tubing mentioned earlier . Looking back at problems with my T bucket that may be an issue as it had a long skinny drag link.
Tie rod bent when the car hit the pot hole. That happened on my 48 coming back from Pleasanton in 1998 somewhere on the east side of SF bay. By the time I got to Bend Oregon the tires were pretty well chewed up. That was more than a pot hole though as the truck dropped over a foot as did every other rig on the road when they passed over it.
Wheel weight knocked off when the tire hit the pot hole.. A small weight might not cause a problem but a couple of ounces may cause trouble on a light car. I've driven down the freeway and watched the tire on a car in the next lane bounce several inches off the pavement as the car drove down the road with the driver showing no concern.
Bent rim from hitting the pot hole.
It is possible that greasing really dry king pins and rod ends got them working a bit too smooth and free but the tightness would have masked another issue before.