Death Wobble

Yes I should’ve used the word trigger.
Combine excessive scrub radius with excessive positive camber is a recipe for disaster.

The Scrub triggers the reaction with a bump , then the caster does what it is designed to do and correct this by turning that side wheel towards the centre of the car [ then the opposite wheel tries to counteract this ]
Too much caster is a cause of death wobble [ shackle wobble ], the wheels start oscillating from side to side trying to correct each other

If I am seeing things correctly here,the King Pin Inclination really needs sorting out

 

I think at the root of the problem is that the car was built sort of for one purpose and is now trying to be used for another. It is pretty obvious with the tube frame, engine, cage, tubs, etc, that the car was built to be more of a drag car than a street driven hot rod. There is a huge difference between driving 1/8 or 1/4 mile at a time and driving down the interstate at 70 mph for hours.

The components used and the way they are used are very much on the light side to handle things like irregular pavement, pot holes, and all the other stuff our cars are subjected to on the street. My opinion is that as it is constructed this car will never be a great driver on the road unless a lot of things like steering and front suspension are beefed up and altered.

The car reminds me a lot of ones I see at shows where the owner trailers them there, drives it a half block, wows the troops, then puts it back in the trailer when the show is over.

Don

 

I'm with Don here, what you've got is a really nice high dollar show/drag car not intended for street use. Somebody put a lot of cash into a beautiful car without following the basic rules needed for a good street car, Either you use it for what it was intended, or build a new chassis from the steering box forward. Too bad, its really nice, but not safe enough to drive to a cruise nite, good luck.

 

All of the above.

Start again and get all geometry correct, dampers are only a cover up for poor geometry.

Get some good books about engineering in relation to levers and ratios this will give you a better understanding where your forces are going, and don't forget the relationship to the rear end as well, it's all one system, Ackerman angle is part of this whole equation.

If you have some part that is minutely sloppy, it will be magnified, if you have many parts that are sloppy it will drive like a bucket of shit.

Good luck with your rebuild, then again it could be trailered to shows as state of the art and all the young guys could replicate it as how it's done, with that big showy set up it must drive great !!! Being a bit sarcastic but this could never happen could it, OH that's right it's been happening since 1948.

Just pointing out that you need proper reference material, because not all info supplied is correct, even this advice, check it out for yourself.

 

i brought mine to a reputible alignment shop that does a lot of straight axel cars and its true as new runs down the road real good,, sometimes it has to go on the front end table

 

I agree with those that say its a collection of setup problems.Ackermans angle looks close to what it should be with the tierod in front but that shaft from the box to pitman arm are what concerns me the most as it would bend and flex when load is applied to it. I would like to place a DTI against it ,lock the wheels and turn the steering wheel back and forth. To have that kind of shaft it would need a support bearing in the centre and the outer one moved next to the pitman arm. But i would start again from scratch and get all angles and joints right. JW

 

Yeap, you all were right. The car didn't like the fix. I'm sure the spool rear end is putting more stress on the steering too.

You've given me some good things to look at. Keep em coming.

 

Few more pics.

 

little present for all the answers.

 

I would tend to agree with Don here but another thing which is worth considering is the tierod, it looks like a 7/8" OD, Last year I had a guy that kept coming to me with a death wobble, he had tried to cure it for a couple of years, I suggested changing the 7/8" tierod for the 1" diameter I use on all my cars, instantly it transformed the car to have perfect road manners, its worth considering.

 

Another car that will never be fixed. Dont put a steerig damper on it then. Put it in a show room and polish it. Certainly would make no sense to try a proven method of dealing with similar problems. What if it worked?

 

I agree, but how to fix the scrub? The tie rod up front requires it to be almost touching the tire to get proper ackerman. So, you can't just swap on different offset wheels.

If you put the tie rod to the rear, then the rod ends would be away from the tire, but with the disc brake swap, I doubt there is a correct offset wheel to fix the scrub.

and yes, I'd be looking at caster spec. Seems like the 7 to 8 degree myth will never die in the hotrod/streetrod world.

 

In all honesty I'd rather NOT see this car fixed with a steering dampener...too much other stuff that needs fixing first but might never get done if a dampener worked.

All things considered I'd live with less Ackerman and get the scrub closer to perfect. In general driving on the street or highway Ackerman really isn't a big concern as the steering never goes far off center...except on a 90* right turn at an intersection or driveway of course...but scrub is a potential problem at all speeds and steering position.
A taller wheel and tire combo can help with the scrub as well...but of course that also increases trail...and now we're into the caster question! LoL

Bottom line for me is this frontend needs a total makeover...and apparently NOT in a drag race chassis shop!
I'm actually shocked that "professionals" put this together!!!

 

Make sure everything is done correctly. The tie rod in front has nothing to do with wobble. That said it does change your steering geometry. When you turn your inside wheel should turn a tighter arc than the outside wheel. With tie rod out front it is just the opposite. T buckets run this way but because they are so light in the front end you don't notice the tires scrubbing as much. As for the wobble I have found that when running a dropped axle I run a steering Damper and that takes care of the wobble.

 

I agree with all of that except that ackerman is kinda important too

But, how to redo that front end with the high horsepower/high performance needs of proper brakes? I have yet to see any disc kits that don't mess up the scrub? If you could get discs and get the scrub correct, that's a huge improvement on this particular car.

I just am not sure how I would go at it, if it was mine.

 

I have to say that you are exhibiting a hell of a great attitude and thick skin with the way you are accepting some very strong critiquing of your car. A lot of guys would be getting all defensive and snotty about it, but you are really taking all of this very well. By the same token, we are all just trying to get you some resolution to this problem and some very good suggestions have been mentioned so far.

The biggest offender in that whole setup is the steering box and longish output shaft. Those are really never going to work well and I am not sure how you can get around that as the car is basically designed around that sort of setup. It could be that everything in front of the firewall needs changed to ever make this car one you can drive down the road with acceptable road manners.

Don

 

F&J...Ackerman is important. It can be compromised a little to help get a different issue under control though.
Sometimes a little compromise here and there can improve the overall big picture!

Very true Don!
I'm wondering if a different box with a longer factory shaft couldn't replace the setup there now and even pull the pitman arm closer to the frame itself.
No good reason for the arm to even be that far out away from the chassis!
No matter what...the mounting structure MUST be a stronger design than whats there now.
Then he can work on the other things like getting rid of the clevis ends and addressing the scrub radius.
Overall the car is very well built so once this stuff is taken care of its a beast!

 

I honestly have to agree with Kerry on this particular build; meaning that he sees the scrub as the biggest problem.

Yes, that steering box setup could/should be beefed, but if the scrub was correct, it would not need as much beefing, if you think about it.

Same is true when some wobble cars can be "fixed" by using a heavy duty mega tie rod. In effect, the wobble is eliminated as the rod no longer can flex/bow, but the root cause is still there with some other part not being correct. This is why 1/2 the guys here say that a damper is a band-aid...and I agree with that.

So, if we redo the box, beef the tie rod, and whatever else, but don't adress the scrub being so far out, I feel like you are taking a very much longer path to try to get it drivable.

The only way I can think how to describe the forces put on the steering with wrong scrub is; put your hand slightly out the window at 60 MPH and feel the force, then extend your hand out as far as you can. With the tire way out past the imaginary king pin line where it intersects the road, anything you hit with one tire sends a massive load into the steering arm on that side. That's why beefing a tie rod can help "some" wobble cars. That is also why a damper can "fix" some cars.

If the scrub was perfect, just imagine the decrease in loads on the steering, no matter what size object the tire hits.

 

1- The unsupported sector shaft needs to be addressed. You need robust support as close to the pitman arm as you can get it.
2- The disparity in length between the drag link and the four link needs to be changed. This will cause bump steer, which results in the car darting when you hit a bump at speed.
3- All joints need to be spherical, not clevises.
4- The root cause of your death wobble is the large scrub radius. Think of it as too much leverage between the tire contact patch and the steering axis (kingpin centreline). Move the tie rod behind the axle, shorten the steering arms if necessary, install wheels with enough negative offset that the steering axis falls within the tire contact patch.

I'd bet a paycheck you'd get this same response from a "real" chassis expert, such as El Polacko. Lots of well- intentioned people here are offering you lots of suggestions, some more well informed than others. Contact a pro, your life could depend on it.

 

few more pics to keep ideas coming.

Picture 1 shows the pitman arm with the wheels turned far to the right as possible. I have about 1.5" of clearance from the body.
How far would you shorten the shaft?

or the real question how close to the body to I get with the pitman arm?


Pic 2 and 3 are more top down shots of the steering box

Pic 4 is a rudimentary angle of the king pin center and tie rod end bolt.

 

I think the plan will be:

1. Shorten the pitman arm shaft.

2. 1 or 2 more anchor points for the shaft.

3. Maybe strengthen steering box tabs.

4. Experiment with different toe, chamber, caster.

5. Change out clevises.

 

Suggestion:
It looks like you could unbolt your steering arms from the spindle and swap them 180 degrees, end for end, so the tie rod ends up behind the axle and between the four link rods. Toe would have to be reset (suggest min. 1/16" toe in), you might have to shorten the tie rod to achieve this.
Install some wheels with more negative offset.
Regarding the steering box, I'd be using a bigger, stronger box behind a 600hp? hemi. There's always going to be flex in the frame between the mount at the box cap and the sector shaft support. I think the current arrangement will always be problematic.

 

MII? Ok, I'm not helping...

 

Even if it is not the cause of the death wobble, if the wobble causes it to fracture, it could be the cause of a death!

I have been criticised for building my stuff like a Land-rover, (a British 4x4) but I'd rather be 500% too strong than 1% too weak.

 

The first thing that needs to be done is to put the front tires on some turn plates and go through the motions of both steering and suspension travel, so you can actually see what is going on. Throwing internet fixes at it based on a dozen pictures is not the way to go about your repairs.

 

Not to have a critical tone, but purely in the sense of safety, the components and design thats there are better suited to a golf cart or lawn tractor not something thats guiding a hemi down a less than glass smooth roadway.....at speed.
Even for track use only, it's a poor design for reasons already posted.

You really need much beefier parts and a 'start from scratch' re-design of that steering system......to be safe.

A builder thats familiar with whats needed will keep you (and any passengers you carry) safe, which should be your top priority.

 

Henry Ford never progressed to the steering damper, in all the 38 years he had axles under Fords.

But then, Pete Chapouris wasn't around to guide him...

 

I did notice a considerable change in front end behavior, when I went from a mono-leaf, to a Posies multi-leaf. Dampened bump response, softer initial hits, due to the multi leaf, etc.

 

I agree the single leaf was always suspect, it does give the car a clean look.

I still would like to know how close to bring in the pitman arm to the body?

and

The proper name of the triangle that the drag link attaches to?

 

I suspect the box is flexing as well as the end where steering arm is attached. Ik there are other issues but that's a major issue.