Death Wobble - speedway tie rod may be suspect

Yeah I hear ya. I got into it on the Jockey Journal with folks that swore SugarBear Springer front ends were strongest because they were made out of solid ROD. WTF? Over.

Tubing in our applications is stronger.

 

Tubing in our application is *better*. It provides the best balance between ultimate strength and strength weight ratio, but I'm not convinced that for a given diameter, tubing is more bend resistant than solid bar. Try the tube bender test I cited. Still, solid is not a good choice for most automotive applications because of the diminishing returns of increased wall thickness.

Bob

 

Just dug out my scrap Speedy tierod. It was cut off years ago PAST the threads. The center is a full 1/8" wall and the threads are also that thickness. Nothing funny here. Does yours measure up to this?

Also flexed an F-1 Tie rod, no flex, most movement was int he tierod ends. Also flexed a model A tierod, unscientifically stiff as a freshman boy in a girls locker room

 

Yes, in the end it comes down to weight vs. strength. AND, DOM is a better choice for many of the things we do than mild steel(more consistant wall thicknesses)

 

This is not correct.

Tubing is stiffer for a given mass (or rather cross sectional area, but if you're dealing with the same material for both parts, the greater CS area yields the greater mass). A solid bar of equal diameter to the tube in question is obviously of greater mass, so it is also as stiff or stiffer.

It's all about moment of inertia. Think of the moment of inertia as the measure of the geometric stiffness of a given cross section.

For that 7/8" dia x 0.140" wall tube, the moment of inertia is equal to:

[Pi(D^4-d^4)]/64 = 0.049(D^4-d^4) = 0.049*(0.875^4-0.595^4) = 0.023

Where:

D = Outside Diameter of Tube in Inches
d = Inside Diameter of Tube in Inches
Pi = 3.15159

For the 7/8" dia round solid, the moment of inertia is equal to:

(Pi*d^4)/64 = 0.049*d^4 = 0.049*0.875^4 = 0.029

See that? Not a huge amount, but the solid is stiffer than the tube for the given 7/8" outside diameter. The closer that the tube gets to becoming a solid (IE the thicker the wall becomes), the closer those two numbers will get to being equal, but the tube will never overtake the solid. To do that, you need to increase the outside diameter of the tube.

What makes tubing great is that for a given weight, you get more stiffness (or, for a given stiffness, you get less weight). You can take that same mass of steel making up that 7/8" solid rod, and instead turn it into a 1 1/2" OD x 0.140" wall tube (very nearly identical cross sectional areas).

That 1.5" tube will be just a fuzz lighter than the 7/8" solid but nearly five (5) times stiffer. THAT is where tubing really shines.

To the OP - I would be tempted to go to a 1.25" OD x 0.120" wall tube and use machined bungs to weld into the ends. That will significantly stiffen the rod, without adding a great deal of mass to the system.

An earlier poster had it right, I believe that this rod is essentially acting as an undamped spring when it gets loaded sufficiently to deflect. When the road loads are not high enough to deflect it, all is well, but as soon as you hit a bump big enough to cause a deflection, the spring action causes the tube to oscillate (inducing toe-in, then toe-out, then toe-in, IE a "death wobble").

The tires being very low on air stops this because: 1) the softer tire absorbs more of the blow from a given road induced load, so the rod is seeing lower loads than it has been, and 2) the flex in the tire acts to damp out the oscillation when a large enough bump is encountered.

I believe this to be why the axle damper so often prescribed as the remedy to "death wobble" was ineffective in this instance, because the damper requires lateral tie rod movement to do any damping. If the simple deflection of the rod is sufficient to cause the "death wobble", the damper would not help, because its mounts would never move in relation to one another.

Very interesting thread. I've suspected for some time that the "death wobble" phenomenon was part mechanical dysfunction (loose parts, worn out parts, etc), and part harmonics.

It looks like your version is mostly centered around the harmonics of the situation.

 

BINGO!

Snuck that one in on my while I was posting my novel.

Very correct.

 

Josh,I was gonna ask about caster but I see it's at 7 degrees so that's good,,,maybe a crazy idea but have you checked the wheel bearings to make sure they are snug? HRP

 

No, very WRONG! If a wheel toes in, it puts the tie rod in TENSION, not compression.

 

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Pal, if I had a new tie rod, I'd send it to you. Bet it ain't bending, something else is going on. If you don't have 6-7 degrees of caster, dial some in.

 

I don't know why nobody is wondering how the tierod could flex when you try to spread the tires by hand?

How is that possible with heim joints when you are pulling against the 2 heims ?

The contact area inside the heim should, in theory, be in a direct C/L of the tierod itself...with normal looking heim joints.

I don't undertand how it can flex on a pull.

 

Depends on which way the steering arms face. The original poster never did say, or if so I missed it.

Bob

 

I think he's spreading the tires at the front and the car has the centerlink in the rear. I'm also not so sure about it seeing tension, regardless of whether it is toed in or out. At speed the tires are going to try to move rearward and depending on wheel backspacing they may want to do one or the other depending on the tires center in relation to the kingpin. Most hot rods tend to have the tires center slightly outboard of the kingpins which would tend to want to compress a rear mounted centerlink. A lot of the cars I've seen with this problem had dropped or bent centerlinks, long dropped aftermarket bolt on steering arms along with skinny wheels w/very little backspacing.

 

Why would DOM reduce flex, not followin you?

 

The key term here is "Oscillation".

Tension or Compression are irrelevant, because it ends up doing BOTH as it cycles.

If you stretch the rod, you get the same spring effect as if you'd compressed it.

If it's front steer, he's right, if it's rear steer, he's got it backwards, but the mechanism he's explaining is valid regardless.

Even though the heim aligns the force along the long axis of the rod, the slenderness ratio is such that when loaded in compression, it will tend to want to buckle the rod, and the predecessor to buckling is deflection.

It may stretch in a straight line, but when it rebounds into the back half of the cycle, it will deflect one way or another, and tend to aggravate the oscillation.

The same thing will happen with a stiffer rod, but by making it stiffer, the frequency of the oscillation will be increased, hopefully to the point that it will transition from a death wobble a simple road vibration.

I think his problem is that this particular combination of parts oscillates close to the natural frequency of the chassis, which sets up a feedback system, turning what should be just a teeth rattling bump into a death wobble inducing bump.

If we can increase the frequency of the oscillation (by stiffening the members involved) enough, we can move the vibration created outside the natural frequency band of the chassis. If you can do that, the problem should go away.

 

I had the exact same symptoms/problem on a car I had driven for five or six years and then one day it developed a death wobble, which would only go away by slowing to below 30 mph. I replaced the kingpins - problem solved.

 

I had this problem with my 37 pick up and the speed wobble would be really bad at slow speeds but got better when my speed incresed. the fix was the toe in and toe out. the correct toe out is .25. take a measurment of the front center tires and then the center of the back of the front tires and the front tires should be toe in .25

 

you could also put a steering stabilizer on the front tie rod...

 

I have chased this problem around for the past 40 years of wrenching and it always comes down to worn, loose steering linkage or king pins/trunion bearings or not enough positive caster.

 

Danny, I pulled the caps and made sure. I could not go to the next hole in the nut and there is not any play in them. There is also no slop in the king pins. Nothing is sloppy.

The tie rod is behind the wheels.

Thanks for the awesome input Coolhand. You describe what is happeneing very well. Like you said, it's all about the Moment of Inertia!

I think you are right about why the damper doesn't do anything too.

I think I will try the idea about the 1.25 tie rod with bungs in the end.

 

Josh, we used to have that problem with "set up" four wheel drives. Only way we fixed the problem was a perfect ballance on the tire and a steering damper shock on the tierod. Wish I had more .

 

Tubing is stronger than solid... its all about trianglation (sp?) At least thats what I was told.

 

im struggling to see how it can bend the tie-rod with pushing the tyres out. then you say you welded a piece of angle onto the bar along its length and it STILL bends with pushing the tyres - you must be mighty strong if you can bend a piece of bar with angle iron welded to it along its length which is bolted in between two links by pushing out the tyres.

I could see the rod with angle welded onto it bending if one end was in a vice and you were pulling it down but not when its bolted into the front end and all you are doing is pushing the tyres and not touching the bar.

 

It is and I can. That is what's alarming is it doesn't take that much force. The wheels do act as a 25" or so lever on both sides to pull on. That's will multiply some force.

The angle stiffened it up considerably, but I can still see if moving slightly when I pull on the tires.

 

Going to a 1.25 tie rod isn't necessary, most use 7/8 and have no problem. Sounds like his tie rod is defective (weak). Otherwise wobble is caused/cured as many have mentioned in the area of:
loose parts (kingpins, rod ends, etc)
tire balance (dynamic balancing is the only way)
caster (too much is as bad as too little)
toe (toe out often cures it)
scrub radius (deep or offset rims or the use of spacers/adapters can cause it)
tire size (affects scrub radius)

 

Josh, try sleeving the speedway rod and weld around the ends. I'll bet that will eliminate any flex.

Also, alot of people just don't understand that the tie rod is behind the axle and as you are driving the natural thing is the tires want to separate from the front which in turn is pushing the tie rod together and if the rod is insufficient it will flex. When it flexes, thats when you would get the death wobble.

In turn you could run a lighter duty tie rod in front of the axle but once you go behind you really need a stouter one.


I hope that makes sense.


BloodyKnuckles

 

That's one for the books.
The isolated tie rod would seem to be the problem.

When you say "bowing" I'm taking that to mean the tie rod is bending and not simply rising up.

If you have standard 11/16-18 threads, a 5/8" drill is the tap drill.
That would give you a .120 wall after drilling.

More than likely your Heims are 5/8-18 and that takes a 37/64" (.578) tap drill for 75% threads.
Which would give you a .148 wall thickness after drilling.

Going to 1 1/2" tubing, even as a test strikes me as a lot of work for nothing.
And you may run into clearance problems with other components.

DOM tubing, 7/8" OD and .156 wall thickness is what most everyone uses.

It looks like your tie rod is the correct size, but iIf you're truly flexing the tie rod it may be right on the edge of failure.

Buy or make one from the correct size tubing.

If you deem Speedway as the problem, there are plenty of other tie rod sellers/manufacturers out there.

What type of steering arm are you using?
Tapered or straight drilled hole for the Heim bolts?

I'm still betting the problem is, something is moving and it may be simply the Heim bolt moving in the hole.

Don't overlook loose mounting bolts if your steering arms are bolt-on.
I know you sid they were tight, but if you have an overly long shoulder (non-threaded portion of the bolt) the Heim mounting bolt could be bottoming on that and although torque on the bolt is correct, there is virtually no clamping force.

A quick check could be to add a flat washer underneath - between steering arm and nut - and see if that clears things up.

 

An additional thought.

How are you clamping the threaded Heim joint end where it screws into the tie rod proper?

Nuts or clamps?

Either works well and if your Heim threaded end doesn't go too far into the tie rod and has a loose nut or clamp the Heim will rock back and forth within the threaded tie rod end.

 

C9 - Please give me a little credit. My steering arms are tight! There is no slop in the heims and the steering arm holes. I hae shouldered bolts through the heims and no the shoulders do not prevent the heim from being held flat against the steering arm. The steering arms are very stout flat plate and are not flexing. I watched them too. Yes, the rod is bowing up in the center. The whole thing isn't raising up. Everything is tight man. I went out and put a wrench on the steering arm nuts last night just for you. No BUDGE. I also have new lock nuts on everything. Mcmaster Carr loves me.

 

There are jam nuts and they are tight.

 

I have fought with the "death wobble" several times and one thing that I have found on my roadsters is that any toe in over .030 inch and it will wobble. I added some toe in a while back and I couldn't get out of my drive way it wobbled so bad. I have my wheels at very near zero toe in right now and it is the best it has ever been.

I was talking with a friend that builds some really first class rods, Jeff Esichen, and he had some pretty good wobble on his latest rod and he fixed it by adding a piece of larger tubing over the center section of the tie rod and it really helped.

Going to a one inch tube with at least .095 wall should go a long way to fixing your problem. It really doesn't matter what grade the tube is, EWR, DOM, 4130 it just needs to be steel .

Rex