Death Wobble - speedway tie rod may be suspect

Sorry, when I wrote this I didn't realize the post was 14 pages long. The tie rod is behind the axle and the steer is fron to back.

 

First off, double check that everything is tight, especially the bolt on steering arms and make sure there isn't any play in the wheel bearings and king pins. I see a few things that are a bit unconventional, but nothing really screams trouble. Beefing up the tie rod couldn't hurt though. One quick experiment you could do is flip the steering arms around the other way so that the tie rod is in tension rather than being compressed and go for a test drive. If it still does it, then start looking elsewhere. Caster may have a lot to do with it, but I'm no expert on that particular subject.

 

What is your caster set at?

 

OK... to the best of my ability using an angle finder of the front surface of the axle I am measuring 12 degree's and the floor has a 2 degree slant making the caster 10 degrees. Seems excessive to me but I have never set up a straight axle before.

 

Ok lets explain this one more time, First you have too much caster, you should have adjusted this to between 4-7 degrees maximum tipped rearward the less the better.

The reason that your tie rod bends is a little more descriptive in the picture without the radiator. The tie rod bends due to the forces placed upon the tie rod as the arc swing of the steering arm rotates in an upward circle arc based upon the severe caster angle. Rolling out the caster will bring the steering arm ends more parallel with the ground and eliminate the bowing forces.

Now if you have never set up a straight axle before you should be doing a search on the procedure before you go blasting around and accidentally use a tree or another car as the brake.

 

Thanks for the explanation and I am learning allot. However I had the frame built by a reputable car builder so I assumed the caster was set correctly. I will make he change and post the outcome. Thanks again...

 

Dick, man, all due respect, but with a flexible joint on the end of the rod (Heim, Tierod End, etc), the arc that the steering arms go through will have zero effect on the load the rod is subjected to. Unless, of course, the arc is so severe as to bind the joint at some point.

So long as those rod ends are self-centering, the arc makes no difference to the loads that the rod itself sees, because no matter how the bolt is loaded, the rod end aligns the force vector with the CL of the mounting stud on the rod end.

Now, if the CL of the rod end and the CL of the tie rod are not the same line, you will induce a bending moment on the rod, but that is the only way to get a bending load into a member with self centering end joints. They just have too many degrees of freedom for it to be otherwise.

I agree about the caster being too high though. Pretty good chance that's the cause of your low speed shake.

Also, make sure you have the same caster on both wheels. Having uneven caster from one side to the other can cause some wicked low speed shimmies.

Take a look at video of dirt sprint cars under caution for a taste of what I'm talking about. Those guys stagger their caster to help the car turn-in, and that produces some pretty wild wheel shake at pace speeds. It clears right up at race speed, but until you get there, it can be pretty severe.

 

Hey Cool hand, I've got a sprint car and my axle doesn't wobble at any speed. Caster is set on the right front on a 3 bar car at 8* with 1/8" toe out because the car turns on the left front, initiating a tighter corner radius for better turn in on a small track. Caster wobble on a race car is the sign that the race car has misalignment or some funky front end alignment that the driver or mechanic believes helps the handling but really does not. Rolling excessive caster in to a race car chassis increases a slight diagonal loading during turn in but this is better accomplished with weight jack adjustment. You should not use race car set up for street car application.

Now we have been thru this before, the tie rod arm normally is rotated on corresponding arcs R or L . The tie rod arms swing up slightly and down slightly as they rotate parallel to each other. The force goes thru the tie rod relatively on center of the tie rod because of the misalignment ends and you are not wrong in this assumption but that was not the original question. The tie rod arms are not rotating with each other they are rotating towards each other.

Tudor and 777 wanted to know why they were experiencing a tie rod bow upward when both tires were put in a toe out mode or the steering arms being rotated towards each other. This I believe is computed by using materials buckling equation. But I think easier explained by relating the force angle on the tie rod steering arm being generated upward to the center of the car via the excessive caster upward swinging angle and the fact that the arms inward swing creates a decreasing distance between the two tie rod mounting points. This decrease in distance causes the tie rod to bow in an upward motion to compensate for the lesser spacing and balance the force. When the force is relieved the rod returns to neutral state. The only time this would happen is when severe aligning forces pulled the leading edge tires away from their centerline. Which basically means something is messed up.

Can you bend a tie rod because of excessive caster wobble , yes, because a wobble action from the excessive caster spinning tire can create a force that exceeds the buckling strength of the tie rod with the two wheels coupled. They then could have opposing unequal moments greater than the strength of the tubing..

 

I don't quite understand some of the stuff in all of the 14 pages, but I think some of you guys are going to solve all the mysteries about wobble....like what was said in the above quote. This is a good thread IMO

 

Well, IMO you're strongly verging on arrogance here when talking about race car set-up. You know what's right, and everyone else is doing something wrong. That means you win every race you've ever entered, right? 'Cause if you've got it all figured out, you ought to spank everyone, every time out. If that's not the case, you might dial the sanctimony back a notch or two. Motor racing has the curious ability to evade complete understanding, no matter how great the mind involved. We could all do well to remember that and approach all things racing related with a touch of humility and a pinch of self preserving fear.

Arrogance aside, I believe that the flex seen in the tie rod tube is being caused via the buckling mechanism just as you suggest, but I do not believe that you can discount the magnitude of road forces imposed on a wheel when it hits an obstruction. Your hypothesis about the force vectors changing direction because of varying steering arm angles could be right or wrong (I am inclined to say wrong, because of the mechanics behind the joints, but I am not entirely certain of this), but testing would bear it out, one way or the other.

To really get to the bottom of this, we're gonna need some data about impulse loads from road obstructions, and a couple of free body diagrams (and all the associated chinese arithmetic, as my old man would say).

I'm certain that this is an issue that can be solved using the engineering method, but it's going to be a lot of work for whoever undertakes it.

I would love to be proven right on this, in an absolute manner, but not enough to actually spend the week on figuring it out (especially since I don't own a car with a beam front end on it ).

 

I agree with Coolhand. Very well said.

"verging on arrogance"

 

That car does have low speed death wobble and is getting a new steering setup soon.

 

Whoa, Cool hand I see why you have a nuclear explosion picture, who set you off today. Not me. I didn't think that this was an arrogant post directed at you at all. You brought up the race car analogy I just explained why this happened and some of the resultant. Now you have a hard-on, for what?

I believe you have to look at the forces applied at the tie rod arm in an x,y,z coordinate to determine why the bow occurs. Why the pissing contest?

The whole object of this has to somewhat explain why the tie rod bows outside of doing a bunch of unnecessary math that nobody will understand to prove a point.

Now in your post you admit that you don't even own a car with an I beam so why are you taking me to task on something you have never worked on?

The whole object of this tedious post has the get the people that read this to think more about the importance of simple engineering requirements so that they can build a car that they can drive safely, easily and not worry that something is going to self destruct at any minute putting them or someone else in harms way. If you interpret this as arrogance so be it, would you rather I say hi to you on the weekend and ask how is your car driving or say, "Gee nice guy to bad he's dead from that car accident, someone should have told him that is not a safe way to construct that part.."?.

 

Who said I've never worked on a beam axle? I sure didn't. I said that my car doesn't have a beam, at least not anymore.

I've wrenched on and helped build sprinters, and I've helped build cars and trucks with beams up front. This ain't my first rodeo.

BUT, even if I hadn't, this is an engineering problem, plain and simple. Having never seen a beam axle in real life, a free body diagram and the requisite calculations would tell me everything I would need to know to analyze and understand the system.

Your simplifications are leaving out important parts of the process, and interjecting parts that don't belong.

When undertaking the simplification process (I might add BTW, that your assumption that people here need the information to be prepackaged and simplified is condescending and perfectly illustrates your arrogance), you have to be careful not to simplify things to the point that you no longer fully or properly explain the problem and the solutions to it.

Some of what you have said is correct (when speaking of the physics and engineering of the situation), and some of it is not.

You approach this problem as though there is only one right answer, and that the question is always caused by the exact same thing. In reality though, with a system (or problem) this complex, there is never just a single cause, nor is there only a single remedy.

I'm not angry, nor do I have a "hard on", I am simply speaking the truth plainly, and you don't appear to like hearing it.

Take it or leave it, it's no skin off my ass either way.

 

any info on how or what going to be done...

 

He is getting a Schroeder hot rod steering setup.

 

The caster argument is valid during high speed issues but at 8* positive + this is more a ease of steering.
The other that I think of is the condition of the tie rod ends, as a front end mechanic from years ago we could see if a tie-rod has ended it's useful life based upon the deflection caused by moving the tires while the vehicle is up on rack. Minor problems are amplified with the use of the front wheels and tires as added torque upon the pivot point "fulcrum" to cause the deflection of said tie-rods internal bushings, this can also be done with a large pair of channel lock pliers. The other issue is the coupling sleeve that ties the tie-rod end and the draglink together, if these are not completely tight then a deflection could occur. By and large the main culprit to death wobble is the harmonics of the oscillation of the tire as it increases with force incurred by the "bump" which is why the steering stabilizer was invented (no different than why the shock was invented to prevent up/down harmonics of a bump on a spring)

So...replace the tie rod ends and tighten the piss out of everything else to prevent harmonic flex to amplify the problem.

 

I have a similar wobble form bias ply tires. My tube is from speedway and it seems plenty rigid to me...But I did fix my wobblefor the most part Originally i could go about 65mph on the highway anything over and the car would shake my teeth out. I mounted a VW steering stabilizer shock to the cross shaft and all the wobble is gone...I get a little shake at about 80 mph but it is bearable.

 

Wow - just read all fifteen pages - some damn good info here and a little drama too! Thanks for all the great input! Answered a lot of questions I had after lowering my '40 today.

 

ok here is the deal i need one of the front end gurus to expline to me how my drag link being long and at a angle,would cause the slow speed wobble,my opinion is the drag doesntknow its like that, all it does is push and pull the steering arm, pls explain this to me

 

Stupid question but have you checked to see if the rims are bent or tire has an issue?

 

How's this for long and at an angle? Maybe not real long, but I got the angle part

I had the wobbles last year and fixed it without a damper by following Dick Spadaro's advice on last years wobble threads. My problem turned out to be that the steering box was not centered to exactly dead ahead with the wheels straight. It was off by I think 1 1/2 inches or 1/2 inch, I don't remember exactly, but that was the last thing I did after all the other checks were done and corrected. Including squaring up the axles to the frame, etc, etc, etc.

And I had it all last year, slow speed, medium speed and high speed. At different times as I worked thru the fixes.

I can now run 36 psi in the fronts to keep the heat down and never anymore wobbles.

Frank

 

not a stupid ? all is new rims are not bent, tires good, i just need for all the scientific talk and theroies to explained in plain english...

 

thanks thats interesting, my box is tight and centered, i can drive it but as i go over bumps in the road i put on the brakes a little and it doesnt do it,, othe than that drive great doesnt pull or bump steer...

 

Jeebus!? you fuckers still debating this shit? 15 pages at 99 posts a page..............................As was said before, "It ain't rocket science!"

 

i agree, its not rocket science, i just need to know in plain english, how a drag link, if its long and at a steep angle is the cause of a slow speed wobble, it doesnt know....it pushes and pulls the steering arm....

 

Ok draw a, lets say three inch line angled up from left to right at a 30 degree angle. Now draw a horizontal line that is also three inches and that meets the angled line on the right end. Or more simply draw a 30 degree angle with equal length legs. This angle is going to represent a 3 ft. drag link on. lets say a cowl steering car. The 30 degree angle represents the car at rest and the level line represents a hell of a bump. Now draw two vertical lines from the open ends of the angle's legs. The distance between these two lines is how much that bump moved the steering arm as the drag link moved through it's arc.

 

sorry didn't mean to hijack this thread, i started a new one thanks...

 

777, you have a private message from me. A response would be greatly appreciated.
Dave