Technical The search for a cure-Death Wobble

This has nothing to do with the new BS design thin/low side wall new ugly crap{{{{{{
But tires we should be using.
Tire psi seems too be misunderstood a bit,,in regarde to using a tire on hotrods.
#1 is you can not go by marked on tire psi,or psi of car factory,or any other pre marked down #=
The only way to set PSI is to play some with,the tire is not on any type of normal use,so its up to you to be smart enough to know this;;;
The psi of a tire depends on the load on the tire!!!!!!!!!!!!!!!!!!!!!!
With car loaded and on flat ground,look at the side wall,and tread. If you see no flex in side wall near the ground,then your tire has too much psi for the load,,If your tire has more then about a 1/2in. of flex/bulge showing near the ground=too little PSI//////In between this ,is the ideal setiing< but you do need too play a few psi up or down for best feel n ride for street driving. That goes for both front an rear!! You need a small bulge,about 3/8in. so tire can do what it is made to do.

Not to be for drag race settings though,that's another game,for racing !!!

 

That’s almost exactly what I have minus the unisteer.

 

Usually, a steering stabilizer (frame to tie rod) will solve most death wobble issues, but you MUST have a panhard bar (frame to axle) to first stabilize the axle.

 

I have dealt with this Death Wobble for …well a long time like sixty years! I think there are two different kinds! One that comes in at a certain speed and one that come in only when hitting a bump in the road! If you have the one that comes in every time at the same speed, it is usually your tires or wheels! If it comes in only when you hit a bump it is probably something in the alignment. This is assuming all parts are tight and no slop in the steering or suspension.
I highly recommend stopping the death wobble before adding a damper on any straight axle.
This is a very hard problem to stop! Everything can be be right or measures right, but for some reason some vehicles are prone to it! I had that problem on three identical new trucks one time! One did it the other two didn’t! They were exactly alike and measured exactly the same on alignment! I solved the problem by changing the castor to one degree out of specs. This truck is still running today and that was twenty years ago! Just my experiences!






Bones

 

Hey Chris , you didn't mention what front brakes you have although I assume you have disc's . When I built my 29 It had a horrible death wobble. Tried different wheels and tires, double checked all the adjustments etc. In the end it turned out one of my OE F100 front drums was waaaay out of balance. Changed out the drum all was well.

 

It’s a disc setup. I’m taking the wheels to the shop to check balance soon.

 

My comment about tire pressure came from my experience with my 40 sedan that had a dropped Super Bell, split bones, CE spring and radials. I experimented with tire pressure a bunch in search of a good ride. Never experienced the wobble while trying different pressures.

 

I was just pouring myself a Crown and Coke and it dawned on me, are those front radials Cokers?
I had their bias looking tires on the front of my roadster and the first time I hit 60 I nearly shat myself. Tried balancing them and finally sold them on CL and bought Auburns for the front end. I've had it over 100 and it is smooth now.

 

No, they are some tire made of Chinesium I think. They are a VW sized tire, so my choices were limited.

 

There are some decent imported tires out there, and some that are cheap junk. I've had good luck with GT Radials, and another brand I can't remember the name of right now. Yokohama makes a good truck tire, but may not have anything small enough for you.

 

I don't use a tape measure between wheels to measure toe in. Too floppy, and wont work well enough to get a good measurement. Instead I use two sticks that are about 3/4 length between the wheels. Puah one against each side in front of the axle, and tape them together where they overlap. Then move it to the backside and see if it has 1/8"-1/4" of gap when held between the wheels.
An oldtimer who aligned truck axles locally showed me that trick back in the late 60's and it's what I've used for all solid front axles ever since.

 

If memory serves, @blowby made up something that works pretty good for alignment.

 

Yup. Work great.

https://www.jalopyjournal.com/forum/threads/tech-week-toe-in-tool.1229275/

 

What front wheel size and offset/backspace are you running. Post up a front on pic so we can see the scrub radius.
And, are you running a panhard bar?

 

Roothawg what kind of wheels are you running? I chased a death wobble on my 31 A and like you found a problem that may have been compounded by more than one thing. I had a deflection in my steering box mount, had a used 50 ford F1 steering box and column, wobbles in the 34 Ford wire wheels I was using, out of round Coker bias look radials, wire wheel adapters, no panhard bar, and no steering damper. First thing I did was dump the wires wheels and adapters for new steel wheels. Also Coker replaced the tires for me as well as they wouldn't road force balance. Think pressure was at 32 lbs. I removed the F1 box and installed a new Borgeson vega box and a new column. I installed a pan hard bar on the front axle along with a steering damper and a new tierod. (They can deflect as well!) Did a tape measure alignment. Caster at 7 degrees and toe at 1/8" in. Things have been fine since. I have about 150 miles on it since then and have run it up to 60 mph with no ill effects so far. It made me nuts to chase this down but by working through all the things that could be effecting it I was able to finally solve it.

 

Did you identify one thing that caused it, or did you do everything on your list at once and it went away?
Genuinely curious about the cause. I have a theory that you need a track/panhard bar when you split the wishbone.

 

Root, check to see that your wheelbase is the same on both sides

 

Get yourself a trammel, or make one. Makes things much easier.

On the tires, get them on a Hunter Road Force balancer. This will show if there's an issue with them or not, tested under load, not just rotation. Their website has a location finder.

Chris

 

If you are employing cross steer , you need lateral control ( panhard , watts link ,etc) if you're using side delivery AND your front spring is installed properly ( preloaded) then you don't need the lateral control as that is the function of the preloaded spring .

 

You are thinking bump steer, no?....... Not the same as 'death wobble'.
Are you sure the tensioned spring can still locate everything properly when you lose the triangulation by splitting the wishbone?

 

So I have to ask, Kingpins have any slop?

I would check everything that rotates for balance and not warped or bent also as mentioned above.

Is the shackle angle at 45 degrees roughly?

 

I used the "stick" measuring tool. According to my "stick", I ended up between 3/16 and 1/4 in toe in to stop the wobble, adjusting 1/16 at a time..

 

Thanks guys, good stuff.

I'll see if I can cover the questions.
King pins are the needle bearing style and tight.
The wheel offset is a 15x5 with 2-1/2 offset- Wheel Vintiques
No panhard bar
I haven't checked the wheelbase, but I probably should.
I don't have any bump steer issues, only the dreaded death wobble. It was at 35 mph only, then 35,45 and 60. All the most common usable speeds.

I'll probably removes the stabilizer to see if we really got it. My other 36 p/u has never had a death wobble issue, so this is weird to me.

The cause has yet to be determined. I am waiting on a U joint from Borgeson to finish the steering and it has been raining buckets here for days. My shop doesn't have a concrete drive, so I'll have to wait on the test drive.

I'll try and take pics of the front suspension while it is up on the lift.

I do need to build a trammel of sorts. Chalk lines and tape measures suck.

 

I had the Wobble problem twice. Dropped my axle and found the spring was loose but the u-bolts were tight. The spring spacer was not thick enough. In the process of putting it back together now with a thicker spacer.

 

You know one thing I forgot to mention was that I added 1 medium length Ford leaf back into the spring pack. I thought I might see if that was an issue. I was teaching my nephew how to radius one and taper them. I doubt it will make much difference, but Mine sits so low, it makes it hard to hit very steep driveways. I can't tell that it raised it much.

 

I looked at a set of those online the other day. I was wondering...

 

The following is from this thread: https://www.jalopyjournal.com/forum/threads/scrub-radius.1135446/
It makes a lot of sense, yet is almost always ignored...........

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Positive scrub radius (KPI line or SAI line hits the ground inside the center of the contact patch) imparts a toe out force on the tires. This is typical of RWD cars.
^^^ This is important to understand, as factory toe setting spec was determined by the car manufacturer to work properly with the design of that cars OEM front suspension. If a rod builder has no clue about keeping the original scrub as designed, then the other alignment settings certainly will come into play. More on that below

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Also, the greater the scrub radius, the more the bumps, pot holes and other pavement variations cause the wheel to be deflected from it’s desired path.
^^^ this is what must be understood fully when building the traditional styled solid axle cars. You might think that you are a strong guy and can deal with the steering wheel being yanked as you hit a hole, but think of how many front end components are involved in that chain reaction to get to the steering wheel.

This gets deep. If your build has heavy caster, that caster "tries harder" to keep that tire/wheel from "steering outwards" as it hits a pothole. However, you must then think about that when that tire/wheel does steer outwards, despite that heavy caster, it rams the tie rod in compression, against the opposite side which with it's heavy caster, tries to stay straight. More below on those forces:

Think about the typical OEM Ford spindle arm design. They certainly do flex a bit if the force is great enough. That flexing will increase on both steering arms if the caster is heavy, because that opposite tire/wheel is trying to stay straight due to the heavy caster. Then, you also must think about the tie rod itself, as if it is a thinner wall tubing, it too can flex. This was proven by a hamber here some years ago. He was chasing death wobble and he was big enough/strong enough to lay in front of his beam car on stands and grab both tires to spread them, and pull them, and he saw the tie rod bowing.

That bowing, along with other things flexing with heavy caster can cause death wobble. Death wobble is best understood as a chain reaction, and "any" flexing will cause that reaction to amplify. That is why a steering damper will prevent it, by dampening that chain reaction...and flexing.

What I am trying to get across is to be aware of scrub radius. Too much will amplify the forces transferred through the steering system, causing flex in components. When this flex hits 'resonant frequency', it can cause death wobble. I will get shot down for this, but it is a real problem. Too many just don't worry enough about wheel offset/backspacing and scrub radius, and keep chasing their tail with the wobble.......

 

Those are what we use on our B-mod! They work great, but do not address tire run out.






Bones