Steering Death Wobble

[Michael Dimock]

62 Scout with j@@p wagoneer Dana 44's. Front is a 1975 w/ Ford hubs and rotors, SOA, highsteer, dom tierod and draglink, new 1 ton tre's, new balljoints, and new calipers and pads. I have a 1972 IH 1100 or 1010 pu saginaw power steering box with a wagoneer pitman arm. I am running the stock Scout 80 steering column with a steering ujoint in the shaft that is feeding into the stock belljoint coupling on the power steering box. The wagoneer front axles have 6* of caster built in. Also, the power steering box has a slight bit of play in it. I'm running unbalanced 33x12.5x15's.

Now that I've presented the facts, here is the problem. I've taken the Scout for a drive 4x so far, logging a total of 20 miles. All of these miles have noticeable vibrations from the unbalanced tires. On 2 of the trips, I have been driving at about 45mph when vibrations from the front start to become noticeable, and then the tires whip back and forth violently but not very far in each direction. In each of these cases I braked hard and when I got to about 10mph, it stopped. In each of these instances, I have pulled over and checked everything and nothing is loose or broken. I am not running the power steering at this time, only strong arm steering. Can driving it without the power steering running cause a lack of "hold" on the steering? What other issues could cause this problem? To say the least, I'm not going to be driving it until these problems are resolved.

 

[Michael Mayben]

First off..."caster" is a relative term related to the entire mounting of the axle and related components, in relation to a level plane (the road surface). It might have been six degrees when checked on an alignment machine on the vehicle it was originally manufactured for and mounted on, but I promise ya, it's not six degrees on this morph'd boneyard deal!

You also must look up the original alignment data for the donor vehicle. While that axle assembly might have had six degrees of so-called "caster" designed-in in relationship to the centerline/plane of the pinion, it could have had the spring perches mounted to provide no telling what kind of kingpin inclination relative to a level surface which is the reference point.

Caster is a geometric term used to denote "kingpin inclination" or it's variations which use ball joints instead of kingpins.

This rig needs to go on an alignment rack and have determined what all the numbers are right now. That is a starting point for what to correct.

The problem is not caused by the botched-up steering gear, it's a geometry issue that must be corrected once ya know what to correct and which direction.

Do some research regarding basic motor vehicle alignment vs. Dynamics. And I'm not talking sum kinda four wheel alignment shit for hondas or toyota mcpherson strut systems. Look for truck stuff with a solid front axle then extrapolate for a 4x4 steering axle.

 

[Robert Kenney]

Adding to mm's post, even if everything is tight and to spec your front end will have a natural resonant frequency and adding unbalanced tires can give you the necessary kick to induce the wobble.

Very common on buggy spring early Ford front ends.
Some steering systems are so sensitive that to much air in the front tires can undamp the system enabling the wobble. My model a is exactly that way and it has 5 degrees of caster.

Balance the tires inside and out side not just one or the other.

Lower the front tire pressure 5# at a time and see if that helps. I run 25 psi on my Scout.

Add a steering damper. Many 80's had them for this reason and others.

 

[RobertC]

A couple minor issues to look at as you follow michael's / Robert's suggestions --- particlularly the alignment.

Are the front tires the same brand?

There can be some variation in specs -- particularly diameter -- between brands / tire makes.

I do not think there is as much variation as there used to be because of the computers / abs / etc on new vehicles.

Also, did you check tire pressure to see that both front tires are the same or close to the same air pressure?

 

[Michael Dimock]

All the tires are the same brand. I ran them on the Scout last winter and there was some noticeable tire vibration, but if I have 2 of the worst tires up front, then it would definitely be more noticeable. I have not checked the tire pressure since I took them off 6 months ago, but at that time they were all an equal 18 psi. These tires max out at 20psi believe it or not... I made the new tierod the same length as the old one, but I haven't checked toe in/out. The guy at napa runs j@@p j-20 and wagoneers and said to run the axle with 1/4-1/2" toe in. I had the axles set up by dan hayes when he was running tractractor, so the caster angle better be right! Eyeballing the knuckles, you can see the steering arms sloping backwards, therefore inducing that there is some negative caster. I have a year old steering dampener that I ran on the Scout before that I will put on. The highsteer is going to make mounting it a slight bit more difficult, but I think I will weld on a bracket to the diff cover.... We will see what my mounting options are. Doing some reading, it seems that running the power steering box without the pump running can add a bit of play to the steering as the fluid is not holding everything tightly together and cushioning the steel parts like it should. I have been on trial and error to find the correct length belt and I think I finally have the correct one waiting for me at napa, so we will see.

List of to do's:

set front axle toe in to 1/4-1/2".

Add power steering belt to get the power steering system operational.

Have tires balanced.

Check tire air pressure and adjust accordingly.

At this point mm, getting it to an alignment shop would be a little too hazardous to my health and others, so I will at least need to get it in a non/less wobbly state before I attempt the drive up and over the mountain. Here are some attached pics that I took before I left.

 

[Robert Kenney]

Do you know how to set toe using a tape measure? Fairly important and definatly do not want neg. Toe

 

[RobertC]

I think toe-in of 1/4" - 1/2" is way too much.

I think 1/16" would better.

I checked my t/a manual and all the axles specs state 1/16" toe except a couple state 1/8"...

But, I know nothing about h**ps or soas...

Afaik, toe-in is used to keep the wheels pointed straight down the road.

When I had the suspension on one of my corvairs rebuilt by a suspension expert (he worked on porshes, ferraris, race cars -- had a d-type jag in the shop one time.), he set the toe-in on the corvair at the minimum spec (1/16") with a statement that radial tires had less drag than the original bia-type tires that the specs were based on.

Also, you need to have the alignment shop check the camber -- as well as toe-in / caster...

As robertk stated, you definitely do not want "toe-out"...

Stay safe...

 

[Robert Kenney]

Robert,
a large diameter tire like md has will measure a larger toe at the same toe angle. 1/2" is still a bunch. I use 3/16-1/4 on 31-32" tires. A small tire like the corvair will like 1/16- 1/8 and of course
to each his own.

 

[Michael Dimock]

As I understand it, to set toe in I need to jack up the front axle and place it on jackstands. Then, get a tape measure and find a reference point on the tires half way up them height wise and measure the distance apart in the front and then the rear. The front should be the desired toe in distance smaller than the rear. I will have to do some more reading to determine what the proper toe in for my axle is.

 

[Michael Mayben]

Did dan set the knuckle twist with the weight of the vehicle on the axle with the wheels mounted??? I think not! No doubt it was a bare axle housing in the axle jig.

Actual king pin inclination has to be set with the axle mounted exactly on the vehicle...with weight on it just like you were gonna drive it. Vehicles don't move down the road with their axles in jigs in some kinda "locked down" position, the suspension is constantly flexing and moving around...especially the weirdazz Scout "forward shackle" design! So the caster/camber/toe is a factor that is derived during oem vehicle testing, then those specs are handed off to spicer to incorporate in the axle spec'd for each vehicle.

So setting the caster (knuckle twist) ina jig is at best a guess...unless someone did this exact same axle, orienting it in the jig in the exact position it would "see" the road surface if the weight of the rig was on whatever kinda SOA/botchinated front suspension you are going to use to replicate normal ride height, with the vehicle exactly level, and then determined the setting for the twist...only then could a derived kingpin inclination "amount" be arrived at. To say..."just twist it six degrees" is relative to what??? Spring perches, pinion centerline, pumpkin gasket surface, etc.? How was the axle jigged when twisted...from the perches? We're the perches located and solidly welded in before this knuckles were tweeked? Or was that done afterword?

Jeff and Darren have derived this kind of data from performing many knuckle twists over time, and knowing in advance the entire suspension pick-up points (such as the spring perches), spring deflection, etc. And having the vehicle suspension "loaded" (using much weight stacked on the bare chassis if need be). And...that factor is determined by what the final suspension design will be regarding this Scout stuff...shackle forward, reverse shackle, coil with links, etc. Then all the final adjustments are made with plumb bobs and exact measurements on a level floor using a very precisely laid out string "grid.

When you set up your front suspension this time during the springover, ya didn't even have the engine weight compressing the suspension did ya?

Paying $50 to an alignment shop to tell ya exactly what ya got going on is money well spent. That makes it real easy to simply set the toe correctly and get that part out of the way. Then you will know what ya gotta do to correct the rest of it.

 

[Michael Dimock]

As far as what dan did idk. All I did was give him cash and instructions to make me a SOA, freshened up set of axles with correct caster to bolt on my Scout 80 using the stock springs. When I put the tierod on the axle, there was no engine in, but the draglink and box were finally mounted the same day the engine was installed. It took a week for the Scout to fully settle, as the pitman arm/ tierod clearance did become smaller. I then had to move the power steering box forward to compensate for this as well as adjust the draglink. I plan on getting the rig to an alignment shop, but I have to be able to at least get it there somewhat safely.... for now I am just trying to get it driving straight down the road before I dial in the steering geometry for good. I am planning on adjusting the toe first thing when I get a chance next weekend, as my guess is this is probably one of the major culprits....

 

[Chris Pucci]

Next time it develops the death wobble. Step on the gas and see if you can 'power' out of it....

if this was my truck I would:

1) check caster as it sits (park it in the levelest spot you have and stick an angle finder on it)
2) get all the tires balanced and aired up to max.
3) check bearings to make sure they are tight (to spec)
4) check to see if one of the hubs is locked accidently.
5) check all joints, rod ends, etc for slop.



My Scout gets some nasty death wobble on pavement at about 30 when the hubs are locked and the tires are really flat. If the surface is loose/rough I don't notice it - but if it is smooth it gets pretty bad and will get pretty violent with the hood shaking side to side.... I prefer to power out of it. As the front end comes up under heavy throttle the death wobble goes away....

 

[Michael Dimock]

Sounds like a plan chris! I was actually contemplating accelerating the last time it happened, but decided to take the safer/wuss alternative and stop.... As far as joints go, all are new and I have tried to shake/move the tre's and knuckles and there is no slop. The bearings, I haven't checked yet. No angle finder available, but I will throw a level on it just to check. Slightly better than eyeballing I guess... I will check the hubs as well. More things to add to my to do list.

 

[RobertC]

as I understand it, to set toe in I need to jack up the front axle and place it on jackstands.

Toe-in is measured with the vehicle on the ground / suspension "loaded".

My t/a manual states moving the vehicle back and forth 15 - 20 feet to get the suspension to "settle" after it has been jacked up and then measure the toe-in.

Manual also states that toe-in is set after camber / caster is correct, but you should correct excessive toe-in or toe-out if it exists as the vehicle "stands".

Since you feel out of balance tires is an issue, I think you should take the tires off and haul them "over the hill" in your travelette and get them balanced. The out of balance tires could be adding to your possible "suspension issues" that mm listed above.

 

[brained]

as I understand it, to set toe in I need to jack up the front axle and place it on jackstands. Then, get a tape measure and find a reference point on the tires half way up them height wise and measure the distance apart in the front and then the rear. The front should be the desired toe in distance smaller than the rear. I will have to do some more reading to determine what the proper toe in for my axle is.

To make your reference point grab a piece of chalk, make sure it has a good point to it. Spin the wheel. With a steady hand touch the point to the spinning wheel - it'll make a perfect reference line all the way around the wheel. By using the point of the chalk and keeping a steady hand you can make a very narrow line, the more narrow the more accurate your measurements can be.

Repeat for the other wheel.