Model A straight axle shake. WHY?

Your recirculating 'ball & nut' Mustang steering box must have a snug feel area in the straight ahead driving position. Remove the drag link at the pitman arm. Turn the steering wheel carefully to full lock (either left or right, doesn't matter). Now turn the steering wheel to the opposite lock counting the number of turns. Let's say you end up with 3 1/2 turns. Divide this number by 2. This equals 1 3/4. Carefully turn the steering wheel back 1 3/4 turns - THIS IS THE STRAIGHT AHEAD POSITION OF YOUR STEERING BOX! Loosen the adjuster lock nut and with a screwdriver (usually) tighten the adjusting screw just snug, and then tighten the lock nut. With the front wheels pointed exactly straight ahead, trial fit the drag link end to the pitman arm. If it does not line up, adjust the drag link length until it nicely fits into the pitman arm. This procedure agrees with factory service manuals and tech-school instruction.

Oh, BTW #55, it's a Ruckstell 2 speed rear axle and a Rajo OHV cylinder head! These parts are not interchangeable!

 

Patrick Congrats on finding some of your problem. I had a similar deal on a 39 ford and corrected the problem with new front tires and ,,,,Did a check on wheel bearing adjustment. I tightened both bearing nuts a quarter turn snugger and that made a big difference .I was told by a front end guy up in his years to try that and for me it worked great. He said once the tires go down the road a few miles to snug the bearings to ,quote set them. Just a suggestion .

 

My front axle does not have a parnhard bar or a steering stabilizer shock.
I am puitting some consideration into installing a stabilizer, but don't feel it's a necessity. As said before, the steering wheel does not shimmy.

 

Not in the strictest sense. Panhard bar prevents lateral ch***is shift in relation to suspension. Stabilizer bar prevents ch***is roll. Subtle but distint difference.

Bob

 

I changed the front spring on my 32 to a super slide. The car now has some shudder on bumps and follows the worn places in the riads. Wish I had have left the TCI spring in it.

 

The shudder is easily explainable. Interleaf friction was providing some degree of damping, much like a friction shock would. Eliminate the friction and you've got a spring that can oscillate in the absence of proper shocks. No reason for a spring change to cause wandering unless you inadvertently reduced the camber during the change. Doesn't seem likely but possible?

Bob

 

Your entire front weight of the car hangs on the spring shackles. If some other factor is introduced, be it out of round tires, bump steer, even out of balance rear tires, the car can begin to swing right to left on the spring shackles. and once it starts, it is self aggravating, and gets faster and faster untill it shakes the royal **** out of your car and scares you damn near to death. This is know as "shackle rock"----also as "Death Wobble". A Panhard rod on the front axle prevents this from happening. The Panhard should be attached to the frame on the same side as your rear Panhard bar, and in a perfect world, that should be the drivers side. Many people get away without a Panhard rod. I always use one, and have never had any front end woes on any dropped I-beam hotrods I have built over the years.---Brian

 

Patrick,
What I hear you describing is what we call a "tramp resonance" where I work. Tramp is the one-wheel-up-other-wheel-down position of a straight axle. It just means that the combination of the the spring rates (tire and leaf spring) and axle m*** (wheels, tires, axle) are such that the axle can go into resonance around it's center point. it just takes a good one wheel bump or even a tire imbalance to set it off. You can change this condition in one of many ways. You can change tires or tire pressure. You can change the front leaf spring rate. You can change the tire/wheel m*** either up or down, or you can try to dampen the resonance by changing shock damping. The tires would be the simplest thing to change. I'd try just subs***uting some different wheels and tires. You could also go back to a multi-leaf spring pack. It will have much more friction which will help to dampen the resonance and will also likely have a different (higher) spring rate which will alter the resonance. It sounds like one or more of the changes you have made have altered this spring/m*** system and changed the tramp resonance already.
Dan

 

try and find a garage that has a on the car tire balancer. you may have a drum that is out of balance. i had the same bproblem this past weekend only no balance machine available. my mechanic buddy did it by eyeball and spinning rim and tire.

 

With the different pair of front tires, this description of the front axle wanting to move left/right with available shackle movement makes sense as the car doesn't have jack*** mode. It has a slight uneasy feeling that doesn't make the radiator wave goodbye at you or to see the top of the front tires.
Another good observation that helps with my car. Thanks!

 

I believe this nails what I have been trying to describe.
Replacement wheels are much lighter than the Jeep wheels being used at first. Shock dampening could also be a minor player to correct too. Tire pressure seems to have a major effect, thank you Turdytoo.
I'll try another pair of new tires on my front wheels to prove tire age to potentially be another major factor in my car having jack*** mode.
Y'all keep thinking out loud and reading, please!

 

Is this also to say that if the car had Vega cross steering to the right front, instead of Mustang steering using a forward pointing drag link, that the car would have a change to steering wheel influenced problems that could be tried to be corrected by a steering damper? Can only be corrected by a panhard bar, then to fix the problem that creates "shackle wobble". Wheels, tires, and related.
Tornado warnings here.

 

I have had this problem and heard about it a million times. i have also heard as many solutions. A dampener solved mine, but after i installed it i was told that shims in your spindles/axle will do the same thing. and it looked stupid! your spindles should be shimmed enough that its hard to turn with your bare hands. im sorry if i missed something in this thread, havent read it all, but ive read these type of threads before. just imagine how easy it was to turn your spindles when you were putting the car together, and then recall your driving experience. anyways, i challenge you to try this, and im sure its the cheapest fix around!

 

I am not sure exactly what you mean. Just my own experiences; I feel that "if" some setting are not correct, a car with cross steer will fare better than the same wrong settings on a side steer car. But I don't know why.

All this stuff seems to be caster related IMO. It seems like if the caster goes a bit too far, then everything else needs to be perfect; no slop, strong tie rods, etc. But if the perfect caster setting is found, you don't need to be so fussy with every little thing.

 

It's not caster. I now know I have less caster than I had thought, it's neighborhood of 3 degrees top of kingpin laid back. The car does not want to dive to the outside of a curve while turning. The car is very comfortable to a point to drive thru a hilly switchback type road. The car is comfortable in a straight line at over 85 mph, based on pace car in mirror.
What I tried to describe with Vega cross steering is the monkey motion of the axle under the shackles moving left/right, to give feedback to the steering wheel that shakes the steering wheel left/right also. The transverse spring/shackle combo allow to have left/right motion.
With steering to the left front spindle from a forward pointing Mustang type drag link setup, the monkey motion is not transferred to the steering wheel as to be fighting a left/'right.
With Mustang steering, you float with road and suspension influenced bump steer as you guide the car. Parrellism is one part of how this geometry can work correctly and not have bump steer.
More tornado damage and warnings in central Arkansas.

 

That was not my experience 40 years ago with a parallel sprung I beam with side steer/drag link box during death wobble. I could not hang onto the wheel.

At 19 years old, I did not have the skills or brains to fix it or understand it. No internet to use, and the rest of the car guys chasing the new muscle cars..no help anywhere.

10 years later, I built a clone to that car but was adament about using cross steer for some reason. I used a 605 P/S box cross steered. No damper, no precise settings were needed. Never twitched a bit. Wish I had caster/camber measurements from both cars to compare.

There is some sort of correlation on the correct caster vs. camber vs.wheel offsets IMO. My old car got the wobble for the first time when I swapped on a set of deep dish Keystone mags. I hit a bump in the road surface from a buried cross culvert; that set it off in a big way. I think I either bent something due to the intense violent shaking, or loosened something up. When I switched the wheels back, it was better, but still would wobble in unique situations, and over time, became undrivable that summer.

i am talking theory mostly when I say caster, camber and offset all need to be figured in.

It seems that the imaginary lines through the kingpin are the most important things. When you look at the front of the spindle, the line though the pin should intersect the center of the tire where the tire touches the road. Wheel offsets change that, as well as adding Buick finned drum IMO. If that offset causes the tire contact point to be far outboard of the king pin line, there is much more reaction force transmitted to the steering when you hit a hole or bump. And I almost think that tire balance is very critical if the offset is too far out.

As far as camber, that changes with adjusting the lean of the axle (caster)

Last years wobble thread, **** Spadaro brought up an interesting subject; I think he called it "caster trail". That, I believe, he said was the area from the imaginary line through the king pin (when viewed from the side of the car) to where the center of the tire touches the road. If you have lots of caster, the caster trail is much longer. It also requires more force to overcome the wheel wanting to stay straight. When a wheel hits an obstacle, caster pulls it back to help keep the wheel straight. A LOT of caster makes the wheel "jerk" back to straight. If you have moderate caster the wheel still wants to go straight, but it won't be so violent. If you have way too little caster, the wheels don't know where to go.

That caster trail **** spoke of makes me understand why sometimes a lower tire pressure helps on some wobble cars with too much caster. As the tire flattens out, the contact point of the tire expands closer to the imaginary king pin line when viewed from the side.

As far as offsets affecting wobble, it's like my old car; my caster must have been borderline before I swapped the deep dish wheels on.

Sorry for the long boring post...after 40 years I still want to learn more about wobble.

 

The Ford designers of the buggy spring set up used the center point attachment of the wish bone on the bottom of the transmission to allow for a twisting motion of the front axle. When the bone is split and two pieces are attached to the frame on each side, it limits the twisting motion to some degree. I was wondering to what point it will stop the rolling/twisting motion of the suspension and then start acting as a spring? Even the 4-bar set ups will do this at some point. Now whether this had anything to do with the problem in this thread or not, I don't know. It might though if the rolling motion is stopped and then loaded with spring action, then it could cause a rebounding effect. Just my 2 cents worth. Ford's idea worked pretty well for a stock model A. To change it adds an uncertainty to the situation.

 

I believe that is a very profound statement

We change many things around without maybe realizing the far reaching effects it can have on the front end geometry.
The first thing most guys do is to toss away those large diameter skinny original wheels, and fit something a lot smaller and wider.
That in itself can screw things up, even without any changes to the actual suspension.
As F&J says:

and:

Wheel (actually tire) diameter has a very large effect on trail. It also effects where the tire steers where it touches the road. Fitting much smaller diameter front tires can make the original trail built into the front end all but disappear. Less trail means reduced straight line stability.
Smaller diameter front wheels require more positive caster than larger diameter wheels for the same amount of trail.
Smaller diameter wheels also requre different back spacing, because the king pins lean inwards and effect where the steering axis meets the road.
So even just swapping out some front wheels and tires can really screw things up if you are not extremely careful, even without touching anything else.
When it comes to bump steer, and steering geometry, that can open up a whole new can of worms when you start moving things around or swapping parts.
It all requires some pretty clear thinking to get a handle on what is going on.

 

My buddys T-bucket had similar problem, all good until 40+mph and then hoo-boy, the front end suddenly started shaking from side to side violently until you dropped back to about 30mph or less.

We tried everything then eventually and nothing worked until we discovered one front rim was 'bent', - we figured it had been backed into from the side at the supermarket local car-park.

New front rim and all was sweet!!

 

This makes a lot of sense.

I'm just typing and thinking at the same time but this is how I see it...could be wrong!

What we call Caster, bikers call "Rake and trail".
The "rake" of the steering neck has to be such that the line thru the neck intersects the pavement in a particular spot in front of the tire contact patch. The distance in front of the tire patch is known as "trail".
It's what allows a bike to be ridden with no hands.

A car is basicly like TWO bikes side by side, in that the kingpin is the steering neck and the line should intersect the pavement just in front of the tire contact patch.
The difference is, that the king pin is off to one side, angled, and not itself directly in line with the tire like a bike setup.
Due to the kingpin inclination however, the intersection point DOES align itself with the road in the proper place.

All this works to perfection with the "as designed" caster angle, tire/wheel size and offset on the stock vehicle.

OK...Changing tire/wheel height on a bike would change the trail part, BUT keep the tire patch directly in line with the intersection point of the neck rake line.

Now...a car is a different story.
If you put smaller tires on the same offset but smaller wheels, you have moved the contact alignment of the tire patch and the kingpin line. Originally, chalk lines drawn on the pavement thru the center of the tire patch and the projected kingpin intersection point would have been parallel to each other...but not anymore!
Your not only changing the effective caster angle by reducing trail...your also misaligning the contact patch and the intersecting kingpin line as the pavement sees it!

Thats interesting!!!
Thats got to have some affect on steering stability.

Kinda like the Ackerman principle, but with INVISIBLE points that are meant to be parallel, but aren't anymore...

With regards to the caster angle.
The original caster angle for the original tires was't enough for the new short tires...but now most of us have gone to taller tires again.
Not as tall as original in many cases, but still a lot taller than was common in the 70's and 80's.
How should the caster be addressed now??? Go a little less than the common 6-7* that has been the norm for years?

 

Bill, I think we are both thinking along very similar lines.

Another small thought.
A typical hot rod fitted with bigs and littles stuffs up the original designed in front trail two ways.
The smaller front wheels reduce trail, and raking the whole ch***is forward reduces it even more.
Neither is going to improve the straight line stability.

 

We are...but its because YOU (and F&J as well) put it in my head and it makes a hell of a lot of sense once you think it thru!
I just rehashed it to clarify it to myself and figured...what the hell...I might as well type it out as I think about it because it could help others to understand too.

Regarding the raking of the ch***is, thats certainly true for a car thats getting some changes without a full buildup. Hopefully anyone building from the ground up will take frame angle changes into account as they set up their crossmembers/suspension.

 

Very few of the guys that have adjusted there way out of a wobble have put up the new caster readings. The few that did, seem to back up your suggestion to reduce caster.

There must be a connection with excessive caster that causes tie rods and steering links to now need beefing up. Hard to explain, but with lots of caster: when one wheel hits a hole and it "steers" that one wheel, that wheel really tries to SNAP back to center as it gets past the pothole. Because the excessive caster makes it snap back harder, that really pushes/pulls on the tie rod and steering link. Is why some guys can "cure" their wobble by beefing up the rods. Before they beefed them, the flex was causing a chain reaction that keeps going??

But why is wobble so affected by just certain road speeds?? I can only guess that it is the gyroscope effect of a spinning flywheel just like a gyroscope?? But that sure does not fit well when talking about the high speed wobble.

 

The fact that death wobble is a self reinforcing oscillation, also implies some kind of resonance may be involved for it to suddenly and violently build up in strength at certain speeds the way it does.

More caster, should in theory improve straight line stability, but just adding caster is not going to fix it if there is another very serious problem elsewhere with the suspension geometry.

Wheel back spacing, bump steer, and shackle rock all play a part in this. Once the whole thing goes completely nuts at some road speed, more caster may in fact cause it to over correct and make the instability worse rather than better.

I don't believe just adding more positive caster is a universal cure for all other problems.
There may be more than one problem to fix, which makes the whole thing a lot more complicated.
But I believe starting off with the correct wheel back spacing, and reducing bump steer as much as possible might be a very good start.

Panhard bars, shocks, and caster add the final finishing touches, but these are not going to band aid over a serious basic geometry problem that lies elsewhere.

 

The wheel itself is a very easy thing to neglect in all this. As you've already said, diameter and offset can really mess up what looks like a good suspension/steering system.

From all this I realize that we've been kinda focused on the WRONG part of "Caster".
We focus on the angle itself when in fact we SHOULD be focused on the amount of "trail" our caster angle creates.
Motorcycle builders are just the opposite.
They set their "caster angle" (Rake) to create the amount of trail they require. If they require 1 1/2" of Trail they set their steering neck accordingly.

We set our Caster to the commonly accepted angle and don't even bother to check the "trail" measurement that results due to our tire diameter, or if the wheel offset is even right for that diameter...

Amazing...

What we NEED are some "trail" measurements that have been proven to work on given tire diameters.
Also...wheel backspacing measurements per given tire diameter, as used on early Ford spindles so that the tire patch is located correctly, as per the projected kingpin line.

Has any of this even been considered in the past? I've never seen it written about in regular sources.
Maybe in suspension/handling books, but not for common use.

 

The way I measure the required wheel offset (or back spacing) is to get a spare steering knuckle and p*** a snug fitting steel tube or bar through where the king pin or ball joints go.
Fit the wheel hub, along with the brake drum or disc, and place a straight edge across the flange where the actual wheel bolts on.
The steel bar and the straight edge will cross over at some distance below the hub, due to the king pin inclination.

The trick then is to know the exact rolling radius of the tires you plan to use.
You can then see on your straight edge how far away from the steering axis (steel bar) the wheel flange is at the required rolling radius of the tire.
It is then a case of figuring out how far the wheel rim needs to be moved off centre, either in or out, so the steering axis ends up exactly right in the centre of the tire tread on the road.
This can also be done on the car, but doing it off the car is a lot easier and probably a lot more accurate.

 

That is proof that my lack of any engineering education prevents me from coming up with any answers. I can say I never even thought about tire diameter affecting tire patch with the exact same offset.

There must be a complicated formula to match the correct caster to all the variables in our different choices of tire sizes, offsets, and many other things. I guess then, that there is no "carved-in-stone" caster setting for a Ford I beam...

 

It is not that complicated to calculate trail for different caster angles and tire diameters. Trail = tire rolling radius, multiplied by the sine of the caster angle. This is not strictly correct, but it is mighty close enough for small caster angles

For instance 26" tire, and six degrees of caster.
Trail = 13" multiplied by the sine of six degrees, which is 0.10452
Trail = 13" x 0.10452
Trail = 1.359 inches

Here is a table for looking up sines of angles: http://chestofbooks.com/architecture/Safe-Building/Table-LI-Natural-Sines-Etc.html

 

Ok, I can understand that formula to measure the trail on a given build...but then how do I know if the length of the trail on my mock up is "perfect, or too long, or too short"?

 

It depends on the application, or more specifically the weight over the front wheels, versus steering effort.

Something like a slingshot dragster, or chopper motorcycle with the front wheels way out in the distance can handle insane amounts of positive caster.

Something like a full sized pickup truck with cast iron big block engine right between the front wheels, with any more than minimal caster, not even Superman could steer it.

So more is better, until the steering becomes just too heavy.
But as said earlier, a lot of caster may also become unstable if something else is drastically wrong. I think just trying a few different settings may be what most of us will need to do. If you can set up your suspension so that caster is easily adjustable, that cannot be a bad thing.