spring behind and death wobble?

Contrary to popular concensus and advice that you have received TOO MUCH CASTER is one of the main causes of "death wobble"
You need caster to self allign the front wheels in a straight line [ 7 degrees+ is the popular number ]

When you have too much caster on a crossleaf then problems happen.

This is usually on a crossleaf with shackles and no other form of lateral location .It relies on the weight of the vehicle to equalize [ or centralize ] the spring shackles

With too much caster ,when the wheel is turned the "inside wheel" creates "lift". Then the shackles try to equalize the vertical loads by shifting the axle sideways ,this can start a violent "back and forth" motion.

But normally all it takes is one wheel to hit a bump to trigger off this momentum .

The unequal load of the bump causes the steering to try an equalize by turning towards the center of the car [from that side] which then transfers lift on the opposite side [ which then tries to correct itself ]
While the caster is trying to equalize the veritical loads , the shackles are also trying to equalize the vertical loads by shifting the axle side to side [ which can cause even more alternating "caster lift" ]

If you like the positive feel of a lot of caster then use a panhard bar or watts linkage [ A steering damper is a remedy but it doesn't fix the cause of the problem ]

Most dragsters, altereds,or sprintcars use torsion bars or solid mount the front and don't usually have this problem

 

I keep hoping a thread will get going on caster someday. There just seems to be something wrong when 7 degrees is always repeated, despite weight of car or engine, w/base, radials vs. bias, scrub radii, offsets, etc, etc.

 

That is exactly what the raven's roadster did. it seemed like the pass side was worse than the driver's side when you looked out over the cowl.

I think that the engine balance was so bad on his mill that it was transmitting a harmonic through the chassis.

 

my coupe has spring behind the axle...........i have more miles on it than the space shuttle with no problems at all...........

 

From the Ford Service School Wheel Alignment book copyright 1936:
"The purpose of caster is to create a tendency on the part of the front wheels to line up in the direction of travel."... "...there must be caster at all times. The amount, however, can vary up to 25% without any noticeable effect on the perfromance of the car. Thus with 8 3/4* caster, a change of caster of up to 2* would have practically no effect of itself on the Ford car, providing the change were equal at both wheels."
"As the Ford axle is constantly held in position by the radius rod, the caster angle is held permanently correct. However, with this construction, the twisting of the axle "I" beam or the bending of the radius rod could change the caster angle."
"In addition to the bent axle, the raising or lowering of the radius rod ball changes the caster angle. The height of the radius rod ball depends on the arch of the springs, which is in turn affected by the load. However, with this construction, the caster angle increases as the load is increased or the spring depressed from any cause." ...
"With Ford construction the caster is increased as the wheel goes up since the movement is pivoted from a point to the rear of the spindle pin."

"Camber is the angle of the wheel out of vertical..." ... "The purpose of the camber angle is to place the load on the large inner bearing."

 

On a hotrod if you move the spring behind the axle with the shackles hanging off the "Bones" it will actually soften the ride by altering the "motion ratio" [ This is desirable in a vehicle that has been lightened ]
Spring location doesn't contribute to "death wobble" , but worn out parts dont help.

If you hang onto the steering wheel tight enough you can hold it straight, yet the wheels are still steering left and right.The problem is axle movement sideways [ this can be even worse in a cross steer ]

[ For example ] If the crossleaf has shackles hanging at 45degrees and the axle shifts sideways, one shackle will try and swivel into a vertical position while the other shackle into a horizontal position resulting in different ride heights [ or vertical loads to be more correct ]

This is what that bouncing from side to side is.

The Caster then tries to do what it is designed to do by trying to equalize the steering "between the 2 different vertical loads" resulting in "steering wobble"


If the front end has the stock wishbone the sideways shifting of the axle has some sort of lateral control [ the axle actually shifts sideways in a radius around the front wishbone balljoint ] .
When the wishbone is split there is no lateral control [ no diagonal bracing ] so the Bones and the Axle try to "parallelogram" itself.

Lateral Control is what is needed, eg: Panhard Bar or Watts Linkage but there are also other creative ways like a diagonal brace on one of the Bones back to the pivot point on the opposite Bone, or a Crossleaf with only one shackle

The purpose of Camber [ positive ] was originally for the wheels to be at 90 degrees to the curvature of the roads at the time , Positive camber caused the 2 wheels to try to drive apart [ like 2 cone shapes ] so "Toe-in" was used to correct this

 

So, I guess just like a bad cut a bandaid is sometime necessary. My '32 was set up by the best frontend man around here with experience going back into the 40's (this car was built in 1990) and after all of his expertise left him scratching his head as to the cause of the wobble a steering dampener has stopped it from happening for the last 21 years.

Frank

 

Well, that's a pile of info I have not seen before on hamb. There is plenty to discuss, but one at a time, like:

^ I have always thought that the "fighting" of the two wheels, to be the reason that some hambers have "cured" wobble by a massive beefing of the tie rod. If you read hamber "Tudor" 's posts on that subject from a few years ago... Another hamber posted on that thread, about his 50s built Bonneville race car with massive caster in the teens(i recall 18 degrees?), and he had to run 1-1/4" dia tie rod tubing..

I have an obsessive interest in wobble because as a very young kid, my 50s built roadster developed an instant, undrivable wobble after only putting mags on the front which changed the scrub radius. It was a twin leaf wavy Mopar axle with side steer. Never did cure it with total lack of education and skills back then (over 40 years ago), and even after re-using the old wheels, it started to wobble again on certain situations.

I am at the point where I need to finish my 32 Ford front setup, and as I am using a very quick ratio Ross box as cross steer with a very heavy early Olds Rocket motor, I want to run minimal caster. My bones are split, but tucked way under the car.

My untouched 40 coupe that I just put the identical engine in, appears to have almost no caster at all, and I can't see how anything was ever altered on the front end...it looks untouched? That car steers like silk while stopped, and in yard driving, but is not fit for the road yet for a real test.

So, I am really thinking hard about the caster setting on my 32. What do you think about 2.5 degrees? ..and why won't it track OK?

 

Personally, I wouldn't reduce the caster too much unless the car had a 4-bar setup. With a 4-bar setup the caster always stays at a perpendicular angle to the frame.
With Bones [incl: A frame wishbone, or any form ladder bars ] if the car lifts the front during accelaration it would result in negative caster. [ It will try to dive side to side ]
This problem is even more pronounced with shortened bones.

When the front "dives" under braking you have more caster, but the brake torque transfered up the bones acts like anti-dive which is a desirable feature.
To think about it, it is the opposite effect to 'ladder bars" on the rear end.

The 7 degree caster that is always recommended on Fords is actually a compromise for the mechanical design limitations

Tracking is usually caused by too much "scrub radius", manufacturers reduce this with king pin inclination, So if you put wider or smaller diameter wheels on you increase the scrub radius and you need increase the backspacing behind the wheels


Because you are using a cross steer you might as well remedy 2 issues at once! Bump steer and Death Wobble.
Fit a panhard bar that is the same length and parallel to the drag link from the steering box. It doesn't matter if it is at different heights or further left or right as long as it is parallel.
The Arc of the axle movement up and down will be the same as the steering, so there will be no bump steer.

The panhard bar also prevents lateral shackle wobble
Then if you're using telescopic shocks , try to incline them inwards

 

Good stuff Kerry. Looks like some real information here....absent of guesses and "I've always done it this way" comments. Even makes sense to a dummy like me.

 

The reason you see so many different ways of setting up front end caster, toe in, etc is because every car and every situation is different from the hot rod next to it. If our cars were coming off an assembly line they could all pretty much have the same specs on those items, but since each one is unique unto itself it will have a certain setup that works best for it.

The 7 degree caster number is a starting point IMO. Some cars work best with less, some with more. Camber is pretty much fixed by the axle, but toe in can be toe in or toe out in some cases. As contrary as it sounds to popular theory, sometimes a little toe out will make your car handle much better than some toe in.

Alignment is also not a "set it one time and never change it" affair. What worked last year on your hot rod may not work this year. Tires wear, bearings wear, the car settles down a little, etc. Last year the car may have liked a certain setup but this year it handles differently and you might have to make adjustments to compensate. Tire pressure is another item that varies.........my car likes 28 front and 26 rear, but some people say their car likes more or less pressure. The point is, there are no absolutes in this hobby.

I find shocks to be a MAJORLY important factor in good front end setup. Wrong angle, weak shocks, no shocks at all, can really make an ill handling car out of one that should be a pleasure to drive. If the shocks can't dampen out the ocillations of the tires as they bounce over bumps the front wheels will progressively go more and more out of control until they finally end up in the DW we get sometimes.

And as for steering dampers...........I will never build another car without one installed. Call them a bandaid, I call them a Godsend. I never have to worry over the worst road conditions or when crossing RR tracks since installing them.

Don

 

Wish I had found the HAMB a few years earlier. Helped a friend finish his bucket and both of us together didn't equal one persons knowledge on here. It had severe tire shake at first and we did the dampner deal tire and wheel change and toe adjust but still had a mild case. He got frustrated and sold it and left rodding and went back to late model Mustang stuff. Now it's like we speak different languages. He talks Whipple Chargers and I talk three deuces.

 

Some don't like to see the caster set higher than 7 degrees but I like to run 10 degrees. More caster results in better high speed straight line stability and increased steering effort. I set mine that way because I like the feel, but some may not like it that way. I have spring behind axle and no wobble or bounce.

 

X2...but more info leads to more wondering

Kerry, when you say that scrub radius is more of why the car tracks, versus caster? My short term duty in the mid 70's in a used car dept, when we had the typical US made rear wheel drive IFS car (mostly on bias) that pulled to one side; we first swapped tires fronts to rear. If that did not work, it was usually a caster problem difference on one wheel.

On scrub; let's say you had a bone stock original 32-40 Ford, but then swapped to Buick drums which push the wheels out, or just put wrong deep dish wheels on... it seems like the toe-in would need to be increased with bias tires that have more rolling resistance?

On caster on ford I beam cars; Is much of the reason for heavy caster, the fact of those cars having a very wide ratio stock steering box with a very large diameter steering wheel? to get back some road feel?

The reason I am trying very low caster, is that I have a very heavy engine, a very quick ratio cross steer box, a very good scrub, and a large diameter steering wheel. If I jack up the center of the I beam to take weight off the tires, my steering can easily hit both stock Ford limit stops. If I set the car down, I can only get to within 1/8"-3/16" on both sides and then the box can't go further. When the car is steered from lock to lock, you can see one fender go up and the other go down, much like what happens with death wobble.

I am redoing the bones as we speak to try the low caster. I am hoping it will get the steering stops closer?

I am thinking I won't have much braking or accel change in the caster, due to the limited spring travel on this car.

 

I was looking for an old Motors Manual to check various cars for caster settings, but the oldest I can find is for 35 to 46 cars, unfortunately most of these would be IFS. Some, like 35 Chev Standard should be I beam?, as well as a few cheaper off brands?

I could not find any caster settings even close to the Fords.

However, I can see where the mythical 7 degree hot rod spec comes from: The book shows all Fords from 35 to 46 as "4.5 to 9 degrees". Do the math, and the middle is 6.75 degrees..

I still don't understand why so much is "needed", can it be steering box ratio? What I also do question, is the random 7 degree setting for "every" ford hot rod built today. It just seems wrong to me.... for example: A 28-32 roadster, with a 392 hemi, using a light duty Vega box, with almost no weight bias towards the rear axle?

I can see the need for more caster on a car with more weight bias, lighter motor, and certain box choice and tire choices, or a car set up for straight line racing.

 

I think we both have a different interpretation of what tracking means ! What you describe is "pulling to one side" in which you're correct about a caster difference causing it [ a camber difference also can cause this, because of "camber thrust" a tendency for wheels to "cone" in one direction ]

Tracking is when the car steers itself on uneven roads as if it was on railway tracks [ I've also heard young people call this "tramlining" ] What I refer to as tracking is caused be too much scrub radius.

When the wheel has drag it causes a torque around the king pin ,the larger the scrub radius the greater the torque [ the steering relies on L & R torques to cancel each other out, but the fun begins when only one side hits a bump ]

This is not bump steer even though the behavoir is similar [ bump steer is caused through the geometry of the suspension movement steering the wheels ]

If you are travelling on a smooth road and the LH wheel hits a bump, with too much scrub radius the car will momentarily steer left but with too much caster the car will try and steer right.

Too much scrub radius is why people reinforce the tie rods.

Some people have used more caster to cancel out the effects of too much scrub radius, but this is a hit and miss method because the 2 different loads can be variable.
It is best to try and reduce the scrub radius [ have a look at modern wheels with lots of backspace behind them ]

Lets look at king pin inclination now...

If you had an axle with ZERO caster, the wheel will "fall over" [ as in positive camber ] in both directions it is turned because of king pin inclination.
On a corner the inside wheel having positive camber is desirable [ leaning in like a motorcycle ] but on the outside of the corner positive camber is undesirable causing understeer.

Caster is needed to counteract the dynamic [induced] positive camber on the outside wheel.

Turn your steering hard lock one way and use a bubble level to see if you have enough camber [ it should be between 0 to 1/2 a degree negative camber with radial tyres ] if you have positive camber on the outside you need more caster to remedy this.

One last thing , when you steer from lock to lock and your fenders go up and down, get somebody to check for sideways movement in the shackles

 

King pin inclination (KPI) will also cause the wheel to "lift" the car as it turns from lock to lock.

 

Kerry, thanks for putting up so much info that is not on hamb (as far as I have seen). There is so much there for me to go through; the more I tinker, the more I realize how much more complicated these front ends are.

The part I quoted above; I now need to get it back on the alignment tables as I did not check "camber at full lock".

As far as one fender going up/down, it is not shackle movement. I have the fenders off while on the tables, and as I reduce caster, I can see less up/down movement as I turn 20* each way.

One thing I noted: I checked "toe in at turns" before I went so low on caster. I was at 22 and book spec is 23.5 plus/minus .5. I rechecked with the super low caster setting and it went to 24?

One other thing about KPI that I noticed, and makes me think more, is the pic below. This is the wheel turned 20*, and you can see that the backing plate is "rotated" more than compared to the wheel at dead ahead. This makes the height of the tie rod change, and if it was a side steer car, it would change the drag link angle. Adding more caster would make even more of a change in angles, and I now wonder how that effects cars that do have a serious low speed wobble?

I need to go over all you have posted, in depth, as I am wanting to learn as much as I can about caster as well as wobble, etc. Thanks

BTW, I also would like to learn a bit more about what Dick Spadaro said about "caster trail" , the line from the imaginary road contact point of the KP under or in front of the tire, to the F-to-R centerline of the tire. It seems like there must be some relationship of that line to the scrub radius, and I wonder how different points could cause or reduce wobble? (So much to learn)

 

Death wobble? are we talking about bump steer?

 

Those two are different;

Bump steer is when an up or down motion of the car, or body roll, causes the car to steer one way or the other, even though you have the steering wheel straight.

Death wobble (to me) is the very violent side to side, and up and down rapid ossilations of both front wheels. It can break parts.. Mine did just that, 40+ years ago. This can come in, at very, very slow speed.

There also is a higher speed wobble, but in my opinion, not the same as low speed wobble, nor the same faults that cause it (just my opinion again)