Technical Unsprung Weight Issues In Hot Rods: School Me

I would say,yes there were more then one engineeing benafit from moving springs in later design Fords,I pointed out just one to keep it simple. As for your note,rethink how levergage works,can you pick up the end of a log from the end,or go a few feet in too the log, an pick up the end.

 

I think I'd take a good look at the tires before I got too carried away. I run light duty radial tires on the front of my small cars with good results. When you don't have any real suspension travel the tire makes a large difference.

 

Lots of really good food for thought here. One factor I hadn't considered was that the adapter kit used on my car moves the rotor / wheel / caliper outward by about an inch - maybe a little bit more - from the stock location on the spindle. This certainly must have an effect, but I'm not sure just what that change will do. The weird behavior of my suspension takes place most noticeably when the wheels are slightly turned as when going around a sweeping turn in the road. Seems to improve when the car is going straight ahead...


This overall photo shows the components and how they are arranged on the car. The rear suspension is a GM 12-bolt with coil springs, ladder bars, panhard bar and P&J shocks mounted vertical. Most definitely over-sprung and over-shocked but surprisingly the least troublesome.


This photo shows how far the rotor is moved outward by the adapter kit.


Another photo showing how far the rotor is moved outward. This can't be a good thing.

I reduced the air pressure in the tires today and that seemed to help somewhat, but not a cure...

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The tread towards the center looks close to minimum thickness but pics are deceiving.

A couple of other things which may or may not make a darn bit of difference...but may...

One is a Road-force balance on the front wheels and secondly is try a set of Firestone Champion Bias plys or a Bias ply other than the type you are running.

@missysdad1 are you running radial on the back. If it is a combo could that be affecting the situation?

 

Moving the wheel mounting surface outboard about an inch, assuming the same wheel or wheel offset, changed the 'scrub radius' , moving it outboard by that amount. Scrub Radius, as you may know, is the distance between the center line of the tire patch on the ground and where a line drawn through the center of the kingpin meets the ground. Ideally they should be at or very close to the same point. If the tire center is offset from the king pin inclination line, that distance forms a 'lever' effect on the spindle. Forces acting on the tire are multiplied by that lever effect. A type of 'bump steer' may, or may not, be a result but the potential is there.

Recently, there was a lively discussion of this topic on the HAMB. A search may bring it up. Some vehicles, typically 4x4 models, have a considerable scrub line offset because of the packaging conflicts of steerable hubs that also drive the wheels, especially if the steering linkage is front steer. The common result is that when turning a tighter radius on blacktop surfaces some tire squeal is heard because of the inaccurate Ackermann effect caused by the geometry compromises.

Ray

 

Dana,
Suspension stiffness is nearly always referred to as "Wheel Rate" not spring rate. And placement of the springs is known as the motion ratio. [laying down shocks or forward mounting them on motocross bikes is a good example]
I generally use the term spring stiffness on forums to simplify things [although technically incorrect]

All the car really cares about is the wheel rate, regardless of bellcranks, rockers, or inboard springs etc

There is a bit of math involved in calculating the spring rate vs wheel rate.
The formula is : Wheel rate x Motion Ratio squared= Spring rate.

So if you want 100 lb/in wheel rate and the spring is mounted 1/2 along the A-Arm [2:1 leverage] you will need a 400 lb/in spring.

example [from Road Racing] a Mustang upper A-Arm has 2:1 Leverage, whereas a Camaro lower A-Arm has 1.5:1 Leverage.
A 650 lb/in spring in a Camaro has the same wheel rate as a 1155 lb/in spring in a Mustang

With a bit of math, altering the motion ratio is an easy way to fine tune suspension if correct springs aren't available.

 

Tube axles front and rear with parallel bars front and rear is going to be adding into the mix of issues. Especially when heading into sweeping turns and some body roll gets induced but can't go anywhere. One tire negotiating a road bump or dip does the same. The axles are super stiff supper strong anti roll bars.

Could use a side shot of the pitman arm too.

What exactly are you feeling in wheel?

 

Ray's discuss of offset & scrub radius helps. Load transfer to the outer front wheel when turning, implies you want only a slight offset.

 

When chasing "a spooky feel", we should never assume that one end of the car is not the problem. Pics of the rear setup will help . Just reading the above quote makes me think you have serious build flaws back there...but that is just an "assumption" at this point...without pictures.

We are all seeing things that are not quite right, so this issue might be hard to sort out with just descriptions from the owner? I'd suggest having a seasoned veteran of this type of hotrod also drive it, to help find out what the feel is like.

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The best I can do in words is that the front wheels feel like they go out of balance slightly causing the steering wheel to shake, not violently but enough so it can be seen as well as felt. Then, with a change of pavement and/or going straight again, it smooths out again.

 

Incipient so-called ‘death wobble’ ?? What are the alignment specs? Toe-in ? Do you have access to an alignment shop that will put your car on the rack and see what sort of Ackermann angles you are getting in turns? Tire pressures? There are probably other considerations, but those are what quickly come to mind.

Ray

 

Death wobble and bump steer
I'm guessing it's going to be a combination of stuff
Side shot of the pitman arm and front rod mount?

 

Following this thread with interest.....
I agree, likely a combination of things, both front and rear.
Also looks like the front shock angle is a bit too much?

 

It could be a "start" of that death wobble, and that scrub error certainly can induce wobble, especially on a side steer car.

There is a lot "wrong" on this car. Tube axle, that cannot twist in a body roll, especially with beefed up hairpins...and then if those are true ladder bars on the rear (also a tube axle, really)and the forward mounts of the bars are out at the frame rails, then that end of the car is also "running in a bind condition" in a body roll. Then if that is true, the chassis is now trying to twist with the limited articulation of the F&R suspensions.

but then.... the described spooky "feel" points to a different problem of possibly getting into hints of a death wobble. So yes, multiple issues, or combinations here to sort out.


We have seen countless builds on here that started off wrong, such as less experienced builders starting off with those disc brake compromises that screw up the scrub radius in a very bad way....then if that builder is unaware of that geometry, then he most likely does not understand the tube axle needs either a true wishbone or a 4 bar, instead of hairpins....then out back, ends up with bars mounted out at the framerails... Then when a specific handling "spooky feel" surfaces, all of these obvious errors at both ends, can cause a fox hunt


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Maybe...but so far nobody has ever said what actually causes "death wobble". Lots of smoke-blowing but no concrete cause.

Tires have lots of miles and are wearing properly with no evidence of improper toe. Car tracks well and does not "hunt".

Ackerman might be off a smidge, but I think the spindles are stock. I'll have to check it. But the problem shows on LONG sweeping turns where the wheels are only slightly off straight ahead so Ackerman problems are unlikely IMHO.

I'll get a photo of the pitman arm and post it in a little while. Drag link end at the steering arm has already been shown. The drag link and the radius rods are parallel and about the same length so it all works together like a 4-bar suspension setup. I'm not aware of any noticeable bump-steer.
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Here's the requested photo of the steering pitman arm and related hardware. Keep in mind that I didn't build this car, it's a very old build which had gone through several/many owners/builders/modifiers before I got it. I have made several changes in an attempt to improve the ride/handling - it was so messed up that it was virtually impossible to drive when I got it - but still don't have it the way I want it because it still isn't "right". That having been said, what do you see that might help me...?

 

It's your tires! I had them on my roadster, changes to radials, night and day! Get rid of the rear Ascots also. You're not running on dirt.

 

A picture is worth a thousand words.
That one speaks volumes

 

I'm here to learn. School me.

 

...again, the inexperienced person fails to comprehend the countless threads on helping a certain hamb cars that have wobble, and if you pay attention to very experienced hotrod chassis guys like the late Dick Spadaro, then you'd know that there never will be a exact "same thing" that is wrong on 100 different solid axle hotrods that came here asking for an "instant cure" or, "ONE answer" of death wobble.. It simply does not exist.

recently, I have changed my old view that a damper should never be used as a bandaid for wobble...but we just see so many builds that have so many incorrect things that can "induce wobble" or.."intensify that wobble, once it begins", that we might just throw our hands up and agree to use the damper...OR..build the car the right way! But, some owners simply are not in the position to rebuild the whole chassis...at that point in time.

We are kind of stuck here on this car, and it is not your fault. The written descriptions from any car owner on how something sounds or feels, is just never going to interpreted the same by each different reader trying to help.

I still would try to find a solid axle hotrod guy in your area to drive it and get an unbiased opinion after he test drives the heck out of it. Every one of us has had a weird problem in life, that we then sometimes get a brain-lock...and then start thinking too far out of the box. I wish I could drive it, but we are 3000 miles apart.

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..the geometry of wishbone/drag link will cause issues, like bump steer.
the wishbone may bee too stiff with those vertical bars to work with the tube axle.
the tire to road contact patch is too far out from king pin.
the grooved bias ply tires will not behave as well as you might like, especially with the above issues.
More pictures will likely show more areas that need to be addressed.

 

I would address the geometry issue first.
The way it is set up now every little bump will affect steering.

 

Actually, I have read many of these threads and they all come to the same conclusion that you quote...without getting down to any specifics. I don't want to simply throw up my hands and walk away from my own situation without at least finding out the cause of my problem. If it's "death wobble" and the "cure" is a damper, why don't so many other cars with the same basic equipment have the same basic problem? And, in deference to Dick Spadaro I can't agree that this problem has no cause, and therefor no solution. Somebody, somewhere, has figured it out. I can only hope that he sees this thread and responds.

 

I know you're trying to be helpful but bump-steer isn't the issue. The issue is the small oscillation in the steering wheel as the car goes down semi-smooth pavement on some occasions and not on others. This oscillation comes and goes but seems to be more likely when at highway speeds going around a VERY LONG AND GENTLE curve. It feels like the wheels go out of balance or out of round temporarily - perhaps one might call it a very mild wobble - then smooth out again.

 

Well, one thing, two actually, are for certain. The wheels DO NOT go 'out of balance' or 'out of round' "temporarily".
The cause is related to one or more of the other issues raised or yet to be discovered in your particular car. By the way, I don't think Spadaro's point was 'there is no cause'....more like there are many causes, too numerous and/or obscure to provide definitive answers to the question....'what causes this'? One suggestion though...do not make more than one change at a time, otherwise you will not know what was effective when an improvement occurs.

Were it me doing this, I’d change the front tires as a first step. They are needed in any case.

Ray

 

I'm not talking about the car changing lanes when you go over railroad tracks,
I'm talking about the way all your front suspension components are configured.
the way it is now even the slightest imperfections in road surface are affecting the way your car behaves.
if you want to fix the issues yourself it might be a good idea to buy a book or two on hot rod chassis fabrication, there are lots of them out there..

here is a snapshot of the first one I picked off the shelf..

your steering is similar to illustration #3
-not the way to set it up-

 

I was going to ask him, "do you think the disc brake setup is heavier than a drum brake?" IME the discs are considerably lighter than a drum. Unsprung weight may or may not be an issue with the car in question, but is it the disc brakes that are causing the excess weight?

 

Possible that damping would absorb it, I would look into the geometry (big picture) & the turn conditions that prompt the oscillating. Reminds me of the shopping cart wheel flutter. Better if you can correct the behavior, it weaves between two limits (an ocillation) which suggests a geometry fix. Caster °, king-pin inclination vs. wheel offset.

 

I appreciate you're situation having bought the car as is and trying to slowly tweak it so it fits you're standards vs tearing it apart and starting over. If, as you say, you are generally happy with the way it drives aside from the effects you describe in some turns I would try a different tire in front first. See if you can borrow some from a fellow car guy just to see if the symptom changes. That tire combination has a great visual appeal but I'd bet a paycheck it is far from the best choice when it comes to ride and handling. Your car doesn't have a strong "Track roadster" vibe so you could use a different style tire and not ruin the old hot rod look.
I just installed Speedway's econo disc brake kit on my '28 Highboy and it does not move the wheels out as much as the kit you have. My car came to me with a cobbled together kit that put the wheels out about 1 1/4" on each side (!!) - I addressed that immediately. It drives better. I also went from low profile radial tires to Firestone bias plys. It still drives great.
I believe Speedway also sells an aluminum version of those calipers that would cut the unsprung weight back closer to stock but I would spend the money to experiment with adjustable shocks before doing that.

 

i am having a similar issue...the rough ride part anyway.
when i got this car it had been driven very minimally since it was finished and it had just come out of winter storage...
i thought the ride was stupid hard...almost like there was no suspension at all.
the PO admitted that they put an "extra leaf" in the front pack because they put a big V8 flathead where a 4 banger used to live. but that didn't explain the rough ride at the back of the car. so bad that "tar snakes" on secondary roads were bouncing and rattling the car to pieces (literally).
long story short, the PO simply painted the front and rear leaf spring with POR15. There was no lube between the leaves. (ever try to open a window that has been painted shut?) anyway, i noticed lines of rust dust forming between the leaves so i got some SeaFoam Deep Creep penetrating lube and hosed down the springs and went out driving... it is not perfect yet but it is already a night and day difference. If the leaves cant slide on each other they can't "absorb"
i also have Bias ply tires and they like to wander over any imperfections that run parallel with the lane of travel...I.E. getting on the wrong side of the hump in the middle of the lane will push me into the on coming lane if i am not paying attention. i am running a lower tire pressue in mine to help with the rough ride (low to mid 20's PSI)
just spit balling and sharing what i have found (so far)
Chappy