Technical Unsprung Weight Issues In Hot Rods: School Me

Let's nail down some points so we don't have to keep going over the same ground.

1. There is no single cause, it's always a combination.
2. Ideal scrub radius is about 2".
3. Scrub radius has a direct effect on the tires thru the road force applied by rolling along. That force is counter acted by the toe adjustment to keep the wheels straight. Increased scrub radius increases that force and a necessary increase in toe to compensate.
4. Bump steer will cause/force the wheels to turn as the suspension moves.
5. New parts doesn't mean good ones. Sad fact.
6. I've personally been delivered spindles with bushings reamed that were way too sloppy.
7. Caster changes


Mix up some of that , like on the video.
Rolling along just fine and Scrub is applying even force to both front wheels and the toe is keeping everything at bay.
Hit a bump and the addition of factors within the above combination starts. the bump steer cause the wheels to turn, toe goes all to one side, the road force is suddenly diffent side to side, and then the tires try to redistribute the forces back to equal side to side. The tires aren't trying to balance out a small amount of forces, they are trying to balance out something within that combination of extrordinarily huge forces. Not only that the forces are dynamic which equates to over compensation in an attempt to re balance forces at the tire and now you have the death wobble.

Throw in a few more in your case. The tube axle with stiff radius rods, the parallel rear ladder bars are causing some stress and loads onto the chassis. Those stresses are induced and then unloaded, stress is energy and creating stress is creating energy. That released energy is going to go someplace and it wants out. Where's it going exactly??? Small parts of it have heat as a safe escape but the majority of that energy doesn't have a safe outlet like heat. With that anti roll bar action the frame rails are getting loaded and then unloaded like a steel rubber band.

 

All good until you jumped the shark somewhere around bump steer. Everybody seems to be eager to complicate the "slight wobble" issue I do have with other issues that I don't have. Take a deep breath, take bump steer out of your analysis and see where that thought process takes you.

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I'm telling you,,,,
You have perfect bump steer geometry. That's a fact jack !!! Like apples? How about them apples

Now your car may not produce any noticeable or remarkable characteristics of bump steer for several reasons. Most of those reasons would contribute into your problems. You may not even notice it till you're in that long sweeping turn. Tell me is it a right sweep or a left sweep turn?

 

I'm here to learn.

The car doesn't exhibit bump steer characteristics to any great degree that I'm aware of. The wobble I feel in the steering wheel can happen in either direction or while going straight ahead. The imperfect pavement can appear quite smooth but still have small bumps in it that come through the wheel as small wobbles that come and go. Like I've said all along, it's weird.

Next time I take the car out I'll put my mindset in bump steer mode and see where that leads. I've been wrong before...

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So one of 2 things happen here,

You will hold the steering wheel from moving which keeps the pitman arm still and then the car steers and darts
OR
The pitman arm moves and the steering wheel moves and shakes your hand while the car remains relatively straight on the road and you post a thread about a wired wobble you feel in the wheel- show us pictures of bumpsteer geometry and tell me not to bring it up again because you don't have it.

 

I think the wobble is caused by movement in the systems, perhaps not due to wear or excessive looseness of joints, but in movement and bending of parts that you are considering as "fixed" or immovable. For example - that curve in the end of the radius rod (both sides?) is going to cause the bars to bend in the lateral direction (where they are not reinforced) due to axial forces (otherwise known as "buckling") that are non-linear with the bars. And perhaps the radius rod bracket on the frame bends a little under "service loads". If it were mine, I would disassemble and closely examine every piece of the front end....and consider a 4-bar setup...

 

Got it. Simple minds need simple pictures.

Now all I've got to do is figure out what I'm going to do about it...

Thanks!

 

If big if too, if you do not get the death wobble then let the steering wheel float in your hand, don't jerk it back or over saw at the wheel correcting it and try to get used to it. The other option is fix it, get whoever is working on Arin's ride to do it. It looks like they know what they are doing.

 

So what I'm feeling is tiny bump steer events being fed back through the steering wheel and not unsprung weight amplified road surface vibrations. Not exactly the good news I was hoping for... I don't think there's an easy fix for this one. Oh, well...

 

The scrub amplifies it, tires closer to the king pin move less than those farther away.

When nobody is looking, try some high offset wheels and go for a little spin. See if it makes it tolerable. If so hunt down some suitable wheels and run moon discs to cover the ugly things up.

 

That's pretty much what I've been doing because the feedback in the steering wheel is more annoying than anything else. I was hoping that reducing the brake weight and bringing the scrub radius back into line would make it go away, but that's not going to happen I guess.

Oh, and thanks for the compliment. I'm doing 100% of the work on Arin's car myself. The difference is that I inherited the problems that plague the black car while Arin's roadster is pretty much my own other than the frame table assembly of the ASC rails and Deuce Factory center section which was done by Kiwi Kev in California.

 

Unsprung weight issues gives a hard upward jolt into the chassis. That feels like someone is pounding on the floor of the car. If it's really bad it feels like you bottomed out however the cars body was going the opposite direction of a bottom out. It can knock you up out of the seat. Like the ball video posted earlier. Now ,, if you have bump steer and an axle that gets bounced by unsprung weight problem, that bounce is going to give more bumps and make the problem bigger.

You may have that problem too as well. Especially in the light roadster with a 12 bolt diff.

These things are like turning the water on in an old house that's been off for years. The biggest leak shows up first. You fix that and then turn the water on and find the smaller ones. Fix those then repeat it again till every little drip and drop is corrected and faucet washer changed. Then you turn the system on and let it get full pressure and hope nothing's gonna blow apart because it's been years since it's seen pressure.
That's why it's better to rip the crap out and start fresh.

 

I'm sorry you bought some trash mixed in with the car.

Try this.
I'm giving you a bag of trash from my kitchen. You hold it as long as you want, till its moldy and stinking and full of flies. Hold it so the raccoons can eat it. I gave it to you so you better not toss my stinking garbage.

That guy gave you some trash, hold it as long as you want.

 

Good points. Now all I've got to do is figure out what I'm going to do about it.

Thanks again!

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You can "fudge" wheel offset by adding a bit more positive caster to counteract the toe-out effects of too much scrub radius. Positive caster tries to toe-in the wheel.
But be careful when doing this on a lightweight vehicle, the caster will prefer to lift the vehicle instead of straightening the wheels [causing death wobble]

The whole steering set-up will cause mechanical bump steer as we have established, but the OP complains about the misdiagnosed feedback through the steering.
A simple steering damper on the draglink [not the tie-rod] will remedy the situation, but it is a "patch up".
The bump steer must be dispersed somewhere, usually through the weakest link [the hands holding the steering wheel]
If a steering damper is added to assist the driver, the bump steer will transfer elsewhere eg: start tracking the car.[or worse still, Trigger death wobble if the car is too lightweight]


This whole thread is a classic example of misdiagnosis of a problem by the OP, and everybody chimes in with good solutions.[based on what they were told]
Now lets see if we can all get our heads together and help him sort it out.

 

There's already a dampener on the rig, helped a bit but not enough.

 

We could know what that feel really is, if we could drive it. I switched my thoughts from the original OP's suggestion about unsprung weights, to have me thinking he has the beginning movements of death wobble...but that idea came only from his descripition of "osillations".

but, we cannot assume the damper is good, or it could be too wimpy like a VW bug. vs the heavy duty VW bus one, or even bigger. When he said oscillations, it sounds like how I might describe that twitching on a VW bug when the damper is going bad. Those end up with an air bubble, and that tiny bit of air, over a certain bump, will give that same feel of a mini death wobble, but cannot go wild, as the damper is still able to keep it from convulsing out of control.. If a damper on a VW bug loses all the hyd oil in there, it will go to a full death wobble.


Ok, how many things can be changed easily? If I knew it was a masked wobble for sure, I'd take one end off of that damper to see if it has a tiny dead spot from air, or if it simply feels far too weak to deal with that much of a scrub error. I do know that major scrub issues will definitely induce wobble.

but, this is all based on worded descriptions of what the car actually feels like. How can we diagnose it properly?, as we all see many things that are wrong. We can sit here and say "build a new chassis", but maybe there is a way to make it useable for now?

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I’m not nearly as experienced as some on here, it looking at your pics you have a few things outta whack.

Number one that I see are shitty tires and your wheels are to far out.

Get some wheels with a better offset ( bring the wheel in over the caliper, or at least closer to the car. With some decent tires.
Get your alignment checked and drive it.

You have a mish mash of new/old and strange on this car.
Not necessarily crap components on there own but when put together just don’t work.

I would try some cheap fixes and cheats to try and work around the problem and hopefully confirm what parts or issues your having and replace only what needs to be.

Or just junk that whole front end set up and start from scratch.
Your not the first guy to build one of these and there are tried and proven systems that work very well

 

This appears to be true, and suggestions on how I might improve my situation - preferably without cutting it all off and starting fresh - would be greatly appreciated.

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In addition to checking the condition of the steering damper, I would remove the front shocks and see how used up they are. My P&J front shocks only lasted 20k miles.


Phil

 

Not on the drag link, so I assume it is on the tie-rod.



A dampener on the tie-rod keeps the wheels from shaking ,but any bumps [however minor] will transfer shaking to the steering wheel due to mechanical bump steer.

If the dampener was on the drag-link it would keep the steering wheel from shaking, but will transfer shaking to the front wheels.

Bump steer is the issue, and it has to go somewhere.

 

https://www.jalopyjournal.com/forum/index.php?threads/1101046/

Take a look at the posts here that were authored by Pete Eastwood. Extensive discussion of side steering pivot points to minimize bump steer. The Pete & Jake's catalog has the drawings of a proper side steer installation.


Phil

 

BOY it sure looks like that rod would hit the box in the pic.

The squirmy rod end doesn't look like it's good at transferring the loads either.

Now I asked before and I'll ask again. Does the other side match the drivers side?

 

All you really need to do is use a reversed steering box with the steering arm pointing down [this will take some fabricating And change the column angle]

NOW if that was my HodRod I would leave the steering "AS-IS" and convert it to 4 bar and the geometry would be correct.
You would need some P & J style 4 bar perches. And you could mount the lower bars from them to the existing frame pivots.
All you would need to do is fabricate upper pivots and keep all the 4 bars and drag link parallel with each other.

 

3" travel would only require about 1/4" clearance.
I've seen squirmier rods on OEM steering [check out 55-57 chevy drag links]

The other side doesn't need to match the drivers side, only the pivot points do.

 

Simple - Easy - Test.

Take off your wheels and tires....

Replace with radial tires from something you or someone has.

Drive it.

Report back.

 

The drag link isn't doing as much work either. Perhaps it's personal but I'd get rid of it for any one of a lot of different reasons and ugly is on list. If they don't match I'd have to set myself on fire.

Also if they didn't match the rods have to have a different bend at the axle for caster to be even. Something is just wrong there.



scrub issues -
On the left is a 64 dodge truck 16x4.5 and on the right is 64 studebaker 15x4.5. . In the junk yard are plenty of spare wheels in the sameish configuration but ugly.
Depends on your current back space if these would suck up the track width and decrease the scrub radius or not.

 

Be Careful, a Match can set yourself on fire [sorry I couldn't resist]
And getting rid of things because they're ugly is the curse of Marriage guidance councillors.

I've done a bit of thinking here.

If the pitman arm off the box was shortened it will reduce some bump steer.
The steering arm off the spindle will also need to be shortened to restore steering ratios.[unless the steering is too quick already]

Then use soft radial tyres as they are more forgiving

 

I'm tempted to lie to you just to see if you really would...

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Yes, the Mustang steering and the P&J 4-bar setup go together like peas and carrots and would probably be the easiest way to "fix" this front end. Just one problem: I don't care for the way the 4-bar setup looks, a little too street roddy to my taste. Nevertheless a solid and practical alternative. Good suggestion. The more I think about it this may be the perfect solution. Thanks!

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