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3Vs...We're Not Dead Yet!

Norm Peterson

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Thanks, Grant.

FWIW, after seeing the test series done by Ehren Van Schmus for Maximum Motorsports and posted up on Corner-Carvers, it seems that all-unmodified-poly-bushings in a Fox Mustang created nearly as much roll stiffness as a small to medium-sized real in-the-steel rear sta-bar (I think it was something like 60-ish lb/in vs 80-ish).


Norm
 

Norm Peterson

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We'll see once it gets nicer enough outside to take a closer look, but at this point I'm planning on keeping the OE bushing. I go pretty gentle on those things as far as impact loading is concerned - no dragrace style launching or powershifting, strictly lift-foot upshifts and nearly all downshifts rev-matched. If it does look like it's getting out to its "best used by" date - keeping in mind that it hasn't felt like it was getting anywhere near there yet - a bearing replacement isn't necessarily off the table . . .


Norm
 

67GTA

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👍 Understood and it shows it well.

Great post above, and I hope it convinces or teaches some people to stop using drag race parts on road race oriented cars.
@Grant 302 - what’s your recommendation for the differential end - bushing or bearing? You mention drag race parts on road race cars - please elaborate on what would be an ideal road race setup - assuming 95% track use (HPDE), and the only real street use to and from the track.
 

Grant 302

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@Grant 302 - what’s your recommendation for the differential end - bushing or bearing?
Depends what’s in the other side. My main recommendation is not poly.


You mention drag race parts on road race cars - please elaborate on what would be an ideal road race setup - assuming 95% track use (HPDE), and the only real street use to and from the track.
Depends on the driver. There isn’t one best setup that fits all drivers, courses, driving styles and driver ‘confidence’. Most of that has to do with the specific geometry and various link arrangements etc.

The problem with poly and ‘drag race’ oriented parts is that they aren’t compliant for axle movement in a turn. They are ‘only’ compliant in level up and down motion. The worst place to put poly is in the third link. And it’s not much better in the trailing arms. This is what is commonly referred to as ‘bind’.

One thing that’s often ignored is that the third link ‘needs’ some longitudinal compliance too. Otherwise it forces the axle to rotate about its axis. That’s why I said it depends what’s on the other side...in the body side of the link.
 

Jbrad

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A lot of good information here. I’m leaning towards oem replacements on both sides. But still not 100% certain lol
 

Norm Peterson

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One thing that’s often ignored is that the third link ‘needs’ some longitudinal compliance too. Otherwise it forces the axle to rotate about its axis.
I don't think axle rotation (in side view) can be avoided with any 'simple' suspension linkage arrangement. It's either going to be rigidly defined (by spherical joints everywhere), not very well controlled at all and subject to compliance effects (soft rubber everywhere), or somewhere in between (when some of the compliances have been reduced or eliminated).

A little pinion angle change with suspension travel isn't the end of the world - with the possible exception of drag racing. It's something you're already considering when setting pinion angle, and why with different kinds of pivots/bushings there are different recommended settings. If you're going to get (say) a degree less PA change with rod ends than with OE rubber (because the rod ends are much stiffer), you'd want about a degree less PA than you would for rubber.

Transmission of differential gear noise is a related matter with a whole different set of preferred solutions.


Norm
 

Grant 302

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Pinion angle change is the least of the concern. More of a symptom of the problem.

Assume 6 rod ends at all trailing arms and stock geometry. With what relative force does the upper arm act with a bump on one side of the axle? How big is the vertical component?
 

Norm Peterson

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Pinion angle change is the least of the concern. More of a symptom of the problem.

Assume 6 rod ends at all trailing arms and stock geometry. With what relative force does the upper arm act with a bump on one side of the axle? How big is the vertical component?
Nominally zero.

For any all rod-ended link, the only force of consequence is axial tension or compression. The ball and socket joints release the ends of the link from developing bending moments, which is where any lateral or vertical translational forces (think in terms of shear forces at the rod ends' attachments).

In reality you would develop small moments due to friction between the ball and socket at each rod end, which would develop bending moments and non-axial forces in the link. Assuming the rod ends to be clean, in good condition and not run out to the very ends of their rotational travel, those forces and moments would be small, negligible even.


When you start adding significant rotational stiffnesses at the end of a link (typically by using a cylindrical bushing of finite non-zero stiffness) that induces end moments at that end of the link when you subject it to off-axis displacements/rotations. And the off-axis forces arise from those moments. A rod/bushing combination will develop moments at the bushed end and off-axis forces at both ends.

We should only consider rod-ended links to be true links (only capable of resisting axial forces), and any other variation of rear control arms that uses a cylindrical bushing to only be approximations of a link. If anything, 'arm' really is a more accurate term for any sort of bushed LCAs and UCAs, and strictly speaking the term 'link' should be reserved for spherical-spherical types only.


Norm
 
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DocB

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Regarding what Norm said in post #67.
Just to put some rough numbers on a historically rule of thumb:
Rubber= 2-3* PA
Poly= 1.5-2*
Heim Joints= 0.5-1.5*
 

Norm Peterson

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No, it would have to be horizontal and remain horizontal through the bump event for that to be true.
Regarding what Norm said in post #67.
Just to put some rough numbers on a historically rule of thumb:
Rubber= 2-3* PA
Poly= 1.5-2*
Heim Joints= 0.5-1.5*
Thanks. More detail than my memory was good for, that's for sure.


Norm
 

Norm Peterson

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No, it would have to be horizontal and remain horizontal through the bump event for that to be true.
I think I see where the disconnect here is . . .

You either do all of your thinking in the link's own coordinate system or you have to do it all in the car's longitudinal-vertical-transverse coordinate system. No mixing and matching.

It's far easier to work in the link's own system, where frictionless rod ends can only support forces in the link's axial direction. If you were to grab a rod-ended sta-bar endlink by the rod end ball centers and push one end sideways, there will be zero resistance (OK, some tiny amount due to the joints' minimal amount of friction) and the pushed end just moves. A rear axle's rod ended UCA is no different except for the axial loads in the UCA being much greater than the amount of tension or compression you could apply to an endlink with your bare hands.

The above is a sort-of description of what structural and mechanical engineers call a "free body diagram" when it's drawn out on paper or on a computer screen. Only the item being analyzed is drawn, and the forces applied to it need to consider any details that "release" any forces or moments from passing into this item. An analogy to what rod ends do to off-axis loads in a UCA here is like what happens when you try to push on a string. The end you push on just goes where you push it, with no resistance and no stress in the string (to speak of).

It would be possible to work in the car's coordinate system as long as you consider the link's orientation in 3D space with the three force components (L plus the V & T forces that you mentioned). But that's way messier than necessary and nobody I ever knew would choose to analyze a link in a coordinate system other than the link's own system.

Remember that we've only been looking at the link as a "pure link" up to this point.


It's after you're done with the link and are moving on to the axle- and chassis-side brackets that you need to get concerned with force components. That's where your lateral and vertical forces in the car's L-V-T system have useful meaning. Strictly speaking, the axle bracket's "L-V-T coordinate system isn't exactly the same as the chassis-side bracket's L-V-T system, and neither of those have to match up perfectly with the car's L-V-T.


Norm
 

Norm Peterson

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Sometimes a picture helps. You have to resolve the forces from the axle end into the link coordinate system. The rod end prevents any forces not in line with the link's axis from passing through it, so the relationship between the axle forces is defined by the link's orientation in 3D space. Not the other way around. Only the magnitude of the forces is defined from the axle.

A similar force resolution (going the other way back to force components) happens at the chassis side bracket.

UCA free body diagram.jpg



Norm
 
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Norm Peterson

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When you step away from the rod end/rod end configuration is when it gets a bit messy and other forces and bending moments do pass across the rubber/poly/delrin/whatever bushing material. The analysis cannot be done statically like in the picture above because the stiffnesses of the bushings and of the link itself now matter.

I spent about 40 years getting paid to understand this sort of thing, and I really don't want to attempt a drawing of the full diagram for what happens when you have finite bushing stiffnesses in all 3 force and 3 moment directions instead of the single force direction for the rod end/rod end case.


Norm
 
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Grant 302

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Well you’re doing a great job at explaining for anyone else who doesn’t understand statics.

But there’s no need to over complicate the model that I’m talking about. I do it in my head.

Your diagram of the upper arm in tension is one good example of what I’m driving at. At shallow angles, small vertical inputs create large, unwanted horizontal reactions. At the stock angle of approximately 15°, those reactions are on the order of 4.5x of the horizontal component and only slightly decreasing through a bump cycle. With rod ends, those reactions are ‘immediate’. Certainly so compared to a big rubber bushing with voids in it. Compliance on the order of 1/8” or 1/4” work wonders for delaying and reducing that grip reducing reaction. And therein lies the ‘need’ for compliance in the upper link, and certainly so in an example where rod ends are used in the lower links.
 

Norm Peterson

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I'm afraid you're still not getting it. A purely vertical input at the axle end is a displacement input (not a force input), and the axle-side pivot point simply moves upward with very little force involved because the rod ends simply rotate to permit that displacement. Rod ends are like hinges, except that they're hinges in all three directions (all three axes) where a door hinge is a hinge about only one axis. This is second, maybe third year civil/structural or mechanical engineering stuff.

I'd still like you to dig up a rod-ended sta-bar endlink - preferably one that's clean but has seen some use so that it's not bound up at all - and see for yourself. Self-demonstration >> pictures (and >>> text).


Norm
 
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Norm Peterson

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Compliance on the order of 1/8” or 1/4” work wonders for delaying and reducing that grip reducing reaction. And therein lies the ‘need’ for compliance in the upper link, and certainly so in an example where rod ends are used in the lower links.
True, but that's a different load case even though it usually happens in conjunction with the displacement case..

This would be for forces arising from axle torque reaction (acceleration or braking), which is a force case rather than a displacement case. In these cases it's the horizontal force at the axle-side pivot that defines the link force and (in conjunction with the link's orientation) the vertical and/or lateral forces in the axle bracket. The link is still only loaded in tension (during acceleration) or compression (braking).

This can happen with or without displacement input, but keep in mind that pivot displacement comes from vertical or lateral movements (no forces) and that link forces only come from longitudinal axle force reactions (not movements).


Norm
 

Grant 302

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I’m just gonna let you think about it. If you don’t believe I understand the various link types and the reactions they allow, we shouldn’t even be discussing subjects on this order of complexity. I understand the 2D and 3D analysis statically and dynamically...and can only break down the analysis so much before it’s essentially worthless.

Have a good day and stay healthy.
 

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