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Cary (tube80z) has been mentioning this idea of the dual ball joint front end off and on for the last couple years. I hadn't really given it too much thought until I autoxed my car and was really lifting the inside rear tire, and Dan (74_5.0L_Z) suggested that the problem might be scrub radius and caster related. I guess the idea is that the outside wheelbase increases significantly enough due to scrub that the car leans over in that direction and the inside rear comes off the ground.

Recently this thread popped up talking about swapping front suspensions and it quickly devolved into a pissing match for reasons beyond my comprehension, but it did get me thinking more seriously about the dual ball joint idea. 

Cary seems to think it's doable with the ball joints in the same plane, and had thought of a modified bumpsteer spacer as a possible way to do it. That got me thinking that it might be easier to just make a square tube knuckle and bumpsteer spacer combo, so that's where I'm at right now.  Thinking square tube with simple clevises welded to it for the ball joints, and then taper the front end and have it open and then use shims above and below the tie rod to adjust bumpsteer.

I have absolutely no idea how to figure out the spacing on the clevises to minimize scrub and that sort of thing but I'm very interested to see if I can make it work, and to see if it would be as easy as I think it should be. 

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I think he mentioned that to me in my control arm post as well, although I had no idea how that would work. 

 

Just to clarify, this would be for the rear correct? Or is this more for the front like the video you posted in the other thread?

 

I think I have a visual for it now though:

Stock upright

Weld two gusseted brackets under the hole for the spindle pin on either side

Replace the control arm attachments into two ball joints

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I want to believe I'm doing something seriously wrong here. Please tell me I am. 

Cary asked for measurements including scrub radius. I looked up how to measure it. Looks like imaginary line through upper strut mount and lower ball joint, and then wherever that meets the tire contact patch.

These are 15 x 14 wheels with 13.75" slicks. If I'm doing this right, and I think I am, I'm looking at about 13" of scrub radius. These wheels have 4.5" backspacing and 1/2" wheel spacer. So this means that a typical 15 x 8 with 0 offset and a 23" ish diameter tire is going to have about 7.5" scrub or so. 

If I have this correct, I will wait to see if Cary can figure out where the mounts would need to be to fix, but I suspect that it's not workable in any event, and the solution will be SLA suspension instead.

holyscrubradiusbatman.jpg

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I can swap rear wheels to the front and the drop center on the wheels is different and won't need the spacers. I'd have to check but I believe those wheels are 5" back space, so that would lessen the scrub to closer to 5". Might have issues in the rear with hitting fenders, and possibly front wheels hitting splitter.

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Typical race car "10 lbs of shit in a 5 lb bag" stuff. Looked again, due to tie rod to rim, steering rack to alternator, and other space constraints I'm going to have to stick to this wheel offset to make everything work, or do a whole bunch of other work to make it all fit.

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No, it doesn't, but the angle of the TC rod doesn't change any of the geometry of the control arm though, having the arm in line with the ball joint just loads the TC rod straight instead of at an angle. You can look at a BMW or Porsche lower control arm and they are shaped kind of like a boomerang. More room for the tire, but the geometry is controlled by the pivot points, not the shape of the arm.

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Geometrically, I don't have a clue how to make that work.  My suspension software does cover it, but it would take some thinking to understand what the trade offs are.  I guess BMW uses some version of that  Probably not that hard to implement.  Officially it is called a "virtual A-arm". The "virtual pivot point" is the imaginary point where an extended T/C rod and control arm would intersect.  Looks like you design for this to be close to the center of wheel to eliminate the scrub.

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If that's all it takes it sounds pretty easy to do, but I feel like there have got to be some complications we're not factoring in.  Looking at that Pontiac video looks like the end links will have to be reworked and made as long as possible. Cary has some software that figures it out too. He's asked me for a few dimensions, but he's gone until next week, so I haven't gotten around to measuring yet. Thread should pick up next week.

 

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Setting the geometry up to get what you want at nominal ride height and steer is simple enough but one of the big complications you're not factoring in is that the virtual pivot/kingpin axis moves around during steer, unlike the single arm setup. If you look back at the Pontiac video, this can be visualized really easily and you can see that scrub increases on the inside tire and decreases on the outside during steer, in this case. Also, caster drops on the outside and increases on the inside. The two things you don't want.

 

However, going to a split-lower arm on an S30 would result in a compression link setup instead of tension link, as Pontiac (Holden) has done, so I bet the resulting geometry would be more favorable regarding caster and scrub. Still, a full kinematic sweep is needed (ideally K&C) to be sure you're getting the effects you want.

 

As an aside, TC rod positioning on the control arm does have an effect on geometry, e.g. caster gain (therefore kinematic anti-dive) and bump steer. It can be seem as a sort-of front semi-trailing (leading) arm. This is minutia in the sandbox that we're playing in but worth noting that it does do something.

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So hypothetically you put two new joints/your fabbed knuckle/arms in that suspension - spread to intersect and give the zero scrub. What will that do to the kingpin angle? Does the wheel/tire end up flopping back and forth because it's steep? Does having the two joints add to a steering load especially the further they are spread or just act like a single longer arm pivoting out at the "imaginary" intersection? Looking at that photo your car has a serious scrub radius. Is the steering super heavy at low speeds? Sorry if I'm missing some technical terms here just trying to grasp how all this relates together.

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On 6/22/2018 at 4:19 PM, clarkspeed said:

Geometrically, I don't have a clue how to make that work.  My suspension software does cover it, but it would take some thinking to understand what the trade offs are.  I guess BMW uses some version of that  Probably not that hard to implement.  Officially it is called a "virtual A-arm". The "virtual pivot point" is the imaginary point where an extended T/C rod and control arm would intersect.  Looks like you design for this to be close to the center of wheel to eliminate the scrub.

If those are the official terms I will try and get that right from now on.

 

As far as "virtual pivot point" you would want that located a bit inboard  of center so that the line like you drew here would go through upper pivot then "virtual pivot point" and land ideally at the center of the tire contact patch for zero scrub.

 

With the strut it might just act more like a SLA with shorter upper arm/longer lower rather than a horrendous kingpin. 

holyscrubradiusbatman.thumb.jpg.12cd8df8afdfff5dd47077549c2330da.jpg

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If I was thinking this there would be two viable options. One, transplant a suitable suspension from another car or, two, get a suspension program that allows you to explore in a practical way what does what. Unless it can be done geometrically on paper a good program is the only real option for this DIY stuff. When designing the trailing arm lateral links rear suspension for my Z31 project a double wishbone (think about it) program was used, no way would any alternative method be considered.

Edited by 260DET
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Any suggestions for the "good program" anyone - particularly one that would allow the double joint lower? I've played with the cardstock models that the suspension design references suggest and they help to get an idea of 2 dimensional dinamics but 3d is lacking and changes to pickup points and lengths are tedious. I'd rather not drop thousands on a single car design but would certainly pay a resonable amount.

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