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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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Does the t\c rod intersect at the ball joint if you draw a straight line? I think that is the point you use for you bottom measurement.  I wonder if there is a factory spindle that would reduce fabrication, the only front steer rear t\c rod spindle I can think of is a BMW e90 awd.

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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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