Roll Center Adjusters Revisited

[SoSideways]

Ok, here are the adjusters in question:


Pic courtesy of Nengun Performance.

Anyway, many have argued that they don't do anything, while giving the illusion of helping the roll center by placing the front lower arm downards, but not actually changing the pivot points.

I am still having difficulties picturing this in my head.

Would someone please draw up (or find) diagrams that explain the part where it gives the illusion of helping with roll center by moving the arm down, while not doing anything?

Also, here is how they look on the arm (Moonface sells brand new OEM arms with the bushings in them already, so it seems):





I have a couple of brand new OEM S13 arms that I can take pictures of for comparison, as mine has brand new OEM ball joints on them, thus we should be able to make a valid comparison between these Moonface ball joints versus the OEM ball joints.

[Epstein]

Some US companies sell these, too. It's not that they don't work. It's that it's really easy to market a part like this that doesn't work as something that does. And no one wants to drop the money to measure the actual part and find out. Those things can work. They just need to keep the pivot in the same spot as the OEM and have a longer pin. It's just hard to tell what'd going on with that boot in there. We're not worried about the angle of the physical arm. We're worried about the angle of the mechanical arm that runs between the inner and outer pivot points.

[Def]

Judging from the length of the lower cylinder/sleeve, I'd say it keeps the pivot the same relative to the stock spindle, and just has a longer "sleeve" section. This doesn't affect the mechanical relationship between the pivots and the contact patch(which determines the instaneous center or "roll center" of the front suspension).

[SoSideways]

So in order for something like that to work, you guys are saying that the pivot on the arm side of the ball joint must be raised to above the arm for it to work positively?

Again, when you guys say that stuff to me, it would be extremely helpful to have a diagram to go with it, as I am more of a visual learner.

[Def]

Do some searches for roll center - I'm sure it'll do a better job of teaching it than I will(I hated dynamics).

But my understanding of the physics involved is that the PIVOT location is what's important. So if the balljoint just has a long sleeve on the bottom and keeps the pivot say 1" below the spindle like the stock balljoint, then it doesn't do you any good. The arm angle vs. the subframe changes, but this actually doesn't do anything, as what's "important" is an imaginary line drawn from the arm pivot on the subframe to the balljoint pivot. You can do whatever you want inbetween, but that imaginary line and the arc it forms is what dictates suspension movement/roll center height/camber curve etc.

[Wiisass]

If they don't relocate the pivot, they don't really do that much, but that's what everyone else was saying.

But changing the angle of the arm would change the way things are moving in relation to each other. I mean say you have an arm that is horizontal and one that is at a 20* angle. And in the front view, look at how each point would move over a +-1" range of travel. I'm not saying these are good parts, or that they will even do anything, but I was just thinking about this the other day.

But it's bad design so it's still stupid.

[Def]

I don't see it changing things by having the balljoint extended from the arm. Yes, the arm to subframe angle changes, but the important part that dictates camber curve is the imaginary line between the subframe pivot and the balljoint pivot.

If you raise the balljoint up, the arm is at a different angle, but the maximum lateral distance of the arm is still reached at the same rideheight.

I pictured an impossible situation of a 1' balljoint extender to see if it would change things. In my mind it really doesn't since it's effectively like having the same arm, but it's just "really bent" to get the pivot in the same location.

[Wiisass]

Def, I'm talking about dynamic camber change. If the arm is horizontal the pivot point is already extended as far outboard as its going to be, but if it's at an angle either above or below, it's going to be further inboard and then travel outboard or further inboard depending on position. So it will change it a little, how much, I don't know.

It's not enough that it would matter. I was just saying for arguments sake, that it would affect the camber curve slightly.

[Def]

Wouldn't extending the balljoint pivot affect where the actual pivot reaches its maximum outboard location? As in the arm tip itself reaches the maximum outboard location when it's parallel with the ground(and the camber curve goes to crap after the balljoint goes perpendicular with the spindle), but doesn't extending the balljoint, possibly at an angle, make the "actual arm" just an imaginary line from the subframe pivot to the balljoint pivot?

Maybe I'd just need to model something simple and rotate it about a point to prove or disprove my back of the envelope thinking...

[SoSideways]

That's what I'm saying guys, please post up something like a simple paint drawing or something lol

Actually, I know of one on FA, let me find it...


Pic courtesy of BlaBla on FA.

Is that pic correct, or not?

[AceInHole]

That pic works.

[WishIHadaSilvia]

How are you supposed to tell what is good and what is bad just from looking at it?



Those look more like the bad drawing, but in all honesty I have no clue.

[AceInHole]

Same. I can't tell if they'll work or not.... but I guess if you had bad balljoints and needed to buy something anyways they might be worth taking a shot.

The thing that sucks is: if they're the "bad" design, they'll be weaker than stock since they'll put a bending stress on the LCA.

[WishIHadaSilvia]

So basically the "ball" part of it needs to be on the bottom? I know someone who has them....would we be able to tell by looking at it installed on the arm?

[SoSideways]

So basically the "ball" part of it needs to be on the bottom? I know someone who has them....would we be able to tell by looking at it installed on the arm?

You could, you'd just have to have the person bend or move the ball joint when it's installed on the arm, then you can tell exactly where the pivot point is.

Looking at the pics though, they look like stock ball joints with a spacer below it, thus looking more and more like the "bad" part depicted in the MS Paint drawing.