onevoice

That is a stupid example, but I'll play your game. If the control arm is pointing STRAIGHT DOWN, and the ball joint is somehow connected to this control arm, and the ball joint is in its original position, the suspension geometry WILL NOT CHANGE. Of course there is no room to do this, but the theory still applies.

Good grief, I am trying to be civil about this. Twist the control arm into any type of pretzel you want, if the inner pivot and the balljoint pivot in the same places, THERE WILL BE NO CHANGE in the suspension geometry. If you cant understand that, you need to go ask an engineer to explain it to you.

The control arm is a rigid connection between two pivots. The pivots DONT CARE how you connect them.

onevoice

Bill, I and everyone here usually defers to your considerable knowledge of these cars, but you are dead wrong. The IC calculation only cares about the pivot points, not the control arm. The pivot point does not move, whether top or bottom mounted.

-nick

Ah, true enough. I was adding an extra pivot in my mind. I tend to thrive on poor examples

After writing it down, I agree, the angle of the arm won't effect the geo. Only the angle of the line between the pivots, as you said. Of course, if we space out the lower ball joint in one direction or another...

Cheers!

Bill Verburg

The instant centers are determined by the extension of the line joining the A-arm inner pivot and the ball joint with the extension of the line perpendicular to the strut that goes through the shock top pivot point. You can easily see that the IC center drops when the car is lowered, this lowers the Roll center. The rate at which the roll center drops is greater than the rate that the mass center drops, this leads to greater roll forces in play on the car unless the suspension is further modified


In the above diagram the line coinsides w/ the arm but in reality the line is a virtual one that is determined by the axis of the A-arm and the ball joint

In this pic imagine a ball joint extender thatthat movs the A-arm on the righ down to the level of the one on the left the IC line would not change it just becomes virtual exactly in the same place it is now.

ToSi

so you agree swapping the ball joint from bottom to top of a-arm doesn't affect the geometry?

onevoice

Agree 100%, but this is NOT what is happening by mounting the joint on top or below the control arm. The example of extending the ball joint moves the PIVOT. Mounting the ball joint on top of the control arm does not move the pivot point at all, so the geometry has not changed.

Bill Verburg

To answer that you need to know what the effect of raising the ball joint has on the ride height
Does raising the ball joint raise the the ride height, I suspect it does but don't know for sure

If it raises the ride height is there a 1-1 correspondence between the 2? I don't know that either

I suspect that the factory knows best, they never to my knowledge mounted the ball joint on top, but did spend a lot of money to come up w/ the 993RS wheel carriers to cure bump steer issues and to allow the cars to be lowered as much as possible while retaining the best possible handling.

ToSi

why would the ride height change? the ball joint is still connected to the upright in the same place, which still connects to the same strut & has the same wheel attached..
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onevoice

Moving the joint to the top of the arm doesn't raise the joint, the pivot is in the exact same place. The distance is fixed by the dimension from the wheel mounting surface to the ball joint mounting hole.

Spyerx

Which of you has measured the caster, camber, and toe curves with this change?

sundog

One thing it will do is change the stresses in the components. I would stick with the factory configuration.

onevoice

The curves will look exactly like the ones Bill posted for the stock configuration, because nothing that can effect them has been changed. Those curves can only be changed by moving the pivot locations. Moving the joint to the top of the arm does not change either pivot, nor does it change the arc in which it travels.

Bill Verburg

How does moving the ball joint to the top not change where the ball is? There are 2 points that determine the locus for the lower instant center line
point 1 is the ball
point 2 is on the A-arm pivot axis


JMO but all else being equal raising the ball raises the ride height so you are back to where you would have been before lowering the car

the only difference is the virtual arm(which is the arm used to determine the IC's) is now further above the actual arm.

The height of the axle center line is fixed wrt the ball. So once again, How does this not raise the ride height up? The old ditty about connections comes to mind
to paraphrase, the A-arm is connected to the ball, the ball is connected to the wheel carrier, the wheel carrier is connected to the bottom of the shock, the bottom of the shock is connected to the top of the shock thru the spring, the top of the shock is connected to the chassis....

Spyerx

Right, so you're being assumptive in this and haven't measured the curves and geometry.

I'm not going to share info more broadly as to the why, but you can look up my setup there is a reason the engineer set it up that way. He used to support factory 964 and 993 race cars. So a bit more experience than the Internet peanut gallery.

-nick

I had to draw it out, but I believe onevoice has got it regarding the geo. Here is one place that flipping the arm does make a difference. Interference at about rs-45mm height. Leveling the arm by flipping it will allow for more compression travel. Alternatively, using 993 longitudinals for a wider front track will space the arms further away from the tub and allow more travel too.



Spyrex, I can't tell if you know the answer and are refusing to share? Or you don't know and are sharing that you paid someone who does know? Are either of those helpful?