Ball joint- above or below a-arm?
#31
Tell me more about this interesting geometry. Let's put a stack of spacers between the ball joint and the arm so that the arm is at a 90* angle. Now let's compress the suspension. Oh wait, the hub won't be able to move up/down because the arm is at a 90* angle...
Just checking, none of us do this for a living I hope?
Just checking, none of us do this for a living I hope?
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.
#32
Top mounting the ball joint doesn't change the A-arm angle but it does change the virtual A-arm angle which is what you care about. The virtual angle determines where the instant centers and thus roll center are/is located as well as the scrub radius and also the virtual arc of the A-arm ball joint, on a lowered car you care about the concentricity of the arc of the A-arm and tie rod ball joints. Raising the A-arm ball joint raises the instant centers and thus the roll center and probably increases the concentricity.
#33
Three Wheelin'
Thread Starter
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!
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!
#34
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.
#35
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.
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.
#36
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.
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.
#37
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.
#38
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.
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.
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.
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.
#39
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.
#40
Rennlist Member
Which of you has measured the caster, camber, and toe curves with this change?
#42
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.
#43
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.
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.
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....
#44
Rennlist Member
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.
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.
#45
Three Wheelin'
Thread Starter
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?
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?