upper control arm zerks up or down?

[dougkirkbride]

i finished fitting up the front suspension, now going back thru and tightening, etc. Is there a preference on the upper control arm zerks? up or down? the geometry is definately different, with the zerks down i think the arms will be more level. With the zerks up it causes the arms to tip down at the ball joint. (see pics) Is there access to the zerks in both locations, up and down? Is either one, up or down gonna give me more room for my sway bar as its a tight squeze? what are others doing?

the ball joint in my arms is not angled, i have seen some that are.

zerk down

26877

zerk up

26878

[Xusia]

FFR Tech Support said zerks up when I asked.

[RM1SepEx]

The actual geometry does not change, zerks up or down as the pivot point is through the center of the shaft. I put the zerks down as it looks cleaner to me... Functionally it doesn't really matter

[Xusia]

Dan, looking at mine, I believe the geometry does change. I'm probably going to explain this horribly, but here goes. Because of the way the arms are angled relative to the pivot center, the axis upon which the upper ball joint moves is different. As the UCA swings through it's range of motion, I think it will affect the location of the upper ball joint, and hence the geometry.

[RM1SepEx]

Nope, look from the side... the arm pivots around the enter of the shaft and for geometric calcs the arm goes from the center of the shaft to the center of the ball joint pivot. The slight physical displacement either up or down depending on how you position the zerk doesn't matter. All it does is change the angle of the ball joint slightly.

[Xusia]

...All it does is change the angle of the ball joint slightly.

And since the angle of the ball joint is fixed (because it's connected to the hub), the arm must be in a different position within it's arc to accommodate.

[RM1SepEx]

You are missing the point

The ball joint rotates through suspension travel, it is not "fixed", that is why it is there.

the geometry is dependent only on the position of the pivot point of the ball joint (center of the "ball") and the center of the pivot at the other end. Location of and any angles of the arms... do NOT matter... The arm is essentially a member connecting these two points, their path getting there is moot. The ball joint has enough travel that is doesn't matter as long as you don't attempt to exceed its range during operation. That's why the angled A arm parts need to be assembled correctly, to avoid exceeding the ball joint's range of movement, bad things happen and stresses would skyrocket if you try to rotate the ball joint too far. Think bent, then broken

Edit: Thought of this over eggs and bacon... does the length of the upper suspension arm change when you flip it over? Since it obviously doesn't, the suspension geometry doesn't change as it is dependent on pivot locations (they don't change) and upper arm length (it doesn't change)

I hope that I've done a better job explaining this. For a good read check out Carroll Smith's Design to win, Engineer to Win books

[Xusia]

Of course, the length of the arms doesn't change. But for those of us with angled ball joint mounts, it changes the position of the ball joint within it's travel arc, and hence the angle of that ball joint mount. I understand your point, but I'm not convinced the geometry remains identical through the suspension travel. <-- This is not an invitation to convince me. I need to take a better look and see for myself.

[TahoeTim]

Xusia,
When I am confronted with little puzzles like this, I exaggerate the dimensions to help me visualize. For this puzzle, I am imaging a large carpenters square connecting the ball joint. I think Rm1 is correct.

[Xusia]

Yeah, I do the same, but I'm not coming to the same conclusion. I actually mounted them on the car both ways, and it was different. Unfortunately, I did not take any pictures. :(

[TahoeTim]

A string directly between the pivot points doesn't stretch or slack. It would be different due to the kink, but once the alignment is set, wouldn't it be the same?

[RM1SepEx]

optical illusion... straight arm, offset arm up or down doesn't matter, just the distance between the points. The center of the ball joint doesn't move, just a very slight change in the rotation of the ball in the joint

read the ref books, I also have a suspension design program that I used for my reverse trike. This is in my area of expertise as a mechanical engineer BTW

[K3LAG]

I can't speak to the geometry discussion, but one advantage of having the zerks up is that it creates a bit more clearance for the sway bar. I had mine zerks down at first and switched them because the sway bar was really close on the drivers side with them down. I also think having them up is going to provide easier access for future maintenance.

Larry

[RM1SepEx]

I worry about that sway bar too... I'd love to come up with a replacement straight bar with adjustable ends... :)