In reply to MarshHoltRacing :
I'm probably restating some stuff in an attempt to be clear, so forgive me if there's some wading...
The stock arrangement has the lower "control arm" (leaf spring) mounted to the chassis 9 degrees from horizontal, and the upper control arm's inner mount axis is the same/parallel (as it would have to given that the spindle pivots on pins, not ball joints)
The new subframe which mounts the lower control arms holds their inner pivot axis 3 degrees up at the front from horizontal (or 6 degrees down at the front from the original orientation of the leaf). The upper arm is at the same angle as stock.
I'm not sure what's meant by "bring the pivot up"... The new front subframe is so different from a leaf spring mount that it's hard to compare; that's where we took inspiration from the Abarth subframe as a starting point (roughly), and then started moving things around in Susprog. The Abarth subframe assumed kingpins, and held the lower control arms at the matching 9 degrees from horizontal, same as the original leaf.
Oh! It dawns on me that the annotation on the linked picture might not be helping... It makes it sound like the anti-dive comes from the raised angle of the upper control arm, but that's only contributing because of its relationship to the lower arm; the angle itself is actually contributing also to pro-dive, inasmuch as it adds rearward motion to the wheel's trajectory. There are two kinds of geometric things going on with the forces:
- There's a linear aspect, just like the forks of a motorcycle. In stock form, the wheels' direction of bump travel is mostly up, and 9 degrees back. If you push the wheel up, it goes back. Conversely, if you push the wheel back, it goes up. The forces are proportional to the distances; 9 degrees gives us a trajectory that goes up a bit more than 6 units for every unit it goes back. But if you push it back (like under braking), it will push upward against the spring with a force of a bit more than 1/6 of that force; 500 lbs braking force becomes ~83 pounds of force adding to the forward weight transfer trying to compress the suspension. This is the only one acting with our stock parallel control arms, and it's not helping!
- There's a torque-based aspect: When you apply the brake, of course the caliper wants to rotate with the disc, but can't, because it's attached to the spindle. The spindle can't rotate because it's braced by the arms, at the ball joint of the upper arm, and the ball joint of the lower arm. That torque tries to drive the upper arm forward and the lower arm rearward. If the arms' pivots are parallel, then the torque is resisted without generating any anti-dive, but if the arms are mounted so that the the forward ends of their pivots are further apart than the rear, then the torque will be acting at an angle to the arms' pivot axes, and just like with the linear aspect above, part of the force will be translated into a motion upward or downward. In the case of anti-dive, pushing forward on the upper arm makes it want to push down, and pushing rearward on the lower arm makes *it* want to push down. I believe the direction of the force applied at each ball joint is actually perpendicular to a line through the ball joints, because each end of that twisting force has to be reacted against something or else it would simply orbit the ball joint; each ball joint provides that anchor for its opposite joint.
So it's not that any arm is aimed up, or down, as such; it's that the pivots are arranged such that they're closer together at the rear than the front. Or more to the point, so that if you push forward on one and rearward on the other, relatively speaking, they both push *down* as a result.
Here's what a side view of our geometry looks like; you can see the beginnings of the instant center lines Susprog drew, as well as the relative angles of the arms:
It seems like the books talk about torque reaction, then mostly use that "instant center" visualization of the virtual arm, but don't really get into how the torque becomes that other force. I have an idea about how to create a better visual aid and model, but I've also got quite a to-do list (Gah! Bark mulch!) so I'm not sure when I'll get to that... Hopefully this A) made things clearer and not muddier, and B) is accurate. Not necessarily in that order.