Here's a post of mine from another forum. Thought it might clear up the questions some of you have:
I'm tired of waiting. Let's bust some more myths! Specifically, these 3:
* Lower link angle is, in itself, significant.
* 4link spread affects weight transfer.
* Instant center distance forward is, in itself, significant.
Consider a competition 4link. To simplify the calculations, rear pivot points will always be directly above or below the axle centerline.
"Out" will always be the horizontal distance forward from the axle centerline and "up" will always be the vertical distance from the strip surface.
The lower link is 6 inches up and is horizontal.
The rear pivot for the upper link is 20 inches up.
The instant center is 50 inches out, which means that, since the lower link is horizontal, it is 6 inches up.
The thrust (horizontal force) at the rear tire patch is 3000 pounds.
The lower link would be in compression and the upper link in tension.
The slope of the upper link is 6 minus 20 over 50 or minus 0.28 . This means that the vertical force component, acting in the upper link, would be 0.28 times the horizontal force component. Since the lower link is horizontal, this would be the only vertical force acting against the vertical force at the tire patch. In other words, this would be the total weight transfer carried through the links.
For a horizontal force balance, the horizontal force carried through the lower link, less the horizontal force in the upper link, must equal 3000 pounds. For a moment balance, 6 times the lower link horizontal force must equal 20 times the upper link horizontal force. This gives us 2 equations and 2 unknowns. Solving, we get an upper horizontal force of 1286 pounds tension and a lower horizontal force of 4286 pounds compression.
So, the weight transfer carried through the links is 0.28 times 1286 or 360 pounds.
Now, we'll discard that lower link and install a new link above the upper link. In other words, what was the upper link is now the lower link. This new link has a rear pivot which is 100 inches up. No, it can't be built, but it CAN be calculated and the results will be interesting. Since the instant center is still 50 inches out and 6 inches up, the slope of this new link will be 6 minus 100 over 50 or minus 1.88 .
Total vertical forces will now be 1.88 times the upper link horizontal force downward less 0.28 times the lower link horizontal force upward.
The horizontal force balance remains the same, but the moment balance is now 20 times the lower link horizontal force less 100 times the upper link horizontal force. Solving, we get an upper horizontal force of 750 pounds and a lower horizontal force of 3750 pounds.
So, the weight transfer carried through the links is 1.88 times 750 minus 0.28 times 3750 or 360 pounds. That number sounds familiar, doesn't it?
But, we're not finished. A line through the tire patch and the instant center is a line of constant percent antisquat. So, let's move the instant center out along that line. We'll move it to 500 inches out, which would mean that it would be 60 inches up. We'll return to the original link arrangement: A lower lower link with a rear pivot 6 inches up and an upper link with a rear pivot 20 inches up.
But, for the link lines to intersect at the new instant center, the slopes have greatly changed. The slope of the upper link is 60 minus 20 over 500 or 0.08 . The slope of the lower link is 60 minus 6 over 500 or 0.108 .
Total vertical forces will now be 0.108 times the lower link horizontal force downward less 0.08 times the upper link horizontal force upward.
The moment balance is as it was for the first case: 6 times the lower link horizontal force must equal 20 times the upper link horizontal force. Not surprisingly, the horizontal forces are the same as in the first case: 1286 pounds tension in the upper link and 4286 pounds compression in the lower link.
So, the vertical force is 0.108 times 4286 pounds downward less 0.08 times 1286 pounds upward or 360 (there's that number again!) downward.
So, the weight transfer carried through the links is the same regardless of extreme changes in lower link angle, 4link spread, or distance out to the instant center SO LONG AS the antisquat percentage remains unchanged.
http://www.racetec.cc/shope