(disclaimer: I'm not an engineer and don't pretend to be one)
This is not a replacement to M-spec's guide, he's got some great stuff in there.
In this post, I am looking to answer a few questions that come up, and to try and provide some knowledge so that as you adjust the settings, you know what is happening.
first up: weight transfer
It appears that many people are confusing body roll and weight transfer. Although we associate body roll & pitch with the same forces as weight transfer, they shouldn't be considered the same thing. Softening a spring does not increase weight transfer.
If you were to be riding on a giant sled, with absolutely no suspension, the sled would still exibit weight transfer in a corner, with no body roll at all.
Turning to the left, our little man leaning into the turn.
In steady state corning, what was 200 lbs on each side might become 150/250. The amount of weight transfer is detirmined by how wide the track is (wider = less weight transfer) and how high the CG is (lower CG = less weight transfer). One can easily see why high performance cars are wide & low to the ground 
The reason why weight transfer is important is because of the relationship between how much lateral force a tire can generate and how much downward force is pushing it into the pavement. Obviously, the more downward force, then the more friction, but it does not increase on a 1:1 ratio. X downard force might result in Y friction, but 2X does not result in 2Y - it results in something less then 2Y.
It is because of this that the greatest traction occurs when there is no weight transfer - i.e. when both tires share the weight equally. Anything other then that, and you are making less then optimal traction. If we could, we would make a car that has a CG at ground level (no weight transfer) but obviously that can't be done.
next: Body roll
as we guide our cars around a corner, the suspension allows the body to roll. The axis that the car rolls on, and the amount of roll, depends on the suspension stiffness, the roll center, and the distance of the center of gravity from the roll center.
The roll center is the instantanous point at which the car rolls on. This is detirmined by suspension geometry. It can be found by extending an imaginary line off of the upper and lower control arms to a point at which they intersect (magenta). A line is then drawn from the center of the contact patch of the tire to that intersection point. The roll center is the point at which the last line, blue, crosses the center line of the car. Its also the point at which the two tire lines cross if you are figuring from the both side, which would be used if the roll center was off to one side or the other (again, remember the roll center is constantly moving as the suspension moves).
The grey box is the tire when viewed from the front. The black lines are the suspension arms. The body has been removed.
If you connect the front and rear roll center, you have a line. This line is the axis that the car rolls on. Generally, the rear is slightly higher then the front.
Viewed from the right. The axis that the car will roll on.
As lateral force acts upon the CG, the distance the CG is from the roll center detirmines how much body roll the car will exibit. If the CG was ON the roll center, then there would be NO body roll. the farther the CG is from the roll center, then the more leverage it has on on the body, and the more it will roll.
It is VERY unlikely that PD had time to compute the roll center of each vehicle, and therefore suspension geometry in general is not simulated. This is unfortunate, but expected.
What this DOES help you do is understand why adjusting the springs and sway bars does what it does.
lets take a car that has a perfectly even weight distribution (each number is the weight on each wheel.)
700 700
700 700
in a right corner, we get weight transfer to the left.
900 500
900 500
total weight is still the same.
When we increase the spring rate, we are not preventing weight transfer. The springs react into the body, reducing body roll, NOT weight transfer Stiff suspension does a few things good for us:
- reduces suspension travel, to prevent dynamic changes in camber and toe.
- improves responsiveness
- allows the car to ride lower without bottoming out
so, we increase the spring rate by 50% - but we are still at
900 500
900 500
with the stiffer spring, we could lower the car more, which would reduce weight transfer a little, but we won't include that just yet. now lets see what happens when we stiffen up the rear end more.
850 650
1000 400
but wait - if the rear got STIFFER, why is there MORE weight transfer in the rear?
The reason is because the spring is, again, reacting to the body roll. As the body rolled to the left, the rear spring held up that corner more, increasing the downforce on that side and unloading the inside rear. This results in LESS of a weight transfer up front. The TOTAL weight transfer is still the same, as it should be - 800 lbs difference between left and right.
the front tires are sharing the weight more evenly, so they will be able to provide more traction.
In a FWD car, with the same weight, it wouldn't be rare to see this:
800 600
1400 0
SWAY BARS:
a sway bar is simply a semi-flexible bar that connects one side to the other. It adds spring rate to one side in roll, but not in bump. So if you have soft springs and a thick sway bar, then the ride quality will still be soft, and you will still have lots of pitch & dive, but the suspension will resist roll. A sway bar increases the downforce on the outside tire by unloading the inside. Thus, a thick swaybar is not a good idea for the front of a FWD race car. usually, production based race cars will not have any front bar at all, and rely stricly on proper spring rates.
Discuss
