Moving the MC left requires...
Moving the MC left requires stiffer springs in order to keep the same roll angle. If we were to move the MC in the previous example to the left, we would need a 530 lb/in spring for a coilover and a 1,400 lb/in spring for a stock clip to achieve the same 2.0-degree roll angle. That’s an increase of 300 lb/in rate in the coilover and 700 lb/in in the stock clip.
Any part of the car that moves vertically when you jump on it has a role in making up the sprung weight. If we calculate the lateral force in pounds and divide that by the sprung weight, we get g-forces. So, if we have 4,000 pounds of lateral force and the sprung weight is 2,000 pounds, then our g-force would be 2.0.
2. The center of gravity of the sprung parts of the car is a point where there is equal weight all around that point. If the car's sprung portion was suspended by that point, it would remain motionless and not move in any direction. We're most interested in the height and width of that point in designing our setup. More about that later on.
3. Our front and rear suspensions have a spring rate. In a double A-arm suspension, the installed spring has a rate measured in pounds of resistance per inch of movement. If we compress the spring 2 inches and it will hold up exactly 400 pounds at that height, then the rate is 200 lb/in. That rate is translated through the suspension to the wheel through a motion ratio to what we call a wheel rate.
For a solid axle suspension such as we see in stock cars at the rear and some Modified cars and Sprint Cars at the front, our sprung part of the car will ride on the two springs. The width of these springs represents the spring base. The wheels and axle are separate and apart from the suspension and any attempt to create a wheel rate for a solid axle suspension is not valid for determining criteria for designing our setup.
4. The locations of the moment center front and rear determine the stiffness of the suspension they are a part of. Here's a better explanation.
Moment Center Influence
For a coilover, the spring...
For a coilover, the spring rate of the installed spring is translated to a wheel rate by using the motion ratio and shock angle. We square both the shock angle from vertical represented as a Cos number and the motion ratio and then multiply that number times the installed spring rate to find the wheel rate. Motion ratio is B divided by C.
So now we have some understanding about chassis roll in our car. When we talked about how the center of gravity was acted on by the g-force when we go through the turns, we can now tell you that there's a resisting point other than the tires in our suspension that also resists lateral movement. This point is called the roll center or what we like to term, the moment center, or MC.
The line between the CG and the MC is called the moment arm, a common term that has been used for a long time. It's the length of this arm that helps determine the amount of force the suspension will have exerted on it, and then how much roll angle we will have in our chassis.
I have, in the past, referred to this line as a sort of prybar. The longer the bar, the more work we can do creating more torque. Torque is a good word because it represents a rotational force and our moment arm is trying to rotate our car and make it roll.
Because of the effect of the geometric layout related to the location of the MC, its lateral and height locations determine the effective length of the moment arm in a double A-arm suspension. And, its location laterally is most important and has the most effect on the length of the moment arm and therefore the roll angle amount. So, when you hear or read about measuring only the height of the MC, you're not getting the most important aspect of MC location.
The Special Case of the Solid Axle
If you have followed me so far as to the double A-arm suspension geometry, let's now get into the solid axle. The part of our car supported by the solid axle suspension is sitting on top of two springs. These springs can be mounted in various ways, by coilover shocks attached to birdcages or solid clamps to the rear axle, or maybe onto the trailing arms themselves to create a motion ratio as the chassis moves vertically.