At this time of year I like to do a setup guide and that idea, combined with a number of requests and communications I have had with racers recently, formed this series. Much, if not all of the following, has been covered before, but, maybe not in the way that could be fully understood by everyone. This is my best effort to date to help everyone understand the parts of setup and the combining of these parts into a winning car.
It's very easy for any expert on any subject to assume that those they speak to will understand certain building blocks of knowledge contained in a technical subject. So, we tend to skip over or not fully explain the "obvious," when in fact it's not so obvious to beginners just as it wasn't obvious to us way back when we all started.
Have you ever had a computer guru explain how to work a particular software program? If so, you'll understand what I'm talking about. He talks too fast, moves the computer keys too quickly for us to follow, and ends up at the end of the explanation with us saying, "Can you go over that one more time, this time slower?" The trouble with printed communication is that I can't go over it again until next time, which might be a year or more coming.
For setup analysis, we need...
For setup analysis, we need to visualize the car as being cut in half with each suspension system able to function and roll independently of the other. When we can setup the car with spring rates, moment center locations, and more whereby the two ends are desiring to roll to the same angle in the turns, we’ll then have a dynamically balanced setup. This is the very foundation of every setup. When both ends of the car are working together, a lot of good things happen.
That is precisely why I'm writing this series. I'll do my best to slow things down, explain every detail, so that in the end, you might have a better understanding of the various parts associated with setup and a good idea of the proper way of combining of those parts.
Frontend Geometry and Chassis Roll
The frontend is where we start and the front moment center is the foundation of all setups because it influences the dynamics in such a significant way. Here's how.
When a car, any car, varies from straight ahead, it's referred to as turning. If it has a suspension, it will roll toward the outside of the turn. The force that causes this roll and also pulls things, like us, toward the outside of the turns is called centrifugal force.
The tires resist this force so that we don't slide away from where we are trying to go. The force at the tire contact patch is called centripetal force, or a force that resists the outward force. We'll only deal with the centrifugal force because that's what acts on the center of gravity and causes the roll in the car.
The location of the front...
The location of the front moment center is critical to how the front end will work. Moving the MC more to the right stiffens the front end much like installing stiffer springs. Moving it to the left softens the frontend and makes it more efficient. In this example, it would take a 230 lb/in spring in a coilover or a 700 lb/in spring in a stock clip car to achieve a 2.0-degree roll angle with this MC location.
Every car that experiences centrifugal force wants to roll, even if it has no suspension. A racing kart has a solid suspension, but it does roll to a degree because of the tire spring rate. The outside tires compress and the inside tires decompress. And even if the tires were solid, it would still want to roll. If the force became great enough and the tires held to the track without sliding, then the kart would roll over. It's this desire to roll that will form the essence of our chassis design.
In a race car, we're almost always on a suspension that has a spring rate, even if the car is running on spring rubbers. Those rubbers represent a very high spring rate, but a rate nonetheless. The amount of roll, measured in degrees relative to the surface we're driving or racing on, is directly dependent on, 1) the magnitude of the lateral g-force, 2) the height of the center of gravity of the portion of the car that is sprung, 3) the spring rates of the overall suspension, and 4) the location of the front and rear moment centers.
I'll explain each of those four items:
1. The g-force represents the force that is acting on the CG and is measured in pounds. It's referred to as the g-force and that number is related to the weight of the parts of the car that are basically on top of the springs.
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.
In the coilover, the spring...
In the coilover, the spring angle is nearly zero and insignificant and the motion ratio is found by dividing B by C. This number is squared and multiplied times the installed spring rate to find the wheel rate.
No matter what the mounting design is, the end result is that the sprung portion of the car over the straight axle suspension is riding on two springs. The roll angle amount for this suspension is determined by 1) the magnitude of the g-force, 2) the height of the CG, 3) the combined spring rates, 4) the height of the rear moment center, 5) the width of the spring base, 6) the difference in spring rates, if different.
1. Same as in AA suspension.
2. Same as in AA suspension.
3. We use the installed spring rate corrected for spring angle and in the case of a spring mounted on a trailing arm, factoring in the motion ratio.
4. The height of the rear MC is usually the average height of the ends of the Panhard/J-bar, in some cases the height of the metric four-link MC, or the height of the Watt's link MC. Width of the MC has no effect on roll angle, but does have some effect on weight jacking as does bar angle, but we won't get into that here.
5. The spring base is the width of the top mounting points of the springs. In a leaf-spring car, it's the width measured to the center of the two leaves.
In a straight axle car with...
In a straight axle car with the rear springs mounted on the trailing arms, there is a motion ratio involved. To find the spring rate that the car will feel, you multiply the motion ratio squared times the installed spring rate. The MR is B divided by C. As in the front coilover sketch explanation, if there is a spring angle relative to the swing arm, then you would square the Cos of that angle (difference from 90 degrees off the arm) times the MR times the installed spring rate to find the true spring rate that the car feels.
6. Spring split has a significant effect. If we install different rate springs in a solid axle suspension, our roll angle will be influenced quite a bit. If the outside spring is softer, the roll angle will increase. If the inside spring were softer, the roll angle will decrease over that of a system with equal spring rates.
Roll Angle Comparison
Once we grasp the influences of the different suspension systems and how each part plays a role in creating and determining the amount of chassis roll, we can now proceed on to looking at the entire car and how all of that affects our setup.
Each axle, or to better explain it, each pair of tires, front and rear, are the points ultimately that resists the centrifugal force that tries to take the car to the outside of the turns. As such, for proper analysis, the suspensions at each end and the sprung weights at each end, should be combined into two separate systems or vehicles unto themselves.
Imagine that the race car were cut in half, sideways down the middle in a line through the CG perpendicular to the centerline. And imagine that we could create a swivel where the ends of the car would rotate so that the front and rear could roll free of influence from the other end.
The truth of the matter is that when we install springs and control arms and J-bars, we're creating a system that will ultimately want to roll to a certain angle. What we create, or have for a front suspension and spring rates will result in its very own roll angle while going through the turns at our local track.
What we have for a rear suspension and corresponding spring rates will determine our rear roll angle in this imaginary cut up car. If we could visualize this car going around the racetrack, we might see where the car looks normal through the turns with no apparent distortion and this we can now call a balanced setup. Both suspension ends are rolling to the same angle, the two halves are inline just like when the car is parked and everything is working well.
The method for finding the...
The method for finding the MC location is to use extensions of the control arms through the centers of rotation of the ball joints and inner mounts to form instant centers. The two lines extending from the instant centers to the center of the contact patch of the tire on the same side cross to create the moment center location.
In another scenario, what if when we visualize this car, the ends are not inline, but rolling different angles? What does that mean? It means the overall race car setup is not balanced and the result is obvious, because the two ends don't line up and the overall look is much different than before the car was cut in half.
It's this equality or difference in the front and rear roll angles that constitutes modern chassis setup analysis. We now design our setups based on trying to achieve a balance in roll angles, and in doing so, a dynamic balance in the setup of the car.
Why Is Balance Important?
When the two ends of the car are in sync, meaning desiring to roll to the same angle our load transfer is predictable, all four tires are working to their maximum meaning that they have the most load possible on each tire. The two tires at each end are more equally loaded and therefore produce more grip resulting in faster speeds through the turns.
When the two ends aren't in sync and not rolling to the same angle, an imbalance exists that will cause less than optimum load transfer. The sets of tires become more unequally loaded resulting in less overall traction and less resistance to lateral force. The car must go slower through the turns than if it were balanced because the tires have less grip.
It's Not So Complicated
To create the measurements...
To create the measurements for finding the MC, in a two dimensional diagram, we measure the distance from the centerline, half way between the centers of the two contact patches, out to the points and the height from the ground to the points as shown.
OK, you might have to re-read the preceding parts of this discussion a few times, but getting a grasp on the above is necessary in understanding how our race car works and what our goals are in setting it up.
To help know what influences the chassis roll angle in our cars, I have created the following list in order of importance, the most significant at the top. The understanding of this principle will influence your decisions when making changes to your car.
1. Moment center location, front and rear. Moving the front MC left or right will have a significant effect on the stiffness of that suspension and moving the rear MC up or down will have the same effect.
2. Spring split on a straight axle system. More than spring rate stiffness changes, a difference in rates left to right has a significant effect on the amount of roll angle, especially on higher banked tracks.
3. Spring stiffness and sway bar stiffness.
4. Center of gravity height. The height of the CG helps determine the amount of roll angle, the higher the CG the more roll angle.
5. Lateral g-force. The more grip and faster we go through the turns, the more lateral force, or g-force we develop. And, therefore, the greater roll angle we see.
6. The track banking angle has an effect on the amount of roll angle. The higher the banking, the greater the g-force, but the less roll angle.
Other Geometry Influences
A solid axle suspension is...
A solid axle suspension is analyzed for roll angle by this method. It’s seen as the sprung weight being supported by the two springs. There’s no wheel rate calculation needed for a solid axle suspension. The car doesn’t know there’s an axle or wheels beneath the springs. All it knows is that it’s sitting on the top of two springs. What influences the roll angle is the spring base width, spring rates and spring split if they are different rates. The moment arm is the line between the center of gravity and the moment center.
There are other geometry factors that affect our race car. Once we have worked out our balanced setup, we need to worry about any influences that might have a negative effect on the balance we have created with the spring rates and the MC locations. In our next installment we'll learn about camber change, toe, Ackermann, alignment, and bumpsteer.
If you have a race car with an AA-arm front suspension and don't know where the moment center is located, both static and dynamically, then when you set up the car, it's just like not knowing your spring rates and just throwing any old spring in each corner.
That sounds dramatic, but it's very true. The difference in front moment center location from 10 inches left of centerline to 10 inches right of centerline is equal to a spring rate difference of 300 pounds per side in a coilover car and 700 pounds per side in a big spring, or stock clip car.
So, not knowing the location of the MC means you don't really know the "spring rate" of your front end. How can we expect to properly setup our cars unless we know the exact spring rates and other factors such as MC location?
Even with manufacturers who have determined the MC location for their cars, each track is different in speed and banking angle, teams may run different ride heights and setups, spindles get replaced with new ones that might not have the same dimensions for heights of the ball joints meaning different MC locations, and all of these have an effect on the MC design.
The only true way of knowing where your MC is located is to measure it yourself. Think about it like your engine timing or valve lash or stagger. The only way to know your timing is to measure it, just like your stagger. It takes a bit of time to measure the MC, but it's essential to have that information.