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.