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.