A chassis designed for moment...
A chassis designed for moment center adjustability will have slotted upper control arm brackets as well as slots in the lower control arm mounts in the crossmember. The upper plate shown is designed to take different slugs with offset holes. By changing the slugs to a different offset, we can easily change the upper arm angle.
The lower control arm angles mostly control how far the MC moves from static location (at ride height and down the straightaway) to the dynamic location (at mid-turn). Excess lower arm angles cause the MC to move farther than we would like. If we can keep the lateral movement of the MC to less than 4 inches, the car will be more consistent.
Most cars are not designed with adjustable lower control arms. In many cases, we need to find ways to move the lower mounting points up to reduce the lateral movement of the MC. If there is enough room above the inner bushing assemblies, we can drill new holes to attach the lower arms. If there is not room, we can cut out the crossmember and weld a hat onto the upper portion to strengthen the support.
Anytime we cut and weld on the chassis, it's a lot of work. Failing to take the time and effort will result in an entire season of frustration and poor handling. It is much better in the long run to take our medicine early and enjoy a car that is faster and more consistent.
Historically, the chassis builders have frowned upon teams hacking up their cars. I always tell the teams that once you buy the car and roll it out of the builder's shop, it's yours to do with what you like. Do what you need to do and don't worry about what anyone thinks of what you're doing.
Nowadays, many of the top car builders on both the dirt and asphalt side of racing are paying a lot more attention to the moment center design on their cars.
Control Arm Lengths
We can usually change our control arm lengths to help make changes to the arm angles. This is the easiest way to manipulate the MC location. If we install a different length arm and retain the ball joint, spindle, and inner mounting heights, we will have changed the arm angle as well as the camber change characteristics.
There are limits to how far we can go in making length changes. We don't necessarily want an arm that is way too short or one that is excessively long. The length can be limited because of engine components and the actual upper mounting plate location.
Changing to a shorter upper control arm will add angle to the arm and affect a move of the MC in the direction of that arm. A shorter arm will cause a greater change in the camber from chassis roll and chassis dive. Installing a longer arm does just the opposite.
General Truths Here are a dozen basic truths about MC design and placement:1. The upper control arm angles, especially the upper-right arm angle, dictate the lateral location of the MC and the camber change characteristics.
2. The lower control arm angles control the amount of lateral movement from static location to dynamic location. Less angle (chassis mounts lower than the ball joint) equals less movement. More angle equals more lateral movement.
3. Taking angle out of the upper-left arm and/or putting more angle into the upper-right control arm moves the MC range to the right.
4. Putting more angle into the upper-left control arm and/or taking angle out of the upper-right control arm moves the MC range to the left.
5. Installing a longer control arm using the same mounting holes and ball joint/spindle combination will reduce the arm angle.
6. Installing a shorter control arm using the same mounting holes and ball joint/spindle combination will increase the arm angle.
7. Higher-banked tracks require an MC range that is more to the right of the centerline of the car.
8. Lower-banked tracks require an MC range that is closer to the centerline of the car, or even left of the centerline in the case of a dirt car.
9. Very high-banked tracks require less upper control arm angles to control camber change. This compensates for the excess vertical movement from the high amount of mechanical downforce in the turns.
10. Low-banked tracks require more upper control arm angle to help control camber that results from chassis roll.