racing aspirations » Moment Center for Circle Track

Moment Center for Circle Track

by Racing Aspirations on November 9, 2017

If you run on the circle track you probably already know the benefits of having an asymmetric suspension setup, but let's have a look at how the lateral location of the moment center is so important.

Roll Center or Moment Center

Racing Aspirations refers to this point as the roll center (RC), but it can also referred to as the moment center.

The front moment center (MC) location plays a huge role in how the front end wants to work. The MC should always be somewhere close to the centerline (i.e., midway between the tire contact patches). The moment center is the point about which the suspension pivots. Like trying to undo a wheel nut without a wrench, trying to rotate the suspension about this point will be pretty much impossible.

The farther left the MC is located, the more efficient the front end will be and will want to roll. The farther to the right of the centerline, the less the front end will want to roll. Low-banked tracks require a location more to the left and higher banked tracks are best set up with a MC more to the right of centerline.

Why Is This?

Consider the diagram:

Centrifugal force is always be horizontal so with no banking it will push the car to the right, but as the banking increases it will push the car into the track. Centrifugal force emits from the center of gravity (CoG).
The moment center is the point about which the suspension pivots. Like trying to undo a wheel nut without a wrench, trying to rotate the suspension about this point will be pretty much impossible.
The resultant force line is one leg of a right angle triangle where the hypotenuse is the line between the CoG and the MC.
The effective moment arm is the other leg of a right angle triangle where the hypotenuse is the line between the CoG and the MC and it is the length of this arm which forms the length of the wrench in the moment center wheel nut analogy.

The farther left the MC is located, the longer the effective moment arm and the more efficient the front suspension will be meaning it will want to roll more. A more left location is proper for low banked asphalt racetracks and dry slick dirt racetracks, and cars with a low CG.

The farther right the MC is located, the less efficient the front suspension will be meaning it will be stiffer and want to roll less. A MC that is located farther to the right of the centerline of the car will be good for all tracks that are high banked including dirt and asphalt tracks. The higher the banking, the farther right the dynamic MC should be located. Because of the higher amount of downforce caused by the banking, we need the front to be somewhat stiffer to resist excessive dive on entry and in the middle of the turns.

Moment Center Movement

Given that the ideal location of the moment center differs depending on the banking angle then it makes sense to setup your suspension to move the moment center to the right as the banking angle increases. As the banking angle increases the centrifugal force will change from pushing the car sideways to pushing the car vertically which will reduce the ride height.