Roll Centers

Every vehicle has a front roll center and rear roll center.   They are independant from each other and are determined by suspension geometry.

The front roll center of a car with the Mustang's MacPherson strut suspension can be found as follows:

  • Draw a line at an angle of 90 degrees from the top of the front strut
  • Draw a second line through the lower control arm.   The point where these lines intersect is the instantaneous center
  • Draw a third line from the instantaneous center to the center of the tire contact patch.   The point where this third line crosses the car's centerline at the roll center.

Note in the diagram below how lowering the car by installing shorter springs changes the angle of the control arm and moves the roll center closer to (or even below) the ground.

The rear roll center of a car with Mustang's non-parallel four-link rear suspension can be found as follows:

  • Find point A by drawing lines through the non-parallel upper links.
  • Find point B by drawing lines through the non-parallel lower links.
  • The roll center is the point where line A-B crosses a vertical plane through the axle centerline.

If you were to add a panhard bar to the above rear suspension, point A would be at the point the panhard bar crosses the the centerline of the car when viewed from above.   If you were to add a watts link, point A would be the pivot of the bellcrank.  By adjusting their mounting points, either of these devices can be used to raise or lower the car's rear roll center height (RCH).

The roll axis of a car can be found by drawing a line that connects the front and rear roll centers.   A roll axis that points down toward the front of the car is an "understeering" roll axis, and it is often found on high performance cars.   Guidelines for building an open track race car usually specify that the front roll center be 1-3 inches above ground and rear roll center 8-12 inches above ground.