Rocking Chairs: Fundamental Design Considerations

When I taught furniture design to college students I was often asked how rocking chairs work, and how to determine the rocker design and radius. Over the years my students designed and built several successful rocking chairs.

Here are the basics:

The center of gravity (of the occupant + the chair) will be directly over where the rocker arc touches the floor when at rest. The distance from the floor to this center of gravity has to be shorter than the radius of the rocker arc. The closer these two values are together the farther the rocker will roll. The farther it rocks the longer the rocking period will be. Ideally you want to use a rocker radius that creates a rocking period equal to the occupant's at-rest breathing rate. You don't want the seat plane of the chair rocking forward past horizontal. 

Let M = distance from the floor straight up to the center of (the occupant's) mass above the floor plane (in inches).  On a more upright rocking chair design, the M may be 23" to 25". R = Radius (in inches) of the rocker arc.  39" is a good R value to start with.

Rocking 5 degrees forward and back

When M is less than R, or your center of mass is lower than the center of the rocker arc, the chair will return to center when rocked forward or back because the C.O.G. has been lifted up and gravity will pull it back down. In the diagram above the C.O.G lifts .0575" with 5 degrees of rocking forward or backwards.  

The center position is when M is directly below the center point of the rocker arc. From a side view, M is normally located near the occupant's belly button when he/she is in a sitting position. For women it may be slightly forward of the belly button. A value for M can be found by positioning the user above the floor in a comfortable at-rest sitting position. Measure from their belly button to the floor to find M.

Centered Position

R > M, and a good dimension for R is 39" or so. Increasing R (flattening the arc) reduces the period of a rocking cycle. Reducing R increases the period of a rocking cycle. By changing R slightly the rocking period can be "tuned" to match the breathing rate of the occupant. The simple act of breathing will shift the occupant's mass, causing the chair to rock back with each inhale and forward on exhale.

Having your feet flat on the floor will impede rocking motion. Ideally the occupant's feet should cycle from touching toes to touching down the heel as the chair rocks. This suggests the height of the front edge of the seat should be 16" to 18" or so. Less if the intended occupant is of small stature.  

This all works because shifting weight moves the occupant forward or back from being centered. Since the center of gravity is straight down from the center of the rocker arc, moving forward or back lifts the occupant uphill. The incline gets steeper the farther one rolls.  Shift your position on the rocker and you will "roll" to another centered position. Relax back to your rest position and the rocker slows to a stop.

If your rocking chair tried to throw you out when you rocked forward then the center of the rocker arc is too far backwards. If your rocking chair feels like you'll fall over backwards when you rock back then the center of the rocker arc is too far forward. I've seen and sat in student rocking chair designs that suffered from one or the other mistakes. 

Comments and questions are encouraged and welcome!

4D