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

Iterative Progression

In furniture or product design often what you hope will be a good design ends up with obvious room for improvement. This is why initial builds are considered to be prototypes. It takes seeing and testing the first prototype to realize where flaws exist or where there is room for improvement in aesthetics or strength or performance or functionality or simplicity of build.

Shown here is a sequence of Balans style chairs I designed and made. Inspired initially by the original rocking Balans chair my pursuit was to find a design that was simple to build, adjustable, and stable.  In my PhDesk article photos you can see most of an earlier 3 caster perch version done as a class project by my students. The design was static with no adjustability or flexibility. Link:  PhDesk Article

Imbuia Wood Collapsible 

My Imbuia and leather prototype above improves on that earlier student design with knee pads that could rotate to meet your shins at a whatever was the most comfortable angle.   Initially the frame post beneath the seat was intended to be moved to different positions along the lower rails. This prototype revealed that changing the angle of the seat would also tilt the caster stems off vertical and reduce the ease of rolling the chair around. The frame could collapse by lifting the center post off the pin it rests on. Collapsed it would  fit in a smaller box for shipping or storage.

3 wheels Adjustable Height

Highest Perch Position

The 3 wheeled version above could be adjusted in height/angle.  This design isolates the caster base from the adjustability of the seat and knee rest. An aluminum push button  releases the aluminum post when pushed in and locks the post position when released. Knee pads pivot to meet shins at the most comfortable angle.

4 Wheels Adjustable

In use.

While there is an economic benefit to using 3 casters rather that four, a 3 point footprint comes with a flaw discovered in use. They could tip and roll out from under the occupant when leaning to the back right or left.  This four wheeled version eliminated the tipping flaw of all the 3 wheeled versions.  This version stretched the frame back so the back caster beam was behind foot clearance. It had the same push button height adjustment and pivoting knee rest as the 3 wheeled version above.  

The sharp bend in the center frame of the 3 and 4 caster versions above required making them from 80 very thin veneer layers of wood. A  later version used far fewer and thicker wood layers by changing the center frame to a smooth arc from under the seat down to the rear caster beam. Below is a simplistic rendering of the arced frame. It also had a seat that could be slid forward or back and locked in position with a cam lever.  The arc made room under the frame for occupant heels to meet or cross. This final version was gifted to the International Woodworking Fair management office in Fall 1988.  I'm still looking for any photos I might have of it.

Arced Frame Render

The final version was the simplest build, the safest to sit on, and had adjustable height, seat position adjustment, and pivoting knee rests.  It was a design that only came about after making and using the previous designs. They were all built in a university fab lab and benefited from being tried out by several students and other professors. Feedback gained from each version led to advancements in later versions.  This sequence shows iterative progression in action.

An even later iteration I designed is my rocking Balans.  You can read about it HERE.   

Comments and questions are encouraged!

4D

Ze Chair. A Chair Design that Failed.

I designed and built Ze Chair in the Spring semester of 1980 while in college. It won first place in the quick assembly category of the 1980 IWF Design Emphasis Student Furniture Design Competition. With padded seat and back it was very comfortable to sit in. 

Ze Chair

"Ze Chair" was an obvious name for the zig zagging frame of this chair design. "Gravizy" was my second choice. It is gravity that holds the design in its Z shape. The design bolts together and can be taken apart for compact storage, packaging, and shipping. It can also be unfolded, although when stretched out flat the length is impractical to hang or store. 

The back of Z pivots to meet the back of the occupant at whatever angle is most comfortable. This meant that most who sat in it, no matter their posture, found it comfortable. 

I made two prototypes of this design.  The first had thinner parts and was made from locust.  The wood split at the bolted half lap joints when first put under load. This should have been a clue. I (temporally) solved that problem by increasing the part size and using red oak instead. My second prototype survived a competition (where it won 1st place) and 2 years of use before failing the same way the first prototype had.  It eventually started to split at the half lap corners. 

I also won an award for the poster design I designed for Ze Chair. It was a first place award from the National Association of Furniture Manufacturers, 1980 local student furniture design competition / poster division. Sadly I no longer have this poster or any record of it.   

I often revisit this design with the hope of finding a way to reproduce it without the potential flaws. I know now far more about joinery, the forces involved, and production methods for making the parts. The chair doesn't need to unfold, and that it did made it a challenge to pick up. That it can be quickly put together and taken apart is a nice feature though which I'd like to keep.

I've thought about trying this design one more time.  One idea is to trim away 3mm from each side of the half-lap corner joints and glue in a piece of 3mm Baltic birch plywood.  The plywood shouldn't split. Glued to the oak it should help keep the oak from splitting.  I can even pocket out a recess for the plywood so it doesn't show when the joint is together. First task would be to find/buy material to make the parts.  

If this was an initial design of a student of mine I would encourage them to keep iterating, perhaps toward some triangulation in the design that would remove splitting stress on the folding corners.   The bottom piece running from front to back could be eliminated and a rear leg could run up and connect to the angling front leg, seat rail and the arm rest for example. That would triangulate the structure and lock all the pieces together.

Questions and comments are encouraged!

4D