The first rocking chair I made did not rock well because the rocker radius was too large and it took me a bit to figure out what was wrong. When I fixed it I left it alone (If it ain't broke!) for several years, then as more and more students began to ask why the rocker radius was what it was, I thought I should be able to provide a better answer.
I know most rocking chair makers out there are in love with the "Center of Gravity" hypothesis but in my opinion that just does not wash. You can do a simple "mind experiments" and discover that COG is not the answer. I remember as a young boy, five or six years old rocking in my grandmother's rocking chair. Well, my feet stuck out straight and my COG was about six inches above the seat surface. Did the chair rock well?? YES, it rocked GREAT! Did it rock well for my six foot three cousin?? The answer is another yes. The reason is that "how a rocker rocks" is a function of rocker radius over leg length, not the center of gravity of the individual sitting in the chair. This is America and the last time I checked I was still entitled to my personal opinion!
How Not to Fall Off Your Rocker
or
Rocking Chair Radiuses as Easy as Pi
When Evan Eschmeyer, the smiling, six foot
eleven inch NBA player drafted in the second round of the 1999
season by the New Jersey Nets, came to my shop looking for a
rocking chair to fit him perfectly; I was prepared! Although I
did not know Evan until he walked through the door of my work
shop, I had been studying how to make his chair for the past
thirteen years. This is the story of my learning to make Evan’s
rocking chair.
Since the making of my first rocking chair
for my daughter Rachael, sixteen years ago, I have been intent
on making every chair I make fit their new owner perfectly.
Following the adage that “form should follow function” I believe
it is self evident that the function of a rocking chair is to be
exquisitely comfortable. If it does not fit properly then it can
not be as comfortable as it should be.
As important as the fit, is the way the
rocker rocks. This is determined by the radius of the rockers.
As people go beyond average height the radius must change. How
much it should change is the question that is addressed here.
When I made my second rocking chair (the
first one I intended to sell) I found it did not rock worth a
hoot! After much head scratching and gnashing of teeth I
decreased the radius of the rockers a good bit and to my
surprise it rocked beautifully. Thus, I learned early on, the
importance of the rocker radius. I continued making rockers,
using the same radius for several years. You know, “If it ain’t
broke.” In making rocking chairs fit their new owners perfectly
I have changed all dimensions of my rockers many times over. In
the process I developed three (I now have six) standard sizes of
chairs for folks who order over the internet.
It was in the making of a rocking chair
for a very long legged fellow that required me to lengthen the
legs of my tallest rocker by a full inch and a half that I
learned another important lesson about rocker radiuses. When I
completed this chair and sat down in it I felt compelled to HANG
ON to the arms as the rock was a little on the scary side! I had
to increase the radius of the rockers on this chair in order for
it to rock properly. The lesson here was that the ideal radius
of the rockers appeared to be a
function of the
leg length. After this experience I began to investigate rocking
chairs on the basis of the rocker radius (RR) over the leg
length (LL) I have adopted the term RR/LL to express this
relationship. The leg length I use for these calculations is the
total apparent leg length, the distance from the sitters butt to
the floor when the chair seat is parallel to the floor.
I took Calculus in college and don’t
remember much of it now but the one thing that stuck with me was
the value of looking at minimum and maximum values in order to
evaluate functions. The
min and max values of a rockers leg length would look something
like this:
MINIMUM
If you had a rocker with a 42 inch rocker
radius and made the legs zero inches tall would it rock well?
No, because a forward or backward
force applied where the legs would be would simply result in the
rocker scooting forward or back, not rocking.
MAXIMUM
Now, conversely, imagine a rocking chair with
a 42 inch rocker radius and legs 100 feet long. Yes, I know 100
feet is extreme but you need big numbers for the min max thing
to work, the bigger the better. What will happen to this guy?
Well, the lever arm of 100 feet applied to a circle with a 42
inch radius will result in a one way ticket to the ground!
Why is he going to crash? Because the leg length exceeds the radius of the rockers! This would be exactly analogous to a person on a unicycle removing their feet from the pedals. Why is this? Well, if we take a look at circles (what grown up radii are) it becomes a little more clear.
If you apply a weight and a horizontal force
towards the bottom of a circle it will be fairly stable and
resistant to moving. This RR/LL would be very high, 10 to 20.
This would be like trying to move a bicycle wheel by pushing on
the valve stem when it is on the bottom. It will move a little
then ‘scoot’, exactly like a rocking chair with too large a
rocker radius.
Now, apply a weight and a horizontal force at the center of a circle. The circle will be free to roll where ever it likes. Once an individual started to rock (by applying a horizontal force) the circle would keep rolling in what ever direction the initial force was applied because the weight, in this case, does not influence the position of the circle. This would not be ideal for a rocking chair. This situation represents an RR/LL of ONE. Clearly too small for a stable rock. Again, if you applied force at the axle of a bicycle wheel, it would continue to roll until it ran into something.
So far, I believe we can intuit that a RR/LL
of one (weight in the center) will not work and also that a VERY
small RR/LL would not be suitable either, so the answer must be
somewhere in between. I believe it is safe to assume that all
circles behave the same. Therefore I will also assume that there
is only one RR/LL that provides an ideal oscillation back and
forth, always returning to center and never over rotating, that
is perfect for rocking. (Disregarding personal taste for the
moment) No matter how large or small the circle, the ratio will
remain constant. I also conclude that the function is clearly of
a linear nature.
|
Large |
Leg Length = 14.33 |
RR/LL = 2.93 |
|
Medium |
Leg Length = 13.42 |
RR/LL = 3.13 |
|
Small |
Leg Length = 12.5 |
RR/LL = 3.36 |
Two point ninety three to three point thirty six is fairly close. Every size rocking chair, I would think, should have a specific rocker radius. If I am using the same radius rockers on three sizes of chairs then any one of them is perfect, the other two must be off a little. Taking the average of all three sizes results in a number strikingly close to Pi. I find that fascinating! Wouldn’t you know it would be a number that every rocking chair maker knows about but didn’t associate with rocker radii.
During this same time frame I was commissioned to make a chair for a Major in the Air Force, David Hoopes, that was six foot seven inches tall AND long legged! He required a rocker with a seat that was an inch and a half taller than my current largest rocking chair. I knew this chair was going to require a new rocker form in order to rock properly from my work thus far. So what radius should I use? With a projected new seat height of 15.8 inches, multiplying that by 3 (I didn’t use 3.14 at this time because I had not made the connection to Pi) resulted in a new radius of very close to 47 inches. I made the form with a 44 inch radius, expecting the usual spring back after removal from the gluing form to result in a radius very close to a 47 inches. It worked perfectly!
David Hoopes and his new rocking chair
Combining the data on all of the chairs I make, the data looks pretty interesting.
|
Extra Large |
Seat Height 15.87 |
RR/LL = 2.96 |
|
Large |
Seat Height 14.33 |
RR/LL = 2.9 |
|
Medium |
Seat Height 13.42 |
RR/LL = 3.12 |
|
Small |
Seat Height 12.5 |
RR/LL = 3.36 |
|
Child’s |
Seat Height 10.74 |
RR/LL = 2.9 |
Evan Eschmeyer, with shoes, is 7 feet tall
What about using “Center of Gravity”? It is my opinion that RR/LL trumps COG calculations in a rocking chair. Why? Well, in the beginning I thought it was odd that people of ALL sizes could sit in my small rocker and exclaim that it rocked beautifully and was the most comfortable chair they ever sat in. Did the COG change with each person? Absolutely! Did it make any difference to the comfort or the quality of the rock? It didn’t seem so to me! Each individual would, of course, be more comfortable in a rocking chair that fit them perfectly but it was the RR/LL that seemed to be the most important factor.
What has happened? I have lengthened the "lever arm" and also changed the COG by two inches. Which one effects the change in rocker action? If someone five feet tall sits in my large rocker they will think it rocks well. Another person six feet four inches can sit in the SAME CHAIR and exclaim that it rocks beautifully. Has the COG changed more than two inches, caused by the extreme difference in the height of the sitter?? Absolutely! Did it affect the rock? Absolutely NOT it stayed the same! Did the "lever arm" (leg length) change? Absolutely NOT.
It looked to me like I had figured it out. Now, did I have the courage to put my theory to the test? Not having the time or space to make a 100 foot rocking chair I settled on a six foot tall version. That is six feet from the top of the seat to the ground.
Getting on was not straight forward. Finally on!
Rocking Chair Maker