
What I am talking about here is when the bottom surface of
the rocker is radiused so that the rocker resembles a bent broom
stick. (the part that contacts the floor is rounded)
The reason this is done, I believe, is that it makes alignment
of the rockers a "no brainer" in that you do not have to worry
about it. If the rockers are not aligned exactly it will never
be noticed. Although this does make the construction of a
rocking chair easier, this method is not without it's problems.
The real dilemma here is that the area of the rocker contacting
the floor is quite small, a little less than 1/4 square inch.
When an average sized person sits in this chair the force
exerted from the rocker to the floor will be around 300 pounds
per square inch. This is going to cause a fairly rapid
compression and wearing away of the rocker where it contacts the
floor.
Given that the portion of the rocker used when the chair is
being rocked is about 8 inches, when as little as
one eighth of an inch of the rocker bottom is worn away in the
center of the rocking surface, this will cause an increase in
"apparent" rocker radius from 42 inches to 59 inches.
Fifty nine inches is an unacceptably large rocker radius and
will not provide a good rock. BUT as the change will be somewhat
gradual it will not, in all probability, be noticed by the chair
owner. However if a new rocking chair was placed next to the
chair that had been rocked for a year or so, the difference
would be immediately evident.