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 from side to side)

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 1/8 square inch. When an average sized person sits in this chair the force exerted from the rocker to the floor will be over 1000 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 that is used when the chair is being rocked is less than 12 inches, that means that 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.