Radiusing the top and back of the guitar is done for two separate reasons. On the back, the arched shape is thought to help increase the reflection of the vibrations coming from the top in order to increase the instrument’s volume, tone, and projection. On the top, that arched shape provides greater structural support where the strings are pulling against the soundboard at the bridge.
What is the ideal radius to choose? On the top, this corresponds to the amount of string pull. On nylon-stringed guitars and ukuleles, where the string pull is far less than on steel-string guitars, it is not uncommon to find a flat soundboard.
The steel-string guitar plans we carry, for the most part, mention the radiused top and depict it, but fail to mention which exact radius is used. There may be several reasons for this. One is that the tops on the older guitars portrayed in these plans have subtly warped over time. The other might be that there were subtle differences in the radii used in the factories. It’s an interesting question, and we would love to know more from any customers who have insight into this!
So, what do we recommend? It’s widely believed that the majority of Martin guitars (the standard-bearer for almost all things steel-string) used a 25’ radius on the top and a 15’ on the back. LMI offered our radiused dishes in just these two radii for many, many years—so it’s a good, solid recommendation. Now for the last few years, we have also offered a 12’ , a 30’ and a 50’ dish. For a slightly higher price and a short and reasonable wait time, we can also custom make a hollow form for you to the radii of your choosing.