Resonator “Leaf Spring” Bridge

It started out as a client question, “Is it possible to build an 8 string ukulele with a resonator cone”?  Well, that started a conversation about resonator ukuleles and are you sure you don’t want a re-tuned resonator mandolin?  I had never seen an 8 string, 4 paired course instrument built on a 6 inch ukulele resonator kit.  The first, major problem to overcome is that a set of 4 nylon ukulele strings tune up to around 40 lbs of tension, and if you double that you would be putting 80 lbs of tension stress on a 6 inch aluminum cone with a biscuit bridge. A biscuit bridge puts the full load on the very center of the cone, and I wasn’t that comfortable with clearly exceeding the structural capacity of a thin aluminum cone. Explaining why the cone keeps collapsing under the bridge is not a conversation I look forward too.  I spent a few days looking for a 6 inch spider bridge (which distributes the tension to the rim of the cone) but was unable to locate a manufacturer.  BUT, in the process I learned about the Schireson Brothers “Flywheel” cone that lost in Patent court against the Dopyera Brothers (Dobro) in 1935. The Schireson Brothers had approached the same problem of distributing the string tension, but using a wooden disk instead of a cast aluminum “spider” array.  I thought it was brilliant, and can’t understand how they lost out to Dobro’s Spider cone. From this research, I developed a few test prototypes and came up with my own design of bridge.

This design addresses the main issues I encountered with creating an 8 string resonator ukulele. The first step was to flip over the aluminum cone so that it is suspended by the rim facing down instead of sitting on the rim facing up.  This way the tension on the center of the cone is stretching it and not crushing it. The biscuit is replaced with a wooden support which provides lateral stability and helps distribute the tension load to the rim of the cone, and acts much like a leaf spring suspension. It was just a matter of adding the saddle supports to keep the bridge saddle in place.

 

2 thoughts on “Resonator “Leaf Spring” Bridge

  1. I have been making 3 String resonators with ss dog bowls, had some aluminum bt they are hard to find. Looking for a bridge that doesn’t oil can the center. This looks like something I can use. Came here looking for just intonation on 3 strings, used 19 limit just intonation on my harps but not the same.

    1. I can help you with Just Intonation, it can be a bit tricky. The fret locations need to be based on the specific tuning of the string course in question. This is where a lot of confusion about (JI) comes from. The fretwork must be centered on the “E” note, as there is only a half step between E and F. Depending on the tuning for that string course, the “E-F” fret spacing can be in different locations. This is why you see a lot of partial frets used in (JI) instruments, because using a full fret can easily cause the intonation issues you mentioned.

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