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.

 

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