Measuring Deflections – Tops and Plates


In the past 20 years or so since I wrote “Left-Brain Lutherie” there have been a number of new ways to measure the deflections of instrument tops and individual plates. In order to appreciate these efforts by others and introduce my take on them, it’s necessary to move a few things around the website and create a new page grouping devoted to measuring deflections of all sorts.

The Earlier Static Approach for Estimating MOE (Modulus of Elasticity or Young’s Modulus)

This approach used a simple wooden frame with fixed distances to rest the wooden plate on. A dial indicator below the place measured plate deflection when metal rods were sequentially placed across the center of the plate.

The Earlier Approach for Measuring Instrument Top Mechanical Compliance

This approach also used a dial indicator to measure deflection. The dial indicator was placed in one of a series of holes in a movable wooden frame that spanned the instrument top. The tip of the dial indicator rested in a circular metal ring attached to the end of a lever arm. A measured weight was placed on  a small platform on the lever arm near the ring to causing the top to deflect.

The  More Recent Deflection Jigs

These jigs seem to have come about as a result of Trevor Gore popularizing the idea of measuring the vertical deflection of a guitar bridge in order to have a sense of the responsiveness of an instrument. Variations of the jig by “jeffhigh”, Craig Bumgarner, Michael Cone, Greg Holmburg and Trevor Gore are on pages 1 and 2. I thank Trevor for introducing the use of a more rugged deflection indicator in the form of a tire tread depth gauge by iGaging and available on Ebay. In general these jigs seem to be focused on measuring only the bridge area. My take is to expand the width of this jig to make it more useful for a wider variety of uses.

The Jig Used for Bridge Deflection to Determine Monopole Mobility (coming soon)

This was my first attempt at making one of these jigs and is loosely patterned after that of an early model of Trevor Gore’s. There is a single position for the depth gauge because only a limited area of lateral movement is required for the measurement.

The  Jig and Setup Used for Measuring Instrument Top Mechanical Compliance (coming soon)

Making a mechanical compliance map of an instrument top requires greater lateral movement than the model above. With that in mind we lengthened the channels for the carriage bolt side adjustment units so that measurements more near the edges of the instrument could be measured. We also created simpler ways for repeatedly finding the same measuring spot on the instrument.

The Recent Jig and Setup for Static Determination of MOE (coming soon)

In this case, we used a simple framework identical to the one shown in my book, made from 1×2’s, PVC pipe and aluminum angle. Either of the jigs immediately above can be used for this purpose. Although the measurement is relatively simple, a number of precautions are both necessary and outlined.