Simple top deflection measurement

[hilo_kawika]

Whenever an instrument is strung up to tension, the top deflects downward on the soundhole side of the bridge and upward on the tailblock side of the bridge. No news here. But if we knew the top deflection of an instrument which we deemed to be successful - neither turning into an ashtray within a few months nor being so stiff as to be a hollowbody electric rather than an acoustic instrument - perhaps we would be closer to being consistent in our building.

If you have a dial indicator at your disposal, you're not very far from being able to make such a measurement. And as a bonus, no calculations - not even long division!



Here I show a jig which will do the trick. The top horizontal board is a couple of inches wider than the width of the lower bout and has a hole drilled along one edge that will snugly fit the base shaft of a dial indicator. Two additional pieces of wood below each end of the board with leather padding on the underside support the board above the soundboard. For maximum deflection, the dial indicator is positioned midway between the brace immediately below the soundhole and the bridge for one measurement and analogously between the bridge and the tailblock for the second measurement.

The difference between the amount of top deflection with the strings tensioned up and strings slacked is what's to be measured in each case.

The actual amount of deflection is a function of many parameters, but if a builder makes essentially the same or similar instruments with nominally the same string tensions, then "successful" instruments will in all likelihood have similar deflections. At least all of mine have...

It would be interesting to see what the deflections of various builders' instruments are. Any volunteers?

aloha,

Dave Hurd
http://www.ukuleles.com

[jeffhigh]

So David, when are you measuring it.
Immediately before first stringing? and after a few weeks?
What sort of figures are you getting for post stringing deflection?
regards,
Jeff

[kiwigeo]

It would be interesting to see what the deflections of various builders' instruments are. Any volunteers?

aloha,

Dave Hurd
http://www.ukuleles.com

Ive just fretted up a classical so Im might just rig up your deflectometer and see what results I come up with.

[Dave White]

Dave,

Interesting. and I know you say that many other parameters are included in the measurement but as you show and describe the measurement technique you are indexing from the "feet" of the jig at two different positions on the rimset, and so the deflection measurement will include any differences in lateral curvature in these two positions which could differ from instrument to instrument. Wouldn't it be better to have an instrument that rested on the top of the bridge with dial indicators at the two points of measurement in front of and behind the bridge - the difference in measurement relating directly to the deflection you describe.

[hilo_kawika]

Jeff,

I have deflection measurements for four different nylon string guitars. The instruments had been strung up for varying periods of several weeks to several months. Measurements are in mil (thousands of an inch).

1. Kenny Hill Palo Escrito #3027 Classical -- 28 mil

2. Ramirez R31993 Classical -- 8 mil

3. Tom Harper guitar #13 Classical -- 21 mil

4. Tom Harper guitar #9 Flamenco Negra -- 12 mil

I also have ukulele deflection measurements but felt that since this was a mostly guitar making community...

There are also instrument sound descriptions associated with each guitar if desired.

Dave,

The problem with putting any part of the resting frame for deflection measurement on the bridge is that the bridge also deflects downward as a result of a downward pressure from the combination of string angle and string tension. By having the "legs" rest on the edges and making the reasonable assumption that the sides are relatively incompressible, I think you have a more repeatable set of measurements.

aloha,

Dave Hurd
http://www.ukuleles.com

[Dave White]

Dave,

The problem with putting any part of the resting frame for deflection measurement on the bridge is that the bridge also deflects downward as a result of a downward pressure from the combination of string angle and string tension. By having the "legs" rest on the edges and making the reasonable assumption that the sides are relatively incompressible, I think you have a more repeatable set of measurements.

aloha,

Dave Hurd
http://www.ukuleles.com

Dave,

Perhaps I'm not understanding what the deflection is that you are measuring - I thought it was the difference in relative soundboard height in front of and behind the bridge while the instrument is under string tension due to the tops distorting under the tension. I thought that you were measuring in the two places you described and taking the difference in measurements. Hence my point about the sides curvature differences between the two points and between two different instruments in those positions.

Are you measuring under string tension and with the strings slack and looking at differences due to string tension?

If you are using the side rims as "reference" then top arching comes into the equation - two instruments that deflect the same amount but were built with different top arching would register different readings on your device I think. However, measuring with the strings slack and then under tension and taking the difference and doing this at your two points plus over the bridge (using the side rims as reference) would give an indication of the deflection change for each instrument pretty much taking arching out of the equation.

[hilo_kawika]

However, measuring with the strings slack and then under tension and taking the difference and doing this at your two points plus over the bridge (using the side rims as reference) would give an indication of the deflection change for each instrument pretty much taking arching out of the equation.

Dave,

I apologize for not having made myself more clear in the initial write-up. Yes the deflections represent differences between the top under string tension and with the strings slacked.

I should also note that a top properly optimized for high tension strings will be stiffer than one optimized for lower tension strings. For example my ukuleles are optimized for Savarez Alliance Red tension strings which are about 50% higher tension than a typical nylon ukulele string set.

aloha,

Dave Hurd
http://www.ukuleles.com

[Allen]

There are getting to be a fair number of us building ukuleles here now David. I'd be interested is seeing the deflection measurements that you have for some ukes.

[hilo_kawika]

Hi Allen,

No problem. Here are some ukulele measurements:

1. Royal Hawaiian 4-string soprano ukulele -- 10 mils

2. Martin 4-string soprano ukulele -- 13 mils

3. Kawika (me) 4-string tenor ukulele -- 12-13 mils

4. Gibson 4-string tenor ukulele -- 13-14 mils

aloha,

Dave Hurd
http://www.ukuleles.com