Simple method for locating body-drop by sight

[Georg]

Guess most players can hear body-drop detuning when pedaling a PSG, but pin-pointing mechanical weak-spots in the construction so the phenomena can be fixed or counteracted, can be a bit problematic since the actual/visual flexing, bending or drop is so small. Some, like me, don't like the audible effect of body-drop, and want to do something about it. So, thought I'd share how I went about locating weak-spots in my PSG 2 decades ago. I'll use the same method today, both to check and fix existing PSGs and while building/checking new ones.


Need a few small pieces of mirror - the smaller the better, some putty, and one or more flashlights.

The PSG must be steadied sideways against something that can't move at all, a heavy workbench, solid wall or something like that. The floor better not move or vibrate either, so a concrete-floor is fine. The whole idea is to keep the PSG-body perfectly stable since we'll be looking for very minute flexing/bending that affects string tension.

BTW: better take the rubber-feet off the legs while testing. That rubber doesn't cause body-drop, but it'll make the instrument slightly unsteady on the floor and the test less accurate.

One or more flashlights are fixed to something equally stable - a horizontal 2 by 4 will do fine, 5-10 inches above the steel, pointing down towards the mirrors.

Fix the mirror-pieces at various points along the PSG's top-side with putty. Adjust the angle of the mirrors in the putty, so the reflected light from the flashlight(s) spread out nicely as separate light-points on the ceiling in the room.

Push pedals and pedal-combinations slowly, while observing how the light-points in the ceiling move relative to each other. Any tendency to flexing/bending at and between mirror-points will show up, even if the unwanted movement is smaller than the width of a hair.

May have to use more than one flashlight to cover mirrors along the entire body-length of the PSG, and the closer the flashlight is to a mirror the more visible the body-drop movements will be - on the ceiling.

Audible body-drop may be caused by weaknesses in the changer-construction itself or the way it is mounted, so fixing very small mirror-pieces at individual bridge-rollers may be necessary. A bit more problematic to angle them right so they show the detuning movements that matters there, but putty is very adaptable


Don't jump to conclusions too early, as flex doesn't necessarily affect string tension and cause audible detuning. Depends very much on where what part of the construction flexes.
Needless to say I fixed my PSG, based on what I found out from such a "mirror test". Took me a while - a couple of weeks - to pin-point the real weak spots though, despite the fact that I could see them clearly from the moment I started the test.

Now, what to do about audible body-drop once the weakest points are located, is another matter - and depends very much on how the PSG is constructed. Maybe we can go into that in other threads, if "eliminating body-drop" is interesting.

[Bent]

This is indeed interesting, and innovative like only Georg can think up.
Just a few questions for clarification.
-"steadied sideways" meaning that the end plate is steadied against a wall? In which case i would think that both end plates would have to be steadied, or the guitar would just push itself sideways
- Also, is it really enough to just take the rubber tips off the legs? The least little bit of "walking" when the pedals/knees were engaged would result in invalid testing due to the effect this would have on the light/mirrors.
For sure a great idea to expand on.

[Georg]

Not very innovative really; it's an old method for detecting and measuring minute physical flexing/displacement, from before really sensitive and more compact mechanical and electronic measuring-devices could be built.

For me (and my old Dekley) it was enough to steady the keyhead end against a workbench and take those rubber-feet off. I wasn't playing the instrument, only pushing pedals/levers very, very, slowly while watching the "amplified" read-outs on the ceiling. I took my time - all in all I spent more than 24 hours over a 2 week period looking for minute flexing and making notes, so general instrument-movements became interpreted as such and evened out.

Others may want/need to fixate the instrument-body better, so they can focus on interpreting the read-outs. Use your imagination, but don't fixate the instrument in such a way that it detunes differently from when it's played.

Practical procedure:
• By having a mirror at each end of the PSG as control-points, the line-up and overall stability of the instrument can be continuously checked. On really weak PSG bodies the control-points may move relative to each other when pedals are pushed, so use the light-point from the one at the keyhead end as "absolute reference".
• Then place a few mirrors at various suspected weak-spots along the top-plate and on the changer-parts, and see if, and how, the light-spots from these get displaced relative to one or both control-points when pedals are pushed. You probably do not want to draw up lines for exact measurements on the ceiling, so eyeball those light-point displacements as best you can.
• Make notes and move all but the control-mirrors around until the entire string-rigging is mapped. Repeat until you're sure you have a reasonably good "map" and understanding of how the top-plate and entire string-rig behaves under stress.
• Note that soft putty isn't perfectly stable over time, so some adjustment of those mirrors may be necessary now and then.

This testing-method makes for an interesting "light-show"

[Bent]

Great explanation Georg. I'll stick some big white construction paper sheets to the ceiling. That way I can make all the reference point marks I want.
Eldon and all you scroungers, we can look in the dollar stores for those mirrors, as well as a myriad of small flashlights.
Also, I envision the grandkids' play-dough instead of putty. Always seems to be piles of it around here.

[Pat Comeau]

This is very interesting indeed...is there anyway you can draw a picture showing how and where to place those mirror ect..., a good description of the method would be appreciated.

Pat C.

[Georg]

Placing the flashlight(s) 10 inches above and a little behind the backside of the top-plate, pointing at an angle onto horizontal mirror-pieces, will make the light-spots hit the ceiling well in front of the PSG. Easiest to see them there while pushing those pedals, and the lightbeam from mirrors up becomes a little longer which "amplifies" deviation more.
The longer the lightbeam reflected from the mirrors up to the ceiling (or whatever surface the lightbeam can be made to hit), the better, but the angle for the light from the mirror should ideally not be more than around 30°. That'll give max 60° between light-source and target-area on the ceiling.

• You fix those tiny mirrors in place anywhere you suspect bending when pedaling, make notes of how much and in what direction(s) the bending cause the light-point in the ceiling moves if at all, and move the mirrors from position to position along the entire width of the top-plate. Takes time to "map" the entire top-plate with neck and all, but ideally not a single square inch or component should be left unchecked.
• Since it is the variation in string length/tension of non-pedaled strings when pedaling that matters, both the keyhead assembly, the keyhead nut/roller assembly and the bridge/changer rollers and mounting are most obvious places to look for variations in bending.
• Since all stringed instruments will bend when all strings are tuned from completely loose to pitch, it may be worth the time to check how much it bends, and where it bends and give the most during a complete tune-up procedure. The same points/parts are most likely to flex and give a little when pedaling, and it is this flexing and variation in compression in the string-rig we hear as "body-drop".
• The usefulness of such a "visual body-drop check" depends on thoroughness, time and patience while checking and rechecking for those mostly pretty minute deviations. Time spent the first time one performs such a check, will save on time needed for a valid check on similarly constructed instruments later.
This is especially true since probably none of you have performed such a displacement test using "a lightbeam reflection" before and don't quite know what to expect. You may be in for a few surprises on how much this method can tell when performed and interpreted well.

[Bill Dobkins]

Interesting thread, as I am working on a method to adjust the cabnet drop out of the instrument. So far all test have been positive
BD

[Allan]

Georg, this is great stuff. I am currently kicking myself for not coming up with the concept on my own. I used to work at a place where we designed speaker enclosures for public address systems and we used a similar system to check resonance on a new design.

I am wondering if it would be easier to plot the reflected beam if it were produced with a small laser pointer instead of a flashlight. What do you think? I don't know how well the flashlight beam can be focused to a point. Just a thought since these things are so inexpensive these days. I have actually bought a few from time to time for a dollar each.

Really good thread!

Best regards, Allan.....

[Georg]

Allan, those cheap laser pointers would of course work fine - I designed a little gismo using a laser pointer mounted on a pair of safety glasses, to help my chiropractor sort out neck-movement problems in patients. Works the same way but without the mirror

Actually, the lightbeam from a flashlight doesn't have to be focused at all, as those small mirrors will produce narrow squares of reflected light no matter what light-source is used. Just don't place the light-source too close to the mirrors, and use the smallest mirrors you can find/produce. The light-squares you get with unfocused light on small mirrors, are usually easier to see and keep track of as the move, than the extremely narrow point from a laser.

[Georg]

Note that in most cases the house's overhead/ceiling-light (one light-bulb only) shining down on the steel/mirrors, will do just fine. Just not as bright reflections from the mirrors.

So, for a start you can leave out flashlights and/or those mini-lasers, and just crush a small mirror into smaller pieces and fix them with some play-dough or putty. Makes for a really cheap "body-drop test."