Help with bridge rotation

[JRCros]

Hi everyone,

Thanks to Gregory Holmberg for pointing me to this forum on FB, I recently acquired the Gore & Gilet's book and I can't stop reading and trying to understand and learning as much as I can. Thank you guys for putting together such an amazing piece of work.
I am trying to measure the bridge rotation of some of my builds and I am getting inconsistent results.
First I tried with an app, the mobile stuck with putty to the bridge. Guitar held on a vise on the neck and on the tailblock to the bench.
I got results between 2,3º to 2,5º ... which most people seem to think is too much, but I seriously doubt that my guitars have such a flexible top. I think the mobile maybe is quite heavy and it's moving around too much? I got a bit concerned anyway, so I bought the inclinometer in the picture attached and tried more or less the same way. Device stuck to the bridge with putty (not touching the strings), guitar held on a vise by the neck and on the tail and the shoulders to the bench.
The measurements this time seem to be more reasonable, but they go from 0,75 to 1,35 on the same guitar. The device seems to be working fine, but it works kind of funny. I stick to the bridge, zero it, and then untighten the strings slowly one by one. The reading does not change until I've slackened four or five strings, and then suddenly it changes to the final reading.
The most repeated reading for my 12 fret 000 is 1,35, which seems reasonable, but I am not sure if I'm doing something wrong. The device seems to be working fine on other situations.
Sorry for the length and thanks in advance!

Cheers,

Jorge

[JRCros]

In case it helps anyone...
I've been doing more tests and I think I know what happens. I think the device must have a movement sensor, so it reads only when it moves, but the sensor is not sensitive enough to notice when the movement is very small. If you remove the tension from three or four strings sometimes it still shows 0,00. Then you tap lightly on the guitar and it changes to 0,65 or so. So it makes sense to me. I've been able to take consistent readings this way. The trick is to make a light tap on the guitar after removing all the tension. Then you see it move and it gives you the final reading. If you tap again, it changes and goes back to the same reading (or almost). Not ideal but I think it should work.
If anyone has experienced this behaviour with a similar device it would be good to confirm if my theory is correct

[Trevor Gore]

I've got one of those angle cube things. Mine's by iGaging. It doesn't have any "stickiness" like yours seems to have, though.

Three thoughts:
1) There's some damping going on that you may be able to adjust or switch off
2) You have a faulty gauge
3) That's just how it is with that brand

[JRCros]

Thanks a lot Trevor.
The behaviour is the same on other guitars, so I think it's either faulty or it's just the way it works. I think it's a device that's not thought to use for this, but now I seem to be getting more consistent results. I might try another method though even if just to confirm results.
Thanks for your great work, I love it.
Regards,
Jorge

[Dave M]

Not really answering the question but I couldn't get on with the angle device such as yours. I went back to using a stick stuck to the bridge as per the book. I do use a stick twice as long - 600 mm and this gives pretty consistent answers.

Someone, I think Al Caruth, suggested the use of a light pointer such as a lecturer might use. This could be shone on a wall some distance from the guitar giving greater precision.

[JRCros]

Thanks Dave. I will have to try something like that.

[Dave M]

Jorge I was wondering if you resolved the bridge rotation issue.?

[GregHolmberg]

Someone, I think Al Caruth, suggested the use of a light pointer such as a lecturer might use. This could be shone on a wall some distance from the guitar giving greater precision.

This reminds me of an experiment I saw at NIST, the US government agency that defines measurement standards. My brother-in-law is a scientist who works there and he was involved in the recent re-definition of the kilogram.

Any way, they had a mirror mounted to a surface, the movement of which they wanted to measure. Then they shined a laser at the mirror at a low angle and onto the opposite wall, and measured the motion of the laser dot on the wall. If you know the distances involved, the trigonometry is pretty easy, and you can calculate the angle moved. It was extremely accurate, of course, being NIST.

I think it would be pretty easy to mount a small mirror on the bridge and shine a laser pointer on it.

Greg

[Dave M]

Greg I like the sound of that. Mounting a small mirror securely sounds easier than lengths of wood. I have just looked up laser pointers and they are pretty cheap and available at least in the UK.