Why is this bridge not lifting ?

[Otterhound]

I said that I would build a test mule and here it is .
6 strings pulled to tension , no screws , glues or any other method of attaching the bridge to the top surface .
The bridge is sitting firmly in place . I have no doubt that id I were to force something under it at the rear that it would move , but that doesn't happen in the real world .
The paint stick is glued to the plywood and acts only to prevent a forward slide of the bridge . Fear not , it is an old paint stick .
Your opinions please .

[JonWint]

Main reason is the rigid board that the bridge is sitting on. The clamping force is much greater than the rotating moment resulting from the string height over the saddles.

An acoustic guitar top would belly in reaction to the same moment. The belly shape would apply a tensile force which attempts to peel the back of the bridge from the top.

[Otterhound]

Originally Posted by JonWint

Main reason is the rigid board that the bridge is sitting on. The clamping force is much greater than the rotating moment resulting from the string height over the saddles.

An acoustic guitar top would belly in reaction to the same moment. The belly shape would apply a tensile force which attempts to peel the back of the bridge from the top.

So all acoustic guitar tops belly enough for the resulting phenomenon you describe ? What of the bracing ? Isn't the bracing supposed to prevent the movement that you describe and allow for the top to vibrate? How much belly is present in a new guitar top when strung to tune ? I desire to create that very scenario .
What is this clamping force that you mention and where is it present ?
I have seen it written on this forum that the top is actually attempting to shear from the bridge on an acoustic , not the other way around .
Please take note that my inquiries are honest and only intended for me to gain understanding .
Thanks .

[Bruce Sexauer]

On a pin bridge the string are anchored to the inside of the top but this does not eliminate the torsional forces completely. The less lumber-like the guitar top, the more belly, and it is not only not a bad this to have belly, it is essential.

If the strings terminate on a pinless bridge, the bridge takes a great deal more of the torsional force and if there is a glue failure it will become obvious much more quickly. The glue failure in a pin bridge is largely due to the bellying and relative rigidity of the bridge as compared to the top.

Good to see you over the weekend, brief though it was.

[fazool]

The bridge isn't hold any of the string tension. It's a string-thru-body design and the strings are anchored to the other side of the wood (ala strat/tele).

The bridge is only, literally in this case, a bridge, lifting the strings up.

[Alan Carruth]

Although the strings are anchored to the - um - 'top', there is still a torque on the bridge due to the tension and the break angle.

You can divide up the string load on the bridge into two parts; the tension trying to pull the bridge toward the nut, acting as a shear load on the glue line (in this case, between the stick and the 'top') and torque caused by the fact that the strings are above the level of the top. The torque is taken up by the column load of the strings through the 'top' acting at the edges of the mounting holes, and there's enough leverage to hold the back edge of the bridge down. So long as it doesn't slide forward it's stable.

The torque is still trying to lift the back edge of the bridge, of course, but with everything so rigid you don't see as much gap. Wood cold creeps. With a wood bridge on a thinner wood top both pieces would deform over time and start to peel the back edge of the bridge up (or the top down, depending of how you look at it).

[nickv6]

Maybe I'm missing something but it seems straightforward to me. If the strings are anchored through the body, the saddles are acting like a bridge on an archtop, but the angle is sreeper so there's more down pressure than a typical (say) Gretsch so it's more firm.
Nick

[redir]

Draw the vectors.

[Otterhound]

In an idiotic attempt to try to create a belly effect , I placed 1/8" fiberboard under the rear of this bridge .
No change .
The saddles are a fulcrum point , yet no rolling action .
I still want to hear about the clamping action in the first response .
Also , at this point I feel the need to assert that I am not anchoring any strings to the bridge in my pinless concept .
I also want to thank Steve Klein for planting the idea of using a door to access strings that need to be installed from under the top . Thank you sir !
I do intend to use this going forward .

[joe white]

I can't offer any additional thoughts on the bridge scenario but I do think that thing might need a neck reset soon.

Just kidding Ott, cool experiment though.

[yellowesty]

A couple of observations:

1) In his book, Trevor Gore suggested that guitars built with typical soundboard construction and brace dimensions sound best when string tension generates around 2 degrees of bridge tilt. (This can be used as a rough indicator of adequate-but-not-excessive bracing for conventionally-braced steel-string guitars.)

2) the rigid "soundboard" coupled with the paintstick skid preventer in the experiment means that the bridge is not able to rotate around the bridge-soundboard interface, but, rather around the top edge of the paintstick. Viewing the pictures, that suggests that the torque seen by the bridge is around half of what would be seen by a glued-on bridge. Otterhound's experiment can be made more realistic by thinning the paintstick where the bridge presses against it.

[Otterhound]

Originally Posted by yellowesty

A couple of observations:

1) In his book, Trevor Gore suggested that guitars built with typical soundboard construction and brace dimensions sound best when string tension generates around 2 degrees of bridge tilt. (This can be used as a rough indicator of adequate-but-not-excessive bracing for conventionally-braced steel-string guitars.)

2) the rigid "soundboard" coupled with the paintstick skid preventer in the experiment means that the bridge is not able to rotate around the bridge-soundboard interface, but, rather around the top edge of the paintstick. Viewing the pictures, that suggests that the torque seen by the bridge is around half of what would be seen by a glued-on bridge. Otterhound's experiment can be made more realistic by thinning the paintstick where the bridge presses against it.

What thickness would you like ?
Currently , it is around 1/8" . The paint stick does not hinder rotation . It actually provides a fixed axial point . The bridge isn't even held in place in order to prevent side to side movement . Without string tension holding it in place , it will simply fall off if allowed .
What causes you to believe the half torque number ?

[Otterhound]

Originally Posted by joe white

I can't offer any additional thoughts on the bridge scenario but I do think that thing might need a neck reset soon.

Just kidding Ott, cool experiment though.

The neck has been reset back onto my Cherry hardtail Strat copy .

[mirwa]

Originally Posted by Otterhound

I said that I would build a test mule and here it is .
6 strings pulled to tension , no screws , glues or any other method of attaching the bridge to the top surface .
The bridge is sitting firmly in place

A few notes to consider.

All acoustic tops are built light enough to allow some rotation of the bridge, this rotation is typically 0.5 - 0.9 degrees, this rotation allows the production of sound, less rotation leads to lifeless / dull unresposive guitar, more rotation is big sounding guitars but these guitars tend to have tops that deform over time.

Your strings and front glued brace are clamping the .8mm thick piece of steel against the surface so it cannot replicate 8-10mm of wood being loaded from the path of the strings through the bridge pin holes

Acoustic guitars use saddles which apply forces into the base of the block of wood and to the leading front edge of the saddle which cannot be replicated using brass blocks screw anchored to steel.

Steve

[endpin]

Static Equilibrium