Official Luthiers Forum!

Just glued on a bridge and got to wondering.

I sanded the base to match the top radius pretty closely. When I glue it on, the top is domed as it is in a 45% RH space. When the top moves with the humidity, the bridge will then bend with it and the bottom of the saddle slot will no longer be flat.

Is this problem?

Ed

I don't believe it to be a problem. String pressure on the saddle will more than likely keep the saddle in contact with the bottom of the saddle slot. I think the distortion is pretty small, the top will conform to the bridge more than the bridge will conform to the top due the stiffness difference between the the two. All speculation on my part by the way as I have not actually measured it.

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Jim Watts
http://jameswattsguitars.com

I worry about the unusual things also, but when you consider that the bridge is essentially another brace, coupled with the string tension as Jim said, the saddle should remain in pretty good contact with it's slot throughout's it use. (Is that really one sentence?)

But then I have heard of instances of the saddle not making good contact causing problems, mainly when an unsaddle pickup is used.... Turns out it might be a good thing to question

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Joe Beaver
Maker of Sawdust

Nope, the bridge plate counterbalances it so the soundboard curvature doesn't change much right under the bridge.

The bridge plate is typically as thick as the top (or a bit more), and its grain runs more or less crosswise to the top (like the bridge), so the bridge plate's east-west stiffness will be greater than that of the top -- even if it's made of spruce (with a CF overlay). If it's rosewood it will be stiffer. RH changes mostly affect cross-grain, so the bridge-top-bridge plate sandwich will be fairly immune to RH changes.

As an aside, the glue side of the bridge plate should be sanded to the intended top radius before being attached to the top, just like the bridge itself.

If the bridge deforms so as to have the center high and the ends low, then it can be a problem. If an end of the saddle is not in contact, then the sound will be weak on the unsupported end. I actually had this happen on a flamenco guitar that had a flexible plastic saddle. The slot was low in the middle and the saddle conformed to the slot. When I replaced the saddle with stiff bone, I tried to curve the bottom to match the bottom of the slot, but I overdid it, so the treble end of the saddle was not touching the bottom. The unsupported end was flexible and acted like a shock absorber. Those strings were dead. I solved the problem by just using a flat bottom saddle.

So, if the center of the slot is low, then a stiff bridge seems to be OK, such as a violin bridge which does not touch in the middle.

Not so much from the humidity end, but I used to jack up the top under the bridge plate to match string tension, then rout the saddle slot straight. Typically would find some extra material coming out of the middle of the slot...

This is more of a speculative question than anything else: Is the fact that the bridge can (and sometimes does) yield a bit, thereby making the bottom of the slot not quite flat, a reason why it is important to have a saddle that fits snugly in the slot? After all, "down" is not the only direction that vibrations from the saddle are transferred to the bridge. If the vibrations can be transferred via the sides of the slot, that might make it less of a disaster if the bottom of the slot does not make perfect contact with the saddle at all points. Again, I'm asking, not stating.

My saddles used to be pretty snug, like "pull out with fret puller snug". I suppose a sloppy fit wouldn't transmit well, just as a bad fit to the bottom won't transmit well.

Even if the saddle doesn't contact the slot all the way along the string forces on it will be transmitted to the guitar if it doesn't actually move in the slot, and is stiff enough to not bend too much.

There can be a problem when you use an under saddle transducer. I made a nylon string 'crossover' guitar for jazz player Wolfgang Muthspiel some years ago, and installed a home-brew UST. I used a piece of piezo ceramic material that was about 1/4" wide and 1/16" thick, sandwiched between thin brass shim stock for the contacts. The bottom of the (necessarily wide) saddle and the slot were both dead flat. With only a little tension on the strings the P/U worked fine, but as they were pulled up to pitch the two outer string would drop out. The bridge was flexing upward in the center as the top deflected under the string load, and the wide bone saddle could not follow the curve. The solution was to cut the saddle in two places, so that it was in three sections, each carrying two strings. There was no other way to fine-tune the slot and saddle to work with every possible combination of string tensions. I'll note that the acoustic sound did not change noticeably when we cut the saddle.

I have seen classical guitars with individual bridge supports for each string. In one case, it appeared to be like a regular bridge saddle was sawn into six individual pieces that slipped into a continuous slot. In another case, there were six holes drilled partway into the bridge, instead of a slot, and brass pins were set into the holes and slotted to hold the string.

It is pretty much impossible for a rigid saddle to conform to the bottom of the slot--the bridge always deforms with time and if the saddle is really rigid, it will not deflect as much.

It is not speculative that that an elevated, non-contacting end of a saddle is a problem, because I have experienced that.

As far as a non-contacting center, I believe that some of the structural engineers out there might explain the difference in force required to deflect a beam supported at both ends, as opposed to cantilevered at one end.

Try this yourself sometime. Just make a stiff saddle that is curved on the bottom so as to only contact in the middle of the slot, even under string tension. I believe you will find the result terrible. Then make a saddle that is curved on the bottom to only contact near the ends. You may find that OK.

Thanks all

Love this forum when a good discussion gets going. I like the idea of making 2 saddles with intentional low spots in different areas. Might do that.

Ed

Or, make a flat saddle that contacts all the way across under string tension. A 3/8" high bone saddle doesn't bend. If the bridge/top deforms later, that's maintenance.

Carry on to oblivion, guys...