How to make a bridge on a flat-top "stickier"?

[Jim MacDaniel]

I recently purchased a Tacoma M2 as a traveller/beater, and though its tone and volume both leave a little to be desired, it is a blast to play. However, every now and while playing with my wrist resting a bit on the bridge, the bridge has a tendency to move laterally a bit from my wrist pressure.

I've never encountered this before on either arch-tops or matte-finish flat-tops, and I suspect the gloss-finish might be contributing to the issue. The easy fix is apparrent (don't rest my wrist on the bridge but is there a luthier's "trick of the trade" to help make a flat-top's bridge "stick" to a gloss finish? (e.g., roughing up the finish under the bridge and/or the base of the bridge)

(BTW, I am using medium guage strings, and it is tuned correctly

[Jerry Byers]

This is weird - I was going to ask the same question. I've seen two mandolins now with perfectly fitted bridges, but they can wander if given the right pressure or if I take off a couple of strings during a string change.

I'm almost tempted to put a drop of TiteBond under the bridge.

[Frank Ford]

Sometimes a bit of violin rosin can help. Scuff the bottom of the bridge lightly with some fine sandpaper, strike some marks with rosin, and reassemble the bridge in position. That might do the trick. Problem is that flat instruments have much less downward pressure from the strings, so bridges can skate around more easily.

[Jim MacDaniel]

Thank you Frank

(BTW, I was down at Gryphon last week while in the neighborhood, and fell in love with the Moon flat-tops -- as well as with Big Muddy's walnut model -- and if the rosin trick works, I keep it in my toolkit in case I end up with one of them

[Antlurz]

Is the problem with the bridge, or is the REAL problem that the neck or tailpiece isn't lined up with the center line of the body?

If the neck isn't "plumb" with the body, or the tailpiece isn't directly centered with the neck, and you try to line the strings up on the neck by shifting the bridge over, it will always try to find it's own way back to the center between the nut and tailpiece, even if that center doesn't agree with the rest of the mandolin.

One way to check is with a slightly flexible steel ruler if you have one. Line it up on the very center of the nut, the center of the fretboard, the center of the bridge, and the center of the tailpiece, all at one time. If you can't do that, you are fighting the laws of physics. If that is the case, you would either have to reset the neck to center it, or more easily and definitely more practical would be to shift the tailpiece into alignment. I'd not think the bridge should shift under pressure from playing if the string break angle is anywhere near sufficient.

......but maybe I'm offbase.

Ron

[Jerry Byers]

That would be a good theoretical assumption, but those items don't always line up. In fact, the Gibson models don't generally line up - it's part of 'the design.'

On the two mandolins that I mentioned, the break angle is sufficient, but the bridge still skates around. And everything is lined up correctly. The two outer strings have nearly the same angle coming out of the tailpiece. In both mandolins, the bridge wants to shift towards the treble side. I think with all things being equally, the G string is overpowering the E string in the laterally plane. Of course, this is just my observation with my two mandolins.

Both bridges have a good fit, and they have been polish smooth. I'm going to take Frank's advice and introduce a bit more friction, either by scoring and by rosin.

[Antlurz]

You are quite correct, and I'd not suggest that the offered remedies won't work. Obviously, they probably will.

I was only offering a possible reason why the problem exists to begin with.

It would be interesting to know for sure if your example of Gibson's is "designed" into it, or merely an accepted laxity in precision for the sake of "Git 'er done"?

I also agree with your statement about one set of strings overpowering the ones on the other side as being a possible factor.

Ron

[Jerry Byers]

I would be curious to know the history of the floating bridge on the mandolin and other stringed instruments. Of course, it probably has been covered in detail in other topics, so I won't hijack this thread. But if a medium was used between the bridge and top that was acoustically transparent, what harm could it cause? If you think about it, there already exists such a medium, the finish.

[Antlurz]

I've got a couple of very old flat tops that have had very small brads driven into the bottom of the bridge, then nipped off almost flush. They ...very.... slightly project into the soundboard, and have held the bridge in place for a hundred years or so.

That would probaly make most folks today cringe at the thought, but in fact, works quite well.

Would I do it when building a flat top? Probably not, but i'd consider it before I'd glue the bridge down, maybe.

Ron

[Terry Braund]

I had an inexpensive Gibson archtop guitar copy in for fretwork and setup. One thumbscrew post was protruding a tiny bit out the bottom base of the bridge and putting a dent in the finish. Not liking that, I fixed it but then had the bridge pull off to one side a bit when I tuned it up, shifting the strings to one edge of the fretboard. After scrathing my head for a while, I figured out that the tailpiece was out of alignment with the center line and the post had been keeping the bridge from moving. I moved the tailpiece into alignment and the problem was cured.

Just some input to this discussion and further proof to myself that being too picky on cheap instruments can be troublesome.

[Chris Baird]

Gibsons may have an offset neck centerline from what one may deduce is the body's centerline. But, that doesn't mean that the tailpieces are not lined up with the neck's centerline. If the tailpiece doesn't fall inline with the neck's centerline then it will create a lateral force on the bridge.

Having a moveable bridge is a benefit (for those who know how to keep them in the right place, which can change with different strings and actions).

I've built many flat-tops and never had a problem with a bridge slipping around. I'd say either the tailpiece isn't inline with the neck centerline, or, the breakover angle is too shallow.

[Jerry Byers]

I hear what you guys are saying, but what if it isn't the centerline or break angle? I'm looking at the two mandolins as we speak and the alignment is good and the break angle is sufficient. However, if I loosen a couple of strings, the bridge is apt to move. In fact, one of them is a short neck with the bridge closer towards the tailpiece, so it has a greater break angle than a normal f-model. And since the bridge is closer to the tailpiece, the spread angle of the strings is greater.

I'm not discounting your experience, Chris, but the answer you gave hasn't taken all the forces into account. What kind of tailpiece are you using? Type of bridge? Couldn't you agree that a smaller/thinner bridge would offer less contact area, and therefore, less friction between the bridge and plate?

[Antlurz]

And since the bridge is closer to the tailpiece, the spread angle of the strings is greater.

....and that extreme side to side angle is the very reason the strings have enough leverage to do what you are describing.

If they were a straight(er) side to side pull, it wouldn't happen.

Ron

[BlueMountain]

This mandola by James Curtis has an attractive but slim bridge that is held in place by two tiny pins (they may actually be pieces of straight pins--I'm not sure that seem to be glued into the bridge and stick into holes in the top. This does keep the top from sliding, though one has to be very careful that it doesn't start leaning toward the neck when one tunes up.

[markishandsome]

You can put a couple small pieces of scotch tape on the top indicating the proper position of the bridge so if it goes out of alignment, it will be easy to fix.

[labraid]

However, if I loosen a couple of strings, the bridge is apt to move.

If you're loosening up, say, both courses of D and G, and leaving E and A tightened... well, you're creating a new angle of force to the side. Has nothing to do with break angle or nut-to bridge anything, you're creating uneven, lateral force. It's not abnormal, you're just not really supposed to do that. Loosen them all evenly, a couple turns at a time, or just one single string, completely, at a time, and I do believe your problem is solved....

[Jerry Byers]

Thanks Frank, I'll give the rosin trick a try.

[Jim MacDaniel]

...I've built many flat-tops and never had a problem with a bridge slipping around. I'd say either the tailpiece isn't inline with the neck centerline, or, the breakover angle is too shallow.

Thanks for the feedback Chris.

The tailpiece placement is true to the neck's centerline, and the bridge does not wander unless lateral pressure is applied to it. In examining the breakover angle, I wouldn't be surprised if that was the root cause, as the angle does seem a bit subtle, and the action is rather low.

Since I like the action as-is, I think I will try Frank's rosin fix, but perhaps part of the issue might be a less-than-ideal fretboard-to-soundboard angle (I hope my less-than-technical description makes sense -- what do you usally target for this particular angle in your flat-tops?

[Chris Baird]

For my flattops I used a 3 degree neck angle. That is; the fingerboard plane was at a 3 degree angle to the rim's plane. However, the neck angle is not the only factor in the breakover angle. The breakover angle controls how much downward force the strings put on the bridge. You can measure your breakover angle with a protractor. It is the angle that the strings take as they go over the bridge. I can't recall what my flattop breakover angle was.

But, if it is your breakover angle you really don't want to try to change that. Frank Ford's advice is your best bet.