Fiberglass on bridge/bridgeplate?

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Fiberglass on bridge/bridgeplate?

Postby Matthew Lau » Mon Jan 14, 2019 2:34 pm

Hey MIMFers,

I'm planning on doing my first non-kit builds soon, and I tend to overcomplicate stuff.

I've noticed that some of my bridges on my kits have started splitting due to string tension...ditto for a part of a pinless bridge on a guitar.

Has anyone tried using fiberglass reinforcement for some of these stress areas?
I can also do bonded kevlar reinforcement....Ribbond, used in high stress fillings.

-Matt
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Re: Fiberglass on bridge/bridgeplate?

Postby Alan Carruth » Mon Jan 14, 2019 3:45 pm

Where are they splitting? The two most common sites for bridge splits are through the pin holes, and at the ends of the saddle slot. IMO if you have that sort of problem, you're doing something wrong someplace. There are design/build/use changes you can make to reduce or (hopefully) eliminate those issues.

Splits at the pin holes are usually due either to poor wood choice in the bridge or tight pins. Many people use quartered wood for bridges, but skew cut wood will withstand more strain without cracking. You also need to be careful about the grain direction, so that the pin holes are not all drilled into the same grain line.

Bridge pins are not wedges. Well, they are, but that's not how they're supposed to hold the strings in. In use the pins act as toggles: the ball end of the string rests on the plate, and pushes against the side of the bridge pin. If it's working right even pin that fits loosely into the hole will hold the string fine once there's some tension on it. If you're trying to use the friction of a wedged in pin to hold the strings in as they push upward on the pin, good luck: you'd have to hammer the pins in, and even then they probably would not hold. Since they're tapered pushing them in to ake them stay wedges the bridge on the line of the pin holes, and can cause cracks.

Splits at the ends of the saddle slot are due either to inadequate design margins or too much break angle over the saddle. Many bridges have about 1/8" of wood in front of the saddle slot, but some have even less. If you're replacing a bridge on something like a Martin you're sort of stuck with doing what they did, whether it was a good idea or not. If you're making a bridge for your own guitar you can put the slot anyplace you want to. It won't make any difference in the sound. so long as the mass is the same. Moving the slot back by 1/16" or so gives a lot more safely margin.

If you do a simple vector analysis, you'll realize that the static force on the saddle top is on a line that bisects the break angle. This resolves into a downward force on the saddle top pushing the saddle into the slot, and and forward force trying to tip the saddle toward the nut. The greater the break angle the greater the tipping force. If the break angle is 15 degrees, the tipping force is 25% of the string tension, at 30 degrees it's 50%, at 45 degrees it's 71% and at 90 degrees it's equal to all of the string tension. Having taken the time to do the experiment I can say with some assurance that there is no benefit to having a break angle higher than about 15 degrees: it won't change the sound or playabilty of the guitar at all. A high break angle is asking for trouble, with no benefit.

If you insist on using a high break angle, and have the option, try routing the saddle slot at an angle that will come close to bisecting the break angle. This equalizes the tipping forces on both sides of the saddle. Even something of a backward tilt can go along way toward reducing the splitting force on the front edge of the bridge. I've been using a nine degree back tilt. Rick Turner maintains that this will automatically adjust the saddle compensation if you need to raise or lower the string height off the top to make a small action adjustment.

If you find that need to have a very tall saddle in order to end up with playable action, then the solution is to re-set the neck to get a proper saddle height. Even if the break angle is low, having the strings high off the top puts a lot of torque on it, and risks having the bridge pull off, even if the front of the saddle slot can take the load.

It's always possible, of course, to have a bridge split due to a flaw in the wood. Aside from being really picky about the wood you use for bridges there's not much you can do about that. Saddle slit outs and splits along the line of the pins take care of about 90% of the problems that are not tied to wood issues, and most of the rest that I've seen have to do making the bridge too narrow or thin, or just using a wood that's not up to the job. I can't see the need for 'tech' solutions if you design properly and use the thing right.
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Re: Fiberglass on bridge/bridgeplate?

Postby Barry Daniels » Tue Jan 15, 2019 11:48 am

My current build uses the techniques of Trevor Gore's book which includes a layer of graphite cloth laminated through the bridge in a flat orientation. This will definitely resist splitting around the saddle and bridge pins.
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Re: Fiberglass on bridge/bridgeplate?

Postby Matthew Lau » Wed Jan 16, 2019 3:38 pm

Thanks guys!

One case is my first guitar.
I used a madagascar rosewood bridge (quartered) and made the pins really tight.

The other was a pinless bridge that a friend made for me.
One of the little "wings" popped off, but it still works.

Sounds like I'm overcomplicating things as usual.
Maybe I'll get the Gore book.
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Re: Fiberglass on bridge/bridgeplate?

Postby Olivier Vandebroucke » Wed Jan 30, 2019 12:58 pm

I never built a guitar using that, but I've bought and used different instruments (standard and bass guitars) with a fiberglass reinforced bridge. It does work and sound fine, and I haven't heard a noticeable difference compared to instruments of the same price and quality range with a wooden bridge.
Then, I maybe don't have the best hearing out there...
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Re: Fiberglass on bridge/bridgeplate?

Postby Jan Bloom » Wed Jan 30, 2019 11:41 pm

Do not follow everything that Gore does or says he does in his book.
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Re: Fiberglass on bridge/bridgeplate?

Postby Barry Daniels » Thu Jan 31, 2019 11:08 am

Jan, care to elaborate?
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Re: Fiberglass on bridge/bridgeplate?

Postby Bryan Bear » Thu Jan 31, 2019 12:02 pm

Jan Bloom wrote:Do not follow everything that Gore does or says he does in his book.


Which of Trevor's procedures are you doubting? I don't use his methods myself but I know many do with success. He is also active on other fora giving advice/explanations to those following them. The usual warning applies, you can not mix and match building methods without understanding the whats and whys of them as everything will affect something else in guitar construction.
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Re: Fiberglass on bridge/bridgeplate?

Postby Steven Smith » Fri Feb 01, 2019 10:51 am

Lot's of ways to do things and I look at other people's techniques as options to pick and choose from. I've built one falcate braced guitar following Trevor Gore's instructions to the letter and it turned out great. Turned out to be one of my best sounding guitars and the guy that owns it loves it.
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Re: Fiberglass on bridge/bridgeplate?

Postby Barry Daniels » Fri Feb 01, 2019 11:03 am

I am building a falcate now and am looking forward to see how it turns out. I am beginning to think that negative post was trollish.
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Re: Fiberglass on bridge/bridgeplate?

Postby Steven Smith » Fri Feb 01, 2019 2:02 pm

Barry Daniels wrote:I am building a falcate now and am looking forward to see how it turns out. I am beginning to think that negative post was trollish.


Yeah gotta wonder, that was only his second post and didn't actually contain any relevant or useful information.
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Re: Fiberglass on bridge/bridgeplate?

Postby Alan Carruth » Mon Feb 11, 2019 2:51 pm

As with the rest of us, Trevor builds guitars that he likes. Much of his book is advocating for that particular style and sound, and that may not be what you want to make. The beauty of his book is that he lays it all out. You don't have to agree with his objectives to make use of the data and analysis that he presents. Perhaps that's all that Jan means.
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