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.

Matthew Lau
Posts: 442
Joined: Sat Feb 18, 2012 2:03 am

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.
Alan Carruth
Posts: 850
Joined: Sun Jan 15, 2012 1:11 pm

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.
MIMF Staff
Barry Daniels
Posts: 2020
Joined: Thu Jan 05, 2012 10:58 am
Location: The Woodlands, Texas

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.
Matthew Lau
Posts: 442
Joined: Sat Feb 18, 2012 2:03 am

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