MIMF

Hey luthier buddies,

I've noticed that a number of bridges can crack under high tension strain.
I've been considering using some sort of stabilized wood, or reinforcing the fracture points with brass or carbon fiber.

Any thoughts?

Design it so it isn’t overstrained. Whatever you add moves the stress point to the end of the addition.

Where are you getting cracks?

The cut of the wood makes a difference. Flat cut wood is much more likely to crack, and skew cut slightly less likely, than quartered wood.

As Bob says, take a critical look at your design.

Stabilized wood can present some very tough to overcome glueing challenges.

I have made several bridges with carbon fiber in the middle to prevent cracks. I basically followed the guidelines in Trevor Gore's books. One was for a guitar I made using his falcate bracing technique, I believe Trevor's design uses it that way as part of the bracing scheme. The other was for a repair to a 12 string with a bridge that had split along the pins due to a very poor and split-prone bridge design. In both cases I resawed the bridge blank with about 1/8" for the bottom slice, laminated the carbon fiber in with System 3 epoxy and then finished up the bridge normally.

Thanks for the tips

Again, to fix a problem you need to define it. Where do they crack?

If you're getting cracks along the line of the pin holes then making sure they don't all fall on the same grain line helps. I angle them to follow the line of the saddle There is no need for the pins to fit tightly, and if they do they act as wedges and can split the bridge. They're there to act as toggles, in a sense; the ball end of the string pushes them sideways, and helps jam them into the hole. You should be able to pull them out with your fingers once the string tension is on. Note that slotting the bridge, rather than the pins, helps with this.

If the front edge is breaking off in front of the saddle slot then you need to reduce the load or beef up the structure. Most bridges have about 1/8" of wood in front of the saddle, but there's no reason it has to be that close. Making the slot a little further back, say 1/16" or a bit more, helps a lot. The higher the brake angle the greater the tipping force on the saddle, trying to break out the slot, so reduce that. As far as I can tell there's no need for more than about 15-18 degrees of break angle, although I usually use more. Another good way to reduce that stress is to angle the saddle back, as seen from the side. If the saddle bisects the break angle then there is no net tipping force on it, and no load trying to split out the front of the slot. That's how violins get away with their tall, skinny bridges.

That probably covers the most common types of bridge cracks that are not associated with a lifting bridge. It's mostly a mater of thinking about why you have the problem, and designing around it.