Official Luthiers Forum!

Today I entered a ruminative zone of wondering about the load pinned bridges feel compared to the loads felt by pinless bridges and slotted pinned bridges: Seeing as pinned slotted bridges have the string tension taken by the slotted bridgeplate, while the other two styles have the string load acting on the bridge proper, I thought that the slotted bridge is at less risk of separation than the other two.

Am I thinking about this rationally?

Thanks!

_________________
Peter Havriluk

Please see last comment in the thread by Al C. this says it all for me and brings up the idea that there are many variables here as well. For example there is a joke in the repair world that the brand name Cordoba means bridge reglue in Spanish. The remark is referring to how cedar tops don't seem to hold classical bridges very well.

So there is what top wood has been selected, builder's errors such as the scribing that Al refers to. There are different glues what do better or worse with these loads too. There is letting a guitar dry out, damaged bridge plates, worn pin holes that all contribute to a guitar shedding a bridge.

http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=55749&p=731731&hilit=pinless+bridge#p731731

_________________
Ann Arbor Guitars

Thanks, Hesh. I read the link you posted. I think I can return to the regularly-scheduled broadcast. And this is all nicely timed; my current project uses a cedar soundboard, which seems to want serious attention to detail during bridge installation.

_________________
Peter Havriluk

Cool and I don't mean to scare folks about bridges and WRC and such but we do see bridges peeling off and top damage when they do that is far more frequent and consequential with cedar tops. However people rarely take full advantage of all the things you can do to prevent a bridge from lifting in the future such as:

1) Knowing how the glue needs to be used and it's requirements....

2) Clearing the bridge patch very nearly to the perimeter, we go as close as 0.005" if we can.

3) Fitting the bridge so that only light finger pressure is required to have the bridge fully in contact with the top everywhere.

4) RH control while building and during the life of the instrument. Dry guitars shed bridges frequently.

5) After finishing careful scoring of the finish so as to not cut wood fibers.

6) We scrape the bridge bottom and bridge patch 15 minutes or less before we slather on the HHG and clamp it down.

With this offered we keep track of any failures that we have from our work and methods. The only lifted bridges that we ever had happen after we reglued them were glued on with fish glue. We discontinued the use of fish afterwards and recalled all the work we could find that we did and redid it with HHG at no charge to the client. Our fish glue came directly from Norland, was pitched and replaced every year, etc. But I digress.

_________________
Ann Arbor Guitars

Here's some for you to chew on - http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=46993&hilit=pinless+bridge+stress

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

If it's a guitar you're making, rather than a repair, you should consider making the bridge wider; that is, deeper along the line of pull. This goes a long way to reducing the peak peeling stress along the back edge. It's the difference between a 'bar' bridge and a 'belly' bridge. Martin had a lot of trouble with the bar bridges staying down when they started to use steel strings (partly because they were 'toothing' them), and the belly made the warranty situation at least tolerable. Note that the glue footprint for a Martin style belly bridge is actually smaller than that of many Classical bridges, but the Classical bridges are narrower, and pull up under the stress of steel strings.

WRC, along with redwood, is more prone to splitting than spruce, and harder to glue (lower surface energy), so it needs all the help it can get. I generally use walnut for bridges on WRC or redwood tops; the lower density helps keep the weight of the larger bridge down. Just allow a bit more material in front of the saddle slot to avoid split outs, since walnut is not as strong as rosewood or ebony.

My understanding (FWIW) is that the peel load (torque plus shear) is not significantly different between pinned and pinless bridges. What the pins do is introduce a 'column' load along the line of the pins. As the bridge pulls up this distorts the bridge and the top, making the repair harder, but the pins do keep the bridge from flying off catastrophically and killing the cat before you realize the bridge is loose. Bridge bolts do pretty much the same thing, if my memories of old repair experience are correct.