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Bridge Design
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=37100		 Page 1 of 1
Author:  Leanu [ Sat Jul 14, 2012 8:24 am ]
Post subject:  Bridge Design
I keep going back and forth trying to decide what kind of bridge to use on my build. I am building a mandolele with a scale of 15.5.

I was imagining a pinless bridge, but might go with an 8 pin traditional bridge. Of course there is always an option for a tailpiece!

Any thoughts?
Author:  B. Howard [ Sun Jul 15, 2012 5:52 am ]
Post subject:  Re: Bridge Design
I don't like pinless bridges because they place the entire string tension load on the joint between the bridge and top. That's a lot of shear force on that joint and even if the glue holds any run-out in the top can lead to the wood fibers shearing and the bridge coming loose. A traditional pinned bridge places the string load directly on the top, very little stress on the glue joint. If you go with a tailpiece make sure you've got enough break angle on the strings over the saddle to drive the soundboard.
Author:  Leanu [ Sun Jul 15, 2012 9:27 am ]
Post subject:  Re: Bridge Design
Thanks, Brian. That was the reason I was considering another method, so I'm glad you agree. I think I'll go with the pinned bridge instead of the tailpiece so I don't have to mess much with the neck much.

Depending on how this one turns out, I may try a tailpiece on the next one.

Thanks again!
Author:  Todd Rose [ Sun Jul 15, 2012 9:58 am ]
Post subject:  Re: Bridge Design
An instrument with a fixed bridge and an instrument with a floating bridge and tailpiece are two very different beasts. The forces exerted by string tension on the top are very different. The way the top responds to string vibration is also different. I would design the tops very differently for one versus the other, in terms of both desired sound and structural resilience. So, that's the first thing to think about.

Lots of guitars use pinless bridges successfully, so I wouldn't worry too much about the bridge coming off, as long as you make a good joint. My personal beef with pinless bridges is that they make an instrument a pain in the butt to set up. Every time you want the strings out of the way during setup, you have to completely remove them, whereas, with a pinned bridge, you just loosen the strings, pull the pins, pull out the ball ends, and set the strings aside (i.e. the strings can stay wound on the tuners). I've made both pinned and pinless, and I don't see any advantage at all to pinless bridges, so I don't make them any more for that reason.

If you go with a pinned bridge, each pair of strings can share a pin, which makes for less mass on the bridge and also makes it easier to get your string spacings the way you want them. You just have to cut the pin hole slots and ramps carefully, and then get used to putting two ball ends in as you string it up - no big deal at all, just takes a little practice. (Use unslotted pins and slot the holes.)

One more comment about bridges. IMO, a lot of bridges have unnecessary mass due to their blocky contours. Think about how you can design and carve the bridge to reduce mass.

If you go to the photos page on my web site (http://toddroseguitars.com/Photos.html), you will see photos of some of my bridges, showing how they are designed and contoured to reduce mass. You'll also see a couple bridges of double course instruments (a zoukulele and a 10 string guitar/bouzouki hybrid) with each pair of strings sharing a pin.
Author:  ernie [ Mon Jul 16, 2012 9:18 am ]
Post subject:  Re: Bridge Design
Todd nice instruments, good ideas for the bridge.Did you use black locust for one of those instruments ?? cheers ernie.
Author:  Todd Rose [ Mon Jul 16, 2012 2:42 pm ]
Post subject:  Re: Bridge Design
Thanks!

No - the zoukulele is ash, if that's the one you were thinking of.
Author:  ernie [ Tue Jul 17, 2012 8:12 am ]
Post subject:  Re: Bridge Design
Thanks todd, I have both,including a large ash tree felled in our subdivision.I have never tried ash.How does it sound in your opinion ? cheers ernie
Author:  Todd Rose [ Tue Jul 17, 2012 12:05 pm ]
Post subject:  Re: Bridge Design
So far I've only made that one instrument with ash B&S, so I can't tell you much. I couldn't be happier with the sound of that zoukulele, but that doesn't tell me much about what effect ash B&S might have on one of my guitars.

What I will say is that I've heard and/or played lots of outstanding guitars made with all kinds of "alternative" woods for B&S. I have no doubt you could make a superb guitar using ash, just by making a superb guitar, as you could with virtually any other hardwood. Exactly what tonal character a given wood might tend to impart to a guitar of your making can really only be found out by making it. If you have a narrowly defined preconception of what you want a guitar to sound like, then you better stick with a "known" wood, but, otherwise, be adventurous!
Author:  Alan Carruth [ Tue Jul 17, 2012 12:10 pm ]
Post subject:  Re: Bridge Design
B Howard wrote:
"A traditional pinned bridge places the string load directly on the top, very little stress on the glue joint. "

That's funny: I used to do a lot of repairs, and re-glued many a pin bridge that was peeling up along the back edge. There had to have been a fair amount of stress on that glue line. Also, I don't recall seeing a larger proportion of pinless bridges failing, although, of course, there are not nearly as many of those.

Todd Rose wrote:
"An instrument with a fixed bridge and an instrument with a floating bridge and tailpiece are two very different beasts. The forces exerted by string tension on the top are very different. The way the top responds to string vibration is also different. I would design the tops very differently for one versus the other, in terms of both desired sound and structural resilience. So, that's the first thing to think about."

They're not as different as you might think. It's true that there is no static torque on a tailpiece bridge, and that the tension change signal, which acts through that torque, tends to be much weaker or missing in the sound on tailpiece instruments. But that's not all that much energy: it averages about 1/7 of the power of the 'transverse' signal. It certainly adds some 'tone color' to the sound, and that's important, but the tension change signal is not the main driver of the top.

The big thing a tailpiece allows you to do is stop worrying about torque loads. To make up for that, since you usually need a fair amount of break angle over the saddle to keep the string in contact, you have to have a structure that can withstand the download over time. It's usually six of one and a half dozen of the other, with neither system automatically giving you a lighter or more responsive structure. If tailpiece setups were really all that much better than stop bridges, then you'd see very few stop bridges and a lot of tailpiece designs. Instead it tends to run the other way. Cultural evolution at work.
Author:  Jeff Highland [ Tue Jul 17, 2012 6:39 pm ]
Post subject:  Re: Bridge Design
A pinned bridge and a pinless bridge are going to have the same shear force on the glueline.
What the pinned bridge will NOT have is the uplift force at the rear of the bridge taken by the glue, since the string balls are acting on the underside of the bridge plate.
The rear of a pinned bridge is subject to peeling off due to deformation of the plate, but even when it comes loose, there is rarely a catastrophic failure, unlike with a pinless.
Personally I can't see the point of a pinless bridge on a steel string unless the bracing pattern precludes drilling pin holes, and not with a standard x brace where the area at the rear of the bridge is quite flexible.
Author:  Todd Rose [ Wed Jul 18, 2012 6:09 am ]
Post subject:  Re: Bridge Design
Alan Carruth wrote:
Todd Rose wrote:
"An instrument with a fixed bridge and an instrument with a floating bridge and tailpiece are two very different beasts. The forces exerted by string tension on the top are very different. The way the top responds to string vibration is also different. I would design the tops very differently for one versus the other, in terms of both desired sound and structural resilience. So, that's the first thing to think about."

They're not as different as you might think. It's true that there is no static torque on a tailpiece bridge, and that the tension change signal, which acts through that torque, tends to be much weaker or missing in the sound on tailpiece instruments. But that's not all that much energy: it averages about 1/7 of the power of the 'transverse' signal. It certainly adds some 'tone color' to the sound, and that's important, but the tension change signal is not the main driver of the top.

The big thing a tailpiece allows you to do is stop worrying about torque loads. To make up for that, since you usually need a fair amount of break angle over the saddle to keep the string in contact, you have to have a structure that can withstand the download over time. It's usually six of one and a half dozen of the other, with neither system automatically giving you a lighter or more responsive structure. If tailpiece setups were really all that much better than stop bridges, then you'd see very few stop bridges and a lot of tailpiece designs. Instead it tends to run the other way. Cultural evolution at work.


Thanks for your always-enlightening input, Al.

The other thing with a floating bridge and tailpiece is that you've got the effect of the download on the oscillation of the top. Like somebody's thumb pushing down on the middle of the soundboard as it's trying to bounce up and down (in any of the modes that involve a lot of movement in that area).
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