Bridge design for mandocello

[joehendrick]

Hi Everyone- I'm designing the bridge for an F mandocello that I'm building. At the Woodstock guitar show last fall I spoke with a few archtop builders about plate thicknesses etc. and noticed that unlike the mando building community their saddles touched the top of the plate all the way across. They claimed more contact- more sound transfer. I have seen mando saddles that touch anywhere from an inch on each side to almost all the way across with a slight maybe inch long hollow in the center. Anyone have any reasoning that makes sense of all of this from a mando- very big mando perspective?

As always thanks for your 2 cents the value of this community has always added up for me!

Joe

[Stephen Perry]

The two-foot design seems to lead to top damage more that the one big foot.

If you want a uniform strength beam, then that middle stretch where the strings push down would be a uniform loaded section (example: truss bridges) and the end runs would be point loaded and then run out to nothing. I'm not sure what a supposedly ideal shape for that would be, but I can figure it out.

Is this an adjustable bridge with posts or a one piece? I was thinking one piece. Two piece would be different.

If you want me to help design one, email me. I check vtwinlaw at gmail often. Might be a fun project. Or maybe you could find a real engineer rather than a hack!

[joehendrick]

I typically start out with an adjustable design and replace it after a year with a one piece.

[Stephen Perry]

The systems are rather different. How do you design them?

[joehendrick]

The systems are rather different. How do you design them?

No great science or engineering involved in the design. I have some specs for traditional 2 piece bridges that I have pulled off of several friends and jammers instruments. I've also done some one piece styles off of Red Henry's site. Generally I tend to widen the feet and keep the rest of it narrow to cut down on mass and to give it stability. I'm most concerned with getting volume off the top of the mandocello without crushing it or tipping the bridge.

[Stephen Perry]

Full contact with not too narrow should do it. There are some innovative and highly effective bridges up in my power tool shed that are a bit finicky for mandolins, but would be ideal for larger. If you're interested in designs.

[sunburst]

I like a two-foot bridge because wood moves. Also, for that reason, I like to make the part of the bridge base between the feet rather thin. When the top takes on moisture because of increased relative humidity, the arch rises and changes shape. If the bridge is of rigid design with full contact, the bridge-to-top fit changes. Likewise, the arch lowers when moisture content decrease. I like to use the thin center section of the bridge base as a sort of hinge to allow the bridge to more nearly fit the arch when the shape changes.
As for sound, the full contact bridge might add stiffness to the center of the top and make a small change in sound, possibly even a perceivable change, but I have my doubts. As for "sound transfer", that doesn't exactly happen.

[Stephen Perry]

John, how do you account for the offset post position relative to the foot length? I generally see thin center bridges with distortion in the top where the end of the foot towards the CL rests, indicating that the loading is not even across the foot, but rather concentrates at the ends of the foot near the center. I have two mandolins that just arrived that exhibit this very effect.

Having the post in the center of the foot might help this, but that's not the standard design. Could be accomplished by a longer foot towards the center line or by shortening the outside length.

I do like your thinking. I accommodate by making fit under roughly the string force downwards on the bridge. But things do indeed change and a floating bridge foot system makes sense.

[sunburst]

John, how do you account for the offset post position relative to the foot length? I generally see thin center bridges with distortion in the top where the end of the foot towards the CL rests, indicating that the loading is not even across the foot, but rather concentrates at the ends of the foot near the center.

I've only seen top problems at the bridge feet on a few mandolins. From my experience, it usually doesn't happen, and when it does it often indicates a too soft top, too thin top, excessive bridge height (leading to overloading of the bridge) or a very old mandolin, though I've seen ~100 year old mandolins with no undue top distortion.

[Stephen Perry]

I see it all the time. Can feel it, too. Bridge will fairly easily turn while under load, very little load at the ends of the feet. It's been one of the most persistent issues I've dealt with. New mandolins, old mandolins, all of them. Can see it with transfer paper, too. Put some under the bridge, tension up, and remove.

I can't see how the traditional design can avoid this so long as it has two feet, unless there's a substantial preload built in by carving to fit under pressure.

The saddle suffers, too, with the ends supporting the load across the middle. Can see the distortion in some saddles. Poor structural design, adequate production design. Full of stress risers.

Posts would be better between G & D, A&E courses. But then a footed design would be narrow. Doesn't cross the tone bars the same. However, the ones I've made like that are very good sounding. I should continue my oddball design work sometime. Many ways to reduce coupling between courses, cool down a course that's too hot and breaking up.

Even without a couple at the post - and there's generally opposing couples at the posts - the different footprint on each side causes trouble. Imagine a board on mud. Step in the middle of the board and it settles evenly. Step towards one end and that end sinks like mad, denting the mud / top.

Interesting considerations.

[HoGo]

I prefer one footers these dazs as well. The main problem is that (especially bass side) tonebar runs clse to the end of the foot and load is on the bridge post which is closer to center, depending on stiffness of the bridge and humidity cycling (change of shape of arch) the inside edges will slowly dig into the top. Thin top can "help" it, but the fairly new (less than 5yo) mandolin I'm restoring for a friend had pretty thick top 5.5-6mm at center to 4.5+ mm at edges and did have the bridge indentations. I'm redoing the bridge into full contact in this case as well.

[joehendrick]

Thanks for the responses everyone. I think I'll go with a one foot adjustable and give it a year to settle in then maybe switch to a one foot, one piece.

I am also trying something new with this build that may be of interest. I am using a 3/4" hollow "D-tube" carbon fiber rod in place of a truss rod in hopes of being able to take down the neck thickness a bit. It is a little pricey but the added feature is supposed to be some kind of improved tone and volume too.

The parent company is AllRed but the specification information along with some luthiers pictures and movies is at: http://geminimusical.com/

I promised the AllRed engineers I'd put a little movie about the install on youtube. I'm working on it now and will post the link when done.


Joe