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Thread: Adjustable bridge ideas

  1. #1

    Default Adjustable bridge ideas

    I like the concept of the traditional adjustable bridge, and I’m not one who is concerned with super lightweight anything—I think controlled damping is an important part of fine tone. The thumb wheels are a weak point, obviously, and wear on the couple threads supporting the load easily leads to a tipping saddle and eventually stripping (from experience). Looking at past threads I know that certain people have experimented with designs (notably Marty Jacobson and Vern Brekke) and I was hoping to know what worked and what didn’t.

    I am interested at the moment in trying a threaded insert through the saddle that could take the load over many more threads and using a set screw style threaded rod with a hex socket (or even a simple slot for a screwdriver, though that seems much harder to turn under tension). It would be adjusted from above and spin freely in the base. I think ideally it would only be threaded on the upper half of the rod and the lower part would fit snugly enough as to not chew up the hole in the base with turning. I would hope that with both sections of the post deep in the base and saddle tipping would be prevented, but I think taking out more wood in the saddle for the insert could create a weak point. I’d be hesitant to simply tap the saddle and and be relying on wooden threads. I think some bass bridge adjusters function similarly, but have a wheel for the adjustment. Has anyone already tried this? Anything to report?

  2. #2

    Default Re: Adjustable bridge ideas

    Like this, I guess

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  3. #3
    Moderator MikeEdgerton's Avatar
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    Default Re: Adjustable bridge ideas

    Vern Brekke uses Allen head set screws on his signature bridge design but not in the same configuration.
    "It's comparable to playing a cheese slicer."
    --M. Stillion

    "Bargain instruments are no bargains if you can't play them"
    --J. Garber

  4. #4

    Default Re: Adjustable bridge ideas

    Small diameter threaded inserts are readily available, intended for machine screws into softer materials, and so have coarser external threads. Because hard, brittle woods are usual on bridges, that means you should tap carefully. Or use a different type. The insert will have to be glued in or it may rotate.
    Can’t see any reason to not do it this way, but I also don’t understand why the conventional, two post design is acoustically ok.
    You could further stabilize your design with a fixed flange on the post where it contacts the base.

  5. #5

    Default Re: Adjustable bridge ideas

    Most commonly available threaded inserts would require you to fatten up the saddle quite a lot in order to have enough wood to thread into.

  6. #6
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    Default Re: Adjustable bridge ideas

    A nice idea, but I have had no real problem with tipping on several of my mandolins. If the post is not sloppy in the saddle it should stay straight. Having the saddle and wheel lined up so they are supported flat helps too. I wax the bottom of the saddle and can actually, depending on my old hands, raise the saddle under full tension with my fingers. It has to be a good fit, but have done this many times.
    THE WORLD IS A BETTER PLACE JUST FOR YOUR SMILE!

  7. #7
    Adrian Minarovic
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    Default Re: Adjustable bridge ideas

    There is no real problem with standard thumbwheel bridge. The only bridges I've seen with problems with leaning or bending were those that were made and/or installed poorly. Worn threads show hat the material for the wheels was bad but I've never seen such thing so far even on cheap bridges.
    The biggest problem with cheap bridges that want to lean forward is that the thumbwheels are loose on the threads of posts and the saddle holes are also too large.
    Adrian

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  9. #8

    Default Re: Adjustable bridge ideas

    I have to respectfully disagree. The traditional brass thumbwheel design is under engineered. To apply the both then downward pressure and the tilting of the saddle with repeated tuning (round windings do tug even on properly cut and lubricated slots) on the two thin brass threads in the thumbwheel does cause them to warp and strip. I have done it and seen it happen. What is the point of having a longer nut for the truss rod other than more threads engaging?
    I think the two post model of the bridge is a good one, and it allows a certain flexibility found in other instruments with tall bridges (violin family). I just want more threads engaging. Maybe a thicker nut is all it takes, but I’m interested to know anybody else’s experience.

  10. #9
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    Default Re: Adjustable bridge ideas

    A lot of folks fit the bridge with a slight slant back to keep it from pulling forward. None of my mandolins pull forward, nor have I had to replace wheels or posts on any mandolin of mine. Several of the cheap bridges on others mandolins, but not one that is a good quality bridge and fit well. I have owned my Gibson for around 25 years, others more than 10 and no problems.
    THE WORLD IS A BETTER PLACE JUST FOR YOUR SMILE!

  11. #10
    Adrian Minarovic
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    Default Re: Adjustable bridge ideas

    The post is usually of steel and that with combination of fitting brass thumbwheel will hardly wear with the few adjustments needed per year. Cheap bridges often come with a metric 3mm thread, which is slightly less substantial than traditional 5-40 and slightly thinner thumbwheels that have quite a play on the posts, I have no idea what the alloy is but sometimes they are so soft, almost look like aluminum. The holes are often oversized in the saddles etc. I've worked on dozens of these and reducing the hole in saddle and tight fitting in the base make real difference and if you lube the threads they will work OK.
    But good quality bridges don't need this, if you install them correctly with good fit and slightly leaning back so the force of strings doesn't pull them forward too much they will stay that way for a long time. Even if the owner changes the whole set of strings at once the bridge will lean as whole showing gap uned rear of bridge base rather than just "bend" at the thumbwheels, that actually are not bent at all, it's just all the play showing in one spot.
    If the owner just neglects his instrument and causes premature wear of any part is another subject....
    Adrian

  12. #11
    Registered User PT66's Avatar
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    Default Re: Adjustable bridge ideas

    I have heard of (but not seen) bridges with a hex not instead of a knurled wheel. This allows a way to apply more torque when doing a height adjustment.
    Dave Schneider

  13. #12

    Default Re: Adjustable bridge ideas

    I use hex nuts as suggested by David Cohen, and an interlocking beam that allows the moving top part of the bridge to be taller than normal. Taller means stiffer, in addition to preventing the tipping that can happen when players aren't careful with how the bridge is oriented.
    I've been using this design since 2013 and there haven't been any failures yet.
    Though I have gotten a lot of emails from customers asking "how is it adjustable if there's not air between the top part and the base".

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    Registered User geechee's Avatar
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    Default Re: Adjustable bridge ideas

    I contacted Bruce Weber a few months ago and he agreed to make this pair of “nutwheels" to install on a Cumberland bridge on my Tyler White A5. I believe these are essentially installed upside down to “his adjustable bridge method” and allows the nutwheel to screw up against the saddle on the post. Bruce was very nice to send them quickly and remarked this might be an easy way to convert std thumbwheels on Gibson/Cumberland Acoustic bridges to allow for easier adjustment. I did have to sand the lower bridge ankle to allow for some swing of the wrench. So far, all is good and I’m able to make some easy minor adjustments to adjust for temp/humidity swings.
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  17. #14
    Adrian Minarovic
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    Default Re: Adjustable bridge ideas

    I used ordinary hex nuts on several bridges for my first mandolins. I couldn't get any traditional knurled thumbwheels so that was all available to me. They work just like thumbwheels and you can adjust the bridge with a tiny flat wrench under full tension. They are tiny bit lighter than thumbwheels, someone might consider that an advantage.
    Adrian

  18. #15

    Default Re: Adjustable bridge ideas

    And just to continue this exercise a bit, tiny flat wrenches are still commonly available, if you go the nutty route. They’re called ‘ignition wrenches’ and come in sets so you don’t have to know which one you need. An anachronism since ignitions generally don’t have the small parts anymore, but if you get a set, you might find other uses than bridge adjustment. Get the cheapest ones, which are thinner than the serious ones. Make the rest of them into jewelry or little wind chimes.

  19. #16
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    Default Re: Adjustable bridge ideas

    I use my ignition wrenches still. Sometimes on other things, but my 1957 tractor still has points.
    THE WORLD IS A BETTER PLACE JUST FOR YOUR SMILE!

  20. #17

    Default Re: Adjustable bridge ideas

    Quote Originally Posted by Richard500 View Post
    And just to continue this exercise a bit, tiny flat wrenches are still commonly available, if you go the nutty route.
    I think most people use these because they're cheap and look decent. https://www.mcmaster.com/products/mi...nd-wrenches-8/

  21. #18
    Registered User amowry's Avatar
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    Default Re: Adjustable bridge ideas

    Quote Originally Posted by fazio View Post
    Like this, I guess

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    Jayson Bowerman's mandolin bridges are similar to this. I forget the exact mechanism, but you could ask him. I want to say there's a threaded brass insert in the saddle. I definitely wouldn't rely on wood threads for this.

  22. #19
    Registered User amowry's Avatar
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    Default Re: Adjustable bridge ideas

    By the way, I've used the Axiom thumbwheels on hundreds of bridges and I've never heard of one failing. The issue with the saddle tipping is usually because the holes in the saddle are too large, which you see on cheap bridges sometimes. With 1/8" holes, the 5-40 posts don't have enough slop to allow tipping.

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  24. #20
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    Default Re: Adjustable bridge ideas

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    The four attached photos show two designs for a sliding wedge adjustable bridge with no metal hardware. The "exploded" views show how the bridge foot, bridge saddle, and sliding wedge are held in place in each design. These two were done in maple. I also mode a few more, including one in rosewood and one in ebony. These two weigh ~6.2 grams apiece. The ebony one weighs ~11.6 grams, and the rosewood one weighs ~ 10.8 grams. For comparison, the conventionally adjustable bridges I usually make in ebony with 6-32 Al rod and Al hex nuts typically weigh in the 10 - 11.5 gram range. So this design, as I've usually executed it, doesn't have a significantly lower mass advantage compared to the conventional design.

    Full disclosure: I can't take credit for the sliding wedge idea. A similar design was done decades ago in archtop guitars (Monteleone? I forget). All I did was come up with a version of the design for archtop mandolins. I guess I can take credit for originating the use of aluminum for the hardware. I thought of it independently, although I find it hard to imagine that someone somewhere hadn't tried aluminum hardware before I did. Steel and brass have densities up in the 8 - 9 grams/cm3 range, while aluminum has a density ~2.7 grams/cm3, so the lower mass advantage is obvious. I haven't seen any failures w/ the aluminum hardware, although I'm sure it would fail if subjected to enough abuse.

    I made all these bridges, and several more, for some admittance experiments performed by Jim Rae (the banjo guy that worked w/ Tom Rossing for some interferometry papers on banjos). I fitted them all to one of my test mandolins and sent the mandolin + bridges to Jim for the admittance measurements. I also included some conventional bridges, and a couple of non-adjustable bridges from Red Henry. BTW, one of the Henry bridges was ebony, weighed 7.7 grams. All of the bridges, when installed on the same test mandolin. sounded the same to me. When Jim did the admittance measurements, he found little or no differences in admittance between the bridges (all the way up to about 5 kHz). He was very surprised by that. Me, not so much. Compared to violin bridges and banjo bridges, mandolin bridges are all very low and very stout, so I'd be surprised to see any bridge motions as low in frequency as the typical violin "bridge hill" (ca 1.5 kHz).

    I think mass DOES have a noticeable effect. A repair customer once asked about putting one of my lighter weight bridges on her mandolin, which sounded a bit harsh. I suggested we try it first, so I fitted one to her mandolin and we tried it. It sounded noticeably harsher than with the original conventional Gibson-type adjustable bridge. My conjecture is that a lower-mass bridge decreases the impedance at the higher frequencies, allowing for more of the harsher higher-frequency components to be heard. Just a conjecture though, as I haven't done any experiments (other than that one anecdote) on it.

    Finally, I'd like to know what the OP means by "controlled damping". I don't see any evidence for any control of any kind of damping in what he has described, unless I'm missing something.

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  26. #21

    Default Re: Adjustable bridge ideas

    Certainly, adding mass to the top of a violin bridge changes the impedance strongly, as with a mute, so you’re right. This effect might be construed as “damping” although it technically isn’t. Not surprised that various mandolin bridges perform similarly - it might take an extreme change to have a large effect.

  27. #22
    harvester of clams Bill McCall's Avatar
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    Default Re: Adjustable bridge ideas

    Quote Originally Posted by Marty Jacobson View Post

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    That's a beautiful bridge, the detail is wonderful.
    Not all the clams are at the beach

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  29. #23
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    Default Re: Adjustable bridge ideas

    Try it I see a terribly muted mandolin in your future. Maybe that is what you like, if so go for it but don’t expect a big following.
    Also I’ve played mandolins with traditional bridges for 50.+ years never had one strip threads. Granted not the same mandolin but I’ve seen mandolins a hundred years old with what appears to be original. I think you have come up with a solution for which there is no problem

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