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Graphite vs maple bridge on a cello

by torquil
Tags: bridge, cello, graphite, maple
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torquil
#1
Dec21-10, 10:36 AM
P: 641
Hi!

I have a cello with a hard maple bridge, as well as a spare graphite bridge.

The bridge pushes the strings away from the body, as you can see in this picture:

http://upload.wikimedia.org/wikipedi...llo_bridge.jpg

The problem is the following: The spare graphite bridge is too massive, resulting in less treble and less overall sound volume. So, I want to make it thinner. Hopefully, it will be able to do the same job as the maple bridge, while less massive.

What is the approximate thickness needed to make the graphite similar in strength to the maple one?

I don't know anything about the quality of the graphite in my spare bridge, so just assume it is "run-of-the-mill" graphite, if such a thing exists.

Anyone care to make an informed estimate?

EDIT: Btw, the graphite is "pure", i.e. contains no embedded fibres of any kind. Sort of like the inside of a pencil.
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fss
#2
Dec21-10, 11:21 AM
P: 1,185
You should probably ask a musical instrument repair shop and/or a cellist about this. Thinning the bridge is usually a bad idea, as they're more likely to snap under tension if they aren't at spec.
torquil
#3
Dec21-10, 12:05 PM
P: 641
Quote Quote by fss View Post
You should probably ask a musical instrument repair shop and/or a cellist about this. Thinning the bridge is usually a bad idea, as they're more likely to snap under tension if they aren't at spec.
I'm pretty sure the graphite spare I have is significantly overspec'ed, and that I can make it quite a bit thinner. At the top it is much thicker than the maple bridge I'm using, which is a bit strange.

Cello builders and repairists don't usually know much about graphite bridges, since it is very unusual, mostly due to tradition.

I have just sawed off some of the useless ornamentation on it and managed to reduce the mass to 36g, which is still more than a maple bridge. It has an adjustable metal screw mechanism at the bottom of each leg which I'm also going to remove. I'm guessing that this can maybe reduce the mass by a further 4g, since the metal screws weight 6g total.

Also, at the top it is obviously unnecessarily thick, so I'm gonna remove some of that right now.

jamesrc
#4
Dec21-10, 03:59 PM
Sci Advisor
PF Gold
P: 477
Graphite vs maple bridge on a cello

Hi -

As a disclaimer, I'll admit up front that I don't know the answer to your question, but I thought it was interesting since I've never used a graphite bridge before. It sounds like you've already started working on the bridge, so if you couldn't find a luthier to work on it for you, you may as well keep going - worst-case scenario you'll end up with a bridge you can't use (sounds like it's already an un-usable bridge anyway). So let me think out load for a bit...

I assume you're using one of these Moses bridges, which say they are carbon graphite (rather than "pure graphite" as you mentioned. You can get a ballpark estimate of the material properties on MatWeb (though you won't know exactly which type of carbon graphite you have). The carbon graphite is (about 4 times) more dense than wood (so for the same shape and size it will be more massive), but also stiffer and stronger. My bridge is about 10 mm thick at the feet and narrows down to about 5 mm near the strings (and it's a Belgian style rather than French).

If I were going to really do this analysis, I'd probably want to set it up as a finite element analysis problem since the shape of the bridge would make the math really ugly really quickly. One thing I'd be worried about while removing material from the bridge is not creating any stress risers by varying the thickness in a non-uniform way. Assuming we're worried about the bridge buckling under the string tension, the buckling load it would support should scale with the modulus of elasticity of the material and with the cube of the thickness of the bridge - take this with a grain of salt though since I'm just applying what happens with a simple rectangular beam. Anyway, to support the same tension and considering the material change, you might be able to drop the thickness by about 30% from the wood bridge's thickness measurements (again - I'd try to do that uniformly over the length of the bridge - if you accidentally nick a corner in there somewhere, all bets are off).

Since you said it's thicker than your maple bridge, maybe you should start by bringing it down to the thickness of the wood bridge before going any thinner. You shouldn't feel too confident in the number I threw out there in the last paragraph because I'm not - I think it would take more analysis than I'm willing/able to do to predict how far you could go before this would fail.

On second thought, have you mentioned that you're dis-satisfied with the bridge to the manufacturer? They may have better suggestions for adjustments...

One last minor point - if you're seeing a lack of response (as if you had a mute on), should your end goal be to match the weight of the maple bridge or its moment of inertia about its treble foot? Either way, you'd be removing material, but I always thought the main motion of the bridge was as a lever pivoting over the soundpost. You'd want to more or less mimic the way the weight is distributed throughout the bridge.

In any case, make sure you protect your cello when you put the bridge on if you have any fear that it may collapse under the tension of the strings. Good luck - I'm interested in how this works out for you.
torquil
#5
Dec23-10, 05:41 AM
P: 641
Thanks James!

All your comments were very helpful, and thanks for the link to the MatWeb site which I hadn't seen before.

I am used to doing some work on the cello myself, so that's why I was hoping to be able to also work on the bridge. However, it turns out that it is too narrow at the top, so if I'm going to try a graphite bridge I will probably order a plate of graphite and then make a bridge entirely from scratch.

The main motivation would be to make a bridge that is lighter than a maple bridge.

Doing a Finite Element analysis would be fun. It might also lead to a "better" bridge shape. I wonder if this has been done.

Good point about protecting the cello when trying an "experimental" bridge...

- Torquil


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