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Achieving weight balance

Hawkbone

Hawkbone

Mar 23, 2009
942
251
Newfoundland
#1
Haven't been able to find an answer to this one. How are you all achieving good weight balance (no neck dive) for your builds?

For anyone using a full 3D CAD model, it should be relatively easy, but does anyone bother to do a model detailed enough to give accurate weight/COG info?

Anyone doing manual moment calcs, spreadsheets or the like?

Or does everyone just rely on experience?

If anyone does keep an eye on balance in the design phase, what are your acceptance criteria? COG must be at or to the bridge side of the forward strap button? Further back than that? Do you factor in strap forces, including friction and angles, presumably allowing the COG to slide more towards the headstock?

Seen a few threads lately where new builders are advised to balance body and neck mass so it got me curious how the pros tackle this problem.
 
#4
The balance and weight issue is dependant on material and position. The 12th fret theory is a good place to start but not an absolute, and it takes a lot of research and experimentation to find "your" perfect result. A CAD drawing can help with pivot axis and leverage points, but cannot take into account for materials and hardware, which will have a great effect also, and knowing the weights will also help you figure what will work best for you.
 
#5
I could model all of the parts in 3D at a detailed level to account for every internal structure (i.e. pulltruded carbon stiffening bars, air space around the trussrod, etc ...) and then apply different material properties to each solid to obtain the two extremes on a specific body I build, but it would only allow me to account for the established averages for a given wood species ... and it's a load of work that I'd only have further use for if I was going to utilize CNC in the manufacturing process

my personal preference is to build a prototype neck from reference examples of the heaviest materials I'll utilize and a mating body from reference examples of the lightest materials I'll utilize. it is with this prototype unit that I'll work to achieve my 'balance' such that the bass hung freely on the upper horn strap lock will balance so that there is a very subtle lift on the headstock. I also check this unit to ensure that it evenly balances on the leg rest point. once I have this working, I know I have a design that will also work perfectly with heavier bodies and/or lighter necks

all the best,

R
 
#7
Here's how I approximate it in Rhino, a CAD program. I do it this way because I only design in 2D lately, this would be almost automatic if it were 3D:

-Make a plane that is the same shape as the body.
-Make another another the same as the neck., but only the part that sticks out from the body.
-Find the geometric centroid of each shape.
-draw a line connecting them, and find the length.
-find the area of each shape.
-Multiply the area by the approximate thickness to find the volume. In the example below I used 1.5" for the body and .8" for the neck.
-Divide the neck volume by the body volume, you'll get a number less than one.
-Multiply that number by the length of the line between the centroids.

-The number you have left is how many inches from the body centroid to the center of gravity of the bass. As long as that point is on the body side of where the horn strap button, it shouldn't dive....and the further away from the strap button (toward the body) the better. A lot of it depends on how you wear your strap, and if you point the bass up or straight out.

Screenshot2011-01-11at51801.png


This method works for me, but anyone who knows statics, knows that I am making a TON of assumptions, like that all the hardware weight just work out, I'm not accounting for body routs (but I usually use heavier wood on the body anyway) etc.
 
#8
barnaclebeau said:
...
...
This method works for me, but anyone who knows statics, knows that I am making a TON of assumptions, like that all the hardware weight just work out, I'm not accounting for body routs (but I usually use heavier wood on the body anyway) etc.
Click to expand...
One major thing I see missing is wood density. If you build your basses with heavy-wood bodies, fine, but if you have an alder body at 26 pounds per cubic foot and a maple neck at 40 pcf, there a big factor to be applied.




Another thing people neglect is how you you wear the bass. There are four forces acting on the instrument: its own weight, acting downwards; and three forces that are variable according to the guitar shape, your own body shape, and how you strap it on - upper strap support, mostly up but somewhat back and also somewhat towards either the headstock or the tail; lower strap support, mostly backwards but also upwards, especially if you hang it low, and also somewhat towards the headstock; and belly/chest pushing outwards but also on some people also upwards.
 
#9
pilotjones said:
One major thing I see missing is wood density. If you build your basses with heavy-wood bodies, fine, but if you have an alder body at 26 pounds per cubic foot and a maple neck at 40 pcf, there a big factor to be applied.
Click to expand...

An easy way apply that to this method would be to account for it in the thickness multiplier--rather than use the actual thickness, scale it for density. That will affect where the CG sits, but it will still be somewhere along the moment between the two centroids. I'll have to think about how to do that. Not now though, beer time. :hyper:
 
#10
Getting some good feedback here. Like most engineering problems, the solution can be simple if you're happy with a rough result, but can be complex if you really want to refine things.

If I were just starting a design, I would do something similar to barnaclebeau. I would use 2D CAD at 1st and also use a spreadsheet so I could easily update the calcs as my design evolved, possibly into a 3D model. Using a computer wouldn't be essential though - you could do it all on paper. When I get a chance I will do a sample of how I would approach the problem, including densities, hardware and allowances for void spaces.

In the meantime, lets keep the ideas coming!
 
#11
barnaclebeau said:
An easy way apply that to this method would be to account for it in the thickness multiplier--rather than use the actual thickness, scale it for density. That will affect where the CG sits, but it will still be somewhere along the moment between the two centroids. I'll have to think about how to do that. Not now though, beer time. :hyper:
Click to expand...

Couldn't you just account for it by multiplying the volume by the density? If your body was 157.5ft^3 and the density was 26lb/ft^3, then you would have a 4095lb and if the neck was 52.8^3 and it had a density of 40lb/ft^3, then you would have 2112lb (awesome number for a bass). Then you divide the neck one by the body one and you are left with 0.5158 and it is unitless. Then multiply by the moment line which is 20.2in for a final answer of 10.418in from the body centroid. At least that makes sense to me and takes into account the density. I just used the numbers from earlier in the thread.

lowsound
 
#12
iamlowsound said:
Couldn't you just account for it by multiplying the volume by the density? If your body was 157.5ft^3 and the density was 26lb/ft^3, then you would have a 4095lb and if the neck was 52.8^3 and it had a density of 40lb/ft^3, then you would have 2112lb (awesome number for a bass). Then you divide the neck one by the body one and you are left with 0.5158 and it is unitless. Then multiply by the moment line which is 20.2in for a final answer of 10.418in from the body centroid. At least that makes sense to me and takes into account the density. I just used the numbers from earlier in the thread.

lowsound
Click to expand...

Sure, it passes a common sense check, the CG would be out past the strap button in that scenario, and would be slightly divey. I like it.

Edit: you used cubic feet insteat of inches in your math, but like you said, the answer is unitless, so it doesn't matter.
 
#13
barnaclebeau said:
Sure, it passes a common sense check, the CG would be out past the strap button in that scenario, and would be slightly divey. I like it.

Edit: you used cubic feet insteat of inches in your math, but like you said, the answer is unitless, so it doesn't matter.
Click to expand...

Actually, thanks for pointing that out. Technically my answer is wrong because I didn't convert everything to the same units, but it still turns out to be the same because I made the same mistake, in the same spot for both calculations and it ends up being unitless. There are some funky units in there because of it, but since the number we are looking for is unitless it all works out. My Statics prof would kill me right now.

If you were working with multiple laminates and multiple woods, you would just have to do the density calculation for all laminates and add them together before dividing.

lowsound
 
#14
Here is the sample I was talking about.

The 1st pic shows a sketch of a bass with some key coordinates marked in, referenced to the back of the bridge. You can reference from anywhere, as long as you are consistent with +/- distances from the reference point. This shows how you can account for cutouts, different densities, etc. You can make it as detailed as you want. Note I multiplied the weight of the neck by 0.75 to account for the shape. It may be less than that, just a number to work with. Ditto for hardware weights, just a number I typed in, no basis in reality at all. As your design evolves, you can keep the spreadsheet updated accordingly. If you chose to do a 3D model for rendering purposes and did a fairly accurate model of the body, for example, you could ditch the old area and thickness estimates for the body and cutouts and just type the modelled volume from your 3D model directly into the spreadsheet. You could also set this up to check sensitivity to changes in wood densities.

IMG_0002.jpg



The second pic shows how you can calculate the centroid of complicated shape by breaking it down into small chunks that can be represented by geometric shapes for which areas and centroids can be easily calculated using standard formulae found in engineering handbooks, and most likely available somewhere on the web. Actually came out close, good enough for most purposes.

IMG.jpg
 
#15
I dont have anywhere near the experince of you guys so I am doing most my of design work on CAD (solid works). It alows you to assign mass properties to each material. I just throw the timber on a scale and work out the mass density and dial it in to a custom material. Because I work with assemblys, the hardware is drawn once and stored in a library. I do work with this software 5-6 hours a day, so it is relitivly easy for me to drive the package, but i can see that it may not be an easy solution for everyone. A bonus is the great photo renderings that you can produce. The photos are a bass a friend wants me to build that I am in the middle of finalizing. He's in the UK, I'm in South Africa so good photos really help with comunication.
 

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#16
richtea said:
I dont have anywhere near the experince of you guys so I am doing most my of design work on CAD (solid works). It alows you to assign mass properties to each material. I just throw the timber on a scale and work out the mass density and dial it in to a custom material. Because I work with assemblys, the hardware is drawn once and stored in a library. I do work with this software 5-6 hours a day, so it is relitivly easy for me to drive the package, but i can see that it may not be an easy solution for everyone. A bonus is the great photo renderings that you can produce. The photos are a bass a friend wants me to build that I am in the middle of finalizing. He's in the UK, I'm in South Africa so good photos really help with comunication.
Click to expand...

Beautiful modeling, richtea. I'd love to make the jump to SW, wish I had five grand laying around, or an employer who I convince that we needed it. :)
 
#18
richtea said:
I dont have anywhere near the experince of you guys so I am doing most my of design work on CAD (solid works). It alows you to assign mass properties to each material. I just throw the timber on a scale and work out the mass density and dial it in to a custom material. Because I work with assemblys, the hardware is drawn once and stored in a library. I do work with this software 5-6 hours a day, so it is relitivly easy for me to drive the package, but i can see that it may not be an easy solution for everyone. A bonus is the great photo renderings that you can produce. The photos are a bass a friend wants me to build that I am in the middle of finalizing. He's in the UK, I'm in South Africa so good photos really help with comunication.
Click to expand...

You are probably doing it the best way. I have access to solidworks, but I have never used it. For my current build, I just drew it and hoped it worked out, which it is doing.

lowsound
 
#19
richtea said:
I dont have anywhere near the experince of you guys so I am doing most my of design work on CAD (solid works). It alows you to assign mass properties to each material. I just throw the timber on a scale and work out the mass density and dial it in to a custom material. Because I work with assemblys, the hardware is drawn once and stored in a library. I do work with this software 5-6 hours a day, so it is relitivly easy for me to drive the package, but i can see that it may not be an easy solution for everyone. A bonus is the great photo renderings that you can produce. The photos are a bass a friend wants me to build that I am in the middle of finalizing. He's in the UK, I'm in South Africa so good photos really help with comunication.
Click to expand...
Wow, nice ThunderParker! (ThunderFly?)
 

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