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Tune o matic and stop bar adjustment


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Last evening I ended up arguing about TOM bridge and stop bar height adjustments effect on string tension (or making strings feel more or less spagethi) with a friend of mine. I

I don't see a way how the stop bar height would affect the tension, I also tested adjusting the height but did not notice any difference. I agree that in theory there is slight difference if having stop bar max up or max down, but it is more or less insignificant how it affects the feel of guitar.

This is how I see the matter:
The effective string length is the scale length of the guitar, or where the finger pushes the string to a fret. To play a certain note on a string the string needs to be tuned to certain tension. The tension for a note (frequency on which the string vibrates) stays the same as long as the scale length and string cauge stays the same.

When bending a string, the total string length is added to equation, as the material stretches also on the area that is not playing (bridge - stop bar & heasstock). So the theoretical difference I see that stop bar adjustment would have, is that the string length changes slightly when moving stop bar up or down, but I think the length change is so small, it can't really make a difference on feel how strings are bent. I don't see how the angle on the bridge would affect the string stretching. Specially when the angle change here is not very dramatic.

This started bugging me a bit  and I thought that maybe people here would have more scientific aproach to describe the physics of spagetti strings on TOM bridge relation?

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Sounds like something that could be predicted using a string tension calculator and a bit of pythagoras.

No idea about typical spacings and heights of the various TOM components but:

  • Assume horizontal distance between a string breakpoint and the trailing edge of the tail bar is 30mm
  • Assume max height adjustability of tail bar is 6mm
  • Min distance of tail bar from saddle will be at max tail bar height - assume this to be 30mm
  • Max distance of tail bar from saddle will be at min tail bar height, which will just be a case of working out the distance of the hypotenuse of an equilateral triangle with rise of 6mm and run of 30mm = sq.root (6^2 + 30^2) = 30.59mm
  • Extra length of string at max tail bar depression = 0.6mm

Most online string calculators seem to only allow for differening scale lengths with no allowance for string length behind the nut or behind the saddle, but assume the worst and say the extra scale length is simply added to the max tail bar depression:

  • 24.75" Les Paul scale length (628.65mm)
  • top E string strung with 0.01"
  • Tension according to this page suggests 15.45lbs (7.01kg)

Add extra 0.6mm to the scale length at max tail bar depression:

  • Scale length = 24.77" (629.25mm)
  • Same string on top E
  • Tension = 15.48lbs (7.02kg)

I dunno about you, but in a blind test I don't reckon I could differentiate between an extra 10 grammes of tension on the high E from one extreme to the other. In reality it's probably even less of a discernable difference, as the extra length gets added to the total string length rather than the scale length, which would amount to an even smaller percentage change to the overall string tension.

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In terms of string tension an acoustic archtop there is no exception. Acoustic archtops perform better when there is an optimum amount of down force on the arched sound board. This is directly proportional to the angle of the strings over the bridge.

Given the tailpiece height at the same as the bridge the force is all in line with the strings. Lowering the tailpiece (or raising the bridge) will create a vector of the force vertically into the sound board giving the required down pressure on the top.

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Thanks for the reply,

So basically the string break angle at bridge has more to do with how the bridge is pushed towards the top and how the bridge plays together with the top, rather than how the strings behave, right? That makes sense to me.

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