Saddles on deck

[Downunderwonder]

18 hours ago, BigRedX said:

can't see how the neck is going to warp because of the presence of something less than a 1mm in thickness occupying about a quarter of the length of the neck pocket, unless there was a problem with the neck already

That's because you don't do physics.

 

Turn it all on its head.

 

How does a neck get skijumped?

 

Answer. Because force is applied over small distances over a long time, and timber warps under constant bending input.

[Jo.gwillim]

46 minutes ago, Downunderwonder said:

That's because you don't do physics.

 

Turn it all on its head.

 

How does a neck get skijumped?

 

Answer. Because force is applied over small distances over a long time, and timber warps under constant bending input.

I get that a force applied for a long time could cause warping. I have a jazz neck, bought second hand with bad skijimp. The problem isn't around where a shim might, or might not have been placed it's where the neck leaves the pocket. The neck in the pocket isn't distorted it's nice and flat. @Downunderwonder if you can describe your thinking a bit more I'd be genuinely interested. I've got a physics degree so don't hold back on the detail. I'm rusty but will cope!!

As a PS if shim is placed immediately not at the end of the pocket but instead placed so that the bridge neck screws go right through it there wouldn't be any bending moment on the neck. Is that a good "covers all bases" strategy?

[neepheid]

17 minutes ago, Jo.gwillim said:

I've got a physics degree so don't hold back on the detail.

 

Game on, I reckon.  

 

FWIW I "did" physics at school to CSYS level (6th year Scottish qualification - overlap with 1st year Uni course), so I probably can't contribute scientifically to this discussion, but I could probably tell you how fast the wet haddock someone's about to get smacked upside the head with was travelling when it hit them.

[Geek99]

1 hour ago, MrDaveTheBass said:

How much "relief" i.e. bend is there in the neck? You might find that tightening the truss rod to straighten the neck will bring the action down significantly. 

It’s set to fender spec

ill try a full pocket shim 

Edited March 24, 2024 by Geek99

[Downunderwonder]

1 hour ago, Jo.gwillim said:

I'm rusty but will cope!!

As a PS if shim is placed immediately not at the end of the pocket but instead placed so that the bridge neck screws go right through it there wouldn't be any bending moment on the neck. Is that a good "covers all bases" strategy?

Less bending but still the other end bears all the load at the very end beyond the screws and nothing in between is supported.

 

Someone posted a photo of an end shimmed Wal neck pocket the last time ski jumping came up. It was very clear that most of the pocket never made contact with the neck.

[BigRedX]

Wood, especially that used to make guitars and basses, is incredibly strong, despite the fact that lots of musicians appear to think their instrument is made out of twigs or balsa.

 

I've yet to see any direct evidence that ski-jump exists and the anecdotal evidence appears to point to the fact that if it does it occurs elsewhere than in the location that would be caused by a shim. My suspicion is that is most likely cause by poor selection of lumber for the neck blank, in which case it will occur whether or not a shim has been added.

[Geek99]

I had ski jump in an unshimmed neck, it does happen 

[BigRedX]

Just now, Geek99 said:

I had ski jump in an unshimmed neck, it does happen 

 

Do you have photos showing it?

[Geek99]

There’s a thread on it, but this is somewhat beyond the scope of my original topic 

[BigRedX]

7 minutes ago, Geek99 said:

There’s a thread on it, but this is somewhat beyond the scope of my original topic 

 

Not necessarily. Unless the neck is bowed and needs straightening with the truss rod, you will need to add a shim to get a sensible action. Therefore if shims have the potential to cause ski-jump it becomes relevant.

[Geek99]

I meant really all the intense arguments about physics. 

[BigRedX]

3 hours ago, Downunderwonder said:

That's because you don't do physics.

 

Turn it all on its head.

 

How does a neck get skijumped?

 

Answer. Because force is applied over small distances over a long time, and timber warps under constant bending input.

 

I do physics. Can you please produce a diagram showing the various forces at work prove they cause ski-jump.

[neepheid]

2 minutes ago, Geek99 said:

I meant really all the intense arguments about physics. 

 

Did the wet haddock help?

 

Just shim the neck.  Sometimes basses are supplied with a shim.  Ski jump?  Go skiiing then.  End of thread.

[Geek99]

6 minutes ago, BigRedX said:

 

I do physics. Can you please produce a diagram showing the various forces at work prove they cause ski-jump.

enough already. 
 

I’m unfollowing my own thread. Sadly @neepheid was incorrect. 

[dmccombe7]

24 minutes ago, Geek99 said:

I had ski jump in an unshimmed neck, it does happen 

Yep i had that too. Fender P bass. I managed to fiddle with truss for ages to get a decent balance between fret buzz at 3rd/4th fret area and choking at 14th fret area. Compromise for me was string height at 12th is 2.5mm rather than my usual 2mm that i aim for.

At 2.5mm its Fender spec and not a major issue on a P bass.

Ski jump was at the point where the neck left the pocket.

Dave 

[BigRedX]

4 minutes ago, dmccombe7 said:

Yep i had that too. Fender P bass. I managed to fiddle with truss for ages to get a decent balance between fret buzz at 3rd/4th fret area and choking at 14th fret area. Compromise for me was string height at 12th is 2.5mm rather than my usual 2mm that i aim for.

At 2.5mm its Fender spec and not a major issue on a P bass.

Ski jump was at the point where the neck left the pocket.

Dave 

 

That to me suggests that there is something fundamentally wrong with the piece of wood chosen for the neck.

 

One trick I have seen is to slacken off the truss rod and then apply downward force to each end of the neck with your knee behind the neck at the point where "ski-jump" is. Then while maintaining this pressure re-tighten the truss rod. You will either need a second person to help or a build a jig to do this. You need to apply the reverse force at the problem point in the neck or the truss rod will tend act at its centre point, which won't produce the desired result.

[dmccombe7]

18 minutes ago, BigRedX said:

 

That to me suggests that there is something fundamentally wrong with the piece of wood chosen for the neck.

 

One trick I have seen is to slacken off the truss rod and then apply downward force to each end of the neck with your knee behind the neck at the point where "ski-jump" is. Then while maintaining this pressure re-tighten the truss rod. You will either need a second person to help or a build a jig to do this. You need to apply the reverse force at the problem point in the neck or the truss rod will tend act at its centre point, which won't produce the desired result.

Its not a bass i play very often and at 2.5mm string height its ok at that. Took me ages to get the balance between fret buzz and choking tho. A little bit of truss adjustment every few days until i got it as good as i could. If it was a bass i used a lot i might try that but its not worth the effort for me.

Its a MIM one and didn't cost me a fortune just wanted to see how a P bass sounded in my bands.

Dave

Edited March 24, 2024 by dmccombe7

[Norris]

🙄

[Downunderwonder]

17 hours ago, BigRedX said:

 

I do physics. Can you please produce a diagram showing the various forces at work prove they cause ski-jump.

The forces induce bending moment in the neck. As soon as the holding down force of the screws is apparent it is resisted at the pocket end and the shim or the neck would be in motion. A beam force diagram for you:

 

[BigRedX]

What part of the neck is being shown? Is it along the neck or across the neck?

 

What are the forces pushing up from below and what are the force pushing down from above? Because if they are the strings and the screws, the strings pull at an angle with respect to the neck and not vertically

 

What's the curved dotted line? And what are the two triangles above them? Pivot points? Because if so that's not how a shim would act.

 

This could probably be the most useless diagram ever drawn to show a physics problem solution.

[Jo.gwillim]

4 hours ago, Downunderwonder said:

The forces induce bending moment in the neck. As soon as the holding down force of the screws is apparent it is resisted at the pocket end and the shim or the neck would be in motion. A beam force diagram for you:

 

Thx for this, it's helpful to have something to look at. 3 thoughts, first the defection can't be any more than the shim size and as you have rightly said contact between the neck and body can be minimal it's likely to be much less than that. Next if your theory is right we might expect the screws to need tightening after a while as the neck deforms. It would be interesting to hear if people find this to be so. I haven't but I'm not a very good statistical sample! Thirdly I'm not sure from your diagram how the skijump forms after the neck has left the pocket. I'm interested in all of this as I've a lovely fender jazz neck with awful skijump but i haven't plucked up the courage yet to take the frets off and shave a bit off the finger board.

All the best

[Downunderwonder]

35 minutes ago, BigRedX said:

What part of the neck is being shown? Is it along the neck or across the neck?

 

What are the forces pushing up from below and what are the force pushing down from above? Because if they are the strings and the screws, the strings pull at an angle with respect to the neck and not vertically

 

What's the curved dotted line? And what are the two triangles above them? Pivot points? Because if so that's not how a shim would act.

 

This could probably be the most useless diagram ever drawn to show a physics problem solution.

It calls for some imagination. In engineering it's known as a "free body potato". It frees your mind to examine forces at play.

 

Here it's the neck section in the pocket.

 

Screws pull it down. The pivot points push back as described. The free body has forces in balance.

 

You are then free to examine the internal bending forces that are generated as a result of the external forces in balance.

[BigRedX]

31 minutes ago, Downunderwonder said:

It calls for some imagination. In engineering it's known as a "free body potato". It frees your mind to examine forces at play.

 

Here it's the neck section in the pocket.

 

Screws pull it down. The pivot points push back as described. The free body has forces in balance.

 

You are then free to examine the internal bending forces that are generated as a result of the external forces in balance.

 

What you diagram fails to take into account is that the action of the strings on the whole length of the neck is far, far greater than that of the screws acting against two almost insignificant pivot points approximately 50mm apart. For ski-jump to be caused by the presence of a shim it would have to occur between the neck bolts, yet all the anecdotal evidence I have seen points to it happening elsewhere - somewhere between the centre point of the neck and the heel. So while ski-jump might possibly be a thing, it's not being caused by fitting a shim. 

[TimR]

On 23/03/2024 at 09:45, Geek99 said:

The neck and body aren’t from the same place so I expected a bit of trouble - 

probably neck pocket isn’t shallow enough and it’s original neck was thinner 

 

Did everyone just ignore this?

 

Yes, you need to shim the whole pocket in this case. 

[Dad3353]

6 hours ago, Downunderwonder said:

...

 

 

Give the kid her pencils back. 

 

...