Getting the Facts about Ski Jumps

[zuma]

I think this is the video:

 

[Turnaround]

Somewhere on TB Bruce Johnson describes how he seasons the wood for a neck, then cuts it into strips which are further seasoned before gluing up with the wood strips in the same order, but every other one flipped over. If someone finds that thread, post a link to it here.

It is worth noting, however, that this does nothing to prevent ski jumps. A ski jump is a longitudinal stress distortion, and flipping the laminates will not change that.

 

[takeout]

Did Kubicki do the flip technique with his 32-piece headless necks?

 

[zuma]

I read somewhere on the Kubicki thread that he had those laminate blanks made for him. Regardless of who made them, you'd think they'd also do the flip thing. I'm assuming a bunch of skinny strips of grain going in the "same" direction would also be problematic, so why not do what's best while you're at it.

 

[Joshua]

Moderator insert: the Pro Bench is for discussions at a more macro level so I cleaned up the more micro discussion that was taking place. Please feel free to start/continue that discussion in the Setup forum itself.

Gracias!

 

[mngnt]

I just read through this thread as I have a fairly cheap JazzMaster with the issue.

I plan on trying the heat method first, and had 2 questions:

1- I've seen several references to heat-treated necks that were only temporarily fixed; that the issue returned. However, I have not seen any evidence of this posted. Have I missed it?

Another member (I don't recall who), suggested that the process may actually alter the properties of the wood (heat treating wood at higher temperatures definitely does), so I think it's a reasonable hypothesis. If so, the process could very well resolve the issue permanently.

2- when heat treating, is it a requirement to clamp the neck past straight (into a "reverse ski jump" position)? I have a decent sized granite surface, and I'm wondering if clamping to this would be sufficient. If it hasn't been tried, perhaps I'll be the one to do so.

Thanks in advance!

 

[Turnaround]

I just read through this thread as I have a fairly cheap JazzMaster with the issue.

I plan on trying the heat method first, and had 2 questions:

1- I've seen several references to heat-treated necks that were only temporarily fixed; that the issue returned. However, I have not seen any evidence of this posted. Have I missed it?

Another member (I don't recall who), suggested that the process may actually alter the properties of the wood (heat treating wood at higher temperatures definitely does), so I think it's a reasonable hypothesis. If so, the process could very well resolve the issue permanently.

2- when heat treating, is it a requirement to clamp the neck past straight (into a "reverse ski jump" position)? I have a decent sized granite surface, and I'm wondering if clamping to this would be sufficient. If it hasn't been tried, perhaps I'll be the one to do so.

Thanks in advance!

1. We haven't found a properly documented case of a heat treatment being only a temporary fix. It may be that the reason a ski jumped returned after heat treatment was that the neck was once again subjected to the forces that created the ski jump in the first place without any additional reinforcement.

2. What you are aiming for when straightening a neck is to get it flat when the truss rod is slack, or very slightly back bowed. Judging just how much backbow is tricky since it depends so much on the character of the wood and will vary from neck to neck. So a safer approach is to go with flat. To get there you may need to heat with a slight backbow, but I would start by clamping flat. Go through a couple of cycles of heat and allowing to cool, then assess the result. If there is still an upbow, repeat the process with the neck in a slight backbow.

The properties of the wood will not change in any significant way at the temperatures used for straightening (65 Celsius or 150 Fahrenheit). Roasting maple changes its properties but the temperature to do that is far higher - around 160 Celsius (320 Fahrenheit).

 

[vaesto]

I did heat treatment twice. In the first run I've reached that 65C temperature. It helped to some degree, however, the ski-jump returned in several weeks. In the second run, I've made some rearrangement and temperature of the neck reached 85C. Also, I've clamped frets 0-12 to flat beam, and forced some back-bow/roll-down from 12 to the end. Hold this for 4 hours ore even more. The neck took this shape and holds it 3 years until these days...

So, I can make a conclusion: wood needs somewhat higher temperatures (80C+) to go into keep the new re-shaped form. Beware of the finish, though. At these temperatures it starts to soften and may even separate from the surface.

P.S. I don't actually know what happens to maple structures at these temps. But the result speaks for itself. Would be interesting to conduct scientific study, but this needs laboratory and funds. And, honestly, it still would be something what we know as "British/Canadian scientists has recently discovered..." (in comic sense).

 

[Turnaround]

I think the key to straightening a bend is not just the temperature and clamping, but getting the wood heated all the way through. In the process of kinking, the wood was stretched on one side and compressed on the other. Any action to correct the problem must address both the stretching and the compression, and so the wood should be brought to temperature throughout.

 

[walterw]

when heat treating, is it a requirement to clamp the neck past straight (into a "reverse ski jump" position)? I have a decent sized granite surface, and I'm wondering if clamping to this would be sufficient. If it hasn't been tried, perhaps I'll be the one to do so.

yeah, you'll need to "overbend" just a little to account for some springing back

i'd be scared to clamp a sideways force to a granite plate, i could see it breaking

 

[walterw]

I think the key to straightening a bend is not just the temperature and clamping, but getting the wood heated all the way through. In the process of kinking, the wood was stretched on one side and compressed on the other. Any action to correct the problem must address both the stretching and the compression, and so the wood should be brought to temperature throughout.

geoff luttrell at san fransisco guitar works suggests that the "magic" temperature is when the back of the neck (opposite the heating source) reaches about 112°F, with the key of course being to apply the heat gently enough that you're not barbequing the fretboard in the process

in recent years i've taken to using a variation of his approach, using a thick aluminum bar laying over the fretboard, heated with a long silicone heating strip that i plug into a variac to control the temperature. i also make use of my neck jig to do the "clamping", it lets me push the neck into upbow or backbow or whatever i need entirely from underneath the neck so i can just rest the heating bar on top

when i'm just trying to get rid of the dreaded kink i have a shorter aluminum slab i can rest a small travel iron on top of to put the heat where it's needed

 

[Turnaround]

geoff luttrell at san fransisco guitar works suggests that the "magic" temperature is when the back of the neck (opposite the heating source) reaches about 112°F, with the key of course being to apply the heat gently enough that you're not barbequing the fretboard in the process

in recent years i've taken to using a variation of his approach, using a thick aluminum bar laying over the fretboard, heated with a long silicone heating strip that i plug into a variac to control the temperature. i also make use of my neck jig to do the "clamping", it lets me push the neck into upbow or backbow or whatever i need entirely from underneath the neck so i can just rest the heating bar on top

when i'm just trying to get rid of the dreaded kink i have a shorter aluminum slab i can rest a small travel iron on top of to put the heat where it's needed

I have used a heat lamp that will do a pretty good job of heating a neck through, but you have to stay right on top of it. I burned a neck one time when I took my attention away for about 2 minutes to answer the phone. Not really recommended.

 

[walterw]

I have used a heat lamp that will do a pretty good job of heating a neck through, but you have to stay right on top of it. I burned a neck one time when I took my attention away for about 2 minutes to answer the phone. Not really recommended.

yeah, the shiny aluminum bar resting on top of the neck, itself heated with a temperature controlled silicone heater, that lets me heat it slowly and gently enough that i can rest it right on the frets over a maple finished fretboard without scorching and let it go for a while

(some other stuff i've picked up about cooking pans tells me that shiny new sheet pans don't brown food as well as dark, well-used sheet pans, so the shiny aluminum bar doesn't tend to scorch as quickly)

 

[Bruce Johnson]

The important thing to understand here is that "ski jumps" are caused by over stressing the wood. Too much bending load is applied to the neck by the strings, for the thickness/shape/construction of the neck. The wood on the back of the neck gets stretched because it's overloaded externally.

Yes, you can apply heat to soften the wood, overclamp it to a rigid beam, etc., and bend it back to straight. But that doesn't strengthen it. It may even weaken it a bit. When you put the same string load back on it, it will probably just bend again. Maybe immediately, maybe slowly. That's why heat treatment for ski jumps is risky.

That's why our recommendation is generally to use heat treatment to straighten the kink of the ski jump, but then do something else to reinforce the area or reduce the string load.

It's a whole different thing from normal warpage, bowing, backbowing, twist. That's caused by internal stresses within the wood fighting against each other. Heat treating relieves those internal stresses, while pushing it back to shape. So, it should stop misbehaving, forever. Nothing to do with the string load.

 

[Turnaround]

For those following my test where I attempt to induce a ski jump in a neck with a shim...

It's been four years now , and there has been no bending of the neck in the pocket. Because I brought the strings up over normal tension there is an indication of a kink forming between the 12th and 15th fret, but the neck in the pocket area remains dead flat.

Go back to post #134 to see about the test.

Five years into this test and still not even a hint of a ski jump. This reinforces my opinion that a shim will not cause a ski jump. I tried my best but I could not induce the bend. I don't think I will pursue it further.

 

[TomB]

Five years into this test and still not even a hint of a ski jump. This reinforces my opinion that a shim will not cause a ski jump. I tried my best but I could not induce the bend. I don't think I will pursue it further.

Thank you for your persistence. This has been a valuable exercise for many.

 

[GeeBass]

To me - if a 'kink' in frets 12-15 area occurs - that IS a 'ski jump'. Or whatever else it is called: 'heel swell', 'heel hump', etc.
The problems at the neck heel includes a lot more than the last 2-3 frets, or the flatness of the bottom of the heel.
Most ski jump issues that need to be addressed happen in that 12-15 fret neck heel area IMO/IME.

 

[Gilmourisgod]

If heat treatment doesn't work long-term, are we better off just leveling (in the case of minor ski jump) or (in the case of major ski-jump) pulling the frets out and planing the board level, assuming there's enough board thickness available? I have a 90's era MIM P-Bass that has some definite ski-jump, but it's not a thin veneer board, decent thickness of solid rosewood. It's unplayable above the 12th fret, so I figured nothing to lose using it as a guinea pig.

 

[sissy kathy]

To me - a 'kink' in frets 12-15 area IS a 'ski jump'. Or whatever else you'd want to call it: 'heel swell', 'heel hump', etc.
The problems at the neck heel includes a lot more than the last 2-3 frets. Most neck issues happen in that 12-15 fret area IMO/IME.

Well treat me like I'm in junior high school woodshop and explain to me how a shim under the 20th fret, behind four screws, can cause a bend 20 cm (8 inches) up the neck. While you're working up your explanation, consider if 54~68 kg (120~150 lbs) of string tension might, just maybe, be the real cause of that warp at the 12th fret.

 

[GeeBass]

Oh man - haven't been a regular on the tread - no ill will meant, apologies on my end for any misunderstanding.

You're of course correct in what you're saying.
But the most common problem encountered is that 'hump' that forms in 12-15 fret area causing fretting-out, even just tuned to standard pitch.
Most people encounter this on bolt-on basses, and I'd thought that is what is commonly referred to as a 'ski jump' - rather than the flatness of the bottom of heel being the issue.

This may well be a lack of understanding the issue and terminology fully. My bad.