Getting the Facts about Ski Jumps

[RJHall]

BUMP... thanks WalterW! Can we get even more updates? I've been driving myself mad here: Heel flip? What is it? | TalkBass.com and here: A tale of two necks (Fender AVRI content) | TalkBass.com trying to 1. understand 2. remedy, and 3. prevent ski jumps in my expensive Fender basses.

Ironically, I never detected this issue on my cheaper, thinner-necked basses is the past: a CIG Geddy, a CIJ Jag, and a MIJ P bass(all incidentally had thin necks). Yet, my expensive, thicker-necked basses: 63 AVRI and 2 2012 Ricks all had varying severity of ski jump-age to them. What gives? Please post more findings!

 

[Eric_Allard]

In my opinion, the ski jump phenomenom could partially be explained using Poisson's ratio, which is the negative of the ratio of transverse to longitudinal strain as a result of an applied stress. After all, a bass neck is essentially a post-tensioned beam-column. In the design of post tensioned concrete, it is necessary to provide stirrups in the post tension anchorage zone to prevent "bursting" in that area.
My background is in structural engineering.

 

[Eric_Allard]

Of course, one hole in my theory is that there are two anchorage zones in the neck: at the heel and at the headstock. Why this phenomena isn't more prevalent at the headstock as well is unknown.
My MIM jazz was starting to have a crack between the truss rod adjustment hole and the nut, which could be the same phenomena manifesting itself differently...
One test could be to observe necks with double action and hairpin truss rods. As they don't feature the same types of anchorage zones, they should be less susceptible to the ski ramp if my theory is correct.

 

[Bruce Johnson]

Eric;
I think the force/load model is simpler than that. It's a basic lever arm. The string load is pulling forward on the headstock, and the heel is fastened to the body. Just by lever arm ratio, the highest stress area is right at the transition to the heel; around the 12th-14th fret area, at the back of the neck. Because of the "D" shape of the cross section of the neck, there isn't much cross-sectional area at the back. The wood there is under high tensile stress all the time, and it slowly stretches. The neck slowly develops a kink right there. The neck from the nut to the 10th, and from the 16th to the end are unaffected, but there's a slight bend between those two zones.

That's why I call this problem the "12th fret kink", to help clarify what's actually happening. When you try to set the bass up, it appears that the heel has lifted up into a "ski jump", but what's actually happened is that the whole neck from the 12th fret out has bent upwards slightly. Working the truss rod doesn't help things. The only real fix is to mechanically level the frets/fingerboard, trimming away the ramp on the heel. Then you generally need to add a shim under the end of the heel to tilt the neck back to being parallel to the body surface.

 

[walterw]

The only real fix is to mechanically level the frets/fingerboard, trimming away the ramp on the heel. Then you generally need to add a shim under the end of the heel to tilt the neck back to being parallel to the body surface.

i gotta say i have had some luck with the heat-treatment method.

last time was on an early '60s P with an obvious ski jump (right around where we're saying they happen, 12 to 14, over the transition from round to square in the back); that kink may have been there since before i was in grade school, but i "clamped and cooked", and the kink just went away entirely.

as luck would have it, the customer also needed a pickup re-wind, and didn't have the spare cash to send off the pickup that i'd extracted for him (when world-class winder lindy fralin is two hours away i'm not gonna bother trying to wind them myself), so the bass sat hanging in my shop, no pickup but strung up to pitch, for a solid year after my heat-treatment. (he was a friend of mine, i knew he'd be good for the job eventually.)

i thus had a rare chance to observe the repair long-term, and the neck has stayed perfectly kinkless for that entire year. (he finally got the pickup fixed and rescued his bass just this march.)

 

[Bruce Johnson]

Well, that's interesting, Walter. I'm skeptical that heating and un-bending a 12th fret kink would work over the long term, but I've never tried it. Maybe it could be used as a repair method. I can picture a fixture to hold the neck correctly and apply a steady load, and then apply a steady controlled heat.

What bothers me, from an engineering viewpoint, is that you are trying to compress wood that's already been stretched out. Heating and bending wood by causing the outer fibers to stretch, I can understand. But trying to force stretched cell structures to shorten in length?

Also, accepting that you could force the cell structures shorter, what's going to keep them from stretching out again when you reapply the string load? The loads are the same, and you haven't made the wood any stronger or more stretch-resistant. Because it's been stretched and forced back, it's probably weaker than before.

Now, if you don't try to un-bend it, and fix the problem by machining off the ramp on the fingerboard, you can make the argument that the wood has already stretched as much as it's going to, and isn't as likely to continue stretching. So, I see that as more likely to be a stable long-term repair.

Obviously, the best thing is to repair the kink, one way or the other, and then also reinforce that wood on the back of the neck with something like the carbon fiber backstrap that I described earlier.

 

[mongo2]

Of course, one hole in my theory is that there are two anchorage zones in the neck: at the heel and at the headstock. Why this phenomena isn't more prevalent at the headstock as well is unknown.

I've had two basses with a warp/"ski jump" at the headstock, a '72 P bass "A" neck and an '80s Guild Pilot "hockey stick" neck.

 

[delta7fred]

Poisson's ratio

Haven't heard that since I was in college back in the 60s, brings back memories of writing up lab experiments. Happy days!

 

[Eric_Allard]

Eric;
The wood there is under high tensile stress all the time, and it slowly stretches. The neck slowly develops a kink right there. The neck from the nut to the 10th, and from the 16th to the end are unaffected, but there's a slight bend between those two zones.

While the change in the neck's moment of inertia undoubtedly contributes to the problem, I'm not sure it's quite as simple as that: A correctly adjusted truss rod counters most of the flexural stresses in the neck (minus the amount required to achieve correct relief) and further contributes to the compressive stresses introduced by string tension. As a result, the cross section of the neck should be under fairly uniform compression (with the fingerboard side slightly more compressed). If any material creep is occurring, it would be under the mechanism of compression and, as the fingerboard side of the neck has a far greater area than the back, unit stresses should be correspondingly lower.

It would be interesting to model this in my structural software, but unfortunately, non-linear analysis is required to simulate creep and that is not included in my current license (and cost an extra $20k/year!).

 

[Bruce Johnson]

Eric;

The thing that you are missing is that, in that critical zone around the 12th fret, the truss rod is right up against the fingerboard. It doesn't have any mechanical leverage in that area. Particularly the Fender design truss rod. There is no support to the wood in the back half of the neck in that "kink zone". That's why it stretches right there, and the neck kinks. It isn't that the fingerboard side of the neck is compressing. It's a slow tension creep of the wood right on the back of the neck.

If you make a Fender neck 1/16" thicker at the 12th fret, there's no ski jump problem. Cut it 1/16" thinner, and you'll really see it kink. Where I really learned about all this was on a project for SWR back in 1994. The necks on those basses were 0.740" at the 1st fret and 0.900" at the 12th. They would develop the 12th fret kink in a month! Then we came up with the carbon fiber backstrap, and that solved the problem. I've been using variations of that design on all my bass necks since then, and I've never had them kink, even real thin necks. I put the carbon fiber back straps in all the neck structures that I build for Mike Lipe guitars and a few other clients, and it also stopped all the kinking. I'm quite certain that I understand the loads and the causes of the kinks. I've been working specifically with this problem for a long time.

 

[Eric_Allard]

Eric;

The thing that you are missing is that, in that critical zone around the 12th fret, the truss rod is right up against the fingerboard. It doesn't have any mechanical leverage in that area. Particularly the Fender design truss rod. There is no support to the wood in the back half of the neck in that "kink zone". That's why it stretches right there, and the neck kinks. It isn't that the fingerboard side of the neck is compressing. It's a slow tension creep of the wood right on the back of the neck.

You're likely correct. If the truss rod is on the fingerboard side of the geometric centroid at that location, it would probably contribute to the problem rather than alleviate it.

Do you have a plan showing the typical truss rod profile, by chance? I've seen them for Strats and Teles, but not Fender's basses. I assumed that it would follow a simple parabolic shape from headstock to heel, but with less than 3/4" to work with (minus the truss rod diameter and thickness of wood at the back of the neck) over the approximate 24" length of the neck, it would be difficult to achieve the theoretically ideal profile.

Thanks for the interesting discussion. I am assembling the shop, tools and knowledge required to attempt my first build and haven't decided if I want to use a conventional fender-style truss rod or a double action. A double action seems like the way to go, but I've always been a little nervous about the amount of wood that needs to be removed from the cross section in order to install them.

 

[David Jayne]

Not a fan of double acting because they add a non trivial amount of weight.

 

[walterw]

Well, that's interesting, Walter. I'm skeptical that heating and un-bending a 12th fret kink would work over the long term, but I've never tried it. Maybe it could be used as a repair method. I can picture a fixture to hold the neck correctly and apply a steady load, and then apply a steady controlled heat.

What bothers me, from an engineering viewpoint, is that you are trying to compress wood that's already been stretched out. Heating and bending wood by causing the outer fibers to stretch, I can understand. But trying to force stretched cell structures to shorten in length?

here's a caveman drawing of the caveman method i used:

heat was applied from the top only, so i'm guessing there wasn't all that much actual compression "deformation" of the wood underneath (though i did cook it long enough for the whole thing to get fairly warm, most of a day under a dedicated heat lamp); i'm thinking it was more about stretching of the wood up top, maybe even in the middle a bit.

people talk about this method "slipping" the glue joint, but i dunno about that; i didn't notice any kind of "seam" appearing at the fingerboard/neck boundary at the end of the neck when i was done, like i'd expect if the fretboard had actually slid any in relation to the maple neck itself.

whatever's going on, it's held perfectly for at least a year now, that much i can confirm.

 

[Bruce Johnson]

That's interesting. If one of us tries that heat-bending method again, it would be useful to get an accurate measurement of the distance from, say, the 8th fret to the 16th fret, before and after the treatment, to see if that distance has expanded. If you see your friends bass again, try putting an accurate scale ruler against it and see if you can see any stretched zone around the 12th fret. I'd still bet that the back compressed, just because there's so much less cross-sectional area back there.

 

[Turnaround]

Hmmm... I am not sure about the idea of stretching the cells. My understanding is that the heat affects the lignin, which is essentially the "glue" between the cells, softening it and allowing the cells to slip a little in relation to each other. It that's the case, it begs the question about how the wood arrived in the bent condition in the first place. Did, in fact, the cells compress? Did the lignin deform?

I have to stop asking all these questions - it leads to sleepless nights doing research!

 

[walterw]

and isn't there a thing where heat sort of "hardens" the wood? there's the whole "roasted/baked/vulcanized/torrefied" neck process, where they cook a board in the absence of oxygen (so it doesn't burn), making it more stable, rigid, maybe even brittle, like an old piece of wood is supposed to be.

there's also a faint memory in the back of my head of a bit in the Odyssey where our hero puts the end of his sharpened stick in the fire to harden it up before using it to poke out the eye of the cyclops.

heating a piece for a long period of time, enough to get the internal bit warm, could that be ever so slightly hardening the wood in question?

there was a show on discovery or some such where they made modern versions of ancient weapons to see if they could improve them; i was struck by how their ancient weapons expert took a twisty, bendy sapling branch he cut down and, just by sticking it into a campfire to heat it and bending it, turned it into a straight spear.

 

[Turnaround]

I found some research I did a few years ago, and am posting a highly condensed summary here. It has to do with the relief profiles of a number of different necks, distilled down to 3 very different profiles. Let me explain...

In my curiosity about the actual geometry of "relief" I measured a number of necks using this methodology:

1. In normal playing condition, adjust the truss told to provide .014" relief at the 7th fret with a straightedge resting on the 1st and last fret.
2. Measure the clearance between each fret and the bottom of the straightedge and plot the result.
3. Compare the resulting "relief curve" to an idealized even end-to-end curve

One of the complications is that the neck of a typical bass is tapered in thickness and width along its length. This means that it is highly unlikely that the neck will form an ideal curve under tension - it is likely to flex more in the thinner areas. This may not be a bad thing since more relief is required in the first 3 to 5 frets than further up the neck. However, exaggerated flex in the area around the 12th fret upwards is not desirable and is the very condition we are discussing here - the dreaded "ski jump". As Bruce has suggested, there may well be a tendency for this flex to occur around the transition from the rounded portion of the neck to the flat section that is attached to the body.

In the following graph of my test results there are three very different relief profiles represented from actual neck measurements. The grey area of the graph represents the idea smooth curve end-to-end. The yellow shaded area is that part of the neck that is rectangular in profile and is fastened to the instrument body. The orange area is where the neck transitions from rectangular to round in profile.

The Red line represents a neck that conforms pretty well to the ideal relief curve. It shows some exaggerated relief in the area from the second to fifth fret - not an undesirable profile for the reason stated above. It also behaves well in the upper area, following the ideal curve fairly closely.

On the other hand, the Blue line shows that though the relief follows the ideal curve fairly well in the lower area, in the area beyond the 12th fret, the clearance from fret to straightedge declines abruptly. This is an indication of the ski jump condition.

The Green line is taken from a thin, overly flexible neck. It shows excessive bending in the lower fret area as well as an abrupt change at the 15th fret, right where the neck meets the body.

 

[walterw]

awesome yet again!

i feel like that "green" profile reminds me of a bunch of '70s japanese and '80s/'90s korean necks, where they'd get a weird upbow at like the first two or three frets, rendering good setup impossible.

anyway, another anecdote! (the plural of which is not "data", i know...)

'80s G&L strat-type

here again, the maximum deflection (.008" feeler gauge) seems to be happening right where it transitions from round to square in the back.

 

[Turnaround]

That's interesting. If one of us tries that heat-bending method again, it would be useful to get an accurate measurement of the distance from, say, the 8th fret to the 16th fret, before and after the treatment, to see if that distance has expanded. If you see your friends bass again, try putting an accurate scale ruler against it and see if you can see any stretched zone around the 12th fret. I'd still bet that the back compressed, just because there's so much less cross-sectional area back there.

I really wonder about the effectiveness of heat treating. You may be able to get the neck straight again, but since it will be once again subject to the same stresses that caused the kink in the first place, what will prevent it from re-kinking? It strikes me that if the wood is prone to bending under stress, it will do it again.

 

[Bruce Johnson]

I really wonder about the effectiveness of heat treating. You may be able to get the neck straight again, but since it will be once again subject to the same stresses that caused the kink in the first place, what will prevent it from re-kinking? It strikes me that if the wood is prone to bending under stress, it will do it again.

Yes, that's exactly why I'm skeptical about the heat bending. I can't see how wood is being made stronger by heating and bending, and it seems to me that it's more likely to be made weaker. Unless you are doing some kind of reinforcement or reduction of the load, why wouldn't it bend again, when put back under the same load conditions?

But I'm interested to hear if people are actually straightening necks by heat and getting good long term results.