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Getting the Facts about Ski Jumps

Discussion in 'Pro Bench' started by Turnaround, Oct 27, 2014.


  1. Turnaround

    Turnaround Commercial User

    May 6, 2004
    Toronto Canada
    Independent Instrument Technician, and Contractor to Club Bass and Guitar - Toronto
    This first thread has been in the works for a while. The "pros" involved in this project are 202dy, walterw, JLS and myself (Turnaround).

    I approached the others a while ago after hearing once again that shimming a neck can cause a ski jump. I've heard lots of opinions from many sources about the cause of ski jumps, but no actual facts concerning the subject - opinions are not facts in my book. I decided to enlist a few of my TB friends to join me on a study of ski jumps to uncover the facts about this relatively common occurrence. The result of this effort will show up in this thread.

    First, a stab at defining what a ski jump is. It is an apparent rise in the fingerboard and/or neck occurring near the body joint of the instrument and continuing to the bridge end of the neck. This is not to be confused with relief which is a gradual, slight, and relatively even curve along the full length of the board. The ski jump is a much more pronounced curve occurring for a short distance at the butt end of the board. Sometimes this distinction gets lost - I have heard excessive relief called a ski jump - it isn't, it's just excessive relief.

    First off, the pro team is going to be examining instruments that exhibit this condition. We are going to MEASURE what is going on. We aren't just going to sight the curves in the neck, we are going to put some tools to work and determine (in thousandths of an inch) just what amount of curvature is occurring, and just as importantly where it is occurring. Important to the study is where that curvature begins to deviate from the normal relief curvature of the neck. Once we have a body of measurements we will be able to start to look for the causes of this deviation.

    My thanks to 202dy, walterw and JLS for agreeing to participate in this study. And my thanks to Joshua and Heavy Duty for agreeing to champion this effort and to enlist the support of the TalkBass team.
     
  2. dStar

    dStar

    Mar 1, 2012
    Finally, some hard facts on TB, instead of opinions (be they informed or more likely ill informed). Hats off you guys. I look forward to your work.
     
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  3. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    alpha-music.com
    here's a quick & dirty ipad sketch of just what turnaround is talking about:

    neckcurveskiramps.

    exaggerated for effect of course, but the top line is the profile of a typical upbowed neck (viewed from the side, headstock to our left); the next line is what we want to happen when we tighten the truss rod, and what we get when there are no problems.

    the second pair underneath, we again have a heavily up-bowed neck, this time with the ski ramp issue; you don't really see anything when the whole neck is relieved like this, but:

    go to tighten the rod, and you get that bottom profile;

    most of the neck responds to the rod and straightens out, but the bit at the body end still has the upward curve, called a "kink", where the wood has essentially given way a little from the pressure of the strings and the truss rod.

    keep tightening, and you see what i sketched out in that bottom line there, it actually backbows towards the nut like it should while still having that upward kink.

    a neck with this issue will never have good low action until it is addressed.
     
  4. Turnaround

    Turnaround Commercial User

    May 6, 2004
    Toronto Canada
    Independent Instrument Technician, and Contractor to Club Bass and Guitar - Toronto
    There may be a misconception drawn from your illustration, Walter. The heel end of the neck does not actually lift up. After all it is bolted to the body of the instrument. I don't think you were implying that it did rise, just that the overall geometry is as you describe, without referencing to the plane of the body.

    Or could it be that under tension (compression actually) the heel's geometry actually changes to a slight wedge shape? I think we would be more likely to see a shearing effect if the forces were strong enough to cause distortion at the heel.
     
  5. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    alpha-music.com
    ha! you're correct of course; imagine instead the bottom two lines rotated about 10° clockwise, so the last bit was still parallel with the body.
     
  6. Turnaround

    Turnaround Commercial User

    May 6, 2004
    Toronto Canada
    Independent Instrument Technician, and Contractor to Club Bass and Guitar - Toronto
    Here are some measurements we might use in our study. Hope they're useful. Measurement on the truss rod is between the anchor points. I will follow up with similar measurements on a Fender neck with the adjustment at the heel when I can. Any other measurements you think might be useful?


    tumblr_ncfi1wEeQO1tml43io1_500.

    Instrument Technician, Toronto
     
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  7. Turnaround

    Turnaround Commercial User

    May 6, 2004
    Toronto Canada
    Independent Instrument Technician, and Contractor to Club Bass and Guitar - Toronto
    This image illustrates in what manner the trussrod will affect the curvature on a typical 20-fret bolt-on neck. The trussrod in the illustration is shown in light blue running from the rod's nut (in this case at the headstock end) to the anchor near the neck heel.

    The red line in the illustration is the point at which the neck meets the body, here labelled as the "Body Joint". From the body joint to the heel, the neck is bolted to the body (Area A in the illustration). Though the trussrod extends into this area, it will not be able to form any curvature here since this entire section of the neck is fastened to the body. If it were to bend in this area, the neck would not sit flat in the pocket. The truss rod will only flex the area beyond the body joint up to its termination point (Area B in yellow). Area C (shaded yellow area) is where any action of the trussrod will begin to have an effect (the 13th to 15th fret). We therefore expect to see the beginnings of a ski jump in this area.

    Actual measurements of basses with the ski jump condition will be done by the members of the Pro Bench and the results will be posted in this thread.


    TrussRodAction.
     
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  8. Turnaround

    Turnaround Commercial User

    May 6, 2004
    Toronto Canada
    Independent Instrument Technician, and Contractor to Club Bass and Guitar - Toronto
    Here's the first set of measurements of a bass that exhibits the ski jump condition. The bass is a 4-string Schecter P/J in a regular Fender shape with 21 frets, bolt-on neck -

    Body joint at 15th fret
    Outside bolt location at 16th fret
    Inside bolt location at 19th fret
    Transition from square mounting block to rounded neck at 14th fret
    No twist in the neck
    Point of maximum deviation from trueness occurs between 13th and 14th fret. In this case we can say that the start of the ski jump is centred here.

    Referring to the diagram in the last post, you can see that the deviation is occurring around the point where the transition from the square heel to the rounded neck completes. This may be significant, but we will need more data before we can establish that this is a common location for the deviation.
     
  9. Turnaround

    Turnaround Commercial User

    May 6, 2004
    Toronto Canada
    Independent Instrument Technician, and Contractor to Club Bass and Guitar - Toronto
    These measurements from an Ibanez GSR100 4 string with a ski jump. 22 fret neck with fingerboard extension, 34" scale. No shim.

    Body joint @ 16th fret
    Outside bolt location at 17th fret
    Inside bolt location at 20th fret
    Transition from square mounting block to rounded neck at 14th fret
    No twist
    Point of maximum deviation from trueness between 13th and 14th fret
    End to end deviation from flatness 0.09"
    Truss rod adjustment at headstock - anchor point 1" from nut, roughly midpoint between nut and first fret
    Truss rod anchor 1.125" from heel, between 20th and 21st fret

    Thickness of neck including fingerboard at butt: 1.027", past transition to roundness 0.934", at nut - .0.794"
    Width of neck: at heel - 2.41", at transition to round - 2.25", at nut - 1.625"

    Though, compared to the Fender, the neck joins the body a little further along and the neck bolts are shifted further inward, the transition from square mounting block to rounded neck occurs at about the same location. Also the point of maximum deviation from trueness is occurring at the same place relative to the transition. The heel-end anchor point of the truss rod is closer to the end of the neck than the Fender-type example above, but the point of maximum deviation of the neck is still near the transition point.
     
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  10. Turnaround

    Turnaround Commercial User

    May 6, 2004
    Toronto Canada
    Independent Instrument Technician, and Contractor to Club Bass and Guitar - Toronto
    To help visualize what's going on, I cut a neck lengthwise to show the truss rod imbedded in the neck. I have indicated on the picture the key points identified in the measurements.

    Labelled Cross Section.
     
  11. Turnaround

    Turnaround Commercial User

    May 6, 2004
    Toronto Canada
    Independent Instrument Technician, and Contractor to Club Bass and Guitar - Toronto
    Using the neck in the above illustration, the critical measurements in the area of transition from square mounting block to rounded neck are as follows:

    Width of neck, midpoint between mounting bolts: 2.38"
    Thickness of neck at same point: 1.0"
    Cross sectional area, same point: 2.38" sq.

    Width of neck past transition point: 1.125"
    Thickness of neck, same position: 0.875"
    Cross sectional area at this point: 1.55" sq.

    There is about a 35% reduction in cross sectional area between these two points, with a corresponding reduction in stiffness.
     
    Last edited: Feb 2, 2015
    AGH likes this.
  12. zac2944

    zac2944

    Dec 28, 2004
    Rochester, NY
    I'm loving the detail in this thread, but still can't quite put it together. How exactly does all this lead to the draded ski jump?
     
  13. Turnaround

    Turnaround Commercial User

    May 6, 2004
    Toronto Canada
    Independent Instrument Technician, and Contractor to Club Bass and Guitar - Toronto
    It's a case of examining the factors that may contribute to a ski jump. We have discussed this and have decided upon a methodology we will use to determine the facts surrounding the ski jump condition. The fist step is to gather pertinent information, including measurements of instruments that exhibited the condition. The next step will be to determine if there is in fact a common condition we can label "ski jump" or are there different conditions, as seen from the measurements, that are lumped together under this nomenclature. We are assuming for the moment that there is in fact a common condition.

    After that I think we might perform some tests to see if we can duplicate the condition. If we are able to duplicate it, we are probably well on the road to determining the cause and possibly better ways to treat or even prevent it.

    We all have our opinions about the condition and its cause. Here we are trying to determine the facts. Thanks for your interest, and stay tuned... more to come.
     
  14. AGH

    AGH

    Jun 18, 2013
    Long Island NY
    All of the stuff I got for free wore out!
    Sub'd. I'm here to learn.
     
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  15. zac2944

    zac2944

    Dec 28, 2004
    Rochester, NY
    Awesome. Thanks for doing this and sharing the info.
     
  16. bassdude51

    bassdude51 "You never even called me by my name." Supporting Member

    Nov 1, 2008
    Central Ohio
    Can't wait for your findings!

    Sometimes, a "ski jump" can be experienced but playing the last 2 or 3 frets on the fretboard and hearing the strings buzz on the next higher fret (of course this can also be from un-level frets too.)

    Yeah! What causes "ski jumps"? We'll stay tuned in!
     
  17. Turnaround

    Turnaround Commercial User

    May 6, 2004
    Toronto Canada
    Independent Instrument Technician, and Contractor to Club Bass and Guitar - Toronto
    No the thread hasn't died.

    Part of the problem when you enlist the response of professional repair folk is that they often get very busy. And certainly in my corner of the planet things are busy. Once the cold sets in and the central heating gets turned on, everyone's instrument needs a setup. And it usually takes a couple of months after school starts up for the music departments to get in gear and decide to fix all those busted instruments. Folks are back into the regular swing of things, so the open mic's are flourishing (more repairs as a result) and the pros are working the circuit again (and yes, that's more work).

    I'm not complaining. It's what keeps gas in the tank. But it's in those drier spells that you come up with the idea of doing a study such as the one I instigated here. But that means we may have to wait for a drier spell again before we can add to this discussion.

    I hope you will be patient.
     
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  18. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    alpha-music.com
    and i'm in the midst of a bizarre dry spell where nothing has come in with this particular problem! as soon as i get something to report, i'll report it :)
     
  19. Bruce Johnson

    Bruce Johnson Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    Sorry I'm late.....I've been writing posts on some other subjects....

    Here's a post I wrote a while back, about the Dreaded S-Curves that happen in many Fender-style bass necks, and whether this is related to the type of truss rod chosen. The discussion gets into the "ski jump" condition, which I'm calling the "12th Fret Kink".

    Well, no, an S-curve in a neck isn't caused by having a vintage-type single-rod truss rod vs a modern dual rod. You'll more often see S-curves develop in Fender-style necks, which usually have the vintage-type truss rod. But don't blame the truss rod.

    The S-curve is the result of two things happening, one causing the other.

    The first thing is what I call the "12th fret kink". It's a very common problem on guitars and basses. The neck essentially bends a small amount in the area right around the 12th to 14th frets. It's as if you'd bent it over your knee at that point. If you put a straight edge on the neck, you'll see that it's flat from the nut to the 12th, then takes a little bend from the 12th to the 14th, then is flat from the 14th to the 20th. This is also often called the "ski jump" effect. It's often described as the heel kicking up, but what's really going on is that the neck has bent slightly right where it transitions to join the body. The net effect is a wedge-like ramp from the 14th to the end of the heel, and fret buzz at the 20th.

    So, why does the 12th fret kink happen? Because that's the most highly loaded part of the neck. The wood on the back of the neck, right behind the 12th-14th fret area is under higher tensile stress than anywhere else on the neck. If you think about it, the neck is a long lever handle, and the strings are yanking on it, trying to pull the headstock forward. The stress point is where the handle joins the body, because of the leverage ratio.

    What happens is that, over time, the wood at the back of the neck gradually stretches from the continuous tension. So the neck bends right there. And the truss rod generally isn't much help in counteracting that load. Most production truss rods, either vintage-style or modern dual rods, don't do much of anything beyond the 12th fret, because they are right up against the underside of the fingerboard at that point.

    The truss rod is designed to support and adjust the neck, in the area from the 1st to the 12th fret. When you tighten it, the load it applies to the neck is centered around the 5th fret, and has tapered off to almost nothing at the 12th.

    And this is how the S-bend happens: First the neck develops the 12th fret kink from stretching over time. This causes the action to go high, with too much relief and buzzing down at the 20th, etc. So, the owner gets out a very large wrench and cranks on the truss rod, trying to get the neck to flatten out. But it doesn't, because the truss rod isn't pushing against the area that's bent. But the owner keeps cranking, causing the neck to backbow between the nut and the 12th. But it still has the kink at the 12th and the ski jump.

    There's the classic S-bent neck. Low at the nut, high at the 5th, low at the 12th and high at the 20th. It's a common problem on old Fenders and basses with that style of neck, but it can happen on other style necks too, even neck-thru designs.

    The heart of the problem is the neck being too thin (front to back) at the area where it transitions to the body. We Luthiers have various tricks and ways to counteract the kink, from better truss rod geometry, to carbon fiber inserts, to multilaminate construction. Lots of different solutions. But that's the area you need to look at in your design.
    Here's a follow-on post, about what to do to counteract the dreaded 12th fret kink, in the design of a new neck:

    The main thing is to look at that transition area, where the neck goes from the thick part of the heel, down to the main part of the neck. That's the critical area. Look at the depth and cross section right there. You want to keep that spot as deep (that is, thick front to back) as you can. Ideally, from a structural standpoint, you'd like the neck to have a smooth taper, increasing in thickness from the 1st fret up to the heel thickness. But, most players don't like that feel. So, we cut most necks so that they are only a little thicker at the 12th than at the 1st, with a curved transition to the heel. That's what players want. But the thinner it is at the 12th, the higher the stresses and the more likely that it will kink over time.

    And obviously, the more strings you have, the higher the load will be.

    Things you can do to beef it up:

    Make the transition longer, more gradual.

    Play with the truss rod geometry. One of the cool things about the old-fashioned single rod in a curved channel is that you can cut the shape of the channel to modify how it takes the load. In this case, sink the anchor end down deep in the neck, so the rod curves down from the 1st to the 5th, and then runs almost parallel to the back of the neck, all the way to the end of the heel. That reinforces the critical area.

    I'm not crazy about carbon fiber bars myself, but if you are going to use them, put them where they are really needed. Run them from the 5th fret down to the heel, and set them in deep. That's where you need the strength and stiffness.

    Going with a laminate of quartersawn strips doesn't really increase the strength of the neck, but it does help a lot with the stability. And the more highly stressed the neck is, the more likely you are to have stability problems. So it's a good idea. Thin little ribbon strips look pretty, but they don't do anything for the structure.
    Then, of course, someone else asked why I don't like carbon fiber bars. So I replied:

    I personally don't like the carbon fiber bars because they are essentially a block of plastic. The bars are not solid strips of carbon fiber; they are bundles of carbon finder stranding which are formed into a rectangular shape by pulling them through a bath of polyester resin. The process is called pultruding, which is like continuous casting. My point is that the cool looking black bar is basically a block of plastic with some carbon fiber stranding inside. It's made in rectangular strips to be real easy to install.

    So, when you install them, you are routing out strips of wood from the neck and gluing in bars of plastic. I don't like to do that. If you need to stiffen up the neck, there are better ways to do it which leave more of the wood in place. And there are many good ways to build necks that stay stable and straight, without using those bars.

    Don't get me wrong; I like carbon fiber, and I use it quite a bit. But I buy it in rolls of the raw stranding, which is called TOW. When I want to reinforce an area of a neck, or a repair or something, I make up a bundle of the TOW strands and set them in epoxy in a small groove or hole. I can put the carbon fiber right where I want it to have the most effect, and it's very strong and compact. A little bundle of carbon fiber, less than 1/8" diameter, placed down deep in the neck will increase the stiffness more than one of those big rectangular bars.

    That's one of the ways that I prevent the 12th fret kink in my necks. I set my truss rod down deep in the neck, where it passes through that area, and I fully surround it in epoxy. I also lay in two bundles of carbon fiber TOW down in the epoxy, under the truss rod. The bundles are about 12" long and 1/16" diameter, running from about the 6th fret to near the end of the heel. The important thing is that the carbon fiber ends up within 1/8" of the back surface of the neck. That's where the highest tension loads are in the wood, and the carbon fiber is perfect for keeping it from stretching over time.

    The bars are popular because they are easy to install. Rout or saw a slot and glue 'em in. And you have instant Carbon Fiber Reinforcement for Increased Stability! Which is great for marketing. I mean, Carbon Fiber is cool and high tech. And everyone wants Increased Stability, right? That's why the manufacturers like them. It lets them be sloppier in choosing and processing the wood, with fewer rejects for warpage. Plus, it's a new feature to sell.

    The truth is, if you want to reinforce a neck to make it stiffer and more stable, you can do a better job with aluminum. A strip of 7075 aluminum, 1/16" thick x 1/2" deep, standing up vertically, will increase the stiffness more than the carbon fiber bar. And it's lighter weight and cheaper. I've done it, and it works very well. Like I said, there are many better options.

    But Carbon Fiber Bars are just so cool, I guess.
    Back to the topic of Ski Jumps and 12th Fret Kinks, that's my quick rundown on what causes the condition and how I prevent them in my necks. All of the necks that I build for my own basses, and many of the neck structures that I build for other Luthiers, include what I call the "back strap". It's a bundle of carbon fiber strands in epoxy that's about 10" long. It starts at the 5th fret and runs almost to the back of the heel. It's located down under the truss rod, within 1/8" of the back surface of the neck. It runs at a slight angle, from the side view, from near the heel surface, tapering up until it meets the truss rod in the middle, at the deepest point of the rod's curvature.

    That's how I solve the problem. A hidden stiff cable to keep the neck from stretching at the back. No stinkin' ski jumps on my necks! I first started using these carbon fiber back straps back in 1993, on a development project that a group of us were doing with SWR. Those basses had unusually thin (front to back) necks at the 12th, and the first prototypes had terrible, immediate ski jump problems. The back straps were mandatory on those necks.

    So, can this strap be added to repair an existing Fender-style neck? Sure, I've done it. Rout or mill out the skunk stripe, about 1/4" deep, from the 5th fret back to the middle of the heel surface. Lay in a bundle of carbon fiber stranding in an epoxy bed, covering it with a replacement skunk stripe. That will stop the stretching and kinking. Of course, you still have to resurface the heel fingerboard to take out the existing kink. I don't think that it's practical to bend the kink back.

    Someone may ask, well, why doesn't Fender just slip a little carbon fiber stranding bundle under the skunk stripe, in the factory? And eliminate the problem from the start? I don't know. They haven't called.
     
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  20. marko138

    marko138

    May 24, 2013
    Perry County PA
    Great thread. Thanks for sharing.
     

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