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Getting the Facts about Ski Jumps

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


  1. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    alpha-music.com
    Hell, I’ve discovered that with banjo bridges I can basically run it under hot water for a moment or two then nuke it for 60 seconds and it straightens right out, or at least becomes pliable enough for to clamp it straight and let it cool that way.
     
  2. sissy kathy

    sissy kathy Back to Bass-ics Gold Supporting Member

    Apr 21, 2014
    Arbutus, MD
    img_20180801_084027-.
     
  3. Karl Kaminski

    Karl Kaminski Supporting Member

    Aug 26, 2008
    NYC
    hOh bouy. What happened there? Looks like it sat in the back window shelf of the sedan on a cross country tour.

    Is that an Eside positive twist (fret 5-15)? or just a dip with a shim at the heal and the truss rod too tight? Looks like a job for WalterW's jig. Whats it look like with no string tension?
     
  4. sissy kathy

    sissy kathy Back to Bass-ics Gold Supporting Member

    Apr 21, 2014
    Arbutus, MD
  5. Karl Kaminski

    Karl Kaminski Supporting Member

    Aug 26, 2008
    NYC
    thanks for link. yeah after reading that and knowing its an inexpensive instrument, it'd be more practical to find a cheap Squire replacement neck. But still a great "reinforcement project" as a learning tool.
     
  6. Turnaround

    Turnaround Commercial User

    May 6, 2004
    Toronto Canada
    Independent Instrument Technician, and Contractor to Club Bass and Guitar - Toronto
  7. Bruce Johnson

    Bruce Johnson Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    Yes, that picture really shows what a ski jump is. The neck has taken a distinct kink right in the 12th-14th fret area. In this case the bend looks to be centered on the 14th.

    Look at how the neck from the 14th to the heel is basically flat. It's not that the end of the heel curls up. The entire heel stays still on the body, while the rest of the neck bends up. That's why you run out of vertical travel of the bridge saddles. The main length of the neck has bent upward.

    And look at the main length of the neck, from the nut to the 12th. It's actually somewhat backbowed, with a hump at the 5th fret. This is because the owner has over-tightened the truss rod, trying to fix the ski jump. It doesn't work. It back-bows the main part of the neck, and doesn't correct the ski jump at all.

    Ski Jump 1.

    So, there's your classic S-bend neck. Low at the nut, high at the 5th, low at the 14th, high at the 20th.

    And I'll repeat it again: This is not caused by natural warpage, or leaving it in the sun, or changing the climate it's in.

    The ski jump (aka 12th fret kink) condition is caused by structural overload. The neck design and/or the piece of wood is only marginally able to handle the load of the strings under tension. Over time, the wood at the back of the neck behind the 12th-14th frets stretches, causing the kink.

    It's a weakness in the design of the instrument. The neck needs to be a little thicker in that zone, or have some internal reinforcement. If the piece of wood happens to be on the strong side, it'll be okay. If the wood's a little soft, it kinks. It's a marginal situation.

    Most Fenders are okay and don't kink through their lifetime. But many do. Fender has known about this problem for decades, and they've done nothing about it. Don't touch Leo's design, you know. They just continue to allow a certain percentage of their necks to fail over time. It's a shame. A small design change would fix the problem.

    This is why, when you are shopping for a bass, particularly a Fender or Fender-clone, you've got to sight down the neck. Action or relief out of spec aren't a big worry. But you need to look carefully at the 12th-14th zone to see if any ski jump condition is developing. That's a serious problem that's going to require some serious work to fix.
     
    Last edited: Aug 3, 2018
    wraub, AGH, lowplaces and 4 others like this.
  8. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    alpha-music.com
    yep, this is textbook.
    big +1

    anybody who fancies themselves ready to buy used instruments needs to know how to spot and avoid this very thing
     
    wraub likes this.
  9. Frankiebass59

    Frankiebass59 Supporting Member

    Feb 1, 2008
    Kansas
    Call me crazy but I just got a Highway One Jazz bass with a ski jump. Notes were fretting out above the 14th fret. It wasn't too bad when the neck was arrow straight but it was still not right in my hands. Checked the frets and did some mild leveling but it was still fretting out. I measured the string height when holding down the string at 14 and the last fret and there was a difference as suggested by Turnaround. I decided to try Walterw's method of clamping and heating. It Worked! The neck no longer exhibits the difference in string height per Turnaround's instructions and it no longer frets out. I was even able to actually put some relief into it per my usual setup. Bass plays well and now easily tolerates my over-aggressive right hand. Amazing. It's only been 3 days so we'll see if it holds but right now I'm happily amazed.
     
    walterw and Willicious like this.
  10. craigie

    craigie

    Nov 11, 2015
    calgary
    So would it be beneficial to loosen strings when not playing to reduce the tension and creep on the neck?
     
  11. Turnaround

    Turnaround Commercial User

    May 6, 2004
    Toronto Canada
    Independent Instrument Technician, and Contractor to Club Bass and Guitar - Toronto
    My take on it is that loosening the strings would introduce other problems, like a S curve. The problem is that loosening the strings without slackening the truss rod puts the neck in an unbalaned state. So the prblem might be lessened at the kink while simultaneously introducing backbow below the 12th fret- the dreaded S-curve
     
    AGH and craigie like this.
  12. craigie

    craigie

    Nov 11, 2015
    calgary
    I’m talking about a good neck without a ski jump but the logic still applies.

    So leave tension as is. If loosening the strings for longer term storage also loosen the truss rod. Makes sense.
     
  13. Turnaround

    Turnaround Commercial User

    May 6, 2004
    Toronto Canada
    Independent Instrument Technician, and Contractor to Club Bass and Guitar - Toronto
    Yes. Keep the neck in balance.
     
    craigie likes this.
  14. Turnaround

    Turnaround Commercial User

    May 6, 2004
    Toronto Canada
    Independent Instrument Technician, and Contractor to Club Bass and Guitar - Toronto
    I have recently communicated with a noted builder who is of the opinion that shims can cause a "ski jump" at the heel. He acknowledges that the "12th fret kink" happens, but is also quite certain that he has encountered bends that occur in the heel within the area that mates with the neck pocket in the body.

    When I am working on an instrument with a ski jump, I often remove the neck and lay a straightedge on the heel to check for flatness. I have not seen a distortion in this area except on a couple of really cheap instruments and they didn't have a shim - they were just poorly made.

    It's time to run a test. This one may take some time (a few years perhaps). I am going to take a neck I have and screw it tightly to a hardwood block with a shim at one end. This is a simulation of the neck being bolted to a body with a shim:

    Neck shim test.
    I will start by ensuring both the neck heel and the body block are truly flat. The shim (in red) will be the typical thickness I install (between 12 and 20 thou) and will be of a hard material that won't compress. Once a month I will take it apart, look for deviation in the neck and body and then screw it back together. Let's see what happens.
     
    60bass, Willicious, Joshua and 5 others like this.
  15. Karl Kaminski

    Karl Kaminski Supporting Member

    Aug 26, 2008
    NYC
    nice. will the simulation be under string tension as well?
     
    sissy kathy and lowplaces like this.
  16. Turnaround

    Turnaround Commercial User

    May 6, 2004
    Toronto Canada
    Independent Instrument Technician, and Contractor to Club Bass and Guitar - Toronto
    Well, I didn't think of that. The leverage from the string tension could be a significant factor. I'll incorporate that into the test. Thanks.
     
    60bass, Joshua, TomB and 2 others like this.

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