Getting the Facts about Ski Jumps

[bdplaid]

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.

I was thinking this too. Dovetailing with Bruce's comments, which I took to mean the jump may likely return unless other things happen.

 

[Turnaround]

I was thinking this too. Dovetailing with Bruce's comments, which I took to mean the jump may likely return unless other things happen.

Precisely the point. The ski jump is the symptom. The root cause is a lack of strength in the neck under the tension of the strings. So to prevent it from happening again you need to do one of two things:
1. Reduce the string tension
Or
2. Reinforce the neck
Treating the neck by flattening it only deals with the symptom, not the root cause, so there is nothing to prevent the return of the kink.

 

[Matt Liebenau]

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.

A kink in that area is undoubtedly an issue and needs to be dealt with and could be called a ski jump or, obviously, 12th fret kink, no argument there. The original premise of the thread was whether or not shims cause a ski jump. No harm and no worries on your end.

 

[Turnaround]

The original premise of the thread was whether or not shims cause a ski jump. No harm and no worries on your end.

As the originator of the thread I can say that the premise of this thread was to bring some well-seasoned knowledge to the table about ski jumps. That included what they really are, what causes them, what the treatment is and other related info. I think we covered it pretty well thanks to the input of the other professional luthiers and techs that I invited to join the discussion, and the input of many players. I'm hoping that this will become the definitive source for information about ski jumps.

 

[Matt Liebenau]

As the originator of the thread I can say that the premise of this thread was to bring some well-seasoned knowledge to the table about ski jumps. That included what they really are, what causes them, what the treatment is and other related info. I think we covered it pretty well thanks to the input of the other professional luthiers and techs that I invited to join the discussion, and the input of many players. I'm hoping that this will become the definitive source for information about ski jumps.

And it’s appreciated. I’m sure I could have been convinced of the “shims cause ski jumps” thing if not for you and the other pros on here.

 

[Turnaround]

And it’s appreciated. I’m sure I could have been convinced of the “shims cause ski jumps” thing if not for you and the other pros on here.

Thanks Matt.

 

[Zooberwerx]

Supporting video courtesy of YouTube:

 

[Turnaround]

In another thread the suggestion that a shim can cause a ski jump was raised again. It reminded me that I have not reported on a test I undertook to try to induce a ski jump in a neck by installing a shim. See post #134 in this thread where I describe the test.

It has been more than 5 years and the test is continuing with the shim in place and the strings at tension. Today I checked again for any indication of a ski jump, but there is none. A single test is not proof, but it still is my contention that the use of a shim will not induce a ski jump. Early in the test Bruce Johnson (someone that knows more about neck construction, truss rods and reinforcement that anyone else I know of) suggested that if the neck was not initially distorting under the screw tension, it would never distort no matter how long I ran the test. I believe he is right.

I am probably not going to run the test any longer. I don't think the results will change.

 

[zuma]

Though a test of one, there certainly has to be a lot going for a test of FIVE years, under what seems like controlled conditions to boot.

 

[Gilmourisgod]

This result makes sense intuitively. Try bending a piece of rock maple over something like a 4” span with anything approaching normal string tension, restrained by 4 wood screws. Not happening.

 

[Turnaround]

For completeness, Here is a summary of what I did. Originally I started with the neck srewed to a block of wood, with a shim under the neck heel, like this:

But within a few days at the suggestion of Kark Kaminski, I re-jigged the setup. I used an actual bass body and strung it up so the neck would be under string tension as well. I also narrowed the wood shim so that the inboard screws did not go through it - the shim remained outboard of the innermost screws. This would allow a fraction more potential bending action by shifting the fulcrum further along the lever and allowing the inner screws to apply more levering force. The shim was like the green block in this diagram, rather than the red one above.

As I have described, I ran this test for about five and a half years (started in November of 2019). In that time I occasionally removed the neck and checked for any distortion in the neck heel. The setup was immediately re-assembled, screws tightened and strings brought back to tension. The setup was then stored laying on its back in the collar ties of my shop. The body of the bass was supported by one collar tie, the headstock end of the neck resting on the next collar tie - no support between those two points. In this arrangement, gravity would be adding a bit more bending stress on the neck in the same direction as the string tension.

The shim was a piece of hard maple measuring .023" thick, the full width of the neck, with a rounded end just like the end of the neck. The neck was 3 years old when the test was started and came from well-known brand of bass. It was made of flat sawn hard maple with a rosewood fretboard (fretted). Strings were medium gauge, medium tension, and brought to regular pitch.

The result after 5 years - no neck distortion.

 

[Gilmourisgod]

I'd say that's petty conclusive. Shimming a neck pocket does NOT warp the neck. In other words, not much point wasting money on overpriced continuous-taper shims from Stewmac when a piece of any rigid, reasonably non compressible material will work just as well.

 

[Pilgrim]

There has been a shim under the neck of my 1963 P since the neck was replaced (truss rod broke) in 1972. The neck has been under continual string tension since 1972, without even a change of strings. Hey, I like dead flats.

No ski jump. None at all.

 

[Turnaround]

I think the argument for using a full pocket tapered shim is to avoid that "tone-sucking" or "sustain destroying" air gap in the neck pocket. Frankly I don't believe that tiny bit of a gap affects either tone or sustain to any discernible degree. But as a seller, why not supply the concerned public with a product that addresses their concern, be it a real or imaginary problem? There's profit to be had there. There is plenty of evidence that a partial shim is effective without discernible side effects, and very little evidence to the contrary.

I might note too that I have had more than one customer that complained that they installed a shim and it negatively affected their tone or sustain. I fixed their problems not by removing the shim, but by correcting the way the neck screws passed through the body. If the screws do not pass freely though the body, but instead screw through it, the screw will tighten against the body before the neck is pulled into full contact with the neck pocket. That will reduce the neck-to-body contact and will act as a vibration damper. Enlarging the neck holes until the screws passed through unimpeded allowed the screws to pull the neck tight against the body and shim and the customer agreed that their tone/sustain was fully restored.

 

[dmk]

I came across an interesting post by guitar builder Gil Yaron, Interview: Gil Yaron on re-creating vintage guitars who strives to faithfully recreate vintage Fender and Gibson guitars. He replied to a post on the Telecaster guitar forum, 1961 Stratocaster build regarding the placement of the truss rod and how it can contribute to a ski jump.

He states, “We all know what a ski jump is... and I rarely ever seen one on a pre cbs instrument. There is a small ski jump created from wood expansion around the truss rod nut (humidity gets in...) but I'm talking about the larger one, the one that starts around the 15th fret and can't be fixed by tightening the rod. The old instruments had a 2 degrees angled insertion point in the truss rod nut area (at the heel) and the force was distributed evenly across the neck. Newer designs with a lower insertion point (and degree) don't create enough force in this section of the neck.”

I’m curious as to what others think of this.

 

[Turnaround]

...starts around the 15th fret and can't be fixed by tightening the rod. The old instruments had a 2 degrees angled insertion point in the truss rod nut area (at the heel) and the force was distributed evenly across the neck. Newer designs with a lower insertion point (and degree) don't create enough force in this section of the neck.”

The truss rod cannot have any affect in the area described.

The 15th fret is red. The green area is where the neck is bolted to the body. To have any effect the truss rod has to be able to bend the neck, but it cannot in the green area since it's bolted to an inflexible body there. The angle of the truss rod nut is immaterial.

 

[Bruce Johnson]

I fully concur with Richard; The truss rod is not physically capable of straightening out anything past the 12th fret.

The main reason why you see fewer ski jumps on pre-CBS Fenders is that the necks of that era were slightly thicker around the 12th fret area. An extra 0.060" of maple on the back at the 12th will keep the wood from stretching under load, which causes the 12th fret kink/ski jump.

The angle of the truss rod that Gil is referring to is the downward angle that the rod takes into the neck, looking from the adjusting nut in the heel. It's around 2 degrees on all Fenders that I know of. Inside the neck, the rod is held in a shallow vertical curve, which I call the Droop. It's lower at the center of the neck around the 5th-6th fret, and higher at the 1st fret and the heel. The downward angle of 2 degrees is leading into the Droop curve.

On some of the CBS era necks, particularly the ones with the "Veneer" fingerboards, Fender did raise up the height of the adjusting nut at the heel. This increased the depth of the Droop, which increased the mechanical ratio of the truss rod. It didn't make the truss rod itself stronger, but it gave it more power to bend the neck. But, over the whole length, not in the heel area.

Gil also referred to that other problem that Fender necks sometimes have, where the heel adjusting nut can push up a lump in the fingerboard around the last fret. Usually due to overtightening the truss rod. That'll cause a nasty buzz on the last fret or two. But it's not a ski jump.

 

[JutMitch]

Really appreciating this thread! I purchased a MJT relic'd bass with All parts neck over the weekend and to my dismay i noticed it had a decent little ski jump. It was a cheap bass (@$300) which i really wanted it for the body so i figured the gamble was worth it. It was definitely a neglected bass in that it looks as if the truss rod was never adjusted from new. It was loose with zero marring on the truss rod nut. To boot there was some pretty heavy strings (at least 105/50 and what appear to be EXL strings) and it was stored in what I'm assuming was a tuned state.

It seemed that the truss adjustment wasn't really affecting the 12th fret and all. I noticed from sighting down the neck i would get a dip around the 12th fret and it would continually raise up to the last frets. I stripped the neck, removed the truss nut and placed a 2x4 underneath the 19-20th fret and another 2x4 underneath the 10th fret. I let a hot wet cloth sit on it for about an hour and then i hit the area pretty good with the heat gun and then clamped the area with two large wooden clamps creating a backbow and let set for about 3 hours each session and did this about 5 times.

What i did notice is after maybe 1-2 sessions the neck was able to be adjusted straight as an arrow but would still have relief when the truss was completely loosened. After a few more sessions i noticed the neck is arrow straight even with no tension on the rod.

Now this was all done prior to even seeing this thread. Does it seem like i might be going to the right direction? Should i continue this process over the next few days and then do a few sessions of dry heat? How long to keep this in a clamped position? My plan is to use 95/45 strings which is what i normally use and this should drop my tension from about 185-190lbs to 140lbs. I'm hoping with this drop in tension, combined with proper truss adjustment will keep this bass in ok condition.

Thoughts? Concerns?

 

[Turnaround]

Really appreciating this thread! I purchased a MJT relic'd bass with All parts neck over the weekend and to my dismay i noticed it had a decent little ski jump. It was a cheap bass (@$300) which i really wanted it for the body so i figured the gamble was worth it. It was definitely a neglected bass in that it looks as if the truss rod was never adjusted from new. It was loose with zero marring on the truss rod nut. To boot there was some pretty heavy strings (at least 105/50 and what appear to be EXL strings) and it was stored in what I'm assuming was a tuned state.

It seemed that the truss adjustment wasn't really affecting the 12th fret and all. I noticed from sighting down the neck i would get a dip around the 12th fret and it would continually raise up to the last frets. I stripped the neck, removed the truss nut and placed a 2x4 underneath the 19-20th fret and another 2x4 underneath the 10th fret. I let a hot wet cloth sit on it for about an hour and then i hit the area pretty good with the heat gun and then clamped the area with two large wooden clamps creating a backbow and let set for about 3 hours each session and did this about 5 times.

What i did notice is after maybe 1-2 sessions the neck was able to be adjusted straight as an arrow but would still have relief when the truss was completely loosened. After a few more sessions i noticed the neck is arrow straight even with no tension on the rod.

Now this was all done prior to even seeing this thread. Does it seem like i might be going to the right direction? Should i continue this process over the next few days and then do a few sessions of dry heat? How long to keep this in a clamped position? My plan is to use 95/45 strings which is what i normally use and this should drop my tension from about 185-190lbs to 140lbs. I'm hoping with this drop in tension, combined with proper truss adjustment will keep this bass in ok condition.

Thoughts? Concerns?

You are indeed going in the right direction. Now that the neck is straight without truss rod tension, the neck is in good shape. I would let it sit for a couple of weeks off the bass with no clamps and no truss rod tension and check then to see if it remained flat. If so you are good to go. If not, give it a few more treatments, let it sit out of clamps and without truss rod tension for another 2 weeks. Proceed until the neck remains flat.

 

[JutMitch]

You are indeed going in the right direction. Now that the neck is straight without truss rod tension, the neck is in good shape. I would let it sit for a couple of weeks off the bass with no clamps and no truss rod tension and check then to see if it remained flat. If so you are good to go. If not, give it a few more treatments, let it sit out of clamps and without truss rod tension for another 2 weeks. Proceed until the neck remains flat.

Awesome, I'll pull the clamps off this evening and let it set. I ordered some nitro and plan on refinishing it, but i'll give it a few weeks to settle before finishing. I'm not sure if it's the design but i remember my 1954 reissue sting bass getting extreme relief with only slightly higher tension strings and although the truss corrected it, i went back to low tension Thomastik. Does this vintage design just not work well with higher tension strings. Seems the rest of my basses (2 Warmoth, US PRS, Musicman, Gibson and G&L) were never this sensitive. I know the Warmoth has steel stiffening rods, but maybe others do also.

I'm guessing in my case its probably the combination of everything.....High tension strings, long term storage in tune, high humidity and loose truss rod.