Getting the Facts about Ski Jumps

[craigie]

Remember the basics here: A ski jump (12th fret kink) is caused by the neck having a continuous bending load on it.....which comes from the string tension. If that load is high enough, and/or the neck is marginally weak in design, then the wood on the back of the neck begins to slowly stretch and permanently deform. That's what causes the neck to bend and kink.

The kink normally happens around the 12th-14th fret area, because that's the spot with the highest leverage from the bending load. But that assumes that the heel of the neck is securely seated in the pocket. If the length of the heel is clamped tightly to the section of body wood, that's reinforcing it against the bending load. So, the highest loaded area is just outboard of the body, like the 12th-14th zone. And that's where it stretches and bends.

Now suppose you put a shim under the very back end of the heel, like Richard has shown above. The area between the front screws and the back screws is now unsupported and unreinforced. When the string tension pulls up on the headstock, that area may become as highly loaded, or even more highly loaded, than the 12th zone. Think about the length of the lever arm. So, the stretching and bending may extend partway down the heel, with the neck pivoting on the two front screws. It may develop a mini-ski jump on just the last couple of frets.

Did the shim cause this? It depends how you look at it, I guess. The ski jump, regardless of where it's located, is really caused by the excessive string load vs marginal strength of the neck. That's why it bends. Putting in the shim may just change where it bends.

But, with all that said, I've rarely seen a neck where just the end of the heel is bent up. In almost all ski jump cases, when you look close, the point of bending is just outboard of the body. In almost all cases, that's the weakest spot. The neck transitions from the curved back shape to the rectangular shape of the heel. Through the length of the heel, there's more wood at the back. Helping to resist the bending.

Bruce, I’m a civil engineer and I must say that I agree 100% with your description of what’s happening and it’s very easy to understand.

 

[TomB]

Bruce, I’m a civil engineer and I must say that I agree 100% with your description of what’s happening and it’s very easy to understand.

This is a valuable discussion and research effort. I know it’s not easy to fit into your busy schedules, but know that it’s appreciated out here.

 

[Bruce Johnson]

Bruce, I’m a civil engineer and I must say that I agree 100% with your description of what’s happening and it’s very easy to understand.

Yeah, you CE's are familiar with the idea of beams under load for extended times, and how they stretch and sag.

It's annoying that Fender has been allowing this problem to go on with their instruments for 50+ years, when it would be really easy to fix.

 

[craigie]

This is a valuable discussion and research effort. I know it’s not easy to fit into your busy schedules, but know that it’s appreciated out here.

Bruce sure is a gold mine amongst many others here. It’s so nice that professional builders, players, teachers, techs etc take the time.
Take for example a recent thread about neck grain and stability (turnaround I think?). Ties right in with this one.

 

[bodollar]

This is a valuable discussion and research effort. I know it’s not easy to fit into your busy schedules, but know that it’s appreciated out here.

Great information based on facts not opinions, thank all of you for taking the time and effort in explaining the problem and ways to fix it.

 

[Turnaround]

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:

View attachment 3333366
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.

After 3 months I see no deviation. I doubt that I will check and report every month as I originally stated. But I will look again in another 3 months and see if I can detect any deviation. Stay tuned if you have the patience - this is going to take a while.

 

[Harnett]

Terrific thread. Appreciation for the years of work everyone's put in.

@Turnaround I see you're vested in discovering whether or not a ski bump can happen due to a shim and Id like to explain my specific circumstance atm.

I bought a 2005 squier stratocaster (70s type large headstock) a couple weeks ago for $125. Seemed like a decent guitar for the money and I couldn't wait to set it up. Since then, i've spent more hours than id care to admit trying to figure out this ridiculous buzzing and I think I finally understand what's going on...

When first setting her up I wanted to take everything apart to clean, lube, etc. I sanded down the back of the neck for ease of sliding (which im now slightly regretting after reading this and thinking about the structural integrity at the 12-14th frets vs string tension). Regardless, after taking the neck off I found a .88mm dunlop nylon pick cut in 2 pieces used as a shim.

Ill attach pics of the indents to the pocket from the picks, but one piece was between the two screws closest to the headstock and the other in between the two screws on the low E/left side of the pocket.

When reassembling I wanted to leave the shim/picks out of the equation to see if the guitar actually needed shimmed or not. After tuning her up I heard all kinds of buzz, specifically with the low E, A, D, G strings, especially around the 12th fret and up. The buzz was very harsh so I assumed I needed to check the level of the frets.

I don't have a lot of special tools so generally I just make due with things that I have. After the fret leveling and re crowning I thought I was good to go. I tuned back up but the problem persisted. I took the neck off to shim the pocket to attempt to give the neck a little over 1 degree angle after entering in measurements into a website which showed correct neck angle, but after each time I shimmed, I couldn't get the buzz to leave us. I must have taken the neck off a half dozen times +.

I leveled the frets again and still had issues. I messed with the bridge, action, and relief but there was no hope. I thought i could see a bump in the neck in the pocket, but didnt know what to do about it.

After reading this thread I can def see that I have a ski jump at the 13-14th fret (due to string tension) and a ski bump at the 18-19th frets (imo due to the .88mm pick used as a shim).. Who knows how long the pick was there but long enough to indent the pocket.

At this point in time i'm not too sure what to do about it, but i may try the clamp and heat method as well the counter shim idea that you shared. Im no luthier, but generally pretty handy with most things I get my hands on. I just cant believe i've spent this much time on this issue.

The pictures may be hard to see, but ill try to highlight what I can.

IMHO a ski jump (12th fret kink) is a common problem caused by string tension and a ski bump is an uncommon problem caused by an incorrect/partial shim.

If anyone knows of any other solutions, lmk.

Thanks

 

[Axstar]

Am I reading this right, and the two big luthier guys on this forum disagree on something?

I have almost nothing to add, except:

1) I think shims are over-prescribed and often used by confused players to try and solve problems that are best dealt with my other means.

2) Some necks just want to ski-jump, and I don't really care about the mechanics of why this is. Fender necks can be popped off and fed into the chipper. I owned a Mike Dirnt sig with a neck like a tree trunk, and it had a wee jump in the neck. I've owned thinner Jazz necks that have been straight as arrows. Wood does what it wants to do, ultimately.

3) I've played plenty of basses with manky setups with too much relief or high action, so clearly a lot of bassists settle weirdly set up necks anyway, shim or no shim, ski jump or no ski jump.

4) I know that shims are routine in other walks of manufacturing for dialing in closer tolerances, but surely Fender could machine a sloped neck pocket in this day and age with their infinite CNC power? It seems a bit crappy that it is almost a prerequisite to jimmy a bit of bus ticket, sandpaper or thin plastic into the neck pocket to get the action-to-saddle-height situation somewhat agreeable.

 

[Harnett]

Am I reading this right, and the two big luthier guys on this forum disagree on something?

I have almost nothing to add, except:

1) I think shims are over-prescribed and often used by confused players to try and solve problems that are best dealt with my other means.

2) Some necks just want to ski-jump, and I don't really care about the mechanics of why this is. Fender necks can be popped off and fed into the chipper. I owned a Mike Dirnt sig with a neck like a tree trunk, and it had a wee jump in the neck. I've owned thinner Jazz necks that have been straight as arrows. Wood does what it wants to do, ultimately.

3) I've played plenty of basses with manky setups with too much relief or high action, so clearly a lot of bassists settle weirdly set up necks anyway, shim or no shim, ski jump or no ski jump.

4) I know that shims are routine in other walks of manufacturing for dialing in closer tolerances, but surely Fender could machine a sloped neck pocket in this day and age with their infinite CNC power? It seems a bit crappy that it is almost a prerequisite to jimmy a bit of bus ticket, sandpaper or thin plastic into the neck pocket to get the action-to-saddle-height situation somewhat agreeable.

Shims may indeed be over used but they can also be needed.

If the angles off, you'll need to shim. If the pocket wasnt cut to 17mm (for strats) you'll need a shim. Cut a proper neck pocket and you should never have to shim unless the necks warped or twisted and then you're just more than likely evading the inevitable chipper.

The ski jump/12 fret kink (not to be confused with a what i consider a ski bump, which is in the area of the neck which sits in the pocket ((16-20th frets))) is not caused by wood doing what it wants imo. Its specifically caused by the tension of the strings pulling the neck up while the end of the neck is securely fastened to the body making a short concave area around the 12-14th frets. I assume its more common with bass guitars bc of their longer scale length and thicker strings/higher tension.

It seems hard to believe that a ski bump (16-20th-ish fret) could form from an improper shim due to the thickness of the neck and short surface area of the pocket but i believe thats whats wrong with my neck (shown in a previous post) in combination with a ski jump from tension.

 

[Turnaround]

It seems hard to believe that a ski bump (16-20th-ish fret) could form from an improper shim due to the thickness of the neck and short surface area of the pocket but i believe thats whats wrong with my neck (shown in a previous post) in combination with a ski jump from tension.

Try putting a straightedge on the heel of the neck when you have it off. If it’s flat there is no possibility of a lift caused by the shim.

 

[Harnett]

Try putting a straightedge on the heel of the neck when you have it off. If it’s flat there is no possibility of a lift caused by the shim.

Novice question for you.. I attempted to look this up but didnt find much, possibly due to their being too much info when phrasing the question.

Question:
When using a straight edge on the back of the neck at the heel block, the heel block is sloped in the center (where the truss rod runs). Is this correct? Should it have the same radius as the fretboard but inverted? So maybe a -9.5'' radius? No clue whether im saying that right or whether the heel should be completely flat.

 

[Turnaround]

Question:
When using a straight edge on the back of the neck at the heel block, the heel block is sloped in the center (where the truss rod runs). Is this correct? Should it have the same radius as the fretboard but inverted? So maybe a -9.5'' radius? No clue whether im saying that right or whether the heel should be completely flat.

It should be completely flat.

 

[JD Ronson]

And here we are. I received a 1997 Squier Jazz Bass (MiM) with a rosewood neck. Set it up where I like it, and I get muting at the joint (14/15th fret) on the D string, the rest of the strings all buzz there. Now I'm reasonable, but I do like a little lower action. For our test I set it up to stock fender Jazz bass specifications, and that darned neck just has ever so small a defect at the joint affecting my ability to play over the octave on JUST THOSE FRETS. I'm gonna stick with the 'Kink' theory.

In truth, I can probably file the fret that's most affected here, but the bigger question remains: This is a Squier that was free for the most part to me. I've upgraded the electronics with concentric pots and a series switch, so there's some value in it for me to get this fixed.

In the end, would shop time ever really FIX the problem, or is a new neck going to be a better solution? Adding to the question, what specifications would you add to any neck if you were going to replace it? I'm not married to the Fender branded neck, so really I'm reaching out for any and all advice to a lasting solution for the legions of bassists / hobbyist luthiers like me who just want the best OVERALL recommendations for fixing this once and for all.

A caveat to how I feel about this Squier's neck - it was dropped several times by a previous owner and has damaged chunks of wood that I've since filled and sanded flush. Overall it has a lovely rosewood fingerboard, but really isn't anything extremely special. Afterall - it's on a 90's Squier, which when new wasn't probably exceptionally valuable.

 

[Karl Kaminski]

Well, whats your goal... to get it playing were you want it, or to use/build your skill set to get it playing?

Sounds like a great "self-shopbench" opportunity. There nothing to lose, with skill and knowledge to be gained. Are all the fret in the area securely seated? Id use it as a learning tool, maybe investigate the 'kink' for inventive solutions either via fretwork (ie maybe a board-level/refret, or spot fret work, play with shims, etc)

wow, im heading to ebay to look for a bench baby!

 

[craigie]

And here we are. I received a 1997 Squier Jazz Bass (MiM) with a rosewood neck. Set it up where I like it, and I get muting at the joint (14/15th fret) on the D string, the rest of the strings all buzz there. Now I'm reasonable, but I do like a little lower action. For our test I set it up to stock fender Jazz bass specifications, and that darned neck just has ever so small a defect at the joint affecting my ability to play over the octave on JUST THOSE FRETS. I'm gonna stick with the 'Kink' theory.

In truth, I can probably file the fret that's most affected here, but the bigger question remains: This is a Squier that was free for the most part to me. I've upgraded the electronics with concentric pots and a series switch, so there's some value in it for me to get this fixed.

In the end, would shop time ever really FIX the problem, or is a new neck going to be a better solution? Adding to the question, what specifications would you add to any neck if you were going to replace it? I'm not married to the Fender branded neck, so really I'm reaching out for any and all advice to a lasting solution for the legions of bassists / hobbyist luthiers like me who just want the best OVERALL recommendations for fixing this once and for all.

A caveat to how I feel about this Squier's neck - it was dropped several times by a previous owner and has damaged chunks of wood that I've since filled and sanded flush. Overall it has a lovely rosewood fingerboard, but really isn't anything extremely special. Afterall - it's on a 90's Squier, which when new wasn't probably exceptionally valuable.

Maybe it’s a brutal solution but if you can fix it by filing the upper frets then why not?

 

[JD Ronson]

Maybe it’s a brutal solution but if you can fix it by filing the upper frets then why not?

I'm thinking that's probably best - I just pulled up a YouTube video on leveling / crowning frets. If it's fixable with some elbow grease, count me in!

 

[craigie]

I'm thinking that's probably best - I just pulled up a YouTube video on leveling / crowning frets. If it's fixable with some elbow grease, count me in!

I’m waiting to hear from the more experienced guys (the actual luthiers and techs not a home noodler like me).

I bought a diamond flat file (coarse) and a crowning file from stew Mac. One of my best investments ever. With car sanding sponges finished with steel wool it does a great job.

 

[l_iamsurfbass]

Hello everyone! Awesome thread thankyou to all those contributing.
I have two instruments which seem to be suffering from a similar problem but I'm not sure if what I have here is a ski ramp issue or something else.
The first one is a squier showmaster guitar I tried to show the way the neck is uneven as you can see below

The other culprit is Fender 78' Musicmaster with less of the same kind of uneven plane on the neck but obviously still an issue, shown below.

What are your thoughts about them?
I plan on experimenting with the Squier with using heat and clamps and if all goes well I will attempt the more valuable Fender. I'm no luthier but I do like to learn an tweak my own instruments myself.

Thanks

 

[Zooberwerx]

Hello everyone! Awesome thread thankyou to all those contributing.
I have two instruments which seem to be suffering from a similar problem but I'm not sure if what I have here is a ski ramp issue or something else.
The first one is a squier showmaster guitar I tried to show the way the neck is uneven as you can see belowView attachment 3465601 The other culprit is Fender 78' Musicmaster with less of the same kind of uneven plane on the neck but obviously still an issue, shown below.

What are your thoughts about them?
I plan on experimenting with the Squier with using heat and clamps and if all goes well I will attempt the more valuable Fender. I'm no luthier but I do like to learn an tweak my own instruments myself.

Thanks

Good pics / graphics. To my (un)trained eye, both appear to have kicked into the dreaded "S" curve (~frets 1-8).

Riis

 

[Turnaround]

Just a note of caution...
You shouldn’t use the glue line of the fretboard or the edge of the binding to determine straightness of the neck. In fact it’s not the straightness of the neck that’s the critical factor - it’s the level-ness of the frets. Under ideal conditions both the frets and the neck would be flat, but that ideal is not always achieved. The critical flatness is the frets. Sighting the fretboard glue line or the binding may offer a quick check, but it can be misleading. Best use a straightedge on top of the frets. Don’t have one? Fret a string simultaneously at the first and the last while the string is at tension and it will act as a good-enough straightedge for this kind of assessment.