Getting the Facts about Ski Jumps

[Bruce Johnson]

Yep, the problem is caused by the same thing, and is fixed the same basic way. On a neck-through, I'm more likely to add an insert on the back; a partial skunk stripe. Rout a slot on the back, right down the center, from about the 6th fret to back a little way onto the body. The slot doesn't have to be very deep, maybe 1/4". Lay in the epoxy and the carbon fiber strands, and a neatly fitted strip of matching or contrasting wood. That's the backstrap, which reinforces the neck over that critical area, and keeps it from bending any further. Then, the actual ski jump has to be corrected, by leveling the frets or leveling the end of the fingerboard if necessary. Then, hopefully, there's enough adjustment height in the bridge saddles. If not, that has to be corrected somehow.

If a neck-through has a pretty severe ski jump kink, it's probably best to try to straighten it by heat first, as much as possible. Then add the backstrap; then do a final fret leveling. That will minimize the geometry problems.

 

[dedpool1052]

@Bruce Johnson / @Turnaround
One thing I see continually popping up is that basses with the 3-bolt tilt system can get the neck kink due to either over-tightening of the tilt screw/neck screws. Could you weigh in on this?

 

[Bruce Johnson]

@Bruce Johnson / @Turnaround
One thing I see continually popping up is that basses with the 3-bolt tilt system can get the neck kink due to either over-tightening of the tilt screw/neck screws. Could you weigh in on this?

No, not really. I guess it's possible to over-tighten the neck screws over the tilt screw, distorting the neck's heel and creating a lump in the fingerboard/frets. But that would be difficult to do, because the heel is pretty thick. And a lump like that isn't the same thing as a ski jump/12th fret kink. That's just a rumor caused by misunderstanding what a ski jump/12th fret kink is.

As we described above, the 12th fret kink happens outboard of the heel, around the 12th fret area. And it kinks slowly over time. The cause is long term tensile stress on a weak beam, resulting in the wood on the back of the neck stretching. It has nothing to do with the neck screws or a tilt jacking bolt. Except that the tilt jacking bolt, or a neck shim, are used to help correct for a 12th fret kink.

 

[dedpool1052]

Thanks for the response, Bruce. I figured it was just a misunderstanding, but wanted to get your view on the matter.

 

[narud]

so ive had this neck back after the heat fix for a couple of months now and so far its holding up. i like the strings laying on the board and the upper register is clean. so far im pretty impressed with eriks work at sky guitars.

 

[walterw]

OK, so i just did another neck with the same issue, and so far success!

a warwick bolt-on, and interestingly, the kink was not right at the round-to-square transition but a little ways away (the feeler gauge shows the fattest gauge i could fit under the straightedge and where i could fit it):

here's the heating lamp clamping rig-up in all its ghetto glory:

clamped that spot into backbow, let it "bake" for most of a day, left it clamped up to cool overnight, and it came out great. had i done a fret level afterwards i could have gotten it to "perfect", but as it was it played just fine finally.

the customer even had me lower it from my own standard "medium low" action (2/32" to 3/32", high to low, at the 15th fret) and it still played well for him.

 

[Turnaround]

Since the question of shims and kinks has been raised again in another thread, I though it was time I added some information here that I posted there. In a view to determining the possibility of a partial neck shim causing a "ski jump", I decided total some measurements to see if that could uncover the facts. My though was that if a partial shim could cause the heel of the neck to rise up over time, we should see measurable evidence of that. So when I have encountered a neck with a ski jump, I remove the neck and see if there is a partial shim in place. If there is I lay a straightedge on the underside of the neck heel to check for distortion. In fact, even if there is no shim I do the same thing.

So far I have seen 3 basses with both ski jumps and partial shims, and some more with ski jumps an no shims, and none have a distortion to the neck heel. It's not a large enough sample to draw conclusions - perhaps other benchers could track the same and we might collectively get a decent sample size.

 

[walterw]

So when I have encountered a neck with a ski jump, I remove the neck and see if there is a partial shim in place. If there is I lay a straightedge on the underside of the neck heel to check for distortion.

of course!

if a shim is actually distorting a neck, then the side that the shim is pressing on would be distorted, and since it's supposed to be flat on that side, it should be obvious if it no longer is.

 

[Turnaround]

I have seen numerous postings on the inter web that suggest that a ski jump can be caused by a shim in the neck pocket. Some even have diagrams that show how this occurs. Though the pros on this board agree that a shim does not cause a ski jump, the myth doesn't want to die. So here's another point to consider and something you can try if you think we are wrong.

First here's a the kind of diagram the purports to show what's happening to the end of the neck when there's a partial shim an the butt end of the pocket:

The red block is the shim, and the theory is that the pull of the screws on the neck into that white space bends the neck into an upward kink as shown. If it's possible to kink the neck in this way, then it should be possible to kink it the other way with a shim. If the shim is moved between the two screws and the screws toward the headstock end are clamped down first, the butt end screws should then pull the end of the neck down, causing the opposite of a ski jump. Like this:

Frankly I don't think either of these scenarios are possible, but if you are convinced that you have a ski jump caused by a shim, then you should be able to reverse it as in the second example.

 

[dedpool1052]

Since we're on the topic of shims, I saw this posted in another thread.
Invalid Link Removed
The video shows, in the extreme, the first diagram that Turnaround posted.

 

[Turnaround]

Since we're on the topic of shims, I saw this posted in another thread.
Invalid Link Removed
The video shows, in the extreme, the first diagram that Turnaround posted.

You know, the StewMac video just makes me mad. It explains that a partial shim introduces an air gap and says that that is critical in the sustain and tone of the instrument. But they do not explain in what way it impacts the tone and sustain, nor to what degree. And perhaps as a company they feel that the impacts are decidedly negative and substantial, but I would certainly challenge them on both counts. Where controlled tests have been done, similar claims were explored concerning the neck-to-body interface, and the results proved to be insubstantial - measurable with highly sensitive instruments, but for the listening tests participants could not agree on the differences they thought they heard - some heard one thing while others heard the opposite.

I like a full pocket tapered shim where it's needed. I prefer to reshape the pocket and avoid the shim entirely. But only because I think it's elegant - not that it makes a sonic or structural difference. I use partial shims too. Perfectly fine as far as I am concerned - just not as elegant.

Just my opinion. FWIW.

 

[Turnaround]

Latest patient with the kink came in today. A 1980's era neck-through 4 string. As received, the neck was perfectly straight first to 12th fret. From there to the 21st fret there was a rise of 28 thou making the notes fret out above the 12th. Maximum deviation was right where the neck transitioned into the body.

 

[Showdown]

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.

I have an '86 G&L L2000 that has had a ski jump since I bought it 11 years ago. I can see that it bends right where the neck heel starts, like you said. I just leveled the frets above the 12th fret and that fixed it. But I'm wondering if it will keep bending at that point. When I got the bass the truss rod was maxed out, so I added a spacer under the truss rod nut. But I have noticed over the course of 10 years every time I've adjusted the neck it has needed tightening, never loosening. I'm worried now that it will just keep bending. Do they ever stop?

 

[Bruce Johnson]

I have an '86 G&L L2000 that has had a ski jump since I bought it 11 years ago. I can see that it bends right where the neck heel starts, like you said. I just leveled the frets above the 12th fret and that fixed it. But I'm wondering if it will keep bending at that point. When I got the bass the truss rod was maxed out, so I added a spacer under the truss rod nut. But I have noticed over the course of 10 years every time I've adjusted the neck it has needed tightening, never loosening. I'm worried now that it will just keep bending. Do they ever stop?

Showdown;

Remember that overall bowing of the neck and the "ski jump" effect are two different things. They sometimes happen at the same time to the same neck, but they are caused by different things.

If the truss rod is maxed out and you keep having to tighten it, that means that the neck has developed a forward bow. It may be from a slow drying out, or internal stresses in the board, or something like that. It's just decided to slowly curve forward. The truss rod is fighting to hold it back. If you've had to add a spacer under the nut, that means that the truss rod has been under high enough load that it's starting to crush the wood, under the spacer and/or back at the anchor.

After 10-12 years, the bowing has probably stopped. It may not continue moving, but the damage is there. It's bowed, and the truss rod is trying its best to hold it straight, but it's only marginally able. If it keeps gaining relief, and you keep trying to tighten the truss rod, at some point, something is going to break.

So, that's one problem that you have with the neck. It may hang in there for a while as it is, or you may need to fix it.

The second problem is the "ski jump" or "12th fret kink", which is pretty much unrelated. As we've described ad nauseum in this thread, the ski jump is a bend in one spot in the neck, which is caused by the steady load from the strings. The stress is too high in the wood in that small area on the back of the neck, and over time, it slowly stretches. Unless the loading changes, it's going to keep slowly stretching. It doesn't stop unless you reduce the load (lower tension strings) or reinforce the wood with some repair work, like we've described.

The sort-of good news is that the ski jump usually develops slowly. If it took 12 years to get to where you recently made a correction for it, it may take another 12 or more years before it gets problematic again.

Since you appear to have both problems on this neck, keep watching it to see which of them is getting worse. If the action goes up, is it because the neck is bowing forward some, and the relief in the nut-to-12th zone is increasing? Or is the whole neck continuing to bend at the 12th, increasing the ski jump?

There is one way that the two problems can work together: If the neck bow causes the action to get higher, that increases the loading at the 12th, which may speed up the ski jump stretching. But it doesn't really work the other way. A ski jump bend doesn't by itself make a bow better or worse.

 

[Showdown]

Showdown;

Remember that overall bowing of the neck and the "ski jump" effect are two different things. They sometimes happen at the same time to the same neck, but they are caused by different things.

If the truss rod is maxed out and you keep having to tighten it, that means that the neck has developed a forward bow. It may be from a slow drying out, or internal stresses in the board, or something like that. It's just decided to slowly curve forward. The truss rod is fighting to hold it back. If you've had to add a spacer under the nut, that means that the truss rod has been under high enough load that it's starting to crush the wood, under the spacer and/or back at the anchor.

After 10-12 years, the bowing has probably stopped. It may not continue moving, but the damage is there. It's bowed, and the truss rod is trying its best to hold it straight, but it's only marginally able. If it keeps gaining relief, and you keep trying to tighten the truss rod, at some point, something is going to break.

So, that's one problem that you have with the neck. It may hang in there for a while as it is, or you may need to fix it.

The second problem is the "ski jump" or "12th fret kink", which is pretty much unrelated. As we've described ad nauseum in this thread, the ski jump is a bend in one spot in the neck, which is caused by the steady load from the strings. The stress is too high in the wood in that small area on the back of the neck, and over time, it slowly stretches. Unless the loading changes, it's going to keep slowly stretching. It doesn't stop unless you reduce the load (lower tension strings) or reinforce the wood with some repair work, like we've described.

The sort-of good news is that the ski jump usually develops slowly. If it took 12 years to get to where you recently made a correction for it, it may take another 12 or more years before it gets problematic again.

Since you appear to have both problems on this neck, keep watching it to see which of them is getting worse. If the action goes up, is it because the neck is bowing forward some, and the relief in the nut-to-12th zone is increasing? Or is the whole neck continuing to bend at the 12th, increasing the ski jump?

There is one way that the two problems can work together: If the neck bow causes the action to get higher, that increases the loading at the 12th, which may speed up the ski jump stretching. But it doesn't really work the other way. A ski jump bend doesn't by itself make a bow better or worse.

Ok, thanks for answering so quickly.

 

[Bruce Johnson]

Ok, thanks for answering so quickly.

I just happened to be here, swigging some coffee at the desk, getting ready to head out into the shop for the night shift. First on tonight's list is fitting a truss rod into a neck blank for a client.

 

[Joe Kyle]

I got one for ya:

What about on Rickenbackers and neck-thru body guitars? I can understand the mechanical tension placed by a bolt-on, but the neck through should dissipate some of that force, yeah?
so in that case would it just be poor manufacturing?

 

[walterw]

the neck through should dissipate some of that force, yeah?

nah.

there's still the concentration of force right where the neck goes from round to not round in the back, however it's mounted to the body.

if you took a fender and glued the neck in it wouldn't change this effect.

 

[walterw]

I replace LOTS of cello bridges. They get bent from the string tension and the pull put on the bridge when the strings are tightened to pitch. Recently my supplier ran out of bridge blanks at the same time that the schools en masse decided they needed their cellos fixed. Too many cellos, not enough bridges. Out of desperation I decided to straighten some bridges. I've done it before but was never sure about the long-term effectiveness. My method was to soak the bridges in water for a while, then nuke them in the microwave until they were somewhat flexible, clamp them in flatting cauls and let them dry.

this trick of yours came in handy for me yesterday!

i was setting up an old long-neck Ome banjo and the maple bridge was hideously bowed in the middle from the collapsed head; i soaked it for like an hour, then microwaved it for about 20 seconds, whereupon it straightened itself right back out, no clamping needed.

 

[Turnaround]

If you have enough time you don't need to nuke them. Air-drying works just fine - it just takes longer. I've made it a habit when replacing the bridges on any of the violin family, to pitch the old bridge in a pot of water I keep on hand. By the end of the day, or next day, I take them out and let them dry for a couple of days. Most will have straightened themselves. The few that haven't get tossed in the kindling bin.

I've heard from violin repair folks that what I'm doing is not a good idea. If the bridge warped, it will warp again. Best replace it. But I say it warped in the first place because the students using the instrument have not been instructed on maintaining their bridge geometry, and their bridges develop a tilt as they tune up over time. That tilt will become a warp in time. So my straightened bridges are going to bend again so long as they continue to be mistreated. Same will hold true for new bridges. So I see no particular value in replacing a bent bridge with a new one if the existing one straightens fine. And given the dire underfunding of the music programs in the schools around here, every penny I can save them helps.