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Getting the Facts about Ski Jumps

#61
I picked up a 2013 AmStd Jazz (rec'd last week off eBay) that has a noticeable kink between the 12th and 14th frets, but the kink is a high spot on the E string side of the fretboard, so it seems be bending the other way; as in anti-ski jump.

Is this unheard of? I can't imagine the forces that would cause it to pull this way, but with variability in wood imperfections, who knows, I guess.

Either way, this bass is going back where it came from tomorrow.
 
#62
Bruce Johnson said:
Yes, that's exactly why I'm skeptical about the heat bending. I can't see how wood is being made stronger by heating and bending, and it seems to me that it's more likely to be made weaker. Unless you are doing some kind of reinforcement or reduction of the load, why wouldn't it bend again, when put back under the same load conditions?

But I'm interested to hear if people are actually straightening necks by heat and getting good long term results.
Click to expand...
Interesting (perhaps) sidebar, Bruce. I repair all kinds of string instruments, including violins, violas, cellos and string basses. Most of the work on orchestral strings is done for the school system (my way to give back to the community - they can't pay my usual rate).

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.

Last week was time to do some more - another shortage. I put about 10 bridges in water to soak for a while. Forgot about them as the shop got busy and didn't get back to them until the next morning. I was stunned - even the most warped of them was truly flat - no heating, no clamping! But they were still wet and I decided to see what would happen if I just left them to naturally dry. They are dry now after almost a week, and flat.

It seems that soaking them allowed them to revert to their condition before being subjected to the stress. No heat needed. There has to be some research out there that explains this, but AFIK, it's not part of common lore in woodwork. I've been coached by many a respected luthier not to trust heat-straightened bridges, and I am not sure I do trust them. But when would straightens itself I have to start asking questions. Perhaps we should stick all ski-jump necks in the rain barrel - a sort of trip to the spa for basses.
 
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#63
I suspect that in the case of the cello bridges, cells on the 'short' side have become slightly squashed and compressed. Soaking fattens them back up to their former shape, which is retained once dry. My guess is that once stressed as before, they will eventually revert to the bent state. They weren't strong enough before and they still aren't.
 
#64
I hear you with respect to "if they weren't strong enough before...". But most of the issues with cello bridges aren't a matter of the bridge itself, but how it was (mis)treated. On any of the viols you need to pay attention to the bridge attitude when tuning. Failure to do so will result in a tilted bridge and one that will eventually be bent. You could make one that was strong enough not to bend, but it would be at the expense of the responsiveness of the instrument. With most acoustic instruments, the thinner and more fragile the components, the better the instrument will sound. So, in these cases, stronger is not better.

But conventional wisdom is that once the wood has distorted under stress, you can only active temporary fixes by doing heat bending - that the wood will revert to it's distorted state in time. Seems to me from my initial observation that the wood will revert to it's undistorted state given the right conditions. It's not really a question of a permanent fix, but more of a "natural state".
 
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#65
Ah, I think I see. Not well versed in stringed instruments but I can see that the wide area of the foot would act like a lever and put a bending torque on the bridge under string pressure if it ended up tilted. Makes sense. So, sure, your fix might be permanent if they don't knock it over again.
 
#66
These days, I don't work on many Fender necks, but I have one in my shop now. See the thread below about epoxy coating fingerboards. It's a nice 1998 US Jazz neck, top-load truss rod, rosewood fingerboard. It's been defretted and white-lined, and is here for an epoxy coating. And, sure enough, it's got a perfect example of a mild ski jump. Dead flat from the nut to the 12th, a kink, and dead flat from the 14th to the heel. With a full straightedge, the gap at the 13th fret was about 0.040". And it's only 17 years old.

After all the years that they've been building instruments, has anyone at Fender thought about, you know, fixing this problem?
 
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#69
OK folks, i think i just dealt with the exception that proves the rule!

@Zooberwerx's 5-string G&L bass had a mild ramp so he brought it in for me to just plane down the last bit of fretboard; i noticed that rather than a kink at the transition in the back-of-the-neck contour and then straight on to the end of the neck, it actually curved up all the way to the last fret.

i popped the neck off and planed out the ski ramp, also spying a sizable shim right at the end of the pocket; thing is, when i re-installed the neck, a little bit of the ski-ramp returned :eek:

this bass has 6 individual ferrules for its 6 neck screws, covering a good 4" long area under the end of the neck; it became clear that this neck screw arrangement (unlike a vintage fender neck plate) allowed for enough screw torque to actually warp the square body end of the neck if there was uneven clamping pressure in there!

i ended up adding extra shims, sized to support the middle of the neck pocket while actually forcing the last vestiges of the ski ramp out, and that finally did the trick.

who knew Invalid Link Removed
 
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#71
David Jayne said:
Sure, I could see that. With six screws and supporting the heel at each end, the middle pair of screws can pull the center portion down with a hell of a lot of force.
Click to expand...

"Don't know my own strength" - Bullwinkle.

Kinda wonder if removing the center screws would have allowed the heel to return to some degree of normalcy. Is this a situation where a full-pocket shim would have its benefits?

Riis
 
#76
Nenor said:
Just found this thread, thanks for the enlightening information. I'd really like to hear some opinions regarding ski jump on neck-through basses, as this topic is being ignored for whatever reason...
Click to expand...

Good question! Essentially the same as a bolt-on only with a super-loooooong heel. It's safe to assume that the same physics apply in many respects (i.e. compression, cross-section transition, etc). Ric basses are of particular interest...to me anyway...as I've seen a handful of pics where the relatively-stiffer sidewings have separated from the neck heel.

Riis
 
#77
On many neck through's (not Rics though) the transition from body thickness to neck profile spreads over longer portion of the neck. Do you find the ski jump at the same place? Is it at all as common on neck through's as with bolt-on's?
 
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#78
My preference when it comes to a bass that needs a shim is to re-rout the neck pocket. It requires precision since you need to keep the pocket bottom flat while just deepening one end or the other as required. And we are talking about taking out a few thousandths of an inch, so it doesn't really set the neck significantly deeper.

However, we are talking about a solution that costs quite a lot more to undertake than just adding a shim. So I only do it where the value of the instrument warrants it. In most cases a partial shim works well, though even then I prefer to do a full pocket shim. But costs often dictate what method I use - the cheapest is a partial shim, more costly is the full shim, and the re-rout is the most costly. It's up to the owner.
 
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#79
Nenor said:
Just found this thread, thanks for the enlightening information. I'd really like to hear some opinions regarding ski jump on neck-through basses, as this topic is being ignored for whatever reason...
Click to expand...

Yes, the ski jump condition can happen on a neck-thru too, for the exact same reasons, although you don't tend to see it as often. There are several reasons for this:

First, the transition of the back of the neck on a neck-thru is usually more gradual and deeper than it is on a bolt-on. It's transitioning to the full thickness of the body, rather than just the thickness of the neck itself. The wood is a little thicker at that critical area.

Second, most neck-thru basses are 24 fret, which means they have a longer fingerboard. That doesn't affect the ski jump problem much, but a 24 fret neck will usually be fitted with a longer truss rod. The anchor end is further towards the body. If it's a classic single-rod truss rod, this means that the rod is deeper in the neck as it goes through that critical transition zone. So, it's supporting more of the load.

Third, most modern neck-thru basses use double-rod style truss rods. They definitely support more of the load through that transition zone.....IF the truss rod is moderately tight. If the truss rod is loose and flopping around in there, then it isn't supporting anything, and the highly stressed wood may stretch.

So, neck-thru basses have fewer problems with ski jumps, but they still can develop them in some cases. The bad news is that, if a neck thru develops a ski jump, you can't just trim the jump and reset the neck angle. You have to actually un-kink the neck.

Ski jumps (I call them 12th fret kinks) are primarily a problem with bolt-ons, Fenders and others with similar geometry of the heel transition and the truss rod.
 
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#80
Quick summary of what we have found to date:

1. The ski jump forms where the neck cross-section transitions from square at the heel block to rounded on the back of the neck.
2. The cross-sectional area over the transition may be reduced by as much as 25% from square to rounded. This will have an impact on the stiffness of the neck in this area.
3. The condition can not be corrected by adjusting the truss rod. The exception may be with neck-through instruments that have longer than normal truss rods (this exception is a matter of theorizing at this point - we have no measurable data to support the theory).
4. In some cases where more than four screws or bolts are used to attach the neck and there is a partial shim at one end of the neck pocket, some bending through the heel block may occur.
5. The conventional ways to deal with the condition are -
a) Dress down the upper frets to put them on the same plane as the rest of the fretboard
b) Remove the upper frets, flatten the fretboard in the upper end, re-fret, level and crown the frets.
c) In the case of a neck with a shim and more than 4 anchoring screws, some support at the middle screws may be necessary. Alternatively a full pocket shim or the re-routing of the neck pocket would be effective.
6. Heat treatment to bend the "kink" back to straight may be an option, but it is unproven in our collective experience.
7. Neck-through basses can develop the "kink" though it occurs less often. There are fewer options for treating a neck-thru with this condition.

I've tried to avoid "opinions" in this summary and stick to the facts we can verify. For those who are the participating Pro's in this forum, please correct anything I have put forward in this summary if you feel it does not accurately represent our findings to date.
 
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