I expected Bruce Johnson to clear this up by now, but he either is sitting back and laughing at us, or he is busy making Pythagorean Bruce Bolts to prove his point. I'll wait.
Ha! I'm here, and I've been reading through this thread. I've been planning on joining in this discussion, but hadn't gotten to it yet. Unfortunately, I'm going into the hospital next week for heart surgery, so my time is kind of scrambled up right now....
Here you go:
The reason why we add extra bolts and reinforcements to the neck/body joint isn't to increase
strength. The Fender-style joint with four wood screws is certainly
strong enough. Very rarely does anyone break the neck off of a Fender bass. That's not the issue here.
We add more screws, bolts, metal threaded inserts, internal bars, thicker neck plates, etc., to increase the
stiffness of the joint.
Strength is how much load you can apply before it breaks and explodes.
Stiffness is how much it moves and springs back when you apply a smaller load, and then remove it. Way below the level of breaking it. High stiffness means there's hardly any bending or movement when you apply the load. Low stiffness means it bends more.
The frame of a bass is like an archery bow. It's a wooden frame with a steel string stretched from one end to the other. As you tighten up the string, bringing it up to tune, the wood frame flexes while taking the load. The two ends bend forward and toward each other. That's why the strings lift up off the fingerboard as you bring them up to tune. The wood frame is pulling into its loaded shape.
When you pluck the string and release it, it cycles back and forth as it rings out. The wood frame reacts to that changing load, bending a tiny bit forward and back as the string swings forward and back. Its movement lags the movement of the string, and fights against it. Slows it down. Quiets down the movement and the note. It changes the sound ringing on the string.
How this microscopic flexing of the wood frame changes the sound on the string gets real complicated. A long discussion by itself. Many factors are involved, but they are all related to the
stiffness of the frame. The overall stiffness of the whole thing, and the stiffness of the parts, and the stiffness of the joints between the parts.
As an overall picture, the
stiffer the frame, the less the frame moves under the cycling load, and the less it affects the sound on the string. What you hear is mostly the string's own natural sound. On a bass with a really stiff frame, what the pickup "hears" is mostly created by the string itself. Clear, wide range, cold, a steel sound.
On a bass with a
softer (less stiff) frame, the frame flexes more and it modifies the sound ringing on the string. It can reduce the range, cut down some frequencies, add extra spike frequencies (warmth) in the background, and other things. And this is the sound ringing on the string, which the pickup detects and modifies some more, and sends on down the line to the amp.
And that's a big part of what we Luthiers do: messing around with the stiffness of the frame, to change the sound of the bass. To increase or reduce parts of the sound.
Back to the neck/body joint in a bolt-on style bass. Obviously, the neck/body joint is right in the middle of the frame, so it's a key part in the overall stiffness of the frame. It primarily affects the low end frequencies. If the joint is softer, it quiets down the lowest frequencies on the string and makes the low end warmer and muddier. Less clarity definition and sustain. Fender P-bass. Making the joint stiffer lets the string ring out more. More clarity and definition of the deep lows. More bottom keys of the piano.
Now, the stiffness of the neck/body joint is not the
only thing that affects the bottom end clarity. The stiffness of the neck plays an equally big part. That's why we also play around with truss rods and various reinforcements of the neck. They have to work together. If you want to increase the bottom end clarity,
both the neck and the neck/body joint (and some other things) have to be stiffened up. If one of them is soft, then the whole bass frame is soft overall.
Stiffening up the frame to change the sound means doing all the things. A list of small details. Softening the frame to soften and warm up the tone only requires one thing to be soft.
Anyway, there are many different ways to stiffen up the neck/body joint. One way is to increase the clamping force between the two wood parts by using stronger hardware and tightening it tighter. That's what I do with my brass bars (the infamous BruceBars) in the neck, combined with machine screws and a thicker neck plate. Plenty of surface area and mechanical fastener strength, so I can really clamp down on the wood.
Once again, it's not about increasing the force needed to rip the neck off. It's about increasing the stiffness, so it barely deflects when a mild force is applied.
Machine screws and brass inserts are another way to increase the amount of clamping force you can apply. Another way to increase the stiffness of the joint is to extend the length of the joint. Make the pocket longer, with more screws. Ibanez joints with that extra tang beyond the end of the fingerboard. The MusicMan neck joints. Same reason, to stiffen it up and make the deep bottom clearer. The pickups and preamp aren't the only reason why a MusicMan sounds different from a P-bass.
Yamaha added an extra twist to their neck joint design. They stiffened it up with long length and strong inserts, and then added those two angled bolts at the back. Their purpose is to increase the clamping force horizontally, between the heel of the neck and the back of the neck pocket. All for the same reason: to increase the stiffness of the joint. And, working together with other design features, to make the deep bottom clearer and more defined.
To make it sound
better than a P-bass. Is it
necessary? Well, if you think that the P-bass sound is all anyone would ever need, and all basses
should be made to sound like P-basses, then no, this extra neck joint feature isn't
necessary to make the Yamaha sound like a P-bass. They added the feature as an improvement, among other design features. You can judge whether you think the Yamaha sounds better than a P-bass.
Extra strength to resist Billy Sheehan, and the cool look of the angled bolts are added advertising bonuses.
