Here are some thoughts I have about neck dive:
- It's as much about the bridge end of the bass being pulled up as the neck being pulled down.
- Think of your left shoulder as a pulley and the strap as the cable over the pulley. The weight on the left side of the strap pulls it down, which pulls up the right side of the strap. The right side of the strap pulls up on the body of the bass, and the neck rotates downward.
- The strap button closest to being vertically in line with the center of mass (center of gravity) carries most of the weight of the bass. On my Jazz, the center of mass is around the 19th or 20th fret, so the left strap button carries most of the weight. The right strap button is farther away from the center of mass has less weight pulling it down.
- On many basses, if you tilt the neck of the bass up, the left strap button moves closer to being in line with the center of mass (on my Jazz, it moves closer to being directly over the 19th fret). This causes more of the weight of the bass to pull down on the left side of the strap. If you move the bass neck to horizontal, there is less weight pulling down on the left side of the strap.
To stop neck dive, you can:
1 - Equalize the weight pulling on each side of the strap.
You can do this by moving the left strap button farther away from the center of mass (longer top horn, connect strap to headstock). You can move the center of mass closer to the right strap button and farther from the left strap button (lighter tuning keys, a heavier body, a heavier bridge). Or, you can move the right strap button closer the center of mass (move the right strap button closer to the center of the body).
2. - Have some force to keep the weight on the left side of the strap from pulling up the right side of the strap.
You can do this by wearing a wide strap that grips your shoulder. There is also
this solution.
I use a wide suede strap to tame the neck dive on my Jazz, but I find the pulling on my shoulder can make it pretty sore after a couple hours.
There is often a trade-off between getting a light bass and a balanced bass. A heavier body will move the center of mass closer to the right strap button, and therefore hold down the body side of the bass. But, this would make the bass heavier as a whole. A lighter body would make the bass lighter overall, but also make it easier for the weight of the neck to pull the body up. This can cause pulling on your shoulder from the strap trying to slide over it.