Hi,
Building my first guitar necks (vintage telecaster style) and i'd like them to have single action vintage truss rods. I've researched a bit on the truss rod channel channel and curve and found Bruce's posts here. My Luthier also referred me to Bruce's posts here when I asked him about truss rod curve
The one thing I'm not quite understanding is when we say Truss Rod "droop" of 0.180" and fenders and gibson typically have around 0.12" - 0.15" droop what exactly is that referring to? If I look on typical tele neck plans with the curve, it looks like the deepest part of the curve is genenerally 5/16 - .3125" below the truss rod ends which isn't in the ballpark of what bruce is saying. As I attempt to consider radius or the ratio of length of truss rod to the lowest point, none of the numbers I come up with seem to map onto this measurement range Bruce refers to called droop.
Could someone help me understand what specific measurements yield/determine truss rod droop?
Sure, I'm here. I'll step in and answer that.
The Droop measurement I'm referring to is the distance that the centerline of the truss rod is deflected downward in the middle from the centerline at the two ends. That's how much the rod itself needs to be curved for it to function well mechanically. The Droop determines the mechanical ratio that the rod has; how many pounds of force it can push on the neck, in relation to the torque of tightening the nut.
From my many years of experience with truss rods, I've found that 0.180" is the best amount of Droop to design into a Single Rod style truss rod. That gives it a good amount of power to bend necks, without overloading the 10-32 threads on the rod. Going more than 0.180" makes the truss rod too sensitive. The neck moves too much with small angular turns of the nut, making it more difficult to dial in the relief. Surprisingly, that droop number of 0.180" seems to be the same for all lengths of necks and lengths of truss rods.
When the truss rod is installed in the neck, the rod itself is all below the fingerboard glue line. To figure out the depth of the deepest part of the curved slot in the middle, you have to add up the depth of the centerline at the adjusting end, plus the Droop, plus half the diameter of the rod. Because the bottom side of the rod goes below the centerline of the rod. Those three things added together give you the total depth of the slot in the middle.
At the adjusting end, you have to set or measure how deep the centerline of the rod is below glue line. On most Fenders, it's about 0.100". The Fender barrel nut is cut up into the underside of the fingerboard about 0.080". This applies to both the headstock and heel adjusting versions.
At the anchor end, in most Fenders, the top surface of the rod is almost up at the glue line. The centerline is also about 0.100" below the glue line.
So, on most Fenders, the math is: 0.100" (centerline below glue line) + 0.120" Droop + 0.093" (half the rod diameter) = 0.313" (the depth of the slot in the center). That's how most classic US Fenders are designed. In my opinion, that's not enough Droop, and that's a big reason why so many Fender necks have truss rod failures.
I've seen some newer model US Fenders and Mexican Fenders with 0.160" Droop. I also had to replace the truss rod on a Japanese Fender Lyte bass neck, and was surprised to find that it had 0.220" of Droop. I think that's the deepest Droop I've seen in a production bass neck. But most Fenders, particularly the older ones, around 0.120".
On new necks I build, I deliberately set the truss rods down deeper into the neck, plus I use 0.180" of droop. The deepest part of the slot ends up at 0.560" below the glue line. The center of the rod is almost 1/4" lower in the neck. I do that for a combination of reasons.
If you are making your guitar necks basically classic Tele style, I recommend that you increase the depth of the slot from 0.312" to about 0.372", about 3/8". That doesn't sound like much, but it makes a big difference in the function of the truss rod. Fender should do that on all their necks.
Edit: Okay, here's the 36th post where I've described Truss Rod Droop.
This link to the search might work too: 
