Changer Finger Design Parameters?

[Tim Toberer]

Some of this applies to all different styles of changers, but I am mostly interested in the Pull Release style. The single finger design simplifies the geometry a lot, but I am still a little confused. I will do my best to explain! Sorry if this is confusing

This thread shows 4 different changer fingers. viewtopic.php?t=406875

The top of the finger where the string attaches to the finger is the main area I am looking at. Most fingers are designed with a concentric circle around an axle. The Blanton appears to have the knife edge pivot in the center, and the BMI 1/8 axle is also in the center. The Carter Starter "knife edge" fingers have the "axle-knife edge" set further in, which in my mind would actually raise the strings slightly when the finger was pulled and lower it slightly when the finger was released if I am thinking correctly. This would seem to increase the action on the string causing more of a tone change compared to if the axle was located in the center. Mimicking a larger radius. The Fender is even more unusual geometry, because it is designed with a much larger radius on top. Hard to tell if the Knife edge is located in the center.

My question is how would you increase (or decrease) the action on the string, given the same movement of the finger? I am trying increase the range of tone on the given string, but working in a limited space so the geometry of this seems critical. I am looking to get as much change in pitch with the least movement of the mechanism. If the axle were located in the center would a larger radius (further from the axle) increase or decrease the action? It seems like it should increase it, but I just can't feel sure.

Things I have found that can increase the range of tone (same movement, greater effect) on a pull release.

1 Attach pull rod (or lower rod) higher on the bellcrank (further from the axle)
2 Attach the pedal rod closer the bellcrank shaft (closer to axle)
3 Attach the pull rods closer to the changer finger axle (closer)
4 Attach pedal rods further away from the pedal axle (further)
5 Use larger gauge strings (within reason)
6 Increase scale length

[Ian Worley]

Yes, the larger the outer radius, that is, the farther the string is from the pivot, the farther the string will move for a given amount of axial rotation. However, the larger the radius of the outer finger surface, the flatter it becomes at the apex where the string exits. When the radius is too large and the apex too flat, the string will begin to contact the that surface away from the apex as it vibrates, causing an undesirable sitar-ish sound. This actually happens to some degree at any radius, but when it is small the effect on what we hear is less perceptible or problematic. That's where the other leverage options you mention come into the equation to get the necessary pitch changes.

Somewhere buried in the archives here on the forum is a post from Buddy Emmons discussing how, through some basic trial and error, they arrived at what they felt was the optimal finger radius for the Emmons guitars back in the early '60s. What they come up with as optimal was about R=3/8", or 3/4" diameter, which is similar for most modern PSGs, ~3/4" - 13/16". The radius on the '60s Fenders is a little more, I just measured again (unscientifically), the actual finger surface is ~1/2" radius, but the center of the pivot is only ~7/16" from the apex, so not concentric. I don't know what the radius is on Ross Shafer's Sierras, but I seem to recall they are slightly larger than typical radius too.

It's worth noting also that the most common PSG bar diameter is 7/8", some players use 15/16" or even 1" diameter bars. The round bar interacts with the string in pretty much the same way as the curve of the changer finger, so you could use that as a guide also.