bell crank dimensions

[Tim Toberer]

What is the minimum height above the axle center I will need in order to have the necessary travel for raising or lowering a string 2 semitones? I realize this is a tough question to answer exactly, but ballpark. Space is a serious restriction in the steel guitar I am working on it is very shallow. The ones I made that fit are only about 1 1/8" above the axle center. I guess I may just have to try it and see. I am just looking for insight. I am also curious if 5/16" round bell crank shafts would be rigid enough? The guitar is a short scale 23" so it takes less tension than a long scale to raise a string. I already fabricated a bunch of parts, but now that it is going in there, I am realizing I may not have enough space.

[Ian Worley]

What is the minimum height above the axle center I will need in order to have the necessary travel for raising or lowering a string 2 semitones? I realize this is a tough question to answer exactly, but ballpark. Space is a serious restriction in the steel guitar I am working on it is very shallow. The ones I made that fit are only about 1 1/8" above the axle center. I guess I may just have to try it and see. I am just looking for insight. I am also curious if 5/16" round bell crank shafts would be rigid enough? The guitar is a short scale 23" so it takes less tension than a long scale to raise a string. I already fabricated a bunch of parts, but now that it is going in there, I am realizing I may not have enough space.

There's no precise answer to your question. It's important to think about all of this as a system. Within the context of the system, the behavior of individual components is dependent on all the other components in the system. A 10" long bell crank pulling a 10" long changer finger with connection points the same relative to the axles is pulling at 1:1 ratio, the same as if both components were only 1/2" long (in the real world there are other factors such as friction and mass that would alter the effect but...). There are multiple points of leverage in a typical PSG that affect energy transfer between you and the string - the pedal or lever length, the pedal rod connection point relative to the pedal axle, the length of the pedal crank on the cross shaft relative to the cross shaft, the length and connection point of the bell crank, the length and connection point of the finger. In an all-pull changer there are additional leverage factors that affect how far the the finger radius rotates relative to the linear distance the pull rod moves the scissor. Some designs like the old ZBs, Fender cable guitars, the infamous PS210, Moyo, don't even have cross shafts and bell cranks and use different leverage schemes to transfer the energy.

The simple point is, you need to consider the entire system. Often the best way to do that when designing from scratch is to build a 1:1 functional mock-up, a working model to test how the different interactions work together to achieve the desired result. That would include the cross shaft diameter and length. 5/16" is pretty standard in a lot of older guitars, many builders added center supports on D10s and wider bodies to reduce shaft flexing. Consider also that the guitar body itself may introduce some movement/flex.

Here are some different bell cranks I have laying around from various Emmons and Sho-Bud guitars plus a couple of homemade parts. As you can see there is quite a difference in length, but all work fine within the larger overall system they are designed for. Their max leverage points range from about 7/8" center-to-center on a PP to about 1 3/4" on a LeGrande. For the raises on an early Pro series Bud it was less than 5/8".

[Tim Toberer]

Very useful to see them all lined up. Thinking holistically about the instrument is great advice. I was applying the dimensions from my plan, which is of a more standard guitar, and it just won't work that way. I am gonna have to think outside the box a bit for this one I guess. The 1:1 mock up is a good idea. I may try to just add a couple pedals to start, with the idea that I can add more later if all goes well. Thanks again!

[Bobby D. Jones]

To handle the B to D change on a GFI U 12 it takes a 7 hole bell crank. The guitar has a 24" scale.

This may help in your calculations.

[J D Sauser]

In All-Pull changers, it's all relative to the pivot or actuation point leverages between the pull scissors and also the changer finger (reader).

... J-D.