Can anyone explain the Zum ACS block operation?

[Mark McCornack]

Greetings,
I understand that many of the later ZumSteel guitars have a mechanism for mitigating cabinet drop which Bruce Zumsteg calls an "ACS block". I've seen some superficial discussion on the forum of its effectiveness, but nothing that explains how these actually function.
I was hoping that someone on the forum that is familiar with the details of how this works might be willing to take the time to respond to this post and give a technical explanation. I have a picture (below) that I copped of another forum post, but how it works is not self evident to me from this image. Anybody know the low down and willing to share ?
Mark

[Ross Shafer]

I'd have to see it up close to be sure, but it looks like the set screws are positioned to bear against those bent tabs to limit axle flex. Kinda like the non-adjustable doo- hicky found on MSA Claassic guitars.

just a semi-educated guess

[Mark McCornack]

Well Ross, I'd say your "semi-educated" guess is considerably more erudite than one I could hope to make.
I guess that makes sense to beef up support of that axle centrally as well as at the ends. I've often wondered about the discrepancy in the choice of axle diameter used by different manufacturers. ShoBud vs Emmons for example. Emmons is a good bit wider than the Bud, and I think the stiffness of the axles is largely proportional to the cube of their diameters. You'd think fat bars would have a real advantage there. I don't know which camp the Zum fits in regarding axle diameter though.

[Dennis Detweiler]

I have two MSA U-12 Classics. Both have that similar do-dad. When I re-assembled both guitars, I pried the block against the axle as I tightened the mounting screws. I don't know how effective it is, but I have about 1 hz (439) of cabinet drop. Hardly audible, but it shows on the meter.

[Dan Ray]

I may be mistaken, but I think I've seen a similar "device" on some all pull emmons? Kinda looks like an extra piece of metal and is only the the E9 neck.

[Jack Stoner]

"Axle flex" is where Paul Franklin Sr told me he thinks most of the "cabinet drop" is.

I had a GFI Ultra D-10 with braces so the axle couldn't flex and it had basically zero cabinet drop.

The Emmons Legrande III has something similar to the Zum to minimize cabinet drop.

[Dennis Detweiler]

I still believe some comes from axle flex, but ShoBud changers had casting between each pull finger and still had cabinet drop. When I purchased my first single 12 guitar (1976) I sat down at a single 10 Shobud and a single 10 Push/Pull Emmons just to get a feel for the mechanics of them. The Emmons had very audible cabinet drop. The Shobud had very little. However, I ended up buying a 1976 birdseye maple 12 string MSA. Still have it.

[Ron Hogan]

"Axle flex" is where Paul Franklin Sr told me he thinks most of the "cabinet drop" is.

I had a GFI Ultra D-10 with braces so the axle couldn't flex and it had basically zero cabinet drop.

The Emmons Legrande III has something similar to the Zum to minimize cabinet drop.

Do all the GFI's have that bracing?

[Jack Stoner]

Do all the GFI's have that bracing?

I've had an S-10 Expo and a D-10 Ultra and both have, and actually looks like same changer. Pictures on the GFI web site show same changer for both Ultra and Expo models.

The student model does not.

[Jim Palenscar]

I believe that the set screws in the device that Bruce put on there touch against the raise scissors on the string(s) in question and cause it to ever-so-slightly push the axle back as the raise is at its furthest point.

[Henry Matthews]

Cabinet drop was almost non-existent except to a very select few until the onset of electronic tuners.

[Fred Justice]

Amen Henry, Son I'm going to get your preachen papers next week.
Truer words were never spoken.
folks you think Buddy had a elec. tuner when he cut the black album? and several others

[Chris Lucker]

I do not subscribe to the "axle flex" theory.

Turn a piece of 0.625" hardened drill rod to a 0.5625" sized axle and you do not need tail stock support to guard against side pressure from either a cutting tool or tool post grinder.

If the material flexed, you would machine a taper, but it does not flex. Maybe it would flex if your lathe is fitted with an electronic tuner.

I would look at heavy footed downward pedal rod pressure pulling on the front apron as the culprit. Maybe having the most used E9 pedals more in the middle of the pedal rack with Day setup is worse than Emmons setup in terms of cabinet drop?
An adjustable little third front leg located in the center of the pedal rack may solve the problem?

On old Marlen pull-release guitars, the "teeter-totter" stops on the center pf the cross shaft was a disaster. I don't know why Stadler did it. A heavy foot would simply force the teeter-totter stop to flex the cross shaft away from the cabinet and continue to RAISE the string pitch past the intended target..

[Dennis Detweiler]

I think Chris is on target. Try putting some downward pressure with your thumb on the top center of your guitar and watch the tuner. Maybe two truss rods from the front center of the guitar to the bottom of each end plate would help?

[Chris Lucker]

I . Maybe two truss rods from the front center of the guitar to the bottom of each end plate would help?

Here is what Jerry Blanton did. He had a pair of tension rods from the changer to the keyhead end of the D10 cabinet. these also stabilized expansion and contraction of the guitar.
I don't know if Jerry did this truss rod type thing with all his Blantons, but the feature is on my Blanton. It is a 1964 -- the first one Jerry made. Jerry sent it to me around 2008 after I refused to pay the little amount he was asking for a bucket of Bigsby partisans paid him what the parts were really worth. See, it pays to be honest and Jerry was generous as a result.
The client Jerry made Blanton #1 for in 1964 traded it in on a new model and Jerry played it himself for a few years. Jerry did shorten the scale of the E9 neck (25" to 24 or 24.25" I forgot which).The guitar has solid nuts -- no rollers. Jerry made the guitar an SD10. I have the original parts for the C6 neck, but the two scale lengths will be different.

You can see one of the two remaining truss rods on the underside of the E9 neck here:

[Jack Stoner]

Mr Franklin's theory on axle flex, he says comes from the investigation and testing that he and Duane Marrs did during their days at Sho-Bud.

[Bill Lowe]

Does the Zum acs block work as intended?How about Emmons LG3? Just curios….

[Dennis Detweiler]

If it's the axle, you'd think the guitar would de-tune sharp if lowering the E's or lowering E's and B's at the same time? That's, if it has anything to do with string tension? I did a side by side cabinet drop comparison years ago between a single 10 push/pull and a ShoBud using A and B pedals. The Emmons was worse. The push/pull system didn't seem to help in that regard. I still lean toward downward pressure with pedals as the root cause.

[John Hyland]

I suspect every instrument is different and multiple issues could be at Play and this is why a definitive answer is elusive.
Probably it is a combination of body flex , axle flex and bridge parts movements. But i believe in most cases it is the string tension change that is the problem, although If pushing the pedals after they have reached the stop causes the drop there is a serious body flex issue and is nothing to do with string tension.

Pushing down on the centre of the body is not a good test as it doesn’t replicate a playing condition.

Maybe those with a drop problem could replicate the test above to see if the pedal pressure is the problem. Ie do you get the drop before or after the pedal reaches its stop.

[Jack Stoner]

As has been previously stated I didn't know there was cabinet drop util it was brought up here on the forum.
My Franklin had noticeable on a tuner cabinet drop on string 6 (G#) but it was never an issue playing, either recording where tuning or playing in tune is more critical or in a band.

[Dennis Detweiler]

Try this: unscrew the tuning nuts on pedals A and B and press down on those pedals and watch the tuner. If it shows on the tuner, it's the downward cabinet pressure from the pedals. If the tuner doesn't move, it's more likely the axle.

[Chris Lucker]

I should have added above, where I mentioned my experience machining 9/16ths hardened drill rod with no deflection, I cannot imagine finger pointing to the very short length of drill rod, supported at both ends flexing.

Tuning posts at the other end of the guitar are far skinnier and are typically supported only at the base.

Cross shafts are not hardened drill rod and they are typically skinnier than changer axles and span a longer distance between supports than a changer axle, even when they have a center support on a D10, for example. How can hardened drill rod be the focus and not the less hardened cross shaft?

I believe the day tuning posts and cross shafts are made of 9/16ths hardened drill rod, the culprit will be revealed as the downward pressure of the pedal rods on the pedal rack pulling down the front apron.

[Tucker Jackson]

If it's the axle, you'd think the guitar would de-tune sharp if lowering the E's or lowering E's and B's at the same time? That's, if it has anything to do with string tension?

Absolutely. My guitars all go sharp when lowering a string. Lower the the E's and the 6th string (especially) goes sharp by 4 cents in a typical case. That doesn't seem like it could be cabinet drop, or 'cabinet raise' I guess -- there's no downward pressure when using a knee lever and I doubt the cabinet can 'decompress' from end-to-end to introduce that sharpness to the strings. But the axle does have slightly less pressure on it, both downward and east to west...

I measured one well-known student guitar and the 6th string shot up by a full 6 cents (and it was a wound string)! It's only a big deal if you don't 'tune around' it. Unfortunately, most new players just use a chart and don't make ear-tweaks to intervals that are out. And I know of no charts that have that particular assumption built in that says "the 6th string is going to go maybe 4 cents sharp when lowering another string." Meaning, their G#m chord is always out. Maybe WAY out if their roots and fifths really do land 6 cents off from what was assumed by the chart writer.

I don't think this "Either/Or" debate is an accurate framing of the issue.

Clearly, detuning can be different things on different guitars. A little cab drop on some. Some axle flex on others, and often it's a combination that adds up to something meaningful. And then a few have no detuning to speak of.

An S-12 guitar will usually have more detuning than the same brand/model of the S-10 and D-10 version (the additional detuning introduced over the other models by the longer axle points to axle flex. Meanwhile, if the wider cab of the S-12 over the S-10 was the cause, then the D-10 would have the most detuning of all -- but it doesn't. The one with the most detuning is the one with the longest axle).

Some of the tests to determine how much is being contributed by one or the other have been mentioned: test the cabinet by pressing down on it in the middle and see if the strings go flat.

Then raise a string by pushing on a raise finger with a screwdriver at the changer end. If strings go flat, that's probably axle flex (or loose changer mounting ... or something with the fingers unrelated to a change in the actual cabinet).

[Bill Lowe]

I think the question was how does it work and not where it comes from. No matter where it comes from do these devices on the Zum and Emmmons help the issue? It’s my understanding they work fairly well for their intended purpose. Its another question if you need or want it.

[Dennis Detweiler]

Another factor that I don't think mentioned: The tension of the 10 or 12 or 20 strings above the body of the guitar has to be accounting for some minute bowing of the body? Like an archery bow. Then raise a few strings causing more bowing or lowering a few with a knee lever relaxing the bowing? Another factor into the mix?
Loosen a string at the key head and see if the guitar goes sharp? Loosen the 1st string which would cause less stress on the axle. Then loosen the 6th string and see if it gets much worse from the center of the axle?
After all of it, what about the ShoBuds that had a casting holding the axle and the casting was also between each finger which should have zero flex? But they also had cabinet drop???