Pianotech

Hi James,

That's a common issue our shop deals with quite often, there are several ways measure action ratio, so different techs arrive at different numbers. That makes it difficult or impossible to discuss AR numbers without a common frame of reference. 

Your first formula, AR = (SD-LO)/(KD-AT) is the easiest to discuss since it's objective as the components can be directly measured. And everyone pretty much agrees how to measure them.  Since our shop builds keys and actions, we ask the customer what SD (strike distnace), LO (let off), Key dip (KD) and Aftertouch (AT) they want, and we build against those numbers.

The second formula, while it will give a correct AR number, doesn't reveal the very important ratios in the way you wrote it. A better form of the expression would be AR = KR x WR x SR where the ratios are all Input/Output.  Knowing what the KR especially is, separately is valuable. The KR should be quite close to 0.50 for optimum results.

It should be noted that the most varied measurement is the key ratio. The KR can be measured linearly across the bottom of the key, or radially. And different rebuilders and sources will recommend measuring the key input from the front bottom front of the key, top front, or top front anywhere rom 7 mm to 13 mm from the front.

Obriously, unless the two techs discussing "Action Ratio" agree on how to calculate, taking about AR numbers is worse than useless, it can lead to all kinds of misunderstandings and disagreements.

That being said, our shop measures KR, SR, and WR radially, with key ratio to the top front of the key, not counting the overhang. We find this results in the same action ratio as AR = (SD-LO) / (KD-AT) - the direct measurement method. At least the two method are the same within +/- about 1 or 2%.

The above two methods are similar or the same as used by Renner USA and Nick Gravagne in his PTG articles. These methods measure the AR statically, at the very start of action movement. Typical numbers for this method are 4.7 to 5.2, with the best range usually 4.9 to 5,0.

The third method you mention, Dale Erwin's device gives a higher Action Ratio. The reason is that the AR changes as you press the key down, increasing gradually. The Erwin device is handy, and gives some useful data, but only if you compare to another measurement with the same device.

Whenever I have a discussion on Action Ratios, I ask how the tech is measuring. Then I ask what his number is. If the number is about 5.5 or higher, I know they are not likely measuring it the same way our shop does, but measuring dynamically, later in the action movement, or just using different parameter or methods.

I hope that helps,

Best regards,

-Dean

Hi James,

That's a common issue our shop deals with quite often, there are several ways measure action ratio, so different techs arrive at different numbers. That makes it difficult or impossible to discuss AR numbers without a common frame of reference. 

Your first formula, AR = (SD-LO)/(KD-AT) is the easiest to discuss since it's objective as the components can be directly measured. And everyone pretty much agrees how to measure them.  Since our shop builds keys and actions, we ask the customer what SD (strike distnace), LO (let off), Key dip (KD) and Aftertouch (AT) they want, and we build against those numbers.

The second formula, while it will give a correct AR number, doesn't reveal the very important ratios in the way you wrote it. A better form of the expression would be AR = KR x WR x SR where the ratios are all Input/Output.  Knowing what the KR especially is, separately is valuable. The KR should be quite close to 0.50 for optimum results.

It should be noted that the most varied measurement is the key ratio. The KR can be measured linearly across the bottom of the key, or radially. And different rebuilders and sources will recommend measuring the key input from the front bottom front of the key, top front, or top front anywhere rom 7 mm to 13 mm from the front.

Obriously, unless the two techs discussing "Action Ratio" agree on how to calculate, taking about AR numbers is worse than useless, it can lead to all kinds of misunderstandings and disagreements.

That being said, our shop measures KR, SR, and WR radially, with key ratio to the top front of the key, not counting the overhang. We find this results in the same action ratio as AR = (SD-LO) / (KD-AT) - the direct measurement method. At least the two method are the same within +/- about 1 or 2%.

The above two methods are similar or the same as used by Renner USA and Nick Gravagne in his PTG articles. These methods measure the AR statically, at the very start of action movement. Typical numbers for this method are 4.7 to 5.2, with the best range usually 4.9 to 5,0.

The third method you mention, Dale Erwin's device gives a higher Action Ratio. The reason is that the AR changes as you press the key down, increasing gradually. The Erwin device is handy, and gives some useful data, but only if you compare to another measurement with the same device.

Whenever I have a discussion on Action Ratios, I ask how the tech is measuring. Then I ask what his number is. If the number is about 5.5 or higher, I know they are not likely measuring it the same way our shop does, but measuring dynamically, later in the action movement, or just using different parameter or methods.

I hope that helps,

Best regards,

-Dean

------------------------------
Dean Reyburn, RPT
Reyburn Pianoworks
Reyburn CyberTuner
1-616-498-9854
dean@reyburn.com
www.reyburnpianoworks.com
www.cybertuner.com
www.reyburntools.com
Facebook: www.facebook.com/dean.reyburn
------------------------------

Hi, Dean and colleagues. 

Very good and necessary discussion IMHO. Thanks to James for getting it started. 

Part of the difficulty in communicating technical components is getting mired in acronyms and nomenclature. Should they not be prefaced? I often don't see where acronyms are defined before they are used wholesale. For example, in your reply, you site KR, SR, and WR but never actually explain what they mean. We can only assume. For those who work with these factors all the time, it may be elementary, but it's hard to follow a discussion when one does not understand the terms. It is not my intent to criticize, only to bring this into focus as part of this discussion. KR = key ratio. SR (Someone Rolfing)? WR (Wide Receiver)? As for myself, I recognize many of the more often used acronyms - DW, UW, BW, SW KR, AR, MOI. But not SR or WR. This should probably be the starting point. Could this refining of protocols not include a glossary of related terms and acronyms as well? TIWMDMPC. 

Best regards,

Dave

------------------------------
Dave Conte, RPT

Piano Technician in Residence
The University of Tennessee
College of Music
Knoxville TN
(817) 307-5656
Owner: Rocky Top Piano
------------------------------


Original Message:
Sent: 09-12-2025 03:23
From: Dean Reyburn
Subject: Measuring & discussing Action Ratios

Hi James,

That's a common issue our shop deals with quite often, there are several ways measure action ratio, so different techs arrive at different numbers. That makes it difficult or impossible to discuss AR numbers without a common frame of reference. 

Your first formula, AR = (SD-LO)/(KD-AT) is the easiest to discuss since it's objective as the components can be directly measured. And everyone pretty much agrees how to measure them.  Since our shop builds keys and actions, we ask the customer what SD (strike distnace), LO (let off), Key dip (KD) and Aftertouch (AT) they want, and we build against those numbers.

The second formula, while it will give a correct AR number, doesn't reveal the very important ratios in the way you wrote it. A better form of the expression would be AR = KR x WR x SR where the ratios are all Input/Output.  Knowing what the KR especially is, separately is valuable. The KR should be quite close to 0.50 for optimum results.

It should be noted that the most varied measurement is the key ratio. The KR can be measured linearly across the bottom of the key, or radially. And different rebuilders and sources will recommend measuring the key input from the front bottom front of the key, top front, or top front anywhere rom 7 mm to 13 mm from the front.

Obriously, unless the two techs discussing "Action Ratio" agree on how to calculate, taking about AR numbers is worse than useless, it can lead to all kinds of misunderstandings and disagreements.

That being said, our shop measures KR, SR, and WR radially, with key ratio to the top front of the key, not counting the overhang. We find this results in the same action ratio as AR = (SD-LO) / (KD-AT) - the direct measurement method. At least the two method are the same within +/- about 1 or 2%.

The above two methods are similar or the same as used by Renner USA and Nick Gravagne in his PTG articles. These methods measure the AR statically, at the very start of action movement. Typical numbers for this method are 4.7 to 5.2, with the best range usually 4.9 to 5,0.

The third method you mention, Dale Erwin's device gives a higher Action Ratio. The reason is that the AR changes as you press the key down, increasing gradually. The Erwin device is handy, and gives some useful data, but only if you compare to another measurement with the same device.

Whenever I have a discussion on Action Ratios, I ask how the tech is measuring. Then I ask what his number is. If the number is about 5.5 or higher, I know they are not likely measuring it the same way our shop does, but measuring dynamically, later in the action movement, or just using different parameter or methods.

I hope that helps,

Best regards,

-Dean

------------------------------
Dean Reyburn, RPT
Reyburn Pianoworks
Reyburn CyberTuner
1-616-498-9854
dean@reyburn.com
www.reyburnpianoworks.com
www.cybertuner.com
www.reyburntools.com
Facebook: www.facebook.com/dean.reyburn

Unless I'm seeing the math wrong, based on Mario Igrec's equation, Action Ratio decreases as the the key depresses.  Adding in the cosign angle between the jack and knuckle, the efficiency of the angle increases as it approaches keydip.  His example of 29 degrees decreases to close to 0 at letoff.  Cosign efficiency says that will increase the effective multiplier in that equation SRxWRxKRx1/cos.  

KWYADAWYADI

Peter Grey Piano Doctor