Method and Style:
I recommend the pick-and-add method of weighoff. Key balancing weights are used in pairs, upweight on the bottom, pairing weight on top. Add a pair (downweight) to engage the key and note the speed of hammer rise. Pick the pairing weight off and note the speed of hammer return. Choose downweights and upweights that work well with the action and match speeds by trial-and-error placement of one or more key weights on top of the key (to be drilled for and installed in the side of the key later).
Traditionally, the trade used downweight-only as a method of weighoff. This method had the weakness of obscuring inconsistencies in action friction, which translates to inconsistencies in upweight (arguably more important than downweight for repetition and playability). Center frictions manufactured to be in a playable tolerance produced reasonable results with this system, but the wiggle room in acceptable (profitable) tolerance meant that a predetermined pattern of key weights (strategically cooperating with the center friction tolerance) could produce results that were comparable to the best the downweight method could achieve but with less cost and again more profit.
In the ‘70’s and 80’s, when I was starting out, some technicians (including myself) felt they could do better than the downweight-only method of weighoff. I would choose a series of upweights, starting with the least that was viable in the bass (20g was my minimum) and rising to the most I could get away with in the high treble (generally 25g – other specs being optimized). With sufficient friction in the system, I still find this to be the useful upweight range (although my 20-25g might be someone else’s 21-26g or even 22-27g as I use quite a positive speed of key/hammer response with no action bumping).
I would then select a series of downweights ranging in grams from mid-50’s to mid-40’s, and I would plot out an arbitrary pattern to switch in lighter downweights and heavier upweights as I proceeded from bass to treble. I applied first downweight then upweight and tried key weights on the keys until down and up speeds matched. This worked well (and was more precise than the traditional approach) but contributions from two people significantly improved both result continuity and speed of execution.
David Stanwood’s thorough thinking, calculating, and testing of his Balanceweight System and New Touchweight Metrology nudged the entire trade to better understand and better apply principles of physics and geometry to their piano action work. In particular, I have found his concept of balanceweight (halfway between downweight and upweight) to be useful.
The second influence came from Bruce Stevens, with his stack-of-two-weights approach, shifting my process from manipulating two sets of gram weights and the prospective key weight (a process of picking up and putting down over and over that really asked for a third hand) to just picking and adding the pairing weight with one hand and positioning the key weight with the other. Brilliant!
The Grandwork Weighoff Kit supports two common styles of weighoff with this method. The first works for pianos with a consistent downweight, often around 50g. In this style, the pairing weight decreases 1g for every 1g rise in upweight (20g upweight + 30g pairing weight = 50g downweight, 21g upweight + 29g pairing weight = 50g downweight, and so forth).
The other style provides more downweight in the bass and less in the treble, relating downweight to hammer weight and guided by a balanceweight chosen to offer user-friendly collaboration between each note’s readiness to play and its push-back to the player as played. In this style, the pairing weight decreases by 2g for every 1g increase in upweight (34g pairing weight + 20g upweight = 54g downweight + 20g upweight ÷ 2 = 37g balanceweight, 32g pairing weight + 21g upweight = 53g downweight + 21g upweight ÷ 2 = 37g balanceweight, and so forth).
Working from bass to treble, one style produces the same downweight throughout, while the other offers incrementally lower downweights. But the key is that both styles feature incrementally rising upweights, a consistency lacking in traditional weighoff. And for ease of execution, you don't have to plot switches in downweight and upweight, just use the pair that gives the easiest-to-read symptoms. They will automatically produce a consistent transition from bass to treble.
When I install new hammers, I add to this consistency two other factors. One is smoothly descending strikeweights through hammer taper and tail thickness. The other thing I do is match weight of highest bass hammer with weight of lowest tenor hammer. This adds consistency of inertia to those of balance and friction. Ironing out geometry and regulation irregularities refines this further (custom boring new hammers to match the arc of string heights from a crowned plate, for instance, uncompromises string-height-related regulation discrepancies).
And thank you part manufacturers for developing incremental choices of knuckle and whippen heel placement that facilitate modifying action geometry. And particularly, Wessell, Nickel & Gross, who also brought us composite shanks of consistent weight and stiffness and hard bushings that allow less center friction with more stability. Also, their incrementally-sized modular whippen heels allow using sharp heels that are 2mm taller than natural heels to mediate the unavoidable geometry/leverage differences between sharps and naturals
Trial-and-error proves the fastest route of the finding the best balance. Place on top of the key a suitably sized key weight (use copper if you can since lead is toxic) and find the spot where adding and picking the pairing weight matches speeds of hammer rise and return, respectively. If a key stands out with slower speeds, there is excess friction, faster speeds, less friction.
Caveat. The installation of a key weight won’t work if there is one in the way. Assess the overall add/subtract trend your regulating will induce and pre-remove key weights as needed. Drill out holes with an oversized bit (9/16” works well) and plug with a wooden bung. Consider selling your customer on the removal of all lead key weights from the keys. 9/16” copper key weights weigh about the same as 1/2" lead key weights, so you can remove the lead, drill out the hole, and install copper without materially changing the original weighoff. Similarly, a 1/2" copper key weight is close in weight to a 3/8” lead key weight.
New bungs and key weights in the blacked areas of sharps should be blacked when done. Strategic pre-removal is time-consuming, but it makes weighing off easier and more accurate.
Another notable approach to weighoff comes from Ed McMorrow. As part of his Light Hammer Tone Regulation system, Ed way reduces strikeweight and removes all key weights from the keys, way reducing inertia in the system. The resulting 70g or so low bass downweight feels responsive rather than heavy and the lack of key weights really helps the player in fortissimo playing. He applies this exclusively to Steinway pianos but suggests a version of this method is worth considering for any grand piano.