I received the following question from a technician in response to my 2012 video describing use of the Grandwork™ Squaring Platform and Hammer Square.
Q: Do you also square up the bass hammers? I often see them tilted so that they don't interfere with the neighboring hammers.
A: Thanks for your question - exactly the right one, too! The short answer is yes, I square all my hammers to be vertical at strike.
Tilting positives: Clearance in a flared hammer is improved by tilting it a calculated amount at strike toward the side of the shoulder furthest from the hammer center.
Tilting negatives: The rear shoulder is effectively heavier (being further out the lever arm from the near shoulder) and it travels a greater distance (the arc of its path is further from the center than the near shoulder's arc), consequently moving/accelerating faster than the near shoulder. The natural tendency for the hammer to tilt toward the heavier side when played is worsened by the tilt for clearance (the more tilt, the heavier the hammer, and the harder the blow, the worse it's worsened). A twist is generated, as the hammer is played, in the direction of the rear shoulder. On a hard blow, there will be a naturally-created "soft pedal" affect since the hammer will strike to the side of its intended spacing, and the twist will somewhat undo the tilt's clearance benefits, tilting the hammer further than optimal. The pinning is stressed and the looser the pinning, the further the crown will deflect from its ppp pathway on a hard blow. The strike (tone), the bounce, the reset, and the transfer of power will all be negatively affected by tilting for clearance.
Vertical-at-strike positives: Vertical at strike means that the driving force of the shank pushes from directly under the payload of the hammer. The strings fit to the surface of this hammer will be parallel to the keybed at strike, where tonally it most matters (a place a bubble gauge often can't get to for leveling, at least without damper removal), and both string-hammer fit and string leveling happen in one step. The bounce heads straight back to reset position. And the hammer surface doesn't need to be misfiled or the strings misleveled to achieve the fit necessary for good voicing. Also, that fit will be more stable at different dynamic levels. Strings being parallel to the keybed all but eliminates the need for special soft-pedal voicing, allows the player degrees of soft-pedaling, and does not create changes in the regulation during softpedaling. More power, more clarity, better backchecking, better dampening (because the strings are actually level), and faster reset, together make for greater overall simplicity and a more musical voice.
Vertical-at-strike negatives: Less clearance creates more need for consistently appropriate centerpin friction. And less clearance may mean shaving under shoulders (or choosing thinner hammers when replacing).
Both tilting and vertical at strike "squaring" of hammers are compromises, but I have found that voice, power, repetition, and control improve with the vertical-at-strike solution.