The flexible keyframe is slightly lighter built and less taut than the stiff keyframe. "Stiff" describes a construction that resists collapse. With keyframe fully loaded, glidebolts can be retracted so that none touch. The balancerail may sag and stress a little in this state, but its entire length will remain elevated. The flexible keyframe, however, will relax to the keybed. Most European/American keyframes fall into the "stiff" category. This becomes a desirable feature when studs (glidebolts = studs) are turned down to touch keybed with the pedals depressed. Set in this way, the stiff keyframe will not lose its connection to keybed during pedal engagement. In experienced hands, the traditional thump and bump method quickly executes the touching down of glidebolts, thumping to eliminate tapping, lifting and bumping to confirm connection.
Another technician approach for bedding the stiff keyframe requires less expertise. After backing all studs off the keybed, remove topstack and keys, reattach topstack (and its influence on keyframe shape), and re-install in piano. One-stud-at-a-time, insert a strip of newspaper between glidebolt and keybed, lowering glidebolt until newspaper just pulls out without tearing. This procedure leaves very even, small spaces whose closing with the addition of keys will not threaten the fit from stud-to-stud. And all spaces will be eliminated with enough excess contact pressure to maintain contact during pedaling. A drawback to this approach is the time it takes removing topstack and keys, reattaching topstack, removing topstack again to add keys back on, and finally reattaching topstack again.
The flexible frame's design trims enough stiffness for balancerail to follow the keybed. The problem is setting it up evenly and to the right height. To achieve well-balanced support, manufacturers add extra, pre-set studs in the middle of the longer spans of bass and tenor, effectively stiffening them, while establishing correct elevations and guaranteeing enough vertical space for bedding adjustments. These are made purposefully difficult to access and hard to adjust since they fine-tune the geometry of the action. Leave them as they are unless they are causing trouble.
The 1970's Yamaha C3 action shown in the lead photo has two hidden studs (stud #2 and stud #4 from the bass), placed in the area of the keyframe supporting the majority of hammer mass and compensatory key-lead. When all other studs are retracted, they create a double-fulcrumed see-saw whose dynamic stretch works mostly side-to-side, sagging at the ends. Front-to-back follow happens naturally.
Flexible Style Balancerail Bedding Summary:
Remove return spring (dress bearing surface on keyframe), adjust cheekblocks (for zero bearing), validate backrail and frontrail (action fully assembled), and retract all but hidden studs. Start by turning down stud #1 with the WNG Bedding Tool perched on a key button near stud #2. Turn until indicator moves and back up until it just doesn't. Move Bedding Tool near #1 to validate. The hidden stud (#2) will have been suspended briefly between stud #1 and the other hidden stud (#4) in this operation. Next, with the Bedding Tool beside stud #4, turn #3 down until the indicator moves and back until it doesn't. Then, move Bedding Tool beside #3 and validate. With the Bedding Tool by #4, turn down #5 until #4 moves and back up until it doesn't. Move tool near #5 to validate. Leave tool by #5 to set #6 and so forth. When done, validate all three rails with and without pedals. The flexible frame approach to keyframe bedding is responsive and effective. It works like the soft power and rooting of Tai Chi and other martial arts.
Next time: Edge Cases and the Common No-Time Touchup