back check, a magical mystery tour.

[Richard Brekne]

Grin... Stephen... this is the second time in a row you struck the exact 
chord that brings an appreciative smile to my innards... :)  This time 
its directing us towards the obvious... common sense as it were.  This 
explanation doesnt have a lot of sensuality to it... but it does show 
some promise of being in the right ball park.

Stephen Birkett wrote:

> We've seen quite a few 'second-tier' hypotheses - as Ric says, sexy 
> explanations - for the backcheck phenomenon. These things obviously 
> need to be examined experimentally, but a small dose of Occam might be 
> a good thing first. I'm thinking something is causing this that occurs 
> before the string impact. The obvious candidate is interaction between 
> the backcheck and the hammer tail as the hammer heads toward the 
> string. Backcheck clearance is supposed to be pretty tight, and I've 
> seen enough examples of slight catching on the check  as the tail goes 
> up, enough that power is lost without it being obvious. For a given 
> action configuration, this effect doesn't occur for all blows. It's a 
> function of various action parameters, including softness of the 
> whippen cushion and other felt contacts, hammer shank flexibility, 
> tail geometry, and so on, as well as the type of touch applied to the 
> key. Lowering the check to achieve the magic 2mm separation may very 
> well ensure adequate clearance in all cases.
>
It may very well indeed... especially when the other back check 
parameters are adjusted correctly.

> It's probably possible to check this on a "dud" key before and after 
> making the adjustment of backcheck height, even with just a bit of 
> chalk on tip of the key tail. Static clearance of the check/key tail 
> is certainly no guarantee of dynamic clearance. If it's too subtle for 
> chalk it will need some targeted experiments with high speed images to 
> investigate properly.
>
A good suggestion.. and easy to carry out... most certainly at least 
some what revealing. I'm a gonna give this one a try :)

> From the current discussion, it seems the effect is also apparent if 
> backcheck clearance height is too much more than 2mm, and the 
> explanation above doesn't address that of course.  To those who've had 
> good results with Andre's technique (including Andre himself), in 
> practical circumstances how often do the checks have to be lowered vs 
> raised?
>
Well.. lets see...if the back check is too high or too low.. then it has 
to be moved in the lateral direction and its angle has to be changed to 
maintain the same <<static>> check height and holding strength.  So 
maybe.... the 2 mm simply describes the optimum distance for these other 
two directions to also be at their most efficient ?? 

As far as how often checks need adjusting.  Hmm... you'd be suprised how 
varied back check height is off the factory line... even in pianos like 
Yamaha where <<precision machines>> are specially made to do the job.  
Then there is the hammer change job done by the tech down the 
street...... grin..  In short...  it happens often enough that they need 
addressing.

> Stephen

Cheers
RicB