Touch-Weight Adjustment Systems. Touch or touch weight refers to the pressure required to press a piano key. Too little touch weight, or touch weight that is uneven from note to note, makes a piano action difficult to control; too much touch weight makes a piano tiring to play, and can cause physical problems for the player over time. Touch-weight problems can be caused by poor action design, worn parts in older pianos, or incorrectly dimensioned replacement parts in restored pianos.
Historically, discussions, measurements, and adjustments in this area of piano technology have been about static touch weight — the force needed to make a piano key just begin to move slowly downward. Less well understood, and usually ignored, has been dynamic touch weight — the force required to press a key in actual normal, rapid playing. Here, the rapid movement of the key creates inertia (i.e., the tendency of a moving mass to keep moving in the same direction and at the same speed, and the tendency of a stationary mass to remain stationary.) Unlike static touch weight, which depends on the relative amount and positioning of mass on either side of the key's balance point, as well as on friction, dynamic touch weight depends on the total amount of mass in the system. Attempts to fix problems in static touch weight by adding mass to the front or rear of the key can cause problems with dynamic touch weight by creating excessive inertia.