At 11:39 pm -0400 3/5/07, Frank Emerson wrote: >I have seen technical drawings that clearly indicate that it was the >designer's intent that all of the down bearing should be at the front of >the bridge, with zero back bearing. In a piano built with this type of >bearing design, and if the bridge were supported only directly under the >center line of the bridge, when the string were drawn to tension, the >bridge would immediate roll forward until the front and back bearing were >equal. Hello Frank, Such a bridge as you describe, essentially a closed V with the two pins at the top and the fulcrum at the bottom, would immediately tend to tip forward as tension was applied to the speaking length, however the downward force was distributed over the bridge from front to back, since an increasingly strong almost tangential load will be acting to overcome increasing friction at the bridge. Not until the tension in the back length is equalized with that of the speaking length will the bridge be relieved of the very real tipping force actig tangentially to its fulcrum. Consider now a bridge such as you describe where the 'back' arm of the V is a tiny fraction shorter so that the line of the top of the bridge projected touches the hitch-plate bearing (or string rest) and leaves no gap. The same line projected in the other direction makes an angle of say 1 degree with the speaking length. Suppose that this angle produces a downward force (the down-bearing) of about 3 pounds-force, as it will for regular modern tensions, then the component of that small force that is tending to tip this V-shaped bridge forward is a matter of a few ounces and inadequate to overcome even a small fraction of the opposing tangential force of friction at the top of the bridge, no matter how smooth the bridge and the pins. If we now replace the V-section of the bridge with the normal rhomboid cross-section it can be shown that any tendency for such an arrangement to tip the bridge is so minute as not to be worth considering. At 11:39 pm -0400 3/5/07, Frank Emerson wrote: >My pet peeves on this subject are: Failure to distinguish between >front and back bearing. Failure to distinguish between loaded and >unloaded bearing (before and after the full string tension is >applied). Using linear dimensions to specify down bearing, and >totally loosing sight of the fact that these specifications are only >an expedient to achieve what is really important, the angle of >deflection, front and back. I would go further and say that millimetres, card's thicknesses, bubble gauges etc. are but a means to determine an angle and unless they are so used they are meaningless, and the angle is just as meaningless unless it is used to quantify what is actually down-bearing, and that is a force, measured in units of force. The important question is to determine the proper distribution of weight along the bridges to achieve optimum admittance combined with the desired final arching of the soundboard when the piano is strung and maintained in acceptable conditions of humidity. JD
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