I recently had an a-style mandolin cave-in at the top at the tailpiece, and crack up the top. Has anyone heard of this issue before? Seems it was weak somewhere.
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I recently had an a-style mandolin cave-in at the top at the tailpiece, and crack up the top. Has anyone heard of this issue before? Seems it was weak somewhere.
The top may have been too thin near the tailpiece or the block could have come unglued.
Agree with above. Often both come hand in hand. Weak top allows "folding" of the top at recurve and resulting rotation of bottom block will separate it from back (below tailpiece). Much less often the separation of block from back will cause otherwise sound top break (only if neglected for really long time).
I noticed that the mandolin was flexible seeming when tuning, there was a lot of give (changes) when tightening/loosening to the other pitches. Does this indicate either way? Also could it have been a bad gluing at the 'block'? Most probably weak top wood.
I had it happen to my first build. I sanded and refinished so may time, I got it too thin.
Thin top mandolins are suspectible to changes in humidity or tuning like you describe. Were the block loose you would see the open separation between back and ribs under the tailpiece.Quote:
Originally Posted by toddmars
It's hard to understand why the top would need to hold the stress of tuning. Why there is not enough other structure to brace the instrument. The top is the main sound board correct? thanks for responses!
Just imagine the shape of top and the tension of strings (~150 pounds total) the curve between bridge and tailpiece is S shaped (concave-convex) and the tension is trying to compress the top. In some cases the top will bulge badly behind bridge (sometimes almost touch the talpiece) and the joint pops open widely, in cases where recurve is deep and/or thin the recurve will cave in and often the top will break across grain through botton of recurve. This is hard to see when it starts as the area is hidden under tailpiece and the joint doesn't opentoo much (more visibly inside mandolin), this can take a year or few to develop...Quote:
Originally Posted by toddmars
Thanks, the tension head to tail is 150 lbs, you're saying. The pressure down from the bridge to back is more tension mostly on curved tops. (Violins use a sound post to transmit sound energy from top to back, below the bridge).
What holds the tension from the head to tail? Would the top be free to vibrate more if there was no 'vertical tension', if it was held say by a couple of dowels (or through neck), (from neck base to tail)?...Just wondering if I understand correctly.
Todd.:mandosmiley:
The forces within the whole structure cannot be isolated. It all works as a system. you look at the crossection of mandolin it looks kind of like a bow (shooting) with a tillering stick between bow and bowstring (bridge). The backboard is mainly stretched while soundboard is mostly compressed by the tension of the strings. Neck is quite stiff structure but the body is much less so. The arch of back is thus lowered by the stretching and arch of top is trying to push up against the bridge while the bridge is being pushed by strings down against top. Unless there is a weak spot in the structure the whole will be stable for the life of the instrument. Best instruments should not have weak spots or too stiff spots that will cause inbalance of stresses. IMO, the better balance between all the forces is there the more responsive the instrument is. If the top is weak in some spot the back and sides structure will hold more of the stresses but the top will slowly deform in the weak spot until it fails. The role of soundpost is not in holding the arch of top or transmitting energy. It of course links top and back at the spot but it more of filters what plate modes can occur at given input at both top and back whle on mandolin the vibrational modes of top and back are more independent.
At the risk of too much humor here, think about the construction as follows. Neck- stiff rod. Sides- stiff perimeters. Top and back- variably thick balloons. Strings- lot of force trying to buckle the whole thing and the bridge is contributing to the natural tendencies of the system.