Home Mandolin Structural Failure Hospital


Why are there structural problems??

There are several mandolin repair issues that can only really be explained by looking at the structure. These include split seams; loose or split tone bars; splits in the top; back or sides; warping of the tops around the sound-hole; bent necks; and ripped up tails.

   First, a bit of background. A mandolin is essentially a sound box, with attachments for stretching strings across it to a certain length. It is made of wood, which is protected from the elements to some extent by the finish. Most everything else is purely cosmetic, except where the mandolin is climatically. These three key factors, the stretched wire, the wood used to build it, and where it is kept, are where most of the structural problems originate.


   A mandolin is designed to have a scale length of between about 325 and 355mm. The variety of strings that can be used to create the range of notes played by a mandolin, is enormous. But, put the wrong strings on the wrong instrument, and you have problems. Strings suitable for a scale length of 325mm, are NOT the same as those suitable for one of scale length 355mm, nor vice versa.

   In order to produce the same note over a longer distance, a string must be tightened more, so must be thicker to withstand the increased pressure. Over-stringing is often done because players think it increases the volume, but in many ways it puts the wood under such strain, it is unable to vibrate properly, and the sound is deadened.

   For more information on string choice, see the strings page.


   The other main source of problems, is the wood itself.. A mandolin is a mobile structure; it gains and loses moisture, to a greater or lesser extent, from changes in humidity and temperature. The finish minimises this, but cannot eliminate it entirely. The aim is to achieve the maximum protection with the minimum finish, to allow the wood to vibrate freely.

   These 'flexings' of the instrument cause problems, because wood moves laterally (across the grain) much more than it does longitudinally (along the grain). Imagine the stress for example, in the tone bars, which will not shrink much along their length, attached to the soundboard, which will shrink across its width. As we use wood with the grain going in different directions, for different parts of a musical instrument, this differential rate of shrinkage obviously causes problems. To make matters worse, different types of wood shrink at different rates!


        Mandolins are made with wood, dried to a certain level, (typically 6-8% in the US and 12-15% in Europe) in a place with a certain humidity. Problems arise when the instrument is subjected to different levels of temperature and humidity.  It is to minimise these environmental factors, that professional musicians keep their instruments cased, often with a hydrometer and/or humidifier in the case. 

      Modern instruments tend to fare a little better, often being heavily varnished, though heavy varnish does tend to inhibit good vibration. Below are some of the more typical structural problems caused by shrinkage. Many of the problems below are a direct result of the failure to ameliorate these environmental factors.

     For more information, see the care of instrument page.


General Shrinkage

   The pieces in the back have gradually shrunk. The gap occurs at the weakest point, where pieces are glued together. It is almost impossible to force such pieces back together again as they are now too small to fill the required space.    In this instrument, the intact back was completely detached. When lined up for re-gluing, there was a shortfall either side of about 2-3mm. Lateral shrinkage was much greater than longitudinal. The only recourse here is to make a newer larger back.    In many old bowlbacks, the centre join on the front splits because of lateral shrinkage. Here the centre join was re-enforced underneath, so the split occurred beside the centre strip. If the edges are lifted, and the spilt closed with crack clamps, there are implications for the edge on regluing, as it is then not wide enough. The simplest solution is to fill the gap with a small sliver of topwood.


Structural collapse

With badly fitting strut seatings, differential top and strut shrinkage, and inadequate linings, the instruments are often unable to resist the vagueries of changes in temperature and humidity. Combine this with over-stringing, (old instruments fitted with strings designed for newer bluegrass style instruments, with much longer scale lengths) and instrument collapse often follows, or at best serious warping. The sensitive may not wish to view the following, but I can assure you that each and every one has been carefully rebuilt, and as far as I know is still playing well.

 The above collapses are a result of the pressure of strings or shrinkage (or both) opening up seams and ruining the instruments structural integrity. The back is one of the main areas of collapse and many instruments arrive with several open seams.
   In these the top has given way, most often from shrinkage, but the effect on the mandolin is often disastrous, causing compression of the top from string pressure, warping, loss of the support from struts as they fall out, impossible pressure on the back, and final buckling and collapse of the top around the heel/body join.
   In the above, the top/side join has ruptured as a result of any number of reasons, that are not often evident until the instrument is dismantled. In the bottom left hand photo, the knife blade is inserted into a split in the heel block, but afterwards, it is impossible to tell if this split caused the collapse, or was a result of it. In the photo above it, the side of the top has given way and sunk, and been liberally filled with hide glue. To its right, the warping of the top can be clearly seen, where the structure has given way, and string pressure has pulled the tail towards the head and buckled the top.
   Another area very sensitive to over stressing particularly, is the join at the heel, If that collapses, as it often does, the head is pulled up towards the tail, causing the other end of the fretboard to be depressed into the top, often splitting it open and pushing the sides out as a result. A difficult set of problems to remedy.
  A variety of other probems occur when an instrument collapses making it unplayable. From the left, a head has snapped where the enclosed tuners left it just too thin to cope with the string pressure. In the lute, the heel was far too small and gave way to string pressure, resulting in a playing height of around 6cms. In the waldzither next right, string pressure deformed the top, pulled the tail joint out of place, and thus opened up the join of the sides at the bottom. Next, a neapolitan, which had obviously had previous repair attempts, as evidenced by the old glue round the edges. Here there only remained one support strut! And finally the top of a waldzither, where a combination of shrinkage and string pressure split open the top, and then caused the splitting of the support struts.