The side reinforcement methods used by luthiers vary considerably and each has their own reasons for using what they do.
There are three main type of reinforcement
Cloth strips soaked in glue and secured to the guitar sides – Acoustic Guitar
Small wood reinforcement braces – unshaped or scalloped – Acoustic Guitar
Extensions of the top braces – Often called brace brackets – Classical Guitar
Note that Ukulele sides usually are not wide enough to require side reinforcement. This can change depending on the thickness of the sides, wood being used and size of the ukulele, such as a baritone size.
Cloth Reinforcements – Acoustic Guitars:
One phrase for this method – “Quick and Dirty”. It is the preferred method of the big factory guitar manufacturers. This type of reinforcement does serve its purpose very well and that is to keep the guitar sides from cracking.
I’ve never made a guitar using this method as I like to look inside a guitar and see nicely shaped wood side bracing. You can purchase the cloth material from any number of guitar suppliers. Simply dip the cloth in Titebond Glue and lay them on the sides and you are done – like I said – “Quick and Dirty”.
Small Wood Reinforcement Braces – Acoustic Guitars:
These are my side brace of choice when I make a guitar. On our Guitar Plans at Georgia Luthier Supply, you will see them shaped out of small rectangular pieces of Mahogany with chamfered edges. You could just as easily, round them like a top brace and scallop the ends to make a very dramatic looking side brace that is very functional. I usually make these braces about 4mm wide and 5mm high. I glue them into place as rough-cut rectangular block and scallop and round them in-place. This assure a consistent glue joint for the length of the brace.
Depending on your guitar body size you should use between 7 and 9 of these braces for each side of the guitar. Space them evenly around each side. They can easily be clamped into place using plastic spring clamps with soft rubber faces as shown in the above photo.
Classical Guitar Brace Brackets:
This type of side brace is reserved primarily for the Classical Guitar and is seldom used for Acoustic Guitars. I have used this method of side bracing for a few of my higher end Acoustic Guitars as it is very time consuming to notch out the top brace, shape the side braces and glue them in place. Also if you decide to use this method for an acoustic guitar, plan on adding supplemental side braces to the brackets.
I feel that this method enhances the sound more than the other two types of side bracing, but you do have to deal with the structural issues. As with the above bracing, plan on a total of 7 to 9 bracing points on each side.
GLS Tip: To install the side braces evenly around the sides, measure the total side dimensions between the head and tail block with a flexible seamstress tape. Then divide by the number of braces you intend to use + one. Make a thin template of this to mark the side brace locations on the sides. Be sure to make the marks perpendicular to the top plate and not the back plate.