Choosing balsa sticks for a bridge
How balsa density affects strength and weight, hobby density grades, how to sort sticks with a scale, and where to use light and heavy pieces.
Updated · 2 min read
Balsa is a very light wood, but not all balsa is the same. Two sticks that look identical can differ by a factor of two in weight, and the heavier one is usually stronger. Picking the right stick for each part of the bridge is one of the cheapest ways to improve your score.
Density is the main thing
Hobby balsa is graded by density. Typical bands from model-aircraft suppliers:
| Grade | lb/ft³ | kg/m³ |
|---|---|---|
| Ultra light | 4 to 5.4 | 64 to 87 |
| Light | 5.5 to 6.0 | 88 to 96 |
| Light-medium | 6.1 to 7.5 | 98 to 120 |
| Medium | 7.6 to 9.5 | 122 to 152 |
| Medium-hard | 9.6 to 12 | 154 to 192 |
| Hard | 14 and up | 224 and up |
Stiffness and strength rise roughly in proportion to density for balsa, but with a lot of scatter. In one study of tension strength, density explained only a small share of the variation, so two sticks of the same weight can still differ in strength. Treat density as a good guide, not a guarantee.
How heavy is a stick?
A 1/8 in square stick, 36 in long, at 150 kg/m³ weighs about 1.4 g. A 1/4 in square stick of the same length weighs about 5.5 g. These are calculated from the density, so weigh your own.
Sort your sticks
- Weigh every stick on a scale that reads to 0.01 g, and write the weight on the stick in pencil.
- Divide by length if the sticks differ, so you compare like with like.
- Sort into heavy, middle and light piles.
You will often see a wide spread in a single bag. That spread is a free resource.
Where to use which
- Compression members (the sticks being squeezed) fail by buckling, which depends on stiffness. Give them your stiffer, heavier sticks, or a larger cross-section. See truss basics.
- Tension members (the sticks being pulled) only need to resist being pulled apart, so they can often be lighter, with the caveat that a joint, not the stick, is usually what lets go.
- Check the grain. Pick straight-grained sticks. A stick with a slope in the grain, or a visible knot or soft spot, breaks early. Hold each stick up to a light and sight down its length to look for bends.
Moisture
Balsa takes up moisture from the air, which changes both weight and strength. Estimates suggest about a 10% property change across ordinary humidity. Store your sticks in the same place you will build, and weigh the finished bridge close to test day.
Also check the rulebook
Some contests specify the stick size and even the supplier. Using a stick the rulebook doesn't list can disqualify the bridge. See reading your rulebook.