What size balsa sticks should I use for a bridge?
The answer is in your rulebook, but here is how common stick sizes compare, what they weigh, and how to decide where thicker sticks pay off.
Updated · 1 min read
Short answer: use what your rulebook allows. Many contests specify the cross-section, commonly 1/8 in (about 3 mm) square sticks, sometimes a quantity of fixed length such as 36 in. If the size is free, start with 1/8 in square for most members and use larger only where a stick buckles.
What sticks weigh
At a typical balsa density of 150 kg/m³ (calculated, so weigh yours):
| Stick | Weight per 36 in |
|---|---|
| 1/8 in square | about 1.4 g |
| 1/4 in square | about 5.5 g |
A 1/4 in stick weighs about four times as much as a 1/8 in stick, and is far stiffer. Stiffness against buckling rises much faster than weight does, so a larger section can be worth it on a long compression member.
How to decide
- A member that bows in testing is too slender or too long: shorten it, brace it, or use a larger section.
- A member in tension rarely needs to be large.
- Don't make everything thick. Mass is your score.
Always check the rulebook for allowed sizes and supplier. See reading your rulebook and choosing sticks.