Why did my balsa bridge break at the joint?
The usual causes of a joint failure: a gap, end grain, too little overlap, starved or dry glue and too much load on one joint, and what to try.
Updated · 1 min read
Short answer: most often the joint had a gap, joined end grain with little overlap, or had too little or too much glue. Joints are where most contest bridges fail, so you are in good company.
Look at the broken pieces
- Wood torn off the other stick (a fuzz of balsa on the glue face): the glue held and the wood failed. The joint was good, so look at the wood or the loads.
- Clean, shiny glue faces: the glue let go. Check for a gap, dry glue or a face that was dusty or oily.
- A butted end popped out of the side of another stick: end grain is weak. Add overlap or a second contact face.
Common causes and fixes
| Cause | Fix |
|---|---|
| Gap in the joint | Sand until no light shows |
| End grain butted with little overlap | Overlap against a side face, size the end first |
| Too little glue, or CA that wicked away | Apply a thin even coat, press and hold |
| Too much glue | Thick glue is weaker and heavier |
| Joint moved while setting | Pin it |
| Too much force on one joint | Redesign so the load spreads over more joints |