What if my balsa bridge is over the mass limit?
How to take weight out of a finished bridge, where to look first, and how to avoid it by weighing as you build. Check your contest's penalty rules.
Updated · 1 min read
Short answer: check the contest's rule for what happens when a bridge is over, then take weight out where nothing is likely to fail. Better still, weigh as you build so it never happens.
Check the rule first
Some contests disqualify an overweight bridge, others penalize it or still allow it to be loaded. Read the rule, and when in doubt ask the organizer in writing.
Where to take weight out
- Excess glue. Squeeze-out and thick joints are the quickest gain. Careful trimming with a knife removes it.
- Overbuilt members. Members that never bowed in tests, usually near the supports, may be thinner.
- Unneeded bracing or gussets.
- Heavy sticks in low-force places. Swap for lighter ones in the next build.
Don't sand the faces of joints to lose weight, because it removes little and spoils the fit.
Avoid it next time
- Keep a weight log of every piece. See weighing accurately.
- Weigh after each major step.
- Aim a margin under the limit, since scales differ.
- Use less glue. See gluing joints.
See also making a bridge lighter.