Testing your balsa bridge safely

Testing teaches you more than any amount of planning, so do it early and more than once. It is also the part where something can fall on a foot or a splinter can fly, so set it up with care.

Before you start

• Wear eye protection. Balsa splinters fly when a bridge breaks.
• Test over a soft, clear area, with your feet out from under the load. A bucket of sand falling on a foot hurts.
• Weigh the bridge and write the mass down before it is loaded.
• Match the contest setup. Use the same support gap, the same load point and the same way of attaching the load as the rulebook describes.

A simple rig

1. Rest the bridge across two supports set to the contest's span.
2. Hang a bucket from the load point with a short loop of string or a hook.
3. Add sand, water or weights to the bucket slowly and evenly, in small steps.
4. Weigh the bucket and contents at the end on a kitchen scale.

Add load gradually

Add the weight in steps and stop for a moment at each one. Watch for:

• Bowing in a long stick, a warning of buckling.
• Gaps opening at a joint.
• Creaks or clicks, which often come just before failure.

Take a short video, ideally in slow motion, from the side. Replaying the break shows what failed first, which is hard to see live.

Record every test

Calculate efficiency as shown in the efficiency guide.

What to change next

Change one thing per version, so you know what made the difference. If a compression member broke, strengthen or brace it. If a joint pulled out, redo the fit. If the bridge is far under the mass limit, you may be able to add a bit of wood where it fails. If you are over the mass limit, take wood out where nothing broke.

Remember that a bridge that has been tested to failure is gone. Test a practice bridge, not your contest entry, unless the rules say otherwise.