What is the best angle for the diagonals in a truss?
Why diagonals of roughly 45 to 60 degrees are common, what happens when they are too shallow or too steep, and how to check your own design.
Updated · 1 min read
Short answer: around 45 to 60 degrees from the horizontal is a common and sensible range. There is no single best angle, and we haven't tested it on balsa, so treat it as a rule of thumb.
Why not shallow?
A shallow diagonal has to carry the vertical load at a flat angle, so the force in it becomes much larger than the load, and it is long. Long pushed sticks buckle. In the worked example, a 34 degree slope gave about 0.9 P in each side member, and a 53 degree slope gave about 0.63 P.
Why not very steep?
Steep diagonals carry less force each, but you need more panels or taller trusses to fit them, which adds members, joints and glue.
How to use this
- Pick the truss depth the rules and bracing allow. See why deeper trusses are stronger.
- Choose the number of panels so the diagonals land near 45 to 60 degrees.
- Check the length of every pushed member. If one is long, add a panel or a brace.
- Test a practice bridge and see what bows.
Angles also set how hard your cuts are. See cutting accurate angles.