Rigging & Staging calculator
Bridle Length & Angle Calculator
A two-leg bridle lands a chain-motor point between two beams: each leg length is the hypotenuse of its horizontal run and the vertical drop, and each leg carries more than its share of the load as the bridle flattens. Enter the span, apex position, drop, and load to get leg lengths and tensions.
Where the bridle apex needs to land horizontally, measured from the first point.
Plane geometry and static tension only; hardware ratings, sling angles at the steel, and dynamic loads are not evaluated. Rig to your local standards under qualified supervision.
Formulas
Leg lengths
leg = √(horizontal² + vertical²)- horizontal:
- that leg’s horizontal distance to the apex
- vertical:
- drop from the beams to the apex
Leg tensions
T1 = W × L2 × leg1 / (V × span), T2 = W × L1 × leg2 / (V × span)- W:
- supported load
- L1, L2:
- horizontal distances from apex to each point
- V:
- vertical drop
How it works
The tension formula is the standard entertainment rigging bridle equation: it resolves the vertical load into two legs that also pull horizontally against each other. For a symmetric bridle it reduces to T = W × leg / (2 × drop), which equals W / (2 × cos θ) with θ the leg angle from vertical.
The included angle between the legs is the safety-relevant output. At 60° included, each leg of a symmetric bridle carries about 58% of the load. At 90° it is 71%. At 120° each leg carries the full load, and past that the growth is steep: bridles want depth.
Real bridles add practical constraints the math does not: legs are built from fixed-length steels and STAC chain to hit the computed lengths, baskets around beams change the effective attachment point, and both legs pull inward on their beams, a horizontal force the building must be allowed to take.
Worked example: 20 ft between beams, apex 8 ft from point 1, 10 ft drop, 1,000 lb point
- 1.Leg 1: √(8² + 10²) = 12.81 ft. Leg 2: √(12² + 10²) = 15.62 ft.
- 2.T1 = 1000 × 12 × 12.81 / (10 × 20) = 769 lb.
- 3.T2 = 1000 × 8 × 15.62 / (10 × 20) = 625 lb.
Legs of 12.8 ft and 15.6 ft, carrying 769 lb and 625 lb for the 1,000 lb point.
Symmetric bridle: tension per leg vs included angle (1,000 lb load)
| Included angle | Leg angle from vertical | Tension per leg |
|---|---|---|
| 30° | 15° | 518 lb |
| 60° | 30° | 577 lb |
| 90° | 45° | 707 lb |
| 120° | 60° | 1,000 lb |
| 150° | 75° | 1,932 lb |
Field notes
- Both legs load their beams toward each other; a bridle is also a horizontal force on the building steel.
- Round leg lengths to what your steel and deck chain can actually build, then recompute the apex position rather than pretending.
Frequently asked questions
How do I calculate bridle leg lengths?
Each leg is the hypotenuse: the square root of its horizontal distance to the apex squared plus the vertical drop squared. Rigging tape measures exist because the horizontal distances come from the beam layout, not the plot.
Why is bridle leg tension higher than the load share?
Because a sloped leg must produce the vertical support and a horizontal component that the opposite leg cancels. The flatter the leg, the larger the horizontal component, and the total tension grows as 1/cosine of the leg angle.
What bridle angle is too much?
Practice keeps included angles modest; past 90° tensions exceed an equal vertical split and past 120° each leg exceeds the whole load. Deep bridles are safe bridles, geometry permitting.