Zip Line Sag Calculator

Enter Zip Line Data

Horizontal span

Launch anchor height

Landing anchor height

Cable weight

kg/m

Rider plus gear mass

Dynamic factor

Cable pretension

Cable breaking strength

Trolley position from launch

Ground or obstacle height there

Required clearance

Formula Used

This calculator uses a small-sag cable approximation. Cable weight is treated as a uniform load. Rider force is treated as a point load at the selected trolley position.

Uniform load sag	y_u = w x (L - x) / 2H
Point load sag	y_p = P x (L - x) / HL
Total loaded sag	y = y_u + y_p
Support tension	T = √(H² + V²)
Safety factor	Breaking strength / maximum support tension

L is horizontal span. x is trolley distance from launch. H is horizontal pretension. w is cable weight force per meter. P is rider force including the dynamic factor.

How to Use This Calculator

Enter the horizontal span between anchors.
Add launch and landing anchor heights.
Enter cable weight, rider mass, dynamic factor, and pretension.
Set the trolley position where sag should be checked.
Add the local ground or obstruction height.
Set your required clearance target.
Press Calculate Sag to review sag, clearance, and tension.
Use CSV or PDF export for project records.

Example Data Table

Case	Span	Pretension	Rider Mass	Position	Expected Use
Short backyard layout	25 m	3.5 kN	60 kg	50%	Quick clearance check
Medium recreational line	60 m	6 kN	90 kg	50%	Balanced sag review
Long planning case	120 m	12 kN	110 kg	55%	Support tension comparison

Understanding Zip Line Sag

Zip line sag is the vertical drop of the cable below the straight chord between anchors. It grows when the span is long, the rider is heavy, or the pretension is low. Sag is not only a comfort issue. It affects clearance, landing speed, and support loading.

Why the Estimate Matters

A zip line should leave enough room above the ground, decks, trees, and other obstacles. A small change in tension can move the rider path by a large amount. The calculator uses simple engineering approximations to show the loaded sag, cable angle, and support tension. These numbers help you compare layouts before field review.

Important Inputs

The span controls the main cable curve. Anchor heights set the natural slope. Cable weight adds uniform load along the full length. Rider weight adds a concentrated load at the selected trolley position. Dynamic factor allows extra force from launch movement, bouncing, braking, or rough operation. Pretension represents the horizontal component in the cable.

Reading the Result

Loaded rider height is the chord height minus computed sag. Clearance is that height above the ground or obstacle level at the selected position. Maximum support tension is compared with cable breaking strength to estimate a safety factor. A higher factor is better, but final acceptance must follow local codes and professional inspection.

Practical Planning Notes

Sag changes with temperature, cable stretch, and hardware condition. Wet weather can add weight. Repeated braking can also change tension over time. Measure the actual span and anchor elevations carefully. Do not rely on guessed heights. When the line is installed, verify real sag with a test load. Check that the rider cannot hit the ground, a platform edge, or any side obstruction.

Best Use

Use this tool during early planning, layout comparison, and maintenance checks. Try several rider positions, because sag can change across the span. Test both normal and heavier loads. Keep records using the CSV and PDF options. For permanent installations, confirm anchor strength, cable terminations, braking distance, and rescue access with a qualified person. This calculator supports planning only. It does not replace certified design. Always inspect equipment before riding and keep written inspection notes. Repeat checks after any major tension adjustment.

FAQs

What is zip line sag?

Zip line sag is the downward cable deflection below a straight line between anchors. It depends on span, cable weight, rider load, position, and pretension.

Why does pretension matter?

Higher pretension usually reduces sag. It also increases anchor forces. Always balance clearance needs with cable rating, anchor strength, and professional guidance.

What dynamic factor should I use?

A factor above 1.0 allows for movement, bounce, braking, and load changes. Many planning checks use 1.2 to 1.5, but actual requirements vary.

Does this replace an engineer?

No. This tool gives an approximate planning result. Permanent or public zip lines need qualified review, field testing, and code compliance.

Why check different trolley positions?

Sag changes along the span. Midspan is often critical, but sloped lines and obstacles can make another position more important.

What is available clearance?

Available clearance is the loaded rider height minus the local ground or obstacle height. It should exceed your chosen clearance requirement.

Why is cable weight included?

The cable creates a uniform load over the full span. Longer and heavier cables add sag even before the rider is placed on the line.

What safety factor is shown?

The shown factor compares cable breaking strength with the estimated maximum support tension. It is only one part of a complete safety review.