SAG Calculator: MTB Suspension & Cable Catenary

Switch between MTB suspension sag and cable catenary sag in one clean tool. Enter rider weight or span details, pick units, and get instant percentages, millimeters, clear pass‑fail checks, and sensible guidance. Save results, download CSV, and use the built‑in explainer to understand every variable and formula with confidence. Fast, accessible, Bootstrap five layout.

Inputs

Include gear.
mm
%
mm
Use o‑ring method after sitting in attack position.
:1
Used for rough spring‑rate guidance.

Results

Target sag
— mm
—% of travel
Measured
—%
Enter a measured value to compare.

Sag vs. Percentage (for your travel)

Sag is the amount your suspension compresses under your static riding weight.

  • sag_% = (sag_mm / travel_mm) × 100
  • Target ranges vary by discipline. Trail example: 20–30%.
  • Optional leverage ratio lets you approximate spring rate needs: k_coil ≈ W / (LR × sag_m), where W is rider force (N), LR is leverage, and sag_m is sag in meters.

These are starting points—always test ride and fine‑tune rebound and compression.

Inputs

Height above ground at supports (assumed level).

Results

Parabolic sag
— m
s ≈ wL²/(8H)
Catenary sag
— m
y=a(cosh(L/2a)−1), a=H/w
Clearance check
—
Add attachment height and min clearance to evaluate.

Catenary vs Parabola (centered at lowest point)

We compute both a parabolic approximation and the true catenary for a level, symmetric span.

  • w is weight per unit length in N/m. If you enter kg/m or lb/ft we convert using g = 9.80665 m/s².
  • H is horizontal tension (N). We accept kN or lbf inputs.
  • Parabolic sag: s ≈ w L² / (8 H)
  • Catenary parameter: a = H / w, vertical at midspan: y(L/2) = a (cosh((L/2)/a) - 1)

Always verify against local codes and engineering standards; this tool is for preliminary estimation only.

Understanding SAG: What This Calculator Does and How the Formulas Work

This two-in-one SAG Calculator is designed to help you solve two practical problems that share a similar idea— how much a system “drops” under load. In MTB suspension, sag is the static compression of your fork or rear shock when you sit on the bike in riding position. In cable and powerline engineering, sag is the mid‑span vertical drop of a cable caused by its own weight (and sometimes weather loads) relative to the supports. Both tools here reveal the “right amount” of sag for your goal and make the math transparent so you can validate the results.

MTB Suspension SAG — Concepts & Math

The calculator targets a user‑selected sag percentage and converts it to millimeters based on your travel. The core relationship is sag_% = (sag_mm / travel_mm) × 100. You can enter an optional measured sag to see how far you are from target. For coil‑spring approximation, we provide a rough estimate using leverage: kcoil ≈ W / (LR × sag_m), where W is rider weight (in newtons), LR is the leverage ratio, and sag_m is sag in meters. This is a first‑pass estimate; fine‑tune by test riding, then adjusting spring rate or air pressure and damping (rebound/low‑speed compression) to taste.

A quick field method: slide the o‑ring to the wiper, settle into your attack position, dismount carefully, and measure the band’s movement. Compare the resulting percentage to the discipline ranges below. Running too much sag usually feels “wallowy,” while too little sag feels harsh and reduces traction.

Typical MTB target ranges (guidance, not rules)
DisciplineTypical RangeNotes
XC15–20%Prioritizes efficiency and low bob; better for smoother courses.
Trail20–30%Balanced support vs. comfort; common all‑round starting point.
Enduro/DH30–35%More traction and comfort on rough, steep terrain; tune rebound to avoid pack‑down.

Cable / Catenary SAG — Concepts & Math

For a level, symmetric span, we show both the widely used parabolic approximation and the true catenary. After unit conversion, weight per unit length becomes w (N/m) and horizontal tension becomes H (N). The parabolic mid‑span sag is s ≈ wL²/(8H). The catenary uses parameter a = H/w and mid‑span drop y(L/2) = a (cosh((L/2)/a) − 1). Because the catenary captures large‑sag behavior better, it’s preferable for long spans or low tension; the parabola is often adequate at small deflections.

If you supply an attachment height (support elevation) and a minimum clearance requirement, the calculator checks whether the mid‑span elevation meets your threshold. Clearance at mid‑span is approximately attachment height minus computed sag. Always validate results against local standards and consider additional wind or ice loads when required.

Catenary variables and units used by this tool
SymbolMeaningTypical Units
LSpan length (support to support)m or ft
wWeight per unit length (converted to force per length)N/m (from kg/m or lb/ft)
HHorizontal component of tensionN (from kN or lbf)
aCatenary parameterm; a = H/w
sMid‑span sag/dropm or ft

Assumptions, Limits, and Good Practice

The MTB module assumes your travel is accurate and that you measured sag without stiction or binding; always equalize air chambers on air forks/shocks and re‑measure after short shakedown rides. The catenary module assumes level supports, uniform weight per length, and static conditions; for sloped supports, unequal anchor heights, or significant environmental loads, consult detailed design guides or specialized software.

Use this calculator as a clear, portable starting point: it explains each variable, keeps units consistent, and shows the math so you can trust the outcome. Save scenarios, export results, and iterate—your optimal settings will emerge quickly with a few informed adjustments.

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Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.