Used to calculate theoretical wall thinning factor
Bending Dimensions
Bend Allowance (Arc Length):–
Setback (SB):–
Wall Factor (D/t Ratio):N/A
Note: The Bend Allowance is the actual length of the curved portion of the tube. The Setback is the distance from the tangent point (start of bend) to the intersection of the straight legs.
Mastering Tube Bending: A Guide to CLR and Setback
Whether you are fabricating a roll cage for a race car, building custom furniture, or laying out hydraulic lines, precision is paramount in tube bending. A small miscalculation in the Bend Allowance or Setback can result in wasted material and parts that don't fit.
This Tube Bending Calculator helps fabricators determine exactly where to mark their tubes before placing them in the bender. By inputting your desired angle and the die's radius, you can calculate the arc length and the setback required for precise bends.
Key Definitions
Center Line Radius (CLR)
The CLR is the radius of the bend measured from the center point of the circle to the center axis of the tube. This is typically determined by the die you are using in your bender. It is not the same as the bend radius of the inside or outside wall. You must know your die's CLR to get accurate results.
Bend Allowance (BA)
The Bend Allowance, often called Arc Length, is the physical length of the neutral axis of the tube that comprises the bend. When you bend a straight tube, the material stretches on the outside and compresses on the inside. The Bend Allowance calculates how much "straight" tube is consumed to create the curve.
Formula: BA = (Angle × π × CLR) / 180
Setback (SB)
The Setback is the distance from the start of the bend (the tangent point) to the virtual corner where the two straight legs of the tube would intersect if they continued straight. This is crucial for measuring. Often, fabricators measure to the corner (vertex) and need to subtract the setback to find where the bend should actually start.
Formula: SB = CLR × tan(Angle / 2)
How to Use This Calculator
Identify your Die Radius (CLR): Check your bending die specifications. Common radii for 1.75″ tubing are 4.5″, 5.5″, or 6.5″.
Determine the Angle: Input the degree of bend required (e.g., 45°, 90°).
Calculate: Click the button to see the Bend Allowance and Setback.
Mark your Tube: Use the Setback to measure back from your theoretical corner to mark the "Start of Bend" line on your tube.
Tube Diameter and Wall Factor
While the diameter of the tube doesn't strictly change the geometry of the centerline arc, it is critical for structural integrity. The relationship between the tube diameter and the CLR affects the "Wall Factor." A tighter radius on a larger tube puts more stress on the outer wall, leading to thinning or ovalization. Generally, a CLR should be at least 3x the tube diameter for standard bending without a mandrel.
function calculateBend() {
// Get input values
var angleInput = document.getElementById('bendAngle');
var radiusInput = document.getElementById('centerLineRadius');
var diameterInput = document.getElementById('tubeDiameter');
var resultsDiv = document.getElementById('results');
var angle = parseFloat(angleInput.value);
var radius = parseFloat(radiusInput.value);
var diameter = parseFloat(diameterInput.value);
// Validation
if (isNaN(angle) || angle <= 0) {
alert("Please enter a valid Bend Angle greater than 0.");
return;
}
if (isNaN(radius) || radius <= 0) {
alert("Please enter a valid Center Line Radius (CLR).");
return;
}
// Math Constants
var PI = Math.PI;
// 1. Calculate Bend Allowance (Arc Length)
// Formula: (Angle * PI * Radius) / 180
var arcLength = (angle * PI * radius) / 180;
// 2. Calculate Setback
// Formula: Radius * tan(Angle / 2)
// Note: angle must be converted to radians for Math.tan
var angleRadians = angle * (PI / 180);
var setback = 0;
// Handle edge case for 180 degree bend (hairpin) where tan(90) is infinite
// For 180, setback technically goes to infinity relative to the vertex.
// However, practically, setback is usually irrelevant for 180 bends in this context,
// but we will display "N/A" or limit it for angles = 180) {
setback = "N/A (Parallel Legs)";
} else {
setback = radius * Math.tan(angleRadians / 2);
setback = setback.toFixed(3);
}
// 3. Wall Factor Check (Optional)
// Ratio of CLR to Tube OD.
var wallFactorText = "N/A";
if (!isNaN(diameter) && diameter > 0) {
var factor = radius / diameter;
wallFactorText = factor.toFixed(2) + "x OD";
if (factor < 1.5) {
wallFactorText += " (Very Tight – High Risk)";
} else if (factor < 2.5) {
wallFactorText += " (Standard)";
} else {
wallFactorText += " (Easy Bend)";
}
}
// Update DOM
document.getElementById('resArcLength').innerHTML = arcLength.toFixed(3);
document.getElementById('resSetback').innerHTML = setback;
document.getElementById('resWallFactor').innerHTML = wallFactorText;
// Show results
resultsDiv.style.display = "block";
}