FabLayout

4 point saddle calculator

Enter the obstacle height, its width and the bend angle. You get the mark spacing for each offset, the spacing between the two offsets and the total shrink.

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Enter your measurements

Export from the Truemark app

The PDF cut sheet with this drawing and your marks is made in Truemark. It works offline on site and keeps your jobs.

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Mark spacingMark spacing88″″CenterCenter1212″″ShrinkShrink221188″″
Scale drawing

Mark spacing (each offset)

8in

Exact geometry

Centre spacing (between the offsets)
In the app
Shrink
In the app

Every result is in Truemark

The free web version shows the main result and the drawing. Truemark gives you every mark and measurement, and exports a cut sheet. It is free to download.

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How to lay it out

  1. Measure the obstacle

    Measure how high the conduit has to rise to clear the obstacle and how wide the obstacle is along the run.

  2. Lay out the first offset

    Make the first mark where the rise has to start. Measure the mark spacing from it for the second mark.

  3. Lay out the second offset

    From the second mark, measure the centre spacing for the third mark, then the mark spacing again for the fourth.

  4. Bend up and over

    Bend the first offset up over the obstacle, then bend the second offset back down the opposite way, all four bends to the same angle and in one plane.

  5. Allow for the shrink

    The run is shorter by the total shrink of both offsets. Allow for it before you cut the pipe to length.

Good to know

  • FabLayout runs the same code as Truemark; the result is exact geometry.
  • Source: Plane trigonometry; same calculation as the Truemark app

Method and formula

A 4-point saddle is two offsets back to back: one goes up over the obstacle and the other comes back down. Both have the same depth and angle, so the math is the offset done twice, with a straight between them as wide as the obstacle.

  • Mark spacing (each offset) = obstacle height ÷ sin θ.
  • Centre spacing = obstacle width, the straight piece that runs over the top.
  • Shrink = 2 × obstacle height × tan(θ ÷ 2), one shrink for each offset.

Use it for wide obstacles such as ducts, beams or cable tray, where a 3-point saddle would not clear the full width.

Worked example

The page defaults: a 4″ high obstacle, 12″ wide, with 30° bends. Each offset has its marks 4 ÷ sin 30° = 8″ apart. The second and third marks are 12″ apart, the width of the obstacle. The total shrink is 2.1436″.

Questions

When should I use a 4-point saddle instead of a 3-point?

When the obstacle is wide or square, such as a duct or a beam. A 3-point saddle only clears something narrow at its peak; the 4-point saddle has a flat top as wide as the obstacle.

Do both offsets have to use the same angle?

This calculator assumes they do, which keeps the run parallel on both sides of the obstacle. Mixing angles gives a different spacing and shrink for each offset.

Is the shrink for one offset or both?

Both. The figure shown is twice the shrink of a single offset at that height and angle.

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