Practical tools for measured work
Spiral Easement Template Generator
Print a full-size spiral easement and curve template for model railroad track, with the exact offset from the tangent.
Calculate the layout, check the diagram, then print the result for the shop or jobsite.
Centre-line radius of the circular part of the curve.
Length of each spiral, measured along the track. Must not exceed the radius.
How far the track turns from the entry straight to the exit straight, spirals included.
Overall width to draw. The rail lines are drawn at the scale's track gauge.
Print at 100% or Actual size, join the tiled pages, and check the 25 mm square. TS is where the spiral leaves the straight, SC where it meets the circular arc. Lay the template on the subroadbed and trace the centre line.
How it works
What an easement does: a spiral eases the track from a straight into the full curve, so the curvature builds up gradually instead of jumping. The template uses a true Euler spiral. The tangent angle at distance s along the spiral is s² ÷ (2 × R × L), where R is the curve radius and L the easement length.
Offset: the circular part sits a little inside the original straight line. That gap is the offset. For gentle easements it is close to L² ÷ (24 × R), which is the hand rule some builders use, and the tool shows both values. The template is drawn from the exact spiral, not the approximation.
Using it: print the tiled pages at 100%, tape them to the subroadbed, and trace the centre line. Mark the TS and SC ticks on the wood so the curve can be checked or extended later.
Current-input example
The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.
- Measurement unit
- Inches
- Scale
- HO (1:87.1)
- Curve radius
- 36
- Easement length
- 18
- Total turn of the curve
- 90 °
- Easements
- At both ends
- Roadbed or template width
- 1.5
- Curve direction
- Turns left
Primary result: Offset of the curve from the tangent: 0.374 in.
Before using the result
- Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
- Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
- When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.
Limitations
The template is a plan-view centre line with rail and roadbed lines drawn at nominal gauge. It does not include superelevation, turnout geometry or benchwork clearances. Very long easements relative to the radius are rejected, and templates that need more than 80 printed pages are not offered. Print scaling errors change every dimension, so measure the 25 mm square first.
Frequently asked questions
How long should the easement be?
Longer easements look smoother and are gentler on long equipment but need more room. The length cannot exceed the radius here. Try a few lengths, print the template and test it with your longest car.
Does it replace a curve template?
Use the curve template for plain circular arcs. This tool is for the transition between a straight and a curve, and it also draws the arc that follows.
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