INPUTS
Fills the dimensions below; tweak freely.
Car dimensions (model)
End to end (not couplers)
Bolster to bolster
Truck center → coupler face
How far a coupler can swing before it binds/uncouples.
Your curve
Centerline radius. Use a preset or type your own.
Checks the car against the turnout’s diverging-route (substitute) radius.
RESULTS
Minimum radius (governing)
—
the tightest curve this car tolerates
Coupler swing at your radius
—
per-car offset from tangent
Coupler-limited min
—
from swing allowance
Practical min (length)
—
length-based guideline band
Your curve
—
—
Car bodies (on truck chords)
Couplers meeting at the joint
Track centerline / rails
Turnout diverging-route check
—
—
Substitute radius
—
equivalent diverging-route curve
Frog angle
—
from the frog number
Once a car clears the curve, see how far it swings past the railhead and whether two cars foul on adjacent tracks in the Overhang & Clearance tool.
Coupler-limited minimum radius = coupler overhang / sin(swing allowance); per-car coupler offset on a curve = asin(overhang / R). The practical minimum is a length-based guideline band (longer cars want broader curves), not a manufacturer spec. The governing minimum is whichever is more restrictive. Turnout substitute radii are typical HO values for common commercial turnouts and vary by brand; frog angle = 2 × atan(1 / 2N). Confirm critical fits by rolling your longest car through.