Enter your head tube angle, your fork offset and your wheel, and it computes the ground trail (the table figure) and the mechanical trail (the one that generates self-centering), the wheel flop, and how much trail changes per degree, per mm of offset or per wheel size. Advanced: dynamic trail under braking and under SAG. Closed-form model, validated against an independent numerical oracle. Nothing leaves your browser.
Static trail
Steering geometry · θ from horizontal
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Ground trail (mm)
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Mechanical trail (mm)
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Wheel flop · R used
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Sensitivity (analytical derivatives)
+1° slacker—
+1 mm of offset—
+10 mm of wheel diameter—
Dynamic trail
How trail changes while you ride
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Dynamic angle (°)
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Dynamic trail (mm)
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Δ vs static (mm)
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The model, open.
Ground trail = (R·cosθ − offset)/sinθ · Mechanical trail = R·cosθ − offset · Wheel flop = trail·sinθ·cosθ (θ from horizontal).
Dynamic: fork compression lowers the A2C → the head angle steepens → trail drops (enforced by a sign guard).
Validated against an independent numerical oracle (analytical derivatives ≡ finite differences). Method & validation → · ORCID 0009-0007-5631-7436.
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What trail is, and why there are two
The ground trail is the figure printed in geometry charts: the distance, on the ground, between where the steering axis points and where the tire touches. The mechanical trail is the perpendicular distance to the steering axis, and it's the one that actually generates the self-centering torque. More trail = more stable, planted steering; less = quicker and more agile. To understand what you feel with each number, see the trail entry and the geometry cluster.
BikeLab Studio · Bike Trail Calculator v1.0 / Verified model / Carlos Eduardo Ravello Joo · Trujillo, Peru