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Caster Trail and Offset Geometry

Work out the trail and lead of a swivel caster, and judge whether it will self-centre, wander, or pivot under an offset or high load.

A swivel caster steers by trail: the offset between the swivel axis and the wheel’s contact point is what lets it self-centre behind a rolling wheel. Enter the offset, the wheel diameter, the load, and the direction of travel relative to the caster, and the calculator returns the trail, the lead, and the moment that acts about the contact patch.

The model is one caster in static equilibrium; it does not describe the whole cart’s geometry, the way load moves between casters, or the dynamics of a caster swinging into line after a shove. It is a geometry check, not a fork strength check.

Inputs

Caster geometry

Range: -6–12 in

Positive means the swivel axis is ahead of the contact patch in the direction of travel. This is the trail. Negative means the caster has lead and will not self-centre.

Range: 0.5–10 in

Range: -12–12 in

How far the carried load's centre of gravity sits from the swivel axis. A positive value puts it behind the axis, which is what makes the caster trail.

Range: 0.5–48 in

Range: 10–40,000 lbf

Result

Overturning moment on the fork (lbf-in)
1,200
Self-centring moment (lbf-in)
200
Trail as a fraction of wheel radius
0.600
Trail state
positive trail
Tracking verdict
The caster has positive trail, so a side load produces a moment that swings the wheel back into line. This is the geometry that makes a trolley track straight.
Load position note
The load sits close to the swivel axis, so the self-centring and overturning moments are both small and the caster behaves predictably.
Turning effort note
The trail is large for this wheel size. Expect noticeably more resistance to start turning and a wider turning circle, because the caster has to be swung further to take up the offset.

What to check

  • Measure the offset from the swivel axis to the centre of the contact patch.
  • Check the caster in the orientation it really travels in, including crosswise travel.
  • Confirm the fork's own strength separately; this tool only reports the moment.

Related calculators

A statics model of a single caster. It does not model the whole cart's geometry, the other casters, or how load moves between them, and it does not model friction, damping, or the dynamics of a caster swinging into line. A rigid frame and a rigid wheel with no tyre deflection are assumed, and the moment about the contact patch is not a check of the fork's own strength.