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Caster Threshold and Curb Crossing

Work out whether a caster rolls over a threshold, dock plate, or floor joint, the push force it takes, and whether the wheel locks against the obstacle.

A threshold is a step to a small wheel and a bump to a large one. Enter the wheel diameter, the obstacle height, the load per wheel, and the number of wheels meeting the obstacle, and the calculator returns the lift angle, the force needed per wheel, and whether the caster can cross.

The force depends on the wheel’s shape, the tread compound, the edge radius of the obstacle, and the speed of approach, none of which a geometric model captures. A soft tire deforms over the obstacle and can climb what a hard wheel cannot, which is one reason pneumatic casters handle rough ground better.

Inputs

Obstacle, wheel, and cart

Range: 0.0625–6 in

The step up, the joint width, or the plate lip the wheel has to clear.

Range: 1–20 in

Range: 20–40,000 lbf

Range: 2–12

Push effort

Range: 10–300 lbf

The force a person can sustain, about 50 lbf for a sustained push.

Result

Push force needed per caster (lbf)
90
Obstacle height / wheel diameter
0.150
Total push force needed (lbf)
360
Verdict
The wheel climbs but rides up on the obstacle, which means a jolt and a load transfer onto the leading casters. It will get past, but slowly, and repeated crossings wear the tread shoulder. A larger wheel is the fix.
Obstacle note
A floor joint is a gap rather than a step, so the wheel falls into it and then has to climb out of it. The force is therefore applied twice, once down and once up, which is why joints stop carts even when the gap is small.

What to check

  • Measure the tallest obstacle on the actual route, not the typical one.
  • Count only the wheels that meet the obstacle at the same time.
  • Test the crossing with the cart loaded, because the lift force scales with load.

Related calculators

A geometric screening model. The real force depends on the wheel's shape, its tread compound, the obstacle's edge radius, and the approach speed, none of which are modelled, and the factor used does not include the impact of arriving with speed. Rigid wheels are assumed, so a soft tire's ability to deform over an obstacle is not credited, and all casters are assumed to meet the obstacle at the same moment, which a diagonal approach contradicts.