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Pneumatic Caster Tire Pressure

Work out the contact patch and sidewall squash of a pneumatic or flat-free caster tire, and the inflation pressure it needs to carry the load without folding its sidewall every revolution.

A pneumatic caster carries its load in the air, so the pressure and the patch are two views of the same thing. Enter the wheel load, the tire diameter, the section width, the pressure, and the load rating, and the calculator returns the contact patch area, its dimensions, the sidewall squash, and the pressure the load implies.

The contact patch is modelled as a rectangle, so the true peak pressure at the centre of the patch is higher than the average shown. Carcass stiffness, ply count, and the sidewall load rating still decide whether a correctly inflated tire is also an adequately rated one.

Inputs

Load and tire

Range: 10–20,000 lbf

Range: 1–8 in

The width of the tire, not the rim. A 3.50-4 tire is 3.5 in wide on a 4 in rim.

Range: 4–20 in

Inflation

Range: 5–80 psi

Use the pressure actually in the tire, not the maximum moulded on the sidewall.

Result

Contact patch area (in²)
13.33
Contact patch length (in)
4.44
Pressure needed for a 20% patch (psi)
66.7
Patch length / tire diameter
0.444
Verdict
Badly squashed. A contact patch this long means the sidewall is folding on every revolution, which heats the tire, cracks it at the bead, and rolls the tread under the rim. Raise the pressure, reduce the load, or move to a tire with a higher load rating.
Pressure note
The load needs a higher pressure than the tire is carrying to keep the patch at a sensible length. Check the maximum pressure moulded on the sidewall before inflating: if the required figure is above it, the tire is under-rated for this load, not under-inflated.
Tire type note
A pneumatic tire absorbs shock through its air volume, which is why it is preferred for rough or outdoor ground. That same air volume is what flexes under load, so pressure matters more here than on any solid wheel.

What to check

  • Use the load one tire carries, not the total equipment weight.
  • Set the pressure against the tire's own rating, not the maximum on the compressor.
  • Re-check the patch after any load change, because pressure and load move together.

Related calculators

A steady-state model that ignores the transient when a load is dropped onto a tire and the pressure rise from heat in a long tow. It does not model carcass stiffness, ply count, or the sidewall load rating, so a tire can be correctly inflated and still be under-rated, and it ignores the rolling-radius change with deflection. A hard, level surface is assumed.