Tire Sizing

Tire Sizing (vehicle-design/sizing/tire-sizing)

Use when the task is selecting the tires for the landing gear at the conceptual level: the static load per tire, the tire diameter and width estimates, the number of tires, the inflation pressure, and the footprint and rolling radius that come out of the load.

Domain quick reference

• The static load per tire is the gear load share of the takeoff weight divided by the number of tires on that gear. Worked: a 78,000 kg transport with 0.95 of the weight on the main gear and 4 main tires carries 78000 * 0.95 / 4 = 18,525 kg per tire, which is 40,841 lb.
• Tire diameter and width are estimated with power law fits of the form dimension = coeff * loadexponent on the load per tire in pounds. Representative class-I fit values: diameter = 1.63 * P0.315 and width = 0.40 * P**0.36, giving 46.2 in (1174 mm) diameter and 18.3 in (464 mm) width at 40,841 lb. The fits are estimates: the final tire comes from the tire catalog and the wheel rating.
• The footprint contact area is the load per tire divided by the inflation pressure: 40,841 lb at 200 psi gives 204.2 sq in. Main tire inflation pressures for transports commonly fall in the 150 to 220 psi band.
• The rolling radius is half the tire diameter: 23.1 in (587 mm) for the 46.2 in tire.
• The required number of tires on a gear is the gear load total divided by the maximum load capacity per tire, rounded up: 163,363 lb on the main gear at 45,000 lb per tire gives 4 tires.
• Nose gear worked example: 0.10 of the weight and 2 nose tires give 3,900 kg per tire (8,598 lb) and a 28.3 in (719 mm) diameter tire.
• Tire and wheel selection sits in the FAR-25 / CS-25 context, including the wheel and tire rating and landing gear drop test requirements (25.723, 25.733, 25.735).

Workflow

1. Collect the takeoff weight, the gear load share fraction (for example 0.95 main gear and 0.10 nose gear), the number of tires on the gear, and the inflation pressure.
2. Compute the load per tire with static_load_per_tire(mtow_kg, gear_fraction, n_tires) and convert it to pounds with kg_to_lb.
3. Estimate the dimensions with tire_diameter_inches(load_lb) and tire_width_inches(load_lb).
4. Select the tire count with required_number_of_tires(gear_load_lb, max_load_per_tire_lb) from the gear load total and the catalog capacity.
5. Compute the footprint with footprint_area_sqin(load_lb, pressure_psi) and the rolling radius with rolling_radius_inches(diameter_in).
6. Close with the catalog check: pick the catalog tire whose rating covers the static load per tire at the selected pressure.

Pitfalls

• Routing strut load distribution here: the CG-based nose and main gear load share, the shock absorber stroke, and the tire rating margin check against an existing tire belong to the landing-gear-sizing sub-skill; this leaf starts from the per-tire static load and selects the tire itself.
• Routing ground handling questions here: braking friction, cornering, and traction coefficients belong to the performance leaves, not to tire geometry sizing.
• Treating the power law fit as a catalog value: the fit is a class-I estimate; the final tire must come from the tire catalog with the rating checked against the static load per tire.
• Mixing units: the fits are pound-inch curve fits; convert kg to lb (1 kg = 2.20462 lb) and inches to mm (1 in = 25.4 mm) before comparing with a metric catalog.
• Rounding the tire count down: the required number of tires rounds up; a fractional division result means one more tire is needed.
• A gear fraction above 1: one gear cannot carry more than the total weight; the logic raises ValueError instead of guessing.
• Passing a zero load, a zero pressure, or a non-integer tire count: the logic raises ValueError.

Behavior contract (gate 3)

The load split, power law dimension fits, footprint, rolling radius, and tire count logic are exercised by the gate 3 contract test: scripts/test_tire_sizing.py against scripts/tire_sizing_logic.py (stdlib unittest, offline). Run: python3 scripts/test_tire_sizing.py

Compliance

• Standards referenced, not reproduced: FAR-25 is US government work (public domain) and CS-25 is a free EASA download; the wheel and tire rating and landing gear drop test requirements frame the tire sizing context, and the power law fits are common conceptual design methodology, summary-only per standards-map.yaml.
• compliance: STANDARDS-REF, gated: false.