Johann J Cardenas

Gear3D — Sources

Every number in this app’s library traces to something. This document says what, and — just as importantly — how strongly each number was verified during the build.

Read §1 before using any of these values in a publication.

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1. Verification status — read this first

Three levels are used throughout the data files:

Level	Meaning
verified	Retrieved from the cited source during this build and checked figure-for-figure.
medium	Well-established published engineering practice, reproduced from the standard literature, but not opened against a primary document during this build. Check before publication use.
low	Representative of the vehicle type; not a specific model, not a cited figure. Adequate for illustrating a configuration, not for quoting.

What was verified during this build:

• AASHTOWare Pavement ME Design default traffic inputs — tandem 51.6 in, tridem 49.2 in, quad 49.2 in, dual tire spacing 12 in, tire pressure 120 psi, average axle width 8.5 ft, wheelbase classes 12/15/18 ft with a 17/22/61 % split. Retrieved from the published help manual.
• FHWA Scheme F class definitions and axle topology for all 13 classes.
• Huang’s contact-area coefficient (0.5227) and the shape it derives from.
• 23 CFR 658 statutory limits (20 000 lb single, 34 000 lb tandem, 80 000 lb gross, 102 in width) and the Federal Bridge Formula — these are statutory text and are additionally re-derived arithmetically by the test suite.

What was NOT verified during this build:

• All aircraft gear geometry. See §5. No aircraft library ships in v1.0 precisely because of this.
• Inch-nominal truck tire overall diameters other than 11R22.5. See §4.
• Track widths and wheelbases of individual truck classes. See §3.

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2. Primary sources

FHWA Traffic Monitoring Guide — Scheme F vehicle classification

• Publisher: U.S. Federal Highway Administration
• URL: https://www.fhwa.dot.gov/policyinformation/tmguide/
• Used for: the 13-class definitions, axle counts, unit counts, and the four/six-tire criterion separating classes 3 and 5.
• Status: verified.

AASHTOWare Pavement ME Design — default traffic inputs

• Publisher: AASHTO
• URL: https://aashtowarepavementme.org/MEDesign/help/traffic.html
• Used for: every intra-group axle spacing in the library, the tractor wheelbase classes, and the default inflation pressure of the contact-patch module (120 psi = 827.371 kPa).
• Status: verified — values quoted exactly as published.

23 CFR 658 — Truck size and weight, route designations

• Publisher: U.S. Government
• Used for: 20 000 lb single-axle limit; 34 000 lb tandem-axle limit; 80 000 lb gross limit; 102 in (2591 mm) width limit; the Federal Bridge Formula W = 500[LN/(N−1) + 12N + 36].
• Status: verified. The bridge formula is additionally re-derived in test/run.mjs, which independently confirms that a two-axle group at 4.0 ft is allowed exactly 34 000 lb — the arithmetic origin of the statutory tandem limit.

Yoder & Witczak, Principles of Pavement Design; Huang, Pavement Analysis and Design

• Used for: the classic dual-wheel idealisation — dual tires at 13.5 in centres with the dual sets 72 in (1829 mm) apart — which is the basis for the drive/trailer track width throughout the library; and Huang’s contact-area idealisation.
• Status: the 0.5227 coefficient is verified. The 72 in track is standard in this literature and is used as the library’s stated basis.

Tire and Rim Association Yearbook

• Used for: the three-part aircraft tire designation convention (overall diameter × section width − rim diameter), which lets src/core/tires.js compute aircraft tire dimensions arithmetically; and the inch-nominal truck tire dimension table.
• Status: not consulted directly during this build. The designation convention is implemented from well-established practice. The size table entries are marked individually — see §4.

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3. Truck geometry — what is derived and what is typical

Derived, and therefore reproducible:

• All intra-group axle spacings — AASHTOWare defaults, verified.
• All tractor wheelbases — AASHTOWare wheelbase classes, verified.
• All axle loads — statutory limits or an explicitly stated division of them.
• All trailer-group longitudinal positions — set to the smallest realistic spread satisfying the Federal Bridge Formula over every consecutive-axle subset at the stated loads. This is a real derivation, not an estimate, and the test suite re-derives it. See DECISIONS.md §D7.

Typical practice, confidence medium — check before quoting:

• Steer axle track 2032 mm (80 in) on classes 5–13.
• Drive/trailer track 1829 mm (72 in) — from the classic dual-wheel idealisation above; it is the standard figure of the literature, but it is a modelling convention, not a manufacturer specification.
• Dual tire spacing 330 mm (13 in). Sources genuinely disagree here and the disagreement is recorded rather than hidden:
  • AASHTOWare Pavement ME default: 12 in (304.8 mm) — verified.
  • Yoder & Witczak / Huang dual-wheel idealisation: 13.5 in (343 mm).
  • Library default: 330 mm (13 in), mid-range and representative of current US class 8 practice. A user comparing Gear3D output against a default Pavement ME run should set this to 305 mm; against the classic textbook idealisation, 343 mm.
• Overall lengths, bus and vocational wheelbases.

Low confidence — representative only:

• Classes 1–3 (motorcycle, passenger car, pickup) in full. Their loads are order-of-magnitude estimates from representative kerb masses. They exist so the FHWA classification is complete; they are not pavement-design inputs.
• The class 13 turnpike double’s overall layout, which is representative of permitted turnpike practice rather than a specific carrier’s configuration.

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4. Tire dimensions

Computed exactly from the designation — no table, no uncertainty:

• Metric truck and passenger sizes (445/50R22.5, 295/75R22.5, LT245/75R16, …). Overall diameter follows arithmetically: OD = rim + 2 × section × aspect.
• All aircraft sizes (H44.5x16.5-21, 52x21.0R22, 1400x530R23, …). The three-part designation encodes all three dimensions directly.

Table lookup, in src/data/tires.json:

Size	Confidence	Note
11R22.5	high	279 mm section / 1054 mm OD. The reference truck tire of the flexible-pavement literature; dimensions agree across the major medium-truck data books.
11R24.5, 12R22.5, 10.00R20, 11.00R20, 9.00R20	medium	Reproduced from the standard truck-tire literature. Not checked against a TRA yearbook during this build.

A size that is not in the table is reported by the app as unknown and refuses to produce geometry. It is never guessed. Adding one requires a source and a confidence field, and the test suite fails without them.

Static loaded radius. Two models are implemented and the choice is exposed:

• radiusRatio (default): SLR = ratio × free radius, 0.97 truck / 0.965 aircraft. These defaults come from the build specification and are user-adjustable. Confidence: medium — they are reasonable engineering values, not a cited standard.
• sectionDeflection: SLR = free radius − deflection × section height, with 0.32 for aircraft, following the Tire and Rim Association convention that aircraft tires are rated at a nominal 32 % deflection.

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5. Aircraft — what is sourced, derived and assumed

The aircraft library ships in v1.2 with four Boeing aircraft spanning gear codes D, 2D and 3D: 737-800, 757-200, 767-400ER and 777-300ER.

An earlier attempt was abandoned because the sources could not be reached (the FAA database returned 403 to the fetch tool, and the ACAP PDFs exceeded its size limit). Both were tooling limits, not access limits: the FAA site serves the spreadsheet normally to a browser user-agent, and the PDFs download fine with curl. Every number below was retrieved and read directly.

5.1 Authoritative — taken verbatim

Quantity	Source
Gear designation (Main_Gear_Config)	FAA Aircraft Characteristics Database
Wheelbase, nose to main gear	FAA Aircraft Characteristics Database
Main gear outer width	FAA Aircraft Characteristics Database
MTOW	FAA Aircraft Characteristics Database
Maximum design taxi weight	Manufacturer ACAP, section 7.2
Tire size and tire pressure, nose and main	Manufacturer ACAP, section 7.2
Percent gross weight on the whole main gear (95 %)	FAA AC 150/5320-6G, G.1.3

ACAP editions used: 737 D6-58325-7 Rev C (Oct 2025), 757 D6-58327 Rev H (Dec 2024), 767 D6-58328 Rev K (Dec 2024), 777 D6-58329-2 Rev G (Dec 2024).

5.2 The outer-width trap

The FAA field is not the centreline tread. Its own data dictionary defines Main_Gear_Width_ft as “Distance between outer tires in the main landing gear.” Treating it as the track would push every main wheel outboard by half a dual spacing plus half a tire — for a 777 that is nearly a metre per side, and the figure would look entirely reasonable while being wrong.

So the track is derived, never assumed:

track = outerWidth − (wheelsAcross − 1) × dualSpacing − sectionWidth

Section width comes exactly from the three-part tire designation.

5.3 The cross-check that makes this trustworthy

Nothing in that derivation uses the manufacturers’ separately published tread figures, so agreement between them is real corroboration rather than circularity. With the dual spacings recorded in the data files:

Aircraft	Derived track	Manufacturer published tread	Difference
737-800	5727 mm	5715 mm (18 ft 9 in)	12 mm
757-200	7302 mm	7315 mm (24 ft 0 in)	13 mm
767-400ER	9322 mm	9296 mm (30 ft 6 in)	26 mm
777-300ER	10 963 mm	10 973 mm (36 ft 0 in)	10 mm

All four agree to within a few centimetres, on quantities of 6 to 11 metres. test/run.mjs asserts both the derivation and this cross-check.

5.4 Assumed — declared, and shown in the app

Two quantities are not constrained by any source consulted:

• Nose gear dual spacing. Nothing published pins it down.
• Tandem spacing on 2D and 3D gears. The wheelbase is measured to the main gear centroid, so the spread within a bogie does not move it, and no other figure constrains it.

Every aircraft unit lists these in assumedFields, the schema fails validation if that array is missing, and the app shows an amber notice naming them whenever an aircraft is loaded. Set them from FAARFIELD before using the output for pavement work — and note that changing a dual spacing re-derives the track, so the authoritative outer width is preserved whatever you enter.

5.5 Not included, and why

The 747 (2D/2D2) and A380 (2D/3D2) are omitted. Their wing-plus-body gear layouts need the longitudinal and transverse offsets of the body gear relative to the wing gear. A single outer width closes a two-strut layout; it cannot close a four-bogie one. Including them would mean inventing geometry rather than deriving it, which is the one thing this library exists not to do.

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6. How to check any number in this app

Every axle carries a source string and every load a basis string, both shown in the properties panel when you select an axle. Export unit.json alongside any figure and the citations travel with it.

The test suite (npm test, 74 checks) fails the build if any axle, gear, load, GVW, MTOW, tire pressure or multi-axle group spacing lacks provenance, and includes a negative control that confirms the validator actually rejects a missing source rather than passing vacuously.

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