Add "Hub Motor Measurements"

Hub-Motor-Measurements.md

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+# Hub Motor Measurements
+
+**Date:** 2026-02-28
+**Measured by:** Tee (digital caliper)
+**Motor:** 8" / 10x2.125 hoverboard hub motor (brushless, FOC ESC)
+
+## Axle Profile (Stepped / Keyed)
+
+The axle has a **stepped profile** — wider base near the hub, narrower keyed section for dropout clamping.
+
+```
+        ┌─────────┐
+        │  15.35  │ ← flat-to-flat (top)
+        │         │
+        │  ø13mm  │ ← keyed shaft section
+        │         │
+        │ 43.35mm │ ← keyed section length
+        │         │
+        ├─────────┤
+        │  ø16mm  │ ← base section (measured 16.11mm)
+        │  15mm   │ ← base section length
+        └─────────┘
+        ─── hub face ───
+
+        Total protrusion: 63.5mm
+```
+
+## Measurements
+
+| # | Dimension | Value | Notes |
+|---|-----------|-------|-------|
+| 1 | Axle base diameter | **16.11 mm** | Near hub face |
+| 2 | Axle keyed section diameter | **~13 mm** | Narrower upper section |
+| 3 | Total axle protrusion | **63.5 mm** | Hub face to tip |
+| 4 | Keyed section length | **43.35 mm** | Upper shaft only |
+| 5 | Bearing seat OD | **~37.8 mm** | Hub center collar |
+| 6 | Flat-to-flat width | **15.35 mm** | Anti-rotation flats at top |
+| 7 | Base section length | **15 mm** | Lower stepped portion |
+
+## Tire Specs
+- Inflate to **35 PSI** (240 kPa / 2.5 BAR)
+- Size: 10x2.125 pneumatic
+
+## Design Implications
+- **Dropout clamp** must accommodate stepped profile: 16mm bore at base, 13mm keyed slot above
+- **Anti-rotation** via flats (15.35mm flat-to-flat) — design dropout with matching D-profile
+- **Axle length** (63.5mm) gives room for dropout plate + nut + washer
+- **Bearing seat** (37.8mm OD) — allow clearance on inner face of mount