Wheel Spin-Down Rig: Analytical Interpretation Guide

This document defines the physical principles, data extraction mechanics, and diagnostic patterns for interpreting data generated by the Pi Pico single-pulse revolution logger and visualized on the Vue 3 dashboard overlay.

1. Graph Overview & Data Extraction Profile

Chart 1: RPM Decay Curve

• What it plots: Rotational Velocity (RPM) on the Y-axis against Relative Run Time (Seconds) on the X-axis.
• The Physics: This graph maps total kinetic energy loss over time. The shape of the line represents the combined braking effect of fluid air resistance and mechanical hub friction trying to overcome the wheel’s Moment of Inertia (Inertia).
• Baseline Behavior: A normal wheel shows a smooth, continuous, downward-sloping decay curve. Higher lines that stretch further to the right represent a more efficient overall system.

Chart 2: Time Delta Per Rotation

• What it plots: The time duration of an individual turn in Milliseconds (Y-axis) against the progressive Run Revolution Count (X-axis).
• The Physics: This chart isolates the latency cost of every single physical rotation. Because it ignores the overall time duration and focuses strictly on turn-by-turn deceleration, it maps the mechanical consistency of the internal hardware components.
• Baseline Behavior: A smooth, gradual curve sweeping upward. As the wheel slows down, each turn naturally takes longer.

Chart 3: Instantaneous Power Drag (Watts)

• What it plots: Kinetic Energy Loss converted into Watts (Y-axis) against Relative Run Time (Seconds) on the X-axis.
• The Physics: This calculation translates velocity changes into the wheel’s deceleration rate. By looking at the drop in Joule energy between consecutive turns divided by the time elapsed (Power = Change in Energy / Change in Time), it maps the active power drain.
• Baseline Behavior: Starts at its highest wattage value at 200 RPM and slopes down toward 0 Watts as the wheel approaches a complete stop.

2. Diagnosing Data Patterns (How to Read the Lines)

A. Isolating Aerodynamic Drag vs. Bearing Friction

Because aerodynamic drag scales exponentially with speed while mechanical bearing friction stays mostly constant, you can isolate them by looking at different sections of your charts:

• High-Speed Section (200 RPM down to ~120 RPM): If a wheel plunges steeply downward on the RPM chart and shows high wattage loss on the Power chart here, it is fighting Aerodynamic Drag. This is normal for wide, knobby gravel tire setups (e.g., 44mm tires acting like air brakes).
• Low-Speed Section (100 RPM down to 50 RPM): Air resistance drops to near zero here. If a line continues to drop steeply or stays elevated on the Wattage chart in this window, the drag is Mechanical. This indicates sticky grease, bad seals, or binding bearings.

B. Identifying Internal Hub Compression (Thru-Axle Defects)

To test if your frame’s thru-axle clamping pressure is crushing your hub internals, compare a finger-tight axle run (2 Nm) against a factory torque run (10-12 Nm):

• Healthy Hub Space: The lines for both torque levels sit perfectly on top of each other across all three charts. The bearings are protected by a perfectly sized internal metal spacer sleeve.
• Short Spacer Sleeve / Alignment Defect: The 12 Nm line drops significantly faster on the RPM chart, shows an elevated baseline on the Power chart, and climbs much earlier on the Time Delta chart. The axle clamping force is shoving the inner bearing races inward, jamming the balls sideways into their tracks.

C. Spotting “Jiggly” Lines vs. Real Mechanical Jitter

• Microsecond Clock Noise (Fixed by Software Filter): Minor, uniform jiggles across the entire Power line are usually electronic artifacts caused by slight variations in sensor alignment or microsecond timer resolution. These are smoothed out by the dashboard’s 5-point moving average filter.
• True Mechanical Binding (Notches or Pitting): If you see sharp, isolated spikes or erratic jumps on the Time Delta Per Rotation chart that occur at repeating intervals, this indicates a physical defect. It means a ball bearing is deformed, the cartridge is unseating, or an internal race is pitted, causing the wheel to physically “catch” or drag at a specific point in its rotation.

3. Moment of Inertia (MOI) Reference Matrix

To ensure the Instantaneous Power Drag (Watts) calculation is mathematically accurate, the Moment of Inertia slider variable (Inertia, measured in kg * m^2) must match the weight distribution of the physical wheel setup being tested. Formula: Inertia = Sum of (Mass * Radius squared)

Now that the documentation is locked down in a safe, readable format, we can shift back to your mechanical workflow. Let me know when your bearing press tools land on your workbench so we can tackle the true mechanical baseline calibration for that DT Swiss 240!