Wheel circumference vs loaded radius: choose the right measurement

A wheel-based device can use distance per revolution to translate counted revolutions into travelled distance. A ground-to-axle measurement instead locates an axle above a chosen ground reference under a loading condition. An ideal wheel radius in a rigid geometry model defines a mathematical circle. They can be related in a simplified construction without being identical physical observations on a deformable tyre.

Start the note with the destination field and its unit. If a device expects circumference in millimetres, 700C is not the requested number and 622 is not a circumference. If a geometry model expects a radius in millimetres, a 2160 mm rolling result cannot be pasted directly into that field either. Missing definitions should be resolved from the exact device or model instructions rather than corrected by trial and error.

Schwalbe’s dimension guidance describes a straight rolling test using the valve as a repeatable wheel marker and measuring travel over one complete revolution under a rider-loaded condition. Use that principle only with an already permitted assembly and a secure, controlled arrangement. It is not an instruction to ride an unsupported tyre, reduce pressure, handle moving parts or balance unsafely while measuring.

Record the wheel used, tyre and rim identity, relevant condition and loading method. Mark the starting location of the chosen wheel marker, roll along a straight unobstructed path until it returns to the same orientation after one complete turn, and measure the travel between the corresponding marks. Use suitable help if needed; if the arrangement cannot be controlled safely, retain the limitation and use appropriate assistance rather than forcing the observation.

For a mathematical circle rolling without slip, circumference C = 2πR. A fictional C of 2160 mm gives R = 2160 ÷ 2π ≈ 343.8 mm and diameter ≈ 687.5 mm. Call that an equivalent ideal-circle radius derived from the travel. The calculation does not show where the physical axle sits above the ground when a real tyre is loaded.

Do not use nominal tyre width as the radial extension needed to build an outside diameter from a bead-seat circle. Likewise, do not assume that doubling a separately measured loaded axle height recreates the rolling circumference of a deformable tyre. The definitions differ. The simple equation explains a model, while the real observation retains its actual loading, tyre profile and other conditions.

A circumference note can support an exact device’s manual-entry discussion. A declared ideal radius can support an explicitly hypothetical front-geometry comparison. A physical axle-height observation can support a record under its own conditions. None of those establishes tyre/rim compatibility, a safe pressure, adequate clearance or a better rider position. Device settings change the interpretation of observations, not the bicycle’s physical dimensions.

Use the free worksheet’s blank notes to retain the marker, method, raw distances and conditions. The front sheet is a separate optional model: choose its mode explicitly and read its radius convention before entering anything. Reading these guides needs no camera, account or saved bike. Downloads or paper notes, if you later choose them, remain outside the page; they are not recalled by closing a reading session.