Logging fit measurements (stack, reach, saddle, crank)

Why these matter

A bike fit is only useful if you can reproduce the position after travel, a crash, a component change, or a switch between bikes. Logging the seven core measurements lets you reset the position to the exact state that was working.

All seven measurements live as decimal fields on the BikeSetup record. They are optional, but the more you have, the easier the position is to recreate.

The seven measurements

Stack (mm)

The vertical distance from the bottom bracket centre to the top of the head tube. A frame property. Combined with reach, it defines the basic frame geometry.

Reach (mm)

The horizontal distance from the bottom bracket centre to the top of the head tube. A frame property.

Pad width (mm)

For a TT or tri bike: the distance between the centres of the aerobar pads. Affects shoulder position and aero drag.

Pad stack (mm)

The vertical distance from the bottom bracket to the top of the aerobar pads. Drives the rider's upper-body height in the TT position.

Crank length (mm)

The length of the crank arms, in mm. Common values: 165, 167.5, 170, 172.5, 175. Affects hip angle in the tri position; many triathletes run shorter cranks than they would on the road.

Saddle height (mm)

From the centre of the bottom bracket to the top of the saddle, measured along the seat tube angle. The single most important position number for power and injury risk.

Saddle setback (mm)

The horizontal distance from the bottom bracket centre to the nose of the saddle (or to a specific saddle reference point, depending on convention). Drives knee-over-pedal-spindle position.

How to measure consistently

• Use the same reference points every time. Pick one saddle reference point (e.g. the nose) and stick with it.
• Measure with the bike level (not leaning against a wall).
• Record to the nearest 0.5 mm if you can. Fit work reveals 1 to 2 mm changes.

What to do after a fit session

1. Capture the new measurements before the athlete leaves the fit room.
2. Open the BikeSetup in the Hub and update each field.
3. Log a BikeChangeLog entry noting "post-fit session, [fitter name], YYYY-MM-DD" so the timeline is searchable later.
4. Note any CdA or Crr changes if you measured them.

See also: Bike setup overview, Bike change log, CdA Field Test.