Sagitta needs three groups of numbers: your body (measured with a tape and a book), your bike's geometry chart (looked up, not measured), and your current setup (measured on the bike). Kit list: a metric tape measure, a hardback book, a piece of string with a weight (a plumb line), a phone spirit-level app, a wall, and ideally a helper.
1 · Measuring yourself
Height
- Barefoot, heels and back against a wall, look straight ahead.
- Rest a book flat on your head, spine against the wall, and mark the wall at its underside.
- Measure floor → mark. Enter in cm.
Inseam (the one that matters most)
- Barefoot, back against the wall, feet ~15 cm apart.
- Pull a hardback book up between your legs, spine up, until it presses firmly into the soft tissue — saddle-firm, not gentle. Keep it level against the wall.
- Mark the wall at the top of the spine, step away, measure floor → mark.
- Repeat three times; take the consistent value. Every millimetre here moves the saddle recommendation by ~0.9 mm, so this is worth doing carefully.
Sagitta cross-checks inseam against height — a value outside the human range (a unit mix-up, a soft book press) gets flagged in the Changes panel rather than silently producing a confident wrong number.
2 · The geometry chart — looked up, not measured
Search "<brand> <model> <year> geometry" and read your size's row. These numbers describe the frame and are near-impossible to measure accurately at home — trust the chart.
| Sagitta field | Chart name(s) | Notes |
|---|---|---|
| Stack / Reach | Stack, Reach | Universal on modern charts |
| Seat angle | Seat tube angle (effective) | If the chart lists actual + effective, use effective |
| Head angle | Head tube angle | |
| Head tube | Head tube length | |
| Chainstay | Chainstay length, rear centre | |
| Wheelbase | Wheelbase | |
| BB drop | Bottom bracket drop | Not "BB height" — drop is below the axles |
| Crank | — | Printed on the inside of the crank arm: 165 / 170 / 172.5 / 175 |
3 · Your current setup — road
Saddle height
- Find your sit point: the widest part of the saddle, where your sit bones actually rest — typically 12–14 cm behind the nose. Put a small tape mark there.
- Measure the straight line from the centre of the crank axle (BB) to the mark. Don't follow the seat tube's kinks — straight line, centre to mark.
Saddle setback
- Hang a plumb line (string + weight) from the sit-point mark.
- Measure horizontally from the string to the centre of the crank axle. Behind the BB = positive. A spirit-level app on the top tube first confirms the bike is upright.
Cockpit
- Spacers — total stack height under the stem (measure the rings, or count: most are 5 or 10 mm).
- Stem length & angle — printed on the stem (length is centre-to-centre; angle is usually ±6° or ±17°; negative = pointing down in the normal position).
- Bar reach & drop — from the handlebar's spec sheet, or measure: reach = bar-top centre to the furthest-forward point of the bend; drop = vertical distance bar-top centre to drop-end centre.
4 · Your current setup — TT / aerobars
Both pad numbers locate the centre of the pad's top surface relative to the BB. The floor-reference method is the reliable way to get them:
- Bike level (turbo + front riser; spirit-level app on the top tube or wheel axle line).
- Pad stack = (pad-top height from floor) − (BB-centre height from floor). Two vertical tape measurements, subtract.
- Pad reach = horizontal BB → pad centre: hang a plumb line from the pad centre, mark the floor; plumb from the BB centre, mark; measure between marks.
- Extension tilt = angle of the extensions above horizontal — spirit-level app resting on the extension. Flat = 0°, common risers 5–15°, high-hands setups more.
5 · Wheel outer diameter — for photo calibration
The photo view's mm scale comes from two markers on opposite tyre edges plus the WHEEL Ø value — the outer diameter of the inflated tyre, not the rim. The default 688 mm is a 700×28c road setup. If you run something else, either measure it (tape across the wheel through the hub, outer edge to outer edge) or derive it: rim diameter + 2 × tyre width — roughly 700×25c ≈ 682 · 700×28c ≈ 688 · 700×32c ≈ 696 · 650b×47 ≈ 678. A few mm of error only moves mm-based readings (KOPS) proportionally — angles don't use the scale at all.
6 · Photo markers — the exact landmarks
Angles are measured between joint rotation centres, so each marker goes on the bony landmark that approximates the joint's axis on the camera side — not the pointy bits. These are the same landmarks professional video fits use:
| Marker | Put it on | Not |
|---|---|---|
| SHOULDER | The bony bump where the arm meets the shoulder, just below its outer tip (greater tuberosity) | The top of the shoulder / trap muscle |
| ELBOW | The bony knob on the outside of the elbow (lateral epicondyle) | The pointy back tip (olecranon) |
| WRIST | The bony bump on the little-finger side of the wrist (ulnar styloid) | The middle of the hand |
| HIP | The greater trochanter — the bony prominence on the side of the hip; rotate your leg and it's the bit you feel moving | The waistband or the crease of the shorts |
| KNEE | The side of the knee at its rotation axis — level with the bottom of the kneecap, midway front-to-back (lateral epicondyle) | The kneecap itself |
| ANKLE | The outer ankle bone (lateral malleolus) | The heel or the top of the shoe |
| PEDAL | The centre of the pedal spindle (the axle end-cap) | The pedal body edge |
| RIM ⌀ ×2 | The outermost edge of the inflated tyre, opposite sides straight through the hub | The rim's braking edge |
Snug kit helps (baggy fabric hides every landmark), and a sticker or dot of tape on each landmark before you shoot makes placement trivial — that's exactly what pro fitters do. The app's calibrations assume joint-centre markers: the KOPS plumb line, for instance, already adds the joint-centre-to-knee-front offset for you.
Sanity checks before you trust the numbers
- Bike upright and level for every plumb measurement — lean invalidates them.
- Measure twice, from scratch. Two readings that disagree by >3 mm mean the technique slipped.
- Saddle height + setback should roughly satisfy
setback ≈ height × cos(seat angle)if your saddle is centred on its rails — a wildly different number usually means the sit-point mark is off. - Enter the numbers, look at the 3D render: if the drawn bike looks wrong, a chart number is wrong — wheelbase and chainstay errors show instantly.