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Free local adjustment tool
Enter the positive saddle setback you already measured behind the bottom bracket, choose one forward or backward move from 1 to 10 mm, and get the exact new setback to re-measure on the bike.
This is coordinate arithmetic, not a fit prescription. The assistant does not treat KOPS, a symptom or a body heuristic as an ideal position. It keeps comfort confidence low, requires the real component limits and preserves a concrete route back to your baseline.
One-axis adjustment
Start from a setback you physically measured. This page calculates a new horizontal target; it does not choose a direction from pain, posture, body dimensions or KOPS.
Your target
The result will show the exact positive setback target behind the BB, how to mark the baseline and how to return to it. Comfort remains a ride-test question, not an arithmetic output.
Optional local journal
A saved plan stays in IndexedDB on this device. After 2–3 comparable rides, record keep, revert or a safety stop; the matching bike coordinate updates atomically with that history event.
Repeatable workflow
A useful setback record names both ends of the measurement: the vertical plane through the BB centre and one specific saddle reference. Keep that same reference through the baseline, adjustment and reversal. If you change the saddle, its shape changes the reference and this target no longer transfers directly.
Level and secure the bicycle. Use the BB centre as the vertical datum and name one saddle reference point that you can find again. Measure the horizontal distance between them; enter it as a positive number when the saddle reference is behind the BB.
Photograph both rail-to-clamp boundaries, add removable marks and record the current saddle height and tilt. These records are the path back to the baseline.
Choose forward or backward yourself and enter one movement from 1 to 10 mm. The assistant will not select a direction from symptoms, body measurements or KOPS.
Before using a target, confirm the new clamp position is inside the saddle rail manufacturer's permitted markings, the rail and clamp are compatible, and you have the exact saddle and seatpost torque instructions.
Adjust until the same saddle reference reaches the calculated horizontal target, torque the clamp exactly as specified, confirm it is secure and re-measure. If the trial is worse, restore the recorded baseline and repeat every installation check.
What is exact
Moving backward increases positive setback; moving forward reduces it. The assistant performs only that signed addition or subtraction. The exact target is useful only if you re-measure horizontally from the same BB datum to the same saddle reference after installation.
What remains unknown
No arithmetic can establish your ideal balance, comfort or joint response. KOPS can be recorded as a setup reference, but this tool neither asks for it nor turns it into a verdict. Do not use a self-directed adjustment to work around numbness, severe pain, sudden weakness or worsening symptoms.
Primary and manufacturer sources
Manufacturer fit guide
Trek describes fore-aft adjustment at the saddle rails and gives a concrete warning that rail and clamp compatibility matters. Your exact saddle and seatpost instructions remain controlling.
Read the Trek fitting guideManufacturer torque guide
Trek explains why both over- and under-tightening can damage or loosen a component and directs riders to the exact manufacturer specification rather than a universal number.
Read the Trek torque guidePrimary research - 2018
The experiment shows that fore-aft position can alter knee-joint forces. It does not establish one universal setback, so OpenBikeFit reports the coordinate without prescribing a direction.
Open the PubMed recordRecord saddle height before the move, use an on-device camera re-check after the mechanically safe change, or return to the tools hub without stacking another adjustment into the same trial.