KOPS explained: knee over pedal spindle

Put the crank of your forward leg horizontal (3 o'clock) and drop a vertical line from the bony bump just below your kneecap (the tibial tuberosity). KOPS is the horizontal distance from that line to the pedal spindle: positive when the knee is ahead of the spindle, negative when it is behind.

It is a proxy for how your weight and pedalling forces are distributed. Move the saddle forward and the knee travels forward with it; move it back and the knee follows.

For a road or endurance position, within ±5 mm of the spindle is the usual target, and ±12 mm is still tolerable. Time-trial and triathlon positions sit steeper and further forward on purpose: −2 to +12 mm is typical, and riders at the aggressive end go beyond that.

Saddle fore-aft is the adjustment: loosen the seatpost clamp and slide the saddle on its rails. Move it in 8–10 mm steps if you are just outside the range, 10–15 mm if you are well outside, and re-measure after each change.

Two side effects to remember. First, moving the saddle back effectively raises it (and forward lowers it) because the seat tube is angled — re-check your knee angle after a fore-aft change. Second, fore-aft also changes your reach to the bars, so a big saddle move may call for a stem change.

KOPS is a convention, not physics — there is no biomechanical law that the knee must sit exactly over the spindle. Its real value is as a reproducible reference: it puts your saddle in a sensible starting zone and gives you a number to keep constant when you change other parts of the fit.

Treat it as a starting point plus a red flag for extremes. A knee far ahead of the spindle loads the front of the knee; far behind shifts work to the hamstrings and usually signals a saddle pushed back too far.

Pedalyze measures the offset from a side-on photo with the crank at 3 o'clock, using the detected ankle as the pedal-spindle reference. Because the ankle sits slightly behind the spindle over a horizontal crank, the reading is a touch rearward of a true plumb-line measurement — a small, consistent bias that does not affect before/after comparisons.

Should I use saddle fore-aft to change my reach to the handlebars?

No. Set fore-aft for your position over the pedals (KOPS), then fix reach with stem length and spacers. Using the saddle as a reach adjustment compromises your pedalling position to solve a cockpit problem.

Why do TT positions ignore the classic KOPS rule?

A TT position rotates the whole rider forward around the bottom bracket to flatten the torso. The knee naturally travels ahead of the spindle. That is expected — TT fits use a more forward target range rather than the road ±5 mm.

Do I measure KOPS on the left or right leg?

Whichever leg is forward with its crank at 3 o'clock — measure the side facing the camera or the plumb line. If you suspect asymmetry, measure both sides; a large left-right difference is worth showing to a professional fitter.

What is a plumb line and do I need one?

Any string with a small weight (a key, a nut) works as a plumb line. Or skip it: a level, side-on photo lets you read the vertical relationship directly, which is what Pedalyze automates.

• Knee angle in bike fitting
• Road bike fit angles
• TT & triathlon bike fit
• How to take bike fit photos

• Saddle setback calculator — See how far behind the bottom bracket your saddle sits, and how that compares with the range road and time-trial riders use.
• Saddle height calculator — Estimate your saddle height from inseam with the LeMond method, measured from the bottom bracket to the top of the saddle.