KOPS explained: knee over pedal spindle
KOPS — Knee Over Pedal Spindle — is the classic test for saddle fore-aft position. With the crank at 3 o'clock, a plumb line dropped from the front of your kneecap should fall close to the pedal axle. Here is what the number means, how to measure it, and where the rule's limits are.
By Maxence Poutord·Last updated
What KOPS measures
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.
Annotated example: the plumb line and what the photo sees
| Reference point | Reading in this figure | Meaning |
|---|---|---|
| Pedal spindle (manual plumb line) | ≈ +4 mm | Knee just ahead of the axle. Inside the road ±5 mm band |
| Ankle landmark (Pedalyze photo) | ≈ +20 mm, reported as % of shin length | Same saddle position, different reference point. Judged against its own range, not against the plumb-line one |
What offset is normal
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.
| Position | Target offset | Tolerated |
|---|---|---|
| Road / endurance | −5 to +5 mm | −12 to +12 mm |
| TT / triathlon | −2 to +12 mm | −8 to +20 mm |
How to adjust it
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.
The limits of the rule
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.
How Pedalyze measures KOPS
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 and the detected knee joint centre as the knee. Both differ from a manual plumb line: the ankle sits above and behind the spindle, and the joint centre sits behind the front of the kneecap. The result is a consistent offset from a plumb-line reading for a given foot position, which is why the report gives it as a percentage of shin length with its own range, and why before/after comparisons within Pedalyze are unaffected.
Measurement pitfalls
KOPS is a horizontal distance of a few millimetres read off a body that is a metre tall, so small setup errors swamp it. Each row gives the size of the error where it can be computed.
| Pitfall | What it does to the reading | How to avoid it |
|---|---|---|
| Crank not horizontal | On a 172.5 mm crank, 10° of rotation away from horizontal moves the pedal spindle about 30 mm horizontally (172.5 × sin 10°). The offset swings by centimetres with no saddle change. | Use the crank arm, not the pedal body, as the horizontal reference, and check it against the frame edge of the photo. Shoot several frames and keep the most horizontal one. |
| Camera not level | A 2° tilt of the phone turns a 400 mm shin into about 14 mm of fake horizontal offset (400 × sin 2°). That is the whole road tolerance band from tilt alone. | Turn on the grid, level the phone against a door frame or the trainer, or use a tripod. Pedalyze rejects photos tilted more than about 4° rather than measuring them. |
| Camera ahead of or behind the rider | The knee and the pedal sit at different depths from the lens. A camera that is not square to the bike shifts them by different amounts, so the vertical relationship is no longer vertical. | Stand on a line through the hip at 90° to the bike, back far enough to need zoom. The off-axis gate checks hip overlap for this. |
| Vague knee landmark | Front of the kneecap, tibial tuberosity and knee joint centre lie 20–40 mm apart front to back. Two people with plumb lines can disagree by more than the target band. | Pick one landmark and stick with it. Pedalyze always uses the detected knee joint centre, which is why its numbers should be compared with each other, not with a friend's plumb line. |
| Cleat position changed between shots | Moving the cleat 10 mm back moves the spindle 10 mm relative to the ankle. A manual reading changes; an ankle-referenced reading does not, or the other way round. | Settle cleat position first. Judge saddle fore-aft only after the shoes are final. |
| Perching on the saddle nose | Sliding 20 mm forward on the saddle changes KOPS by 20 mm. A posed photo where you sit where you think you should reads differently from where you actually ride. | Pedal for a minute at your usual effort before the shot and stay where you naturally settle. |
| Reading the far leg | The far knee is estimated behind the frame and the far pedal is hidden. Neither is reliable to the millimetre. | Read the camera-side leg with its crank forward at 3 o'clock. The report shows which side it used. |
Frequently asked questions
- 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.
References and further reading
KOPS is a convention with a history of argument around it. These sources cover the rule, its critics and the fore-aft evidence that outlived it.
- Bontrager K. The Myth of K.O.P.S.: an alternative method of bike fit. Hosted by Sheldon Brown.
The classic essay arguing that KOPS has no biomechanical basis and only works as a reproducible convention. The 'limits of the rule' section above follows its reasoning.
- Silberman MR, Webner D, Collina S, Shiple BJ. Road bicycle fit. Clinical Journal of Sport Medicine 15(4):271–276, 2005.
Clinical fitting protocol for road bikes that includes the knee-over-spindle plumb line as the fore-aft starting point.
- Price D, Donne B. Effect of variation in seat tube angle at different seat heights on submaximal cycling performance in man. Journal of Sports Sciences 15(4):395–402, 1997.
Steeper seat-tube angles (80° versus 74° and 68°) improved efficiency at 200 W in competitive cyclists. Evidence that a knee ahead of the spindle is not a penalty in itself.
- Bini R, Hume PA, Croft JL. Effects of bicycle saddle height on knee injury risk and cycling performance. Sports Medicine 41(6):463–476, 2011.
Review of saddle-height methods that also covers how fore-aft and height interact; moving the saddle changes both.
- UCI. Clarification Guide of the UCI Technical Regulation, 10 January 2024 version (articles 1.3.013 saddle position and 1.3.023 extensions).
The saddle-nose rule for UCI racing: the tip of the saddle at least 50 mm behind the vertical through the bottom bracket, with a morphological exemption procedure.
Related guides
Related tools
- 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.
Pedalyze measures every angle on this page from two side-on photos of you on your bike — and tells you what to adjust. The first analysis is free.