The Mechanism Method
Why most bike fits fail, and what to do instead.
The industry has a thinking problem.
Walk into ten bike fit studios and you'll hear the same phrases. Knee over pedal spindle. Drop the saddle if the hip rocks. Move it back if the knee hurts. Check the angle, match the chart, send the client home.
It sounds like a method. It isn't. It's a recipe book. And the ingredients are wrong.
I call it "if X, then Y" fitting. See a symptom, apply the rule. Hand numbness? Shorten the reach. Anterior knee pain? Saddle back. Hot spots on the foot? Rotate the cleat. It's clean, it's teachable, and it works often enough that a lot of people think it's the job.
It isn't.
The job is to understand why the rider's body is doing what it's doing, and to intervene where it will actually change something. "If X, then Y" skips that step entirely, which means when the rule doesn't work, and it often doesn't, the fitter has no second move. They've either rebooked the client or quietly blamed them.
Certainty feels safer than truth.
The reason the recipe book survives isn't that fitters are lazy. It's that certainty feels safer than truth. "If the knee is in front of the pedal spindle, it causes knee pain" is a sentence you can defend in front of a client. "It depends on cadence, crank length, seat height, force onset, force direction, and the rider's training history" is a sentence that requires you to have done the work.
Most fitters haven't done the work, because no one has made them.
That's what this is.
Three pillars. One method.
Everything I do, every fit, every consultation, every hour of mentoring, every line of software, runs on three pillars.
Variables. What's actually in play.
Mechanisms. Why an intervention changes the outcome.
Forces. How the body and the bike are pushing against each other, in every pedal stroke.
Miss any one of them and you're guessing. Get all three right and fitting stops being a black art and starts being something you can defend to a peer, a researcher, or a court.
Pillar One — Variables
A few years ago I ran a study on ankle movement in time trial cyclists. Before I could even design the protocol, I had to list the variables that could move the needle on the measurement I cared about.
The short version of that list included cleat position, saddle height, knee range of motion, intensity, cadence, calf strength, recent positional changes, whether the rider ran as part of their training, the shoes they wore off the bike, Achilles tendon stiffness, and their natural pedalling style.
Thirteen variables. For one measurement. For one joint.
That's not me overcomplicating it. It's that the ankle is connected to the rest of the body, and the body rides the bike. If you don't know which of those thirteen levers is the one pulling on your rider, you aren't fitting them, you're rolling dice.
When a fitter sees a symptom and reaches for a single rule, they are assuming that twelve of the variables don't matter. They almost never check.
You can't fix what you can't see. And you can't see what you haven't learnt to look for.
Good fitting starts with widening the aperture, not narrowing it. You can't fix what you can't see. And you can't see what you haven't learnt to look for.
Pillar Two — Mechanisms
Variables tell you what could be at play. Mechanisms tell you how they actually cause what you're seeing.
The textbook example is the "knee over pedal spindle" rule. For decades, the industry told fitters that if the knee was in front of the pedal axle at 90 degrees of crank rotation, it caused anterior knee pain. Move the saddle back. Problem solved.
The problem: there is no mechanism.
When you ask, "Why would that cause knee pain?", the answer is usually a hand-wave about patellofemoral forces. But Ericson and Nisell tested exactly that, in 1987. Domalain and colleagues re-tested it in 2016. Both studies showed that patellofemoral forces do not change significantly across a 6 cm fore-aft saddle range, and that the forces in cycling at those positions are lower than the forces a rider experiences running, which most of them do without issue.
So the rule was wrong in 1987, wrong in 2016, and still being applied in fit studios last week.
This is what happens when you inherit an intervention without understanding its mechanism. It lives on, long after the evidence has killed it, because nobody taught the fitter to ask the second question.
Mechanism-first thinking is uncomfortable at first. It forces you to admit that an intervention you've been making for years might be doing nothing, or that it's been working for a different reason than you thought. Moving the saddle back often did help anterior knee pain, just not because it moved the knee off the spindle. It raised the effective seat height, or changed where in the stroke the rider applied force. Same outcome, different cause. And if you don't know which one you fixed, you don't know how to do it again.
But once you start asking "why", you can't stop. And your fits get better, fast.
Pillar Three — Forces
Most fitters will accept Variables, eventually. Some can be taught to think in Mechanisms. Almost none are fluent in Forces.
This is the biggest gap in the industry, and the one that changes the most fits.
Every force has an equal and opposite. If your client is seated on the bike and their upper body isn't falling into the bars, the forces acting on them are balanced. Gravity pulls them forwards and down. Three things can oppose it: weight on the bars, core engagement, or pedal reaction force.
Get the balance right and the rider holds their position effortlessly. Get it wrong and they're either collapsing onto the bars, hanging off their spinal erectors, or sliding forwards on the saddle for the entire ride. None of those are fit problems you can solve by moving a saddle one millimetre at a time.
Here's a 30-second test you can do right now.
Stand against a wall with your heels and bum touching it. Arms folded across your chest, upper body straight. Hinge forwards from the hips. You'll fall away from the wall almost immediately, because the ground reaction force under your heels is purely vertical, so nothing is pushing you back into the wall.
Now move your feet 5 cm forwards, keep your bum on the wall, and hinge again. You can go further, and hold it longer, because the ground reaction force now has a small rearward component pushing you back into the wall.
That's what pedal reaction force is doing during a bike fit. When a rider pushes down and forwards on the pedal, the equal-and-opposite force helps push their pelvis back into the saddle and support their upper body. When they don't produce enough of it, because they're spinning light, or carrying a bit of extra weight, or pushing vertically down, they topple forwards into the bars, load the wrists, and the fit unravels.
This is why a pro can ride bars 8 cm lower than an amateur and still be comfortable. It isn't flexibility, it's forces. A pro at 400 watts generates enormous pedal reaction force, has very little upper body mass to resist, and therefore has almost nothing to topple. Put an amateur in the same position and they collapse, not because they're weak, but because the physics doesn't add up.
Forces explain why the same bar drop works for one rider and ruins another. And most fits never touch them.
Forces explain aggressive positions. They explain why flipping a stem changes a rider's whole experience. They explain why the same bar drop works for one rider and ruins another.
And most fits never touch them.
The method in practice
Put the three pillars together and fitting becomes a thinking sequence, not a checklist.
Take a rider with anterior knee pain. The "if X, then Y" fitter moves the saddle back. The mechanism-based fitter does something else, running five checks in order, each with a mechanism behind it.
One. Cadence and gear. Low cadence in a big gear means higher joint loads. Mechanism: more torque per stroke, more force through the patellofemoral joint. Fix the cadence before you touch the saddle.
Two. Crank length. If the cranks are too long for the rider's leg, knee flexion at top dead centre can push past 115 degrees. Mechanism: above that angle, the patella contacts less of the femur, pressure rises, and the force vector points more directly into the joint.
Three. Seat height. Same mechanism as crank length, different cause. If knee flexion stays below 112 degrees through the stroke, this isn't your culprit.
Four. Force onset. Where in the stroke is the rider actually applying power? My working hypothesis is that smaller riders on proportionally longer cranks may be avoiding pain because they delay force onset until the knee has extended past 110 degrees. Riders who try to produce force earlier in the stroke seem to be at greater risk, regardless of size.
Five. Force direction. Has the rider been taught to "pedal in squares" or push forwards over the top? That turns the pedal stroke into a quad-dominant movement at exactly the knee angle where patellofemoral forces are highest. It's a taught behaviour, and it's a taught injury.
Five variables. Five mechanisms. In order. By the end, you either know what to fix or you know enough to refer. You don't guess.
That's the method.
Why this matters
For clients, it means fewer repeat visits, fewer "maybe try this, come back in a few weeks" conversations, and fits that hold up over thousands of kilometres instead of a single test ride.
For fitters, it means the end of guessing. You stop dreading the complicated cases because you have a way of thinking about them. You stop charging for second and third visits you can't justify. Your repeat business comes from referrals, not from clients returning to re-fix the same problem.
For the industry, it's the line in the sand. We either raise the floor through real mentoring, shared cases, and honest peer review, or we keep blaming clients for fits that were never going to work.
The method, not the man
None of this is about me.
The reason I've published the frameworks, run the courses, built the software, and taken on the mentoring clients is because the method has to outlive any individual fitter, including me. The Mechanism Method is a way of thinking. Once you have it, you don't need me.
If you want the shortest path into it:
Read the rest of the thinking on this site. The writing on variables, mechanisms, and forces is where this starts, and it's free.
Book a Case Consultation if you have a specific client you cannot solve. One hour. One case. We find the mechanism and you leave with a plan.
Join the Mentoring programme if you want to rebuild how you fit from first principles, not patch the gaps with more rules.
Apply for the Mechanism-Based Practice Review if you want your entire practice held up to the light — systems, pricing, process, and outcomes.
Whichever door you come through, the work is the same: stop applying rules, start asking why.
That's the job.