Essay

The Mechanism Method

Why most bike fits fall short, and what to do instead.

Walk into most bike fit studios and you'll hear the same phrases. Knee over pedal spindle. Drop the saddle if the hips rock. Move it back if the knee hurts. Check the angle, match the chart, send the rider home.

It sounds like a method. It isn't. It's a collection of rules that work often enough to feel convincing, and not consistently enough to hold up when things get harder.

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 problem isn't that the rules are always wrong. It's that they skip a step. They jump straight from what you can see to what you should do, without asking why the rider is moving the way they are in the first place. When the rule works, it looks like good fitting. When it doesn't, there's very little to fall back on beyond trying something else and hoping it sticks.

That's where most of the frustration comes from. The rider doesn't understand why anything has changed, the fitter isn't entirely sure what's driving the problem, and the process starts to feel like trial and error dressed up as expertise.

Breaking down the fit

A better approach slows that process down and makes the missing step explicit. Before you change anything, you need to understand what's actually happening in the rider, why it's happening, and what any intervention is likely to do. Obvious enough on paper. It isn't how most fits are run.

Take something as simple as ankle movement in a time trial position. It's tempting to treat it as a single variable and try to correct it directly. It isn't a single variable. It sits inside a much wider system: cleat position, saddle height, cadence, recent positional changes, calf strength, training background, even what the rider does off the bike. Any of those can influence what you're seeing. If you don't know which is actually doing the work in that moment, you're not making a decision. You're narrowing the odds.

That's the first shift: widen what you're looking at before you narrow it down.

The second is understanding mechanism. Knowing that something changes isn't the same as knowing why it changes. A lot of long-standing fitting rules fall apart at this point. The "knee over pedal spindle" idea is the most familiar example. For years it was treated as a cause of anterior knee pain, with the standard fix being to move the saddle back. The trouble is that when you look for a clear mechanical explanation, there isn't one. The forces involved don't change in the way that rule suggests.

And yet, moving the saddle back does sometimes help. The question is why. In most cases it isn't the fore-aft position itself doing the work, but a change in effective saddle height, or a shift in when and how the rider applies force through the stroke. The outcome looks the same. The cause is different. If you don't know which one you've affected, you can't reliably reproduce it.

You stop asking whether something worked. You start asking what made it work.

That's where fitting moves away from rules and towards reasoning. You stop asking whether something worked. You start asking what made it work.

Understanding forces

The third piece, and the one most often missed, is force.

Every position on the bike is a balance. If a rider isn't falling forwards, something is supporting them. It might be weight through the bars, it might be core engagement, or it might be the reaction force coming back through the pedals as they push against them.

When the balance is right, the position feels stable and almost effortless. When it isn't, the rider finds a way to compensate. They load the hands, slide forwards on the saddle, hold tension in places that weren't meant to carry it. Those are the issues that get chased with small positional tweaks that never quite settle anything.

It also explains why two riders can sit on what looks like the same position and have completely different experiences of it. A stronger rider producing more force through the pedals can support themselves in a way another rider can't, even if their flexibility looks similar on paper. That's often the difference between a position that holds and one that slowly unravels over the course of a ride.

Once you start looking at a fit through that lens, the process changes. Instead of jumping straight to an intervention, you work through what's likely to be driving the problem. A rider with anterior knee pain might still end up with a saddle adjustment, but not before you've looked at cadence, crank length, how much knee flexion they're working through, and where in the stroke they're actually producing force. By the time you make a change, it's tied to a specific explanation, not a general rule.

That doesn't make fitting simple. If anything, it makes it more demanding. It does make it more consistent. You stop relying on whether a rule happens to fit the situation in front of you. You build a picture of what's going on, and intervene in a way that matches it.

For riders, that means fewer return visits and fewer cases where a position feels good for a week and then quietly falls apart. For fitters, it takes a lot of the uncertainty out of the harder cases. You still won't have every answer immediately. You'll have a way of working towards one.

None of this is new in isolation. The pieces exist across biomechanics, coaching and applied practice. What's often missing is the connection between them inside a fit. Bringing them together is what turns fitting from a set of rules into a process you can actually follow.

Stop starting with the intervention. Start with why the rider is doing what they're doing.

If there's a single shift to take from it, it's this: stop starting with the intervention. Start with the question of why the rider is doing what they're doing, and let that lead you to what to change.

Where to start

If you want to go deeper into this approach:

Whichever route you take, the work is the same: move away from applying rules, and start asking why.

— Andy Brooke

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