Your Bike Was Built for a Body That Doesn't Exist

source: https://unsplash.com/photos/woman-riding-white-rigid-bike-o2FCfhNSjPo

When you buy a car, it rolls off the lot ready to adjust to your body: seat height, fore and aft, recline, lumbar, wheel tilt, and reach can all be easily adjusted to fit.

But that’s not the case with a bike you buy off the showroom floor. Your bike will have a saddle that you can adjust, and you can flip or adjust the stem, but the rest, well, it’s not quite so simple. How far away the bars are, how long the cranks are, and how far your hip has to close at the top of every pedal stroke are all essentially predetermined.

Worse still, they were all designed for a body that doesn’t exist.

You might not notice it if you’re near the middle of the distribution. But for everyone else, it’s a daily fact of life: the 5’2” rider whose small frame puts the bars out of reach, the rider with long femurs for their height, the rider whose hips stop closing at a certain angle no matter how much they stretch.

Most road frames come in five to seven sizes, covering about a foot and a half of rider height. Those sizes are built on stack and reach (how tall the front end is, and how far forward) and both are scaled against assumptions of how a body of a given height is proportioned.

Those assumptions can cause major problems. Phil Burt, who has fitted riders at the top of the sport, puts it plainly: bikes are designed on a normal distribution curve for height, and that makes a big assumption about the proportions of people’s limbs relative to that height.

Those limb proportions can have quite a spread. One fit clinic measured its own clients and found leg length ranging from 43.2% to 50.1% of a person’s total height, with inseams ranging from 67.5cm to 94cm. That means that two riders of the same height can walk into the shop and need meaningfully different bikes.

These averages come from real measurements, but they’ve been aggregated until the individual disappears. This works well for the people in the middle of that distribution, but everyone at the edge ends up riding a bike made for somebody else.

Now, this isn’t done out of malice. Instead it’s about volume.

The team at Cyclefit, who build custom frames for a living,

You can see this clearly with cranks. Small frames typically ship with 170mm, mediums with 172.5mm, and larges with 175mm. While extra small frames are increasingly getting 165mm, it’s still rare to see anything out of this range specced as standard on any size of bike. That leaves you with a 10mm range of cranks for a 27cm range of legs.

When there’s a mismatch between you and your bike’s geometry, your power, efficiency, and comfort are all at risk.

Take your saddle. Set it too low, and your knee bends further under load, raising compressive force behind the kneecap. Too high, and the hips rock to reach the bottom of the stroke, costing you power and loading the hamstrings and lower back.

There’s a workable target. A review of the saddle height research recommends setting height so that the knee is at 25 to 30 degrees of flexion at the bottom of the stroke, and notes that roughly half of cyclists deal with overuse knee pain at some point.

Cranks that are too long work the other end of the same stroke, closing the hip at the top. This leads to riders hitting a wall on breathing, cadence, and getting low without their knees coming up to their chest.

An hour of riding at 85rpm is around 5,000 revolutions, and over time, the cost of that mismatch adds up.

Even worse, many riders won’t see it as an issue with their bike. Instead, they see it as a “them” problem: not being flexible, fit, or tough enough.

Nobody tells a runner whose shoes are too small that they just need to work on their tolerance. They tell them to get new shoes. But far too often in the cycling world, the response to persistent pain is to question the athlete, rather than the equipment.

The longstanding assumption is that longer cranks mean more leverage and therefore more power, so going shorter means giving up power. You might be surprised to learn that’s not actually the case.

Martin and Spirduso tested trained cyclists at 120, 145, 170, 195, and 220mm, and found power dropped only at the extremes: about 4% at 120 and 220mm, and no meaningful difference at all between 145 and 195mm. Later work reached the same conclusion: once you account for pedalling rate and pedal speed, crank length isn’t an important determiner of maximum power.

The takeaway here is that crank length isn’t about performance; it’s about fit. That means a rider who’d be better served on something shorter isn’t trading away watts to get there, just correcting a decision somebody else made on their behalf.

Now, it’s worth noting that the evidence linking any specific fit measurement to injury is weaker than the fit industry usually implies, and reasonable people disagree about how much saddle height matters. What’s less disputed is that the range of equipment on offer is narrower than the range of bodies buying it.

The belief that comfort and performance work against each other simply isn’t true. You won’t be able to maintain speed if you can’t comfortably hold a position, or experience pain from a poorly fitted bike. Riders will quit over discomfort long before they plateau on power.

If you’ve spent years assuming discomfort was your fault, the reality is, that’s not the case.

The JCOB Review is focused on the parts of this problem the industry doesn’t have much incentive to explain, twice a week, completely free: crank length, saddle anatomy, proportional sizing, and what happens when standard geometry meets a body it wasn’t made for.