What cadence is and why it rules

It is the number of crank revolutions per minute. You can deliver the same power with little force quickly or a lot of force slowly, and everyone has a band where it costs them least: most people are comfortable between 70 and 100 strokes per minute on the flat. Too slow loads knees and muscles; too fast sends your breathing and heart rate up. The drivetrain exists precisely to keep you inside that band while the terrain changes.

How to read the chart

Each line is a gear: for every cadence, how fast you go. The steeper lines are the taller gears, which turn a small change of legs into a lot of speed. The shaded band is the cadence you said suits you, and the part of the line inside it is the range of speeds where that gear works for you.

Duplicate gears

When two lines overlap, you have two combinations doing practically the same thing. They are not a serious flaw — they are unavoidable on any 2x — but they explain why a 22-combination drivetrain does not give you 22 gears: a 50/34 with an 11-34 yields about 17 distinct ones. That is the number to compare when someone tells you one groupset has more speeds than another.

What the real order is for

The real order lists every gear from easiest to hardest, which is not the order they sit in at the handlebar. It tells you which one belongs at each speed and which you can skip. On a 2x, shifting up 'for real' often means changing chainring and dropping two sprockets at once: this is where you see it.

Why MTB accepts steps that would be unacceptable on the road

On tarmac you spend hours near a constant power and cadence is your fine adjustment: a 20% step drops you from 95 to 79 rpm, and your legs notice immediately. That is why a road cassette spends half its sprockets on the first four teeth.

On the mountain, speed changes constantly and you never hold a steady effort long enough to suffer a big step. What does leave you walking is reaching a 20% rocky ramp without a low enough gear. That is why the 10-51 took over with steps of up to 20%: range beats fineness.

1x against 2x

A 1x removes duplicates, the front derailleur and cross-chaining, but the whole range has to come out of twelve sprockets and the steps get fat. A 2x gives you twenty-odd combinations with fine steps, at the cost of repeating a third of them and having to think about which ring you are in. The calculator's "usable gears" column is exactly that argument turned into a number.

The tyre changes the gear too

A 29×2.4 is about 2,300 mm around and a 700×28 about 2,130: an 8% difference. With the same chainring and sprocket, the MTB wheel is 8% harder, as if you had added two teeth to the chainring. When comparing setups across disciplines, look at development, not ratio.

And the resistance of the ground

Switching discipline here is not cosmetic: the climb test starts using the rolling resistance and frontal area of each surface. The same gear, the same gradient and the same watts give you different speeds and cadences on tarmac, on gravel and on soft dirt.

Crank length

MTB has drifted down to 170 and 165 mm for ground clearance and to avoid striking rocks; road arrived there for reasons of position and cadence. The crank catalogue at the end lists the usual sizes of each family.

In the same gear, a longer crank gives you more leverage. Gain ratio captures it: divide the wheel radius by the crank length and multiply by the total ratio. It is the only one of the five measures where the crank appears.

What the number actually measures

The crank is the arm running from the bottom bracket axle to the pedal, and its size is the distance between the centre of the axle and the centre of the hole the pedal threads into. It is usually stamped on the inner face, near the pedal. Common sizes run from 165 to 175 mm in 2.5 mm steps, and manufacturers have started offering 160 and below.

If you change them, the saddle moves too

The crank sets the lowest point of the pedal. Fit one 5 mm shorter and your foot no longer drops as far, so your knee stays more bent through the whole stroke: you have to raise the saddle by those same 5 mm to get back the extension you had. Plenty of people try short cranks without touching the saddle, find it odd and draw the wrong conclusion.

Which length is yours

There is no reliable formula based on your height, however many circulate. What matters is whether your hip and knee feel comfortable at the top of the stroke and what kind of bike you ride. The trend of recent years runs towards shorter cranks even for tall riders, because they open the hip angle, make high cadences easier and leave more ground clearance. If you are fine as you are, changing is not a priority: the chart beside this tells you how small the difference really is.

The force that reaches the ground

This is where the crank becomes tangible. The force with which the wheel pushes on the ground is the force you apply at the pedal divided by the gain ratio. Nothing else: not your weight, not the brand, not the material. Push with 300 newtons in a gear with a gain of 3.0 and 100 newtons reach the contact patch, and that figure decides whether you beat the gradient or stall.

That is why a shorter crank in the same gear leaves less force at the ground: less leverage, higher gain, bigger division. To get it back you have to lower the ratio by the same proportion, which is exactly what the equivalences section works out when you choose to keep leverage.

What you cannot keep at the same time is speed. Shorten the crank by 4% and lower the ratio by 4%, and the force at the ground is the same again, but at the same cadence you go 4% slower. You get the speed back by pedalling 4% faster, which is precisely what short cranks make easier. That is the whole short-crank argument, in two sentences.

What you gain and what you lose

• Longer: less force at the pedal for the same ramp, but your foot travels further per revolution and, at the same cadence, the knee bends more and moves faster.
• Shorter: more compact pedal strokes, easier to hold high cadences and a better hip angle in an aero position. In exchange, each stroke demands a little more force.
• The difference is small: going from 175 to 165 mm is 10 mm out of 172, about 6%. Dropping one sprocket on the cassette often changes your life more than changing cranks.

Only the rolling circumference counts

It is the distance the bike travels every time the wheel makes one full turn. Diameter, inches and the tyre's commercial name are routes to that number, not substitutes for it. When you compare two bikes, compare circumferences.

The tyre counts twice

The circumference is π times the outer diameter, and the outer diameter is the rim plus the tyre height counted twice, because the tyre sits on top and underneath. A 28 mm tyre does not add 28 mm to the diameter: it adds 56, which multiplied by π is 176 mm of circumference. It is the most common mistake in this calculation.

Commercial names mean nothing; ETRTO does

700C, 29", 28", 27.5", 650B, 26"... are names inherited from different countries and eras, and some of them describe exactly the same rim. ETRTO settled it with a single number: the bead seat diameter in millimetres. 622 is both 700C and 29". 584 is 650B and 27.5". 559 is the classic 26". Two wheels with the same ETRTO number take the same tyres; two with the same commercial name do not necessarily.

The standard being written right now: 686

The 32-inch wheel that has appeared in the MTB World Cup is ETRTO 686: 64 mm more rim diameter than a 29", about 10% bigger. With a 2.4 tyre the assembly measures around 2,500 mm of circumference against the 2,300 of a 29×2.4, and that has a direct consequence people rarely spell out: every gear becomes 9% taller. A 32-tooth chainring on a 32-inch wheel develops like a 35 on a 29er, so anyone moving across who wants to keep their low gear has to drop to 29 or 30 teeth. The rim selector now has 686, and also 660, the 750D size circulating in gravel.

Why the geometric calculation always comes out slightly large

Multiply blindly and you overshoot. Nominal width is not real width: the same 28 mm tyre measures 30 on a wide internal rim and 27 on a narrow one. A wide tyre is not as tall as it is wide either, because the profile flattens. And under your weight the contact patch gives. That is why a correction that grows with width is applied here, and why a measured circumference always beats a calculated one.

That said, the wheel does not travel less in the same proportion that it squashes. The tyre casing does not stretch: the tread always unrolls its own length, like a tank track. So the real circumference stays close to the geometric one rather than to what the loaded radius would suggest: we are talking millimetres, not centimetres.

What this means for your gears

Circumference multiplies directly: if a wheel change raises it by 8%, every one of your gears becomes 8% taller without a single tooth changing. A 700×28 is about 2,130 mm and a 29×2.4 about 2,300: exactly that 8%. With the same 34 chainring and the same sprocket, the MTB wheel asks for the force of a 37. And the other way round: moving a gravel bike from 700×40 to 650B×47 lowers your gearing by nearly 4% without buying a single drivetrain part.

Details people forget

• A 29" and a 700C are the same rim: 622 mm. The whole difference in development comes from the thickness of the tyre, not the wheel.
• Width: going from 25 to 32 mm adds about 45 mm of circumference. Not much, but enough to throw off your bike computer if you do not recalibrate it.
• Your bike computer uses this same number: get it wrong and it lies to you not only about speed but about distance and everything derived from it.
• How to measure it properly: mark the valve, mark the ground, roll one full turn with your weight on the bike and measure between marks. That is the number that gives reliable results.