How does crank length affect the number of gears a bike needs (crankset, gears, bike)?

Ah! Bicycle cranks! The most neglected biomechanical device ever! While setting the height and seat-to-handlebars distance can change the variety of body sizes required, commercial cranks vary in length, and only 6% of such 1.60 people will not be able to choose a crank that is more than 6% taller than someone who chooses a shorter crank that is 25% taller.

Conor Dunn is 2.04 meters tall next to Esteban Chavez who is 1.64 meters. I bet they use different crank lengths. Here's why. Partly because the theory behind crank length is not as uniform as the theory behind the choice of frame size, seat height, and seat-to-handlebar distance. Partly because it's an accepted and cemented concept that the ideal crank length is not exactly proportional to body measurements, and finally, because of tradition, custom and standardized components.

So what kind of crank should we choose? The standard crank length used worldwide is 170mm. This is one of those measures so ingrained in cycling technology and history that hardly anyone dares to question it. If you look at the cranks that are available on the market, you'll see that there are other common crank sizes besides 170mm, which means that the most common sizes are:165mm, 172.5mm, and 175mm.

With a special order, you can buy cranks as short as 150mm or as long as 185mm, but that's about it. The difference in length between the longest and shortest cranks on the market is 6 percent, or 23 percent if you include special-order cranks.

Only 30 years ago, almost all on- and off-road bikes used 170mm cranks, except for those with abnormally short or long bodies, and today there are a number of lengths typically available to adult cyclists that have become the standard for specific applications:Single-gear, fixed-gear road bikes and track bikes use 165mm cranks as the standard; 170mm cranks are available for general-purpose bikes and small to medium-sized road bikes; 172,5mm is used for medium and large road bikes; 175mm cranks are standard for large, extra-large road bikes and all off-road bikes, except for the smallest frame sizes.

What is the difference between choosing long and short cranks? In terms of adapting to your body type, the shorter the cranks, the less knee flexion there is, which means less orthogonal force on the kneecap and less traction on the knee tendons. So if you have knee problems, or if you're keen to avoid knee problems, you're better off choosing shorter cranks for your bike, especially if you're a short-statured person (or more accurately: someone with short thighs).

When it comes to mechanical performance, longer cranks offer a larger leverage foot and therefore better mechanical advantage. Going from a 170mm crank to a 175mm is the equivalent of removing one and a half teeth from a 53-ton chain. Of course, you can't physically install a 51/2-tooth chain, but that's the effect of the longer crank: a 53-ton chain will "feel" as 51/2-ton as the rider's leg.

On the other hand, a shorter crank means that the foot will travel in smaller circles when pedaling, so the spin rate can be greater, taking into account your personal thresholds for speed of muscle contraction and extension.

Installing a 170mm long crank in place of the 175mm will increase your RPM by almost 3%, so if your limit is 120 RPM with the longer crank, you will be able to spin almost 124 RPM by installing the shorter crank without crossing your limit.

As you will notice, these are not great differences considering that between the crank lengths the short and long cranks are so modest and any variation in power and RPMs that the different lengths of cranks can provide can be adequately provided by the choice of gearing.

Summarize this information and you get:

Shorter cranks allow less knee flexion and less force to be applied to the kneecaps and knee tendons, so they are healthier

Longer cranks offer greater mechanical advantage, so they allow you to push harder into the gears than shorter cranks Shorter cranks allow for higher RPMs, so they allow you to rotate at higher RPMs than longer cranks. crank rotation.

Longer cranks are useful when you need to overcome obstacles or to provide acceleration at low speeds because lighter gears are usually not an option, as happens in off-road driving Shorter cranks allow you to reach higher speeds with gears that are too light, as happens on single-gear bikes.

Now, to answer the question:

"How does crank length affect the number of gears a bike needs?" It doesn't. For any given application, you need the same number of gears regardless of crank length.