What Is An Adjustable Bike Crank?

The crankset is the component of a bicycle drivetrain that converts the reciprocating motion of the rider’s legs into rotational motion used to drive the chain or belt, which in turn drives the rear wheel.

It’s almost inevitable that every road cyclist will start to wonder about the crank length they are using. But, it’s also a topic that’s long been debated as it’s all a matter of opinion.

It’s long been popular to experiment with cycling equipment and innovate a bike in new ways to increase performance. As a result, re-visiting the design of bike parts and attempting to better them is going to be around as long as bicycles. The two go hand in hand and progress is constantly being made with superior bikes flooding the market every day.

Cranksets are no outlier and have gained some of this attention. Originally made from steel, they have evolved from largely utilitarian creations to become lightweight and elegant. Aluminum alloy remains the most common construction material, however, the last decade or so has seen the introduction of composites.

However, the length of the crank arms has always stayed relatively the same. It has remained around 170mm since the inception of the safety bike in the 20th century. This is because it’s long enough to serve as an effective lever yet short enough to remain within the range of motion of the human leg.

However, there has been a slight change. 30 years ago, 170mm would have been longer cranks whereas now this is regarded as a shorter crank length with 175mm cranks being considered a ‘long’ crank in the bike industry.

What Is The Optimal Crank Length?

As with anything relating to the human body and physiology, it’s hard to put an exact answer to it.

When you look at crank length from a rider proportion standpoint, some athletes are using crank lengths much shorter than others without a loss in performance or maximum power output.

Due to tradition and consumer availability, we don’t accept these shorter crank lengths as options even though the science supports the idea that it would not impact their power output or efficiency. On the other hand, shorter crank lengths could make athletes faster on a bike. Crank length can be used as a tool to improve fit-related issues impacting power, comfort, and aerodynamics. How I hear you ask? Well, let’s have a look:

Power: A shorter crank length alone will not increase your power output, but it can be used to reduce restriction through the top of the pedal stroke by opening up an impinged hip angle and/or reducing knee flexion.

Comfort: A shorter crank length reduces the range of motion at the knee, hips, and lower back.

Aerodynamics: Going to a shorter crank will allow you to ride at a lower back angle which minimizes frontal surface area improving aerodynamics.

It can be frustrating to know that there are many available crank options that would work for an athlete. Trying to decide between a 165 and 170mm crank is a waste of time as there is too small of a difference to matter.

So, an athlete should pick the length that allows them to most comfortably apply force to the pedals without restriction. In the future, we may end up reaching a point where manufacturers begin making crank lengths in 5 to 10mm increments as the 2.5 are too marginal of a difference.

Bike Crank Types

Over the last few decades, there have been many changes to bike design, largely on bottom brackets and their corresponding crank modifications.

One-piece

One-piece cranks are mainly found on older American-made bicycles, and children’s bicycles made for the U.S. market. Most one-piece cranks use a single metal forging as a left crank, right crank, and bottom bracket axle.

Bicycle bottom bracket shells tend to fall into two categories, threaded and threadless. One-piece cranks fit only the unthreaded shells 51.3 mm (2.02″) in diameter. One-piece cranks are the easiest type to service and require no special tools. All you need is a large adjustable wrench and a screwdriver.

Integrated Crank

This one is a tad more complex compared to the simplicity of a one-piece crank, although much lighter and stiffer. The drive side consists of the spider, crank arm, chainring and spindle together in one piece. The non-drive side crank arm slots onto the spindle and is secured by a bolt threaded into the spindle. Alternately the crank arm clamps around the spindle with a pair of pinch bolts.

Square Tapers & Splines

Square taper designs maintain the separate crank arms characteristic of cottered cranks but improve on the way they connect to the spindle. Four faces with a tapered wedge fitting into a corresponding tapered aperture on the crank arm create a tight and reliable fit.

The crank arms are then attached by means of bolts threaded into the spindle; applying the correct amount of torque ensures crank arms are firmly secured to the spindle when installed, and easily removed come maintenance or upgrade time.

Cottered Crank

Cottered cranks are crank arms that slide onto a round axle and are wedged in place with a cotter pin. They got a bad reputation on cheap, bike-boom bikes from the 70s, but there was a time when they were top-line racing equipment as they can be quite lightweight.

Crank arms slip onto the spindle and are secured by tightening the cotter pin nut. Keeping it tight avoids play and thus rapid wear of the contact area. The cotter pin tightens against the machined edge visible on the left, holding the whole assemblage securely in place.

How To Choose The Right Crank Length

Ok, so how do you decide the best crank arm length for you. As with anything related to bike fitting, some general guides exist, but they won’t work for everyone. Formulas for computing crank length do exist though, so try them out as one of them may work for you:

  • Graeme Obree method – crank length = 0.95 x height
  • “Machine” method – crank length = 1.25 x inseam + 65
  • Lennard Zinn method – upper end: crank length = 2.16 x inseam
  • Lennard Zinn method – lower end: crank length = 2.10 x inseam
  • Bill Boston method – crank length = 1.85 x femur height

 There are some specific characteristics of long-distance and ultra-cycling that suggest that crank lengths shorter than suggested by the above equations could be more appropriate.

Riding longer cranks causes more knee compression at the top of the pedal stroke and requires a tighter hip angle. The hip angle is a major determining factor on what handlebar height is comfortable and how easy it is to ride on aero bars for extended periods of time.

Many athletes and professional cyclists particularly, those partaking in triathlons, etc. are starting to use shorter cranks so that they can more comfortably have their bars at a lower, more aerodynamic position without their knees coming up too high or their hips compressing too much.

Frame Sizes & Crank Lengths

Bike frames are typically offered in a range of sizes from about 50 cm to 63 cm for men and about 44 to 56 cm for women. The largest frame size is therefore about 25% larger than the smallest for each gender, and the variety in people’s heights compared to the provided sizes should cover over 90% of

How To Measure Crank length

C2C is a general bike measurement principle, meaning that taking a specific measurement begins and ends at either the actual or the theoretical, center point of a component which is usually empty space.

Locating the centers on non-integrated cranks is simple: the center of the spindle aperture to the center of the pedal aperture—you don’t measure at the end of the crank arm. However, for integrated cranks, locate the drive side crank arm’s center point with reference to the spindle on the reverse side. You can take a guess as to the crank spider’s geometric center with a 2-3mm accuracy.

Is Crank Length Really That Important?

There doesn’t appear to be a strong argument for optimizing crank length for an individual, at least in terms of pure performance. But there is more to cycling than your power output. There’s a demand for maintaining a highly repetitive activity for long periods in the context of fluctuating loads.

Bicycles are symmetrical in design which contrasts us humans that are asymmetrical. This results in injuries while cycling due to uneven loading. In fact, a high proportion of cycling injuries relate to overuse for both recreational and professional cyclists.

The legs are commonly affected but especially the knees, and while the causes vary, the most common prevention method is to change your position while riding.

This is where the optimization of crank length becomes important. While the saddle height can be adjusted to suit the overall reach of the legs, the crank length largely dictates the range of motion.

As a result, the crank length has become an important parameter to be optimized for every individual, no matter if it’s pre-built by an experienced bike fitter from a bike shop or a custom bike. While this optimization is unlikely to increase your peak power, it does have its perks when it comes to comfort and preventing injury.

Frequently Asked Questions

Does the crank make a difference in bikes?

A higher-end crank might have better quality materials for its chainrings and so the chainrings might last longer. Upgrading your crankset will make your bike lighter and perform better.

Why are shorter cranks better?

 Moving to a shorter crank can improve comfort, as a shorter crank length reduces the range of motion at the knee (extension and flexion), hips, and low back. It also benefits your power as it can be used to reduce restriction through the top of the pedal strike by opening up an impinged hip angle and/or reducing knee flexion. Going to a shorter crank length will also allow you to ride at a lower back angle which minimizes frontal surface area improving aerodynamics.

What size cranks do pros use?

Pros will tend to use shorter cranks as these crank lengths allow for more comfort and reduce the risk of injury. It’s a common misconception that larger crank lengths give you a greater power output than shorter crank arms so this won’t necessarily be a big deal for athletes as they’ll instead opt for a crank length that best suits them and their leg length.

How does crank length affect power?

There doesn’t appear to be a strong argument for optimizing crank length for an individual, at least in terms of pure performance. Instead, crank length affects things such as comfort and rising position which reduces the risk of injury while riding.

Summary

The crank length on a bike has long been a cause for debate. For decades it has been a factor considered to improve your power if you have longer crank arms. However, more recently, Science has disproven this belief as it turns out that the range of crank lengths doesn’t really have much effect on your power.

But, that doesn’t make them useless. On the contrary, the choice between having a longer crank or a short crank is still important and depends on your leg length. Comfort and aerodynamics come into play here as the big effects a crank has on you and your bike.

Having the right crank length will prevent injuries and prove for a much smoother ride in general, however, it’s unlikely to give you an edge in races.

Those of you that have been suffering from a recurring overuse injury may find relief with a change in crank length, but this shouldn’t be attempted by yourself. Instead, ask a trained bike-fitter with experiences as they’re going to arrive at a more conclusive solution that saves you time and gives you a better idea of what length to go for.

But, whichever you decide on out of the range of crank lengths be sure it works for you above all else.

So, keep pedaling!

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