Crank arm length is an important factor in the relationship between a cyclist and their bike.  The impact of crank arm length on bio-mechanical function, power output, and performance has been the focus of quite a bit of attention and research over the past few years, both informally and anecdotally in the fit studio, and scientifically in the research lab. Bike fitters should be evaluating the appropriateness of crank arm length in their assessment of the rider and their bike, and either confirming the suitability of a selected crank length, or making a recommendation for a different length. Such a recommendation may be a “soft recommendation”, where the current crank may be satisfactory, but a different length may be more optimal, or a “hard recommendation” whereby a change of crank length is assessed as essential to overcoming a particular complaint or functional limitation. Increasingly, cyclists are more conscious that crank length can impact function and performance, and are asking their fitter if their crank length is appropriate.

Why is Crank Arm Length Important?

The majority of cyclists use cycling shoes with cleats that attach to a pedal.  With the pelvis jammed on a saddle at one end of their legs, and their feet mechanically attached to the pedals at the other end of the legs, the bio-mechanical gait action of cycling is then largely defined by the (slightly elliptical) circle that the legs move through to deliver force to the pedals.  The length of the crank arms determines the diameter of that circle. For the hip and leg muscles to contract through their optimal range to generate force, the diameter needs to be appropriate. Not too big, not too small.  If the circle is too big, the knee and hip joints are forced through a larger flexion range than they may like whilst attempting to generate force. This can aggravate muscles, tendons, and ligaments as well as compress arterial blood supply, and impinge on nerves.  In addition, the pedal stroke can feel lumpy rather than fluid, and there may be a delay in force applied during the power phase of the stroke.  Common symptoms of a crank that is too long are pain in the hip joint or nearby muscles (often on one side only), knee pain, and riding uphill seems unreasonably hard. A crank arm too short is far less common, and has less downside. The sensation can be that the pedal stroke is over too quickly, and the cyclist was only just starting to get a good push.  This will be more obvious to cyclists who prefer a low cadence.

What is Worse – a Crank Too Short or a Crank Too Long?

A crank arm too long is worse than one too short. A crank too short may result in a loss of performance (although research argues this is rarely the case), but is highly unlikely to cause any harm. A crank too long is a common cause of low-grade injury and futile trips to a physical therapist for relief. Futile because it seems to be a body issue not a bike issue, but the PT makes little progress because every ride sets back any progress made from the therapy.

Why Does a Bike Come with a Particular Crank Length, and What are the Options?

Unless you are on a Strider, every bike comes with a crankset, featuring a particular crank arm length. Why this is what it is can be attributed to a mix of tradition (i.e. that’s the way it’s always been), manufacturing cost control, economy of scale, and a bizarre dance between the demand from bike brand product managers who specify the parts on a bike, and the supply options from component manufacturers. Bike fitters and cyclists alike don’t get a say in the matter.

Focusing our attention on adult road, gravel, mountain, and tri bikes, there are 4 common sizes available from the dominant suppliers (Shimano and SRAM):

  • 165mm
  • 170mm
  • 172.5mm
  • 175mm

That’s a tiny 10mm difference between shortest and longest, to suit riders whose legs may vary in length by 200mm!  Do you see potential for inequity here?  My inseam is 88cm and on my road bike I installed 170mm cranks. I see cyclists in for a fit who are on the same crank length with legs 10cm shorter than mine.  Relatively to me, they are turning a much larger circle, subjecting their joints and attending muscles and ligaments to a larger range of motion and forces. Why aren’t there more options? Product managers aren’t demanding them from the component suppliers (they are not bike fitters and aren’t thinking deeply about fit), and component manufacturers are not offering them due to perceived low demand and high production costs (i.e. poor ROI). Product managers try and do their best with what they can acquire and will generally scale the crankset to the bike size: smaller bikes will have shorter cranks and larger bikes will have longer cranks. Although this is conceptually a good idea, the biomechanical reality is there is frequently a misalignment between what is supplied on the bike and what would be optimal for the rider.  This is especially pronounced at the smaller end of the bike size range. That’s because the crank arm size range available does not cater to the range of body types that actually want to ride a bike comfortably and effectively.

What would be a good size range for cranks then?  I’d suggest at least 155mm to 175mm in 5mm increments, and even better would be 145mm to 185mm. The most common default size in the market is 172.5mm for no apparent reason, and although I’m sure it has its fans it’s sincerely a useless non-size in that a 170 or 175 would likely provide a better alternative for the cyclist. Shimano recently added 160mm cranks for some groupsets, which have been a great problem solver, and an easy changeover where the current crankset is also Shimano. Fortunately, several other manufacturers offer a wider range of lengths including Rotor (many choices), Speed and Comfort  (JCOBB cranks down to 145mm), and Zinn (for those looking for longer than standard).  Another option for various crank lengths is Croder Cycling.  For mountain bikes and single chainring gravel bikes, Canfield Bikes offer a great selection of shorter crank options.

In addition, there are a few cottage industry machinists who can take an existing alloy crank and turn it down in size. I have a client who had a 130mm and a 135mm crankset constructed to suit his uncommon dimensions. Check out Bike Smith Design if you need something custom or customized.

Unfortunately, cranksets are neither cheap, nor “plug and play.” Other than buying a new bike, buying a new crankset is one of the more expensive fit-related equipment changeouts available.  This is often compounded by the dizzying number of bottom bracket (frame shell, bearings and axle) variations which can make for a component and labor-expensive swap. This added expense often serves as a disincentive for a bike fitter to strongly recommend a different potentially more optimal crankset, and a disincentive for the cyclist to enthusiastically embrace the idea that another crank length could be that much better – unless they experience significant pain.

A simpler and cheaper alternative can be a “crank shortener“.  These bolt onto the existing crank in place of the pedal, and provide a number of length options for mounting the pedal to that is shorter than the original.  A potential disadvantage is the increase in stance width.  However these are a popular solution for use on stationary exercise bikes with non-changeable cranks, and for temporary crank length shortening to aid in rehab after knee or hip surgery.

As a Cyclist, How do I Know if my Crank Length is Wrong for me?

As a default, it is my opinion that the vast majority of bikes are equipped with crank arms that are too long to be optimal for the person who will buy and ride that bike.  Note the word optimal. Just because they may not be optimal doesn’t mean they are a problem. The human body has a wonderful capacity to accommodate and work around something that is not quite right. Many cyclists are happily riding a sub-optimal crank length without any adverse effects. They are also usually average or above average height. But when that capacity to make do is pushed too far, the body will speak up with aches, pains, inflammation, and aggravation. Some people will be more predisposed to the adverse effect of the wrong crank length due to any number of factors like aging, previous injuries, osteoarthritis, muscle strength and flexibility, riding position and intensity, or just a greater mismatch from the beginning. Here are 5 signs that your crank arm length is too long or wrong:

  • You have recurring knee pain and all other aspects of your bike fit (cleat position, saddle position, etc) have been checked off and ruled out as a cause.
  • You have recurring aches in the front or sides of one or both hips (often only one side)
  • Bodywork (massage, chiro, PT, acupuncture, etc) for any of the above has not resulted in sustained relief.
  • You can keep up with friends on flats but fall significantly behind on hills even though you seem to be of similar fitness
  • The pedal stroke feels lumpy or sticky when getting your foot over the top of the stroke, rather than smooth and fluid

If you think a different crank length may be beneficial but are reluctant to incur the cost without more proof of concept, find a bike fitter who uses a Fit Bike with an adjustable crankset.  With this tool, bike fitters can provide you with the opportunity to easily test different crank lengths. Often the difference is immediately noticeable.

As a Fitter, How do I Assess Crank Length Suitability for a Rider?

There are several layers of information to gather as a bike fitter to determine if a recommendation for a different crank length is appropriate, and how strong a recommendation this should be.

Rider reported symptoms: In your rider interview, reported issues of knee or hip pain/ tightness or loss of power under heavy force demand (hills, seated intervals) should be considered a flag for examining crank length.

Inseam length measurement:  A static inseam measurement will provide a quick reference point for comparing leg length to crank length.  Use the Inseam Measuring Device from Fit Kit Systems for a professional “non-invasive” approach to gathering this data, as compared to the old “book between the legs and back up to the wall” DIY technique. Anyone with an inseam length under 80cm (800mm) is almost always going to be on a crank too long, and the shorter their legs, the more significant the disparity. To draw from and simplify the recommendations made by cycling coach and bike fitter Rick Schultz, I offer the following guidelines for a suggested match for inseam length to crank length.

Inseam Length – centimeters Recommended Crank Length
Under 75cm 150mm or less
75 – 79 cm 155mm
79 – 82 cm 160mm
82 – 85 cm 165mm
85 – 89 cm 170mm
89 – 94 cm 175mm
Over 85cm 180mm if preferred

Some comments on this chart:

  • Inseam length is one of several factors that influence the selection and recommendation of crank length. It is not the only factor. However skeletal length provides a baseline for a recommendation, and injury history and soft tissue factors are then over-layed on this.
  • Most cyclists are probably on a crank too long, but most can get away with it. The shorter the inseam, the greater the mismatch is exaggerated, and the greater the physical cost and consequence.  The shortest commonly available crank arm is 165mm. There are an abundance of cyclists, predominantly female cyclists who have an inseam length under 82cm, that would benefit greatly from being on a crank that is shorter than 165mm.
  • The above guideline would apply primarily to road and gravel bikes, and secondly to mountain bikes. Mountain bike brands often grossly over-prescribe crank length on their extra small and small frame sizes, to the detriment of their customers’ riding experience.
  • For TT and Tri bikes, I would suggest subtracting an extra  5 – 10mm of crank length to keep the hip flexion in a reasonable range (not too closed off)

Let’s take a closer look at inseam lengths:

Inseam Length distribution

Sample of cyclists Inseam Length. Source: Fit Kit Studio data

Note that from this random sample of nearly 400 cyclists, the highest frequency of inseam length is between 83 and 85 cm.  My recommendation would place them on a 165mm crank, or up to a 170mm crank. The most common crank length is 172.5mm. 175mm is also a very common length, especially on mountain bikes. Look how common it is for an inseam length to be under 80cm! Below is a distribution of inseam length by gender, in relation to body height. There are a significant number of adult cyclists with an inseam length under 80cm, and the majority are women, who are not as well catered to by the bike industry as taller, male cyclists.

 

 

Height to Inseam by Gender

Height to Inseam by gender. Source: Fit Kit Studio data

Movement Assessment off the bike: This can be passive movement assessments on a treatment table, to evaluate knee and hip range of motion, or dynamic movement assessments like the Functional Movement Screen.  Limitations in ROM and/or poor scores for knee and hip movements in the FMS can be indicators to pay close attention to the relevance of crank arm length.

Knee angle at maximum flexion and commencement of power stroke: Assessing how flexed the knee joint is at the top of the stroke at maximum flexion is a common method of assessment, either via hand-held goniometer, video analysis or 3D motion capture. A general guideline is that this should be greater than 70 degrees.  Less than 70 degrees can be problematic for the rider.  An additional assessment is to measure the knee angle at the commencement of the power stroke, which is when the crank arm is in line with the downtube. Close to 90 degrees is ideal. Many shorter-legged riders will be under this. Under 80 degrees is a red flag as the vector forces transmitted into the knee during attempted leg extension through the primary power phase will not be well received. This can result in either symptoms of knee pain, or reduced and/or delayed force transfer to the pedal.

Hip flexion at top of stroke: In addition to knee flexion, hip flexion at the top of the stroke can be an indicator of crank length problems. This is going to be more relevant to athletes on a Tri or TT bike riding in an aero position. Aim for a hip flexion angle of greater than 45 degrees.

Fit bike test using adjustable sizing crank: The best way to convince a cyclist that a shorter crank length would be beneficial, is to allow them to feel the difference. This is most easily done on a Fit Bike equipped with an adjustable crank, enabling different lengths to be evaluated in quick succession. The response from the cyclist when trying a more suitable crank length is usually immediate and pronounced. The decision is made and you may rest your case.

Acknowledgment and Resources

Many thanks to Professor Jim Martin for his multiple research papers and presentations on crank length, and to cycling coach and bike fitter Rick Shultz for his strong advocacy in bringing the importance of crank length to the attention of bike fitters. You can find more about crank length from these people here:

Jim Martin

Rick Shultz