Why are bikes sold with crank lengths that are not appropriate for 70% of people? If you are a male whose height is not in the range of 1.77 m to 1.85 m (5’10” to 6’1″) or a female who is shorter than 1.68 m (5’6″) and want to be more comfortable on your bike and avoid over-use injuries then you should read this! Even if you are in that range then the crank length that your bike came with may not be ideal for you, particularly if you like to Ride Far.
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, as shown in the example geometry chart from Cannondale on the right. The largest frame size is therefore about 25% larger than the smallest for each gender, and if we look at distributions of people’s heights then these sizes should cover at least 95% of both males and females.
So, why do cranks offered on men’s bikes only vary between 170 mm and 175 mm, a 3% difference, and on women’s bikes between 165 mm and 172.5 mm, a 4.5% difference?
Saddle height is covered on the Riding Position page of this website; it is normally set to optimize the extension of the leg at the bottom of the pedal stroke, maintaining a small bend in the knee (often recommended to be an angle of about 30-35 degrees). Crank length then determines the knee angle at the top of the pedal stroke. Even in professional bike fittings, the knee angle at the top of the stroke is rarely measured and few guidelines exist. Why not?
If people’s heights, saddle heights, and bike frames vary by 25% then a variation in crank length of 3% may not be sufficient. People shorter than the range that these cranks are suitable for have far more bend in their knees at the top of the pedal stroke than is ideal and those who are taller have far less of a bend in their knees. Using an inappropriate crank length can have a major impact on cycling comfort and may contribute to developing over-use injuries in tendons, ligaments, and the lower back.
Scientific Opinion on Different Crank Lengths
Are variations in the knee angle at the top of the pedal stroke, and so different crank lengths, important? There has been some research looking at the power output possible for a given individual using a range of crank lengths. The general finding is that a similar amount of power is available when using cranks with a reasonably large range of lengths, and only at the really extreme ends of the scale is a significant decrease in performance usually observed.
A lot of people believe that longer cranks = more leverage = more power. This suggests that we should all use the longest cranks possible. The video below includes a discussion of this, explaining that power is determined by torque and cadence; although longer cranks do cause an increase in torque for the same force on the pedals they also tend to cause a decrease in cadence due to the larger pedaling circle. These two factors tend to offset each other, so power is generally unaffected by crank length (but many people fail to understand this because they believe that power = torque).
The lack of an effect on power of most intermediate crank lengths is probably why crank length receives very little attention. However, ultra-distance cyclists are not very concerned with how much power they can possibly generate and instead focus on optimizing comfort and avoiding over-use injuries, so how might it affect those criteria?
Ferrer-Roca and coleagues published research on the effect of crank length in the Journal of Sports Science in 2017. They concluded that the metabolic costs of cycling didn’t change based on crank length, but long-term injuries were more likely to be avoided by using shorter cranks due to the “maximum flexion and range of motion of the hip and knee joints” being higher for longer cranks.
Andy Pruitt is one of the most respected bike fitters in the world and helped to develop the Specialized BG Fit system. Lennard Zinn reported that Pruitt warns that cranks that are too long can cause injuries because “the compressive and shear forces in the knee joints ‘go up exponentially’ due to the sharper knee bend. … Cranks that are too short are not dangerous, however.” In his book Bicycle Design, Mike Burrows warned against using cranks that are too long to avoid knee problems and Sheldon Brown has written about crank length and how riding with cranks that were too long for him caused knee pain.
The scratch rate in many ultra-distance bike races is often at least 30%, with the most commonly-cited cause being knee problems. Most of those who do finish report having some knee pain at some point during the race. There are many possible causes of this, but crank length should certainly be a very important consideration for such riders. Experienced riders often report that their knee problems went away when they tried using shorter cranks.
How to Choose Crank Length
So, what crank length is right for you? Just like everything else with bike fitting, some general guidelines exist, but they won’t work for everyone. Formulas for computing crank length (in mm) from height, inseam measurement (in cm), and femur height (in cm, measured from the floor to the top of the femur bone) include:
- Graeme Obree method: crank length = 0.95 * height
- “Machine” method: crank length = 1.25 * inseam + 65
- Lennard Zinn method, upper end: crank length = 2.16 * inseam
- Lennard Zinn method, lower end: crank length = 2.10 * inseam
- Bill Boston method: crank length = 1.85 * femur height
The method used by Lennard Zinn suggests using far longer cranks than most other sources recommend. The results of the Obree and Machine methods are presented in the graph below, which give similar recommendations within the typical crank length range (170-175mm), but diverge somewhat for shorter and longer lengths.
For the purpose of this graph, I’ve assumed that inseam = 0.475 * height, but this won’t be true for everyone, so it’s better to measure your inseam and use the formulas. These equations are not perfect and will obviously not work for everyone, but they give a useful starting point.
Further Considerations for Ultra-Distance Cyclists
There are some specific characteristics of long-distance and ultra-cycling that suggests that cranks 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 aerobars for extended periods of time, which is something that ultracyclists often do to improve their upper body comfort and aerodynamics (see the Aerobars section of this website). Some triathletes and time trialers are now using 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.
In addition to shorter cranks making riding on aerobars more comfortable, several ultra-cyclists have reported lowering their saddle height by 5-10 mm for ultra-distance events compared to what they would use for short rides to prevent ligament and tendon problems, myself included. Having a lower saddle means that the knee is compressed even more at the top of the pedal stroke. These two characteristics of ultracyclists (extensive use of aerobars and possibly a slightly lower saddle height) suggest that ultra-cyclists should certainly experiment with using shorter cranks than would be recommended by the universal formulas above.
When trying different crank lengths, keep in mind that shorter cranks encourage a higher pedaling cadence (because people tend to keep their tangential foot speed constant rather than their rotational foot speed). This means that people using shorter cranks are better off with lower gearing that allows them to use a higher cadence when traveling at the same speed as someone with longer cranks. For other factors to consider when choosing gears, see the page on Gear Ratios.
Who Are Typical Crank Lengths Suitable For?
In addition to the lines for recommended crank lengths, the graph above also includes typical height distributions for males and females.
The heights of people who are accommodated by the range of crank lengths that are commonly offered (170 to 175 mm) is shown by the area shaded in dark orange when using the “machine” equation (using the Obree equation would yield an even narrower height range). This suggests that bikes are sold with cranks that are not appropriate for the 50% of males who are below average height (i.e., below 1.77 m or 5’10”) and those whose height is in the upper 20% (i.e., above 1.85 m or 6’1″). Just 30% of males are catered for appropriately.
For females, cranks of at least 170 mm are only appropriate for the tallest 10%. Fortunately, some female-specific bikes are equipped with 165 mm cranks, so the region of heights that this is appropriate for is shaded in light orange, but any female who is shorter than 1.68 m (5’6″) is still not catered for, which is 70% of all females.
If a crankset is purchased separately from a bike then crank lengths of up to 180 mm are available in certain models. The range of heights that are accommodated by these cranks are also shown shaded in light orange, and this should accommodate taller males except for the upper 2% (over 1.93 m or 6’4″). Shorter males can reasonably easily obtain 165 mm cranks after-market, leaving only males shorter than 1.68 m (5’6″), the lower 15% of the distribution, to struggle to find something appropriate.
Even though I’m only discussing crank length differences of maybe 5 mm, it should be noted that if the distance from the bottom of the pedal stroke to the saddle is kept constant (as is normally recommended when changing crank lengths) then a 5 mm difference in crank length will cause a difference of 10 mm in the distance between the saddle and pedal at the top of the pedal stroke (i.e., it’s the diameter of the pedaling circle that matters, not the radius). Most people would consider a 10 mm difference in saddle height to be significant and noticeable, so the same should be true about crank length – 5 mm is already quite a lot. However, many people find it hard to notice the difference if changing their crank length by only 2.5 mm.
GCN have made a video on this topic. They present the same formulas as I do above. It’s a good video but you should ignore the graph that they briefly display because their numbers don’t entirely correspond to the formulas that they state. CyclingTips have published an extensive article on the topic of crank length, which includes more useful references.
Producing cranks in a wider variety of lengths costs the component manufacturers more money, which is why only the more expensive cranks are available in a slightly larger range of lengths (165 mm to 180 mm, a 9% difference, see the section below for a list of models). In addition, frame geometries are adapted for a narrow range of crank lengths. Ideally, the bottom bracket height and the distance between the bottom bracket and front wheel should be adapted based on the crank length. This is why bike brands are happy with the current situation of offering bikes with a narrow range of crank lengths, because it makes their job of frame design easier and keeps parts costs down.
Unfortunately, these cost-cutting methods cause bikes to be sold with cranks that are not appropriate for the 50% of males who are below average height or who are in the upper 20%; and all females whose height are not in the upper 30% are also not catered for. On average, cranks must be the poorest-fitting part of a bicycle, but this is rarely discussed and bike shops rarely suggest changing it even when doing a full bike fitting. There is certainly some evidence that ultracyclists should generally use shorter cranks than they are currently using if they want to have the best chance of avoiding knee problems.
Unfortunately, new cranks are not cheap, but people often buy expensive new wheels for their bike that have a very minimal effect on the bike’s performance or comfort. Most cranks are cheaper than most wheels, so I encourage you to consider this.
Personally, I’m 1.73 m tall and was much more comfortable after switching from a mix of 170 and 172.5 mm cranks (which are standard on the 52 and 54 cm bikes that I ride) to always using 165 mm cranks, which I can now see is approximately what is recommended by the above equations. Having got used to the increased comfort, I could never go back to using the longer cranks for the long-distance cycling that I love to do.
What we really need is for the industry to recognize that the current situation is not ideal and equip bikes appropriately, so I’m planning to send a link to this post to many of the major bike brands and component manufacturers to try to convince them that they are doing a poor job of accommodating most of their clients. If you’ve benefited from a change in crank length then please leave a comment below because that will give extra support to these arguments.
Crank Length Availability
Road cranks that are available outside of the typical 170-175mm range are listed here. Many cranks by the major brands like Shimano, SRAM, and FSA are officially listed as also being available in 165mm, but that length can be difficult to find for sale after-market. Shimano makes Dura Ace cranks in a 180mm version and SRAM offers models at a wider variety of prices up to 180mm length. Campagnolo currently only sells cranks in lengths from 170-175mm.
Rotor and Vision offer some shorter cranks due to the recent interest by triathletes. Rotor’s 3D+ and Flow cranks should be available in lengths from 155 or 160mm up to 180mm, and Vision Trimax cranks in 155-175mm. TA Spécialites is well-known for offering a wide range of crank lengths, with the Vega and Carmina models available in 155-185mm lengths. Cranks designed for kids’ bikes are a good source of reasonably-priced short cranks; many of these are very poor quality, but the Stronglight Impact Kid is probably the best of them and is available in 130-155mm lengths.
There are a few manufacturers of custom cranks. A super-light carbon option is made by Lightning in 160-190mm lengths. Da Vinci makes aluminum cranks in 120-200 mm lengths and Lennard Zinn’s aluminum cranks are offered in 130-220 mm. Leave a comment below if you know of other options.