Updated: Sep 2
When setting up the bike for an ultra distance, we make sure that all aspects of the bike are dialled into our functional anatomy for maximal comfort and best performance, such as saddle height, reach, shoe/pedal interface and that all important well fitting saddle. An aspect often overlooked is crank length, most of us happily riding the cranks supplied with the bike. Advice as to the correct length is widely debated though as there are many variables to take into account, but it is, after all, the lever with which we use our mechanical levers, the legs, to drive the bike forward. Is there a correct length and how close can we get to it given the limited length options? This article attempts to cover some of the thoughts surrounding crank length with some opinions based on personal experience, but it won’t make any particular global recommendations though, as advice is very individual.
Height is usually the norm on which length is decided, but it is only a loose guide given the huge differences in human anatomy, function and movement. Hands up if you know the crank lengths on your bike(s). Chances are that you'll be riding different lengths on the mtb and road bike. If you do know the lengths, does it have any meaning to you and do you have any lower limb pain or any issues swapping from the mtb to the road bike? Most of us have adapted well to the length(s) we have and ride comfortably without any issues. Great if you do, continue to pedal happily! For some riders though with ongoing knee, hip or lower back issues, crank length could be a factor. Length is also a worthwhile consideration for ultra distance riding.
My first mtb frame (17.5") came with 175mm cranks and the equivalent sized road bike with 172.5mm. It was just a number however. I just thought that the longer crank on the mtb was because mtbers required extra leverage for slow speed manoeuvres through technical terrain and shorter for the road where cadence was more constant and smoother, with track being even shorter for higher cadences and constant changes in intensity. Nothing has changed. The industry is still supplying bikes for riders of my height of 177cm (5'10") with the same length cranks some 20 years later. What has changed however is a greater understanding of human mechanics in cycling and the upshot of findings is that swapping out to a shorter crank (by at least 5mm) could be of benefit for comfort and performance for some riders without any negative effect on the ability to generate power. Some manufacturers do offer a range of crank lengths, such as Rotor Bike who have road/gravel cranks down to 150mm and 165mm for MTB.
It tends to be only at the point of injury or pain that many of us seek advice or begin to scrutinize our bike set ups. From a personal perspective, I made the change to 170mm cranks across all the bikes in order to open up the closed hip angle to conservatively manage a minor case of reduced blood flow through the iliac artery due to a kinking in the iliac artery. With an 81.6cm inside leg, the 175mm cranks never felt to be a problem in my former years, but over the years of being in a low bike position and age related physical changes, the change has been a positive one. 175mm now feels more of an effort to turn and I feel the extra demand on the knees.
There is the consensus though that by going shorter we lose the ability to produce power due to the shorter lever, but this isn't the case. The physics of levers isn’t valid on a bike with gears. For a single speed bike there would be a change in leverage effect, but with gears we shift to adapt to the leverage, so crank length won’t make a difference when pedalling rate is optimal for an individual. Gears offset the effect of lever length. Cadence increases slightly and my (albeit limited!) power around familiar circuits hasn't been negatively affected, at all. A cyclist's peak force is produced at the top of the stroke in the 12 o'clock position. Steve Hogg explains it as, for a longer crank, the leg has to reach further past the top of the pedal stroke before being able to apply the same mechanical advantage, so a longer crank doesn't mean greater torque (pressure on the pedal x crank length ) or power (torque x rpm).
This is supported by the findings of John McDaniel and Steven Cobb (of Cobb saddles), that more power isn't generated by riding longer cranks. They also found that a shorter crank is more efficient in the form of less oxygen uptake, where pedal speed (the speed of the pedal along its axis of travel - NOT cadence) remained the same. They found that the greater the pedal speed, the greater the oxygen consumption, where pedal speed is higher with a longer crank due to the larger diameter circle the foot goes through. They worked with a rider of 6'5" reducing crank length down to 165mm and significantly increased his power output. Obviously this was a very calculated research to prove a point and they do emphasis that the significant decrease in oxygen was across a large variation in crank length from a 145mm length to the 190mm, so they do suggest that going from 172.5 to 170mm wouldn't give a significantly noticeable change in oxygen consumption. Mechanically though (inflexibility, pain), even this small decrease may have a positive effect due to the 5mm change in peak knee height (shorter by 2.5mm + increasing saddle height by 2.5mm to account for the reduced reach at the bottom of the pedal stroke) and it may also be more efficient through the dead spot when fatigue sets in due to a reduced peak acute knee angle. Every rider is different however in how they respond to change, with some responding to the smallest of alterations in the bike set up, whilst for others it has to be a larger change for any real effect to be felt. So as with all aspects of bikefitting, what works well with a rider of a certain height/leg length proportion won't necessarily work for another of the same!
Personally, pedaling the 170mm cranks has felt more fluid, being able to produce peak force more easily and it has improved comfort at the hip. The 5mm difference may not seem like much, but in terms of biomechanics, it is a significant 10mm at the top of the pedal stroke. It’s certainly made things more comfortable for bike pack racing.
So how do we work out what we need as a rider? As you may have guessed, there is no straight forward answer to that! Bikefitting is as much an art as it is a science due to the many individual variables of leg proportion, foot size, flexibility, comfort and overall movement. The available advice is a little conflicting with various formulas having been developed; Graham Obree suggests a length which is 9.5% of overall height. The folks at http://www.nettally.com/palmk/crderiva.html work on the premise that crank length is proportional to inseam length and suggest a calculation of inseam x 0.216. This can result in a length on the longer side though. Where inseam length is; stand against a wall in bare feet, place the binder side of a thick book up into the groin and make a mark on the wall (probably best not to do this in the dining room!).
It would seem to make sense that crank length should be related to femur length, but the difficulty arises in accurately measuring the femur. Foot length also has to be considered, where a larger foot acts a longer lever and some riders may differ proportional from the average.
Just as with working out optimal saddle height, formulas can be a useful tool as a starting point, but they should not be taken as an absolute. If you work out to be in the middle of 2 crank sizes, you won't lose anything by going for the shorter option. For the inflexible or those with mild joint pathologies, going shorter may be of benefit, reducing tension through the joints and soft tissues.
Where crank length is incorrect, they are most likely too long. Michael Veal of Bike Dynamics put together a list of possible indicators:
You are often out of the saddle on longer climbs,
You have difficulty in maintaining a higher cadence (above approximately 75rpm)
You have a very distinct dead spot.
You experience continued anterior knee pain, hip pain or lower back pain following a bikefit or an assessment by a therapist
Rocking hips may be another one (if saddle height is optimal)
Or conversely, the cranks may be too short if;
You have power on the climbs, but are not so strong on the flat.
Before scrutinizing your crank length however, there are other adjustments to be made first, such as; saddle height, cleat position or improving core strength and/or pelvic posture on the bike. Cleats are often moved rearward by 5 to 10mm which will reduce both the acute knee and hip angles at the top of the pedal stroke. Get the seat height correct, but for the inflexible, saddle height will be lower than the norm to get the lower back into an optimal position, effectively increasing the acute knee angle for a given leg length. Another reason good flexibility is key for a cyclist! If you are experiencing lower back, hip or knee issues, where relevant, look into a programme of improving flexibility, core strength and/or lower limb strength as an initial 'fix' before investigating crank length.
For triathletes where aerodynamics is a major concern, going shorter can help with being able to maintain a lower position and keeping the closed hip angle as open as possible for comfort and performance.
There is some science behind riding the correct crank length, but what it comes down to however is being comfortable. Some other articles even suggest that there is no scientific formula and that we should simply ride what feels best. If you are experiencing lower back or joint issues, get a bikefit (preferably with someone who is appropriately qualified in being able to assess biomechanics) in the first instance, do some guided, regular flexibility, stability and strength exercises and if the issues persist, seek advice and investigate crank length. Changing cranks is not a cheap option of course, especially if the number of bikes owned is n+1, but it could be a worthwhile investment.