There aren't many win-wins in mountain-biking, but shorter crank arms could be just that.
Most bikes have crank arms between 170 and 175 mm long, and it's been that way for a long time. Some brands spec 170 mm cranks on their smaller sizes and 175 mm cranks on the larger sizes, and you can even buy 172.5mm cranks aftermarket, suggesting that the ideal length for pedalling ergonomics, efficiency, power and ground clearance must be somewhere in that range. But there's a fair bit of published science on crank length which tells a different story.
Trawling Google Scholar
served up seven published studies looking into the effects of crank length on pedalling performance. Given the bike industry's near-universal spec choices, you might imagine they found that the ideal crank length was somewhere in the region of 170-175 mm, or maybe that longer crank arms might improve pedalling power to some extent, given that mountain bikes need to compromise between efficiency and ground-clearance. They found nothing of the sort.
In the rest of this article, I'll briefly summarise the methods and findings of each study, then at the end, I'll sum up and give my own take. But first, a little theory.The Theory
A commonly made argument is that longer cranks offer more leverage, meaning more torque can be generated for a given force at the pedal. This is true, but leverage comes at a cost. If you can move your pedal with a certain force and at a certain speed around the circumference of the pedalling circle, a longer crank will generate more torque but at a lower rotational speed (rpm), because the circumference is bigger. Power is just force times speed or torque times rotational speed, so either way, the power is the same (at least in theory).
It's well-documented that riders naturally pedal at a higher rpm when using shorter cranks; so to some extent, the reduced torque is compensated by a higher cadence.
This isn't to say that crank length doesn't matter, but the difference comes in the biomechanics - do human bodies prefer to do a shorter range of motion more frequently (shorter cranks) or a longer range of motion less frequently (longer cranks)? Or more accurately, where does the optimum between those extremes lie? That's a question that needs to be found out through experimentation on real-life people.
I've searched out all the scientific articles I can find on crank length; seven papers in total. They all have slightly different methods and ways of measuring things, but they all come to a similar conclusion: longer cranks are not better. What the Science saysThis 2001 study by J.C. Martin & W.W. Spirduso
is one of the biggest and most comprehensive on crank length. They measured the maximum sprint power output on a static bike with sixteen trained cyclists, each using a wide range of crank lengths (120, 145, 170, 195, and 220 mm).
Perhaps unsurprisingly, the lowest power outputs were recorded with the most extreme (120 mm and 220 mm) crank lengths. The highest power output was recorded with the 145 mm crank, although the difference wasn't large or consistent enough to be considered significant compared to the 170 mm or 195 mm cranks. The difference in power between the "best" (145 mm) and "worst" (220 mm) crank was just 3.9%
As you'd expect, as crank lengths increased, the cadence decreased, from 136 rpm for the 120mm cranks to 110 rpm for the 220mm cranks. In other words, the body adapts by changing cadence to suit the crank length.
Looking at all the data from all sixteen cyclists and all five crank lengths, the authors estimated the optimal crank length for sprinting was 20.5% of leg length or 41% of tibia length. But in either case, the correlation was too weak to draw any firm or precise conclusions. The main takeaway here is that sprint power output varied very little, especially across the middle three sizes (145, 170 and 195 mm) which is a much wider range of crank lengths than typically used.
Here's how the authors put it: "Even though maximum cycling power was significantly affected by crank length, use of the standard 170 mm length cranks should not substantially compromise maximum power in most adults." In other words, within reasonable limits, the ratio of leg length to crank length doesn't matter too much for sprint power.
Is there a disadvantage for kids riding adult-sized cranks?In 2002, Martin and Spirduso
built on the above study by looking at it in another way. They tested maximum power again, this time with 17 boys aged 8–11 years. Their maximum sprint power was tested with a standard 170 mm crank and with an "optimal" crank length which, based on their previous study, was calculated to be 20% of their leg length. The "optimal" cranks were therefore shorter than 170 mm.
Although there was a difference in cadence (129 rpm for the shorter cranks and 114 rpm for the 170 mm cranks), they found no significant difference in the boys' power output in either case.
What about aerobic efficiency as opposed to maximum sprint power?This 2002 study by J. McDaniel et al.
had nine trained male cyclists pedal at a submaximal (aerobic) power output with 145, 170, and 195 mm crank arms, each at four different cadences (40, 60, 80, 100 rpm). This gave twelve different pedal speeds (crank length x cadence). They did this at 30, 60, and 90% of their Lactate Threshold, while the volumes of oxygen they consumed and CO2 they produced were measured to determine the metabolic cost of cycling at each power output and each crank length.
They found that the metabolic cost of cycling was strongly correlated to the pedal speed (crank length x cadence) but wasn't related strongly to the cadence or crank length per se. In other words, riding with shorter cranks requires a faster cadence (and longer cranks a slower cadence); but with the right cadence, the crank length doesn't significantly affect the metabolic cost of pedalling at a given power output.
Here's how it's put in the paper: "even with our wide range of pedaling rates, pedal speeds, and crank lengths, muscles' ability to convert chemical energy to mechanical work was remarkably stable."
What about for mountain bikers?In 2009, Paul William Macdermid & Andrew M. Edwards
tested seven female cross-country athletes with 170, 172.5 and 175 mm crank arms. Their peak power was measured at a constant cadence (50 rpm) and then at maximal aerobic capacity.
No differences were observed in power output, even when cadence was fixed at 50 rpm. However, the time taken to reach peak power in a spint was significantly less with the 170 mm cranks compared to the 175 mm ones.
The authors suggest this could represent an advantage: "The decreased time to peak power with the greater rate of power development in the 170 mm condition suggests a race advantage may be achieved using a shorter crank length than commonly observed. Additionally, there was no impediment to either power output produced at low cadences or indices of endurance performance using the shorter crank length and the advantage of being able to respond quickly to a change in terrain could be of strategic importance to elite athletes."
But do extreme crank lengths lead to excessive strain in specific joints?In 2011, Paul R. Barratt et al.
looked into the relative contribution of different leg joints towards pedalling power, with crank lengths of 150, 165, 170, 175, and 190 mm. Although there were differences in the hip and knee joint contributions when comparing the 150 and 190 mm crank arms and
when cadence was fixed at 120 rpm, there were no differences when cadence was allowed to vary to suit the crank length. "Crank length does not affect relative joint-specific power once the effects of pedaling rate and pedal speed are accounted for. Our results thereby substantiate previous findings that crank length per se is not an important determinant of maximum cycling power."
What about with untrained cyclists?In 2016, Ventura Ferrer-Roca et al.
measured both the aerobic efficiency and the range of motion at the leg joints in twelve amateur cyclists. Heart rate and gross efficiency (pedalling work done per calorie burned) were measured while pedalling at a fixed aerobic power output. They did this with three crank lengths 5 mm apart.
Again, there were no differences in heart rate or gross efficiency at any of the crank lengths tested. However, longer crank arms measurably increased the range of motion of the hip and knee joints which, the authors say, could be a negative. "the biomechanical changes due to a longer crank did not alter the metabolic cost of pedalling, although they could have long-term adverse effects. Therefore, in case of doubt between two lengths, the shorter one might be recommended."
What about for stand-up pedalling?
Finally, in 2021, Sumin Park et al.
looked into standing cycling. Ten participants cycled out the saddle at sub-maximal (aerobic) power while biomechanical parameters, motion data and pedal reaction forces were measured. With longer cranks, more power was absorbed during the upstroke of the pedalling cycle. "Consequently," the authors say, "longer crank lengths require increased propulsion power by the lower limb muscles during standing cycling compared to shorter crank lengths. Therefore, shorter crank lengths are recommended for stand-up bicycles to avoid fatigue."Conclusion
The main takeaway from the published science is that crank length either doesn't affect pedalling performance, or there's a possible advantage to shorter cranks. Although one paper found a small disadvantage with very extreme crank lengths (120 mm or 220 mm), there is no evidence of a significant difference in maximum power output or efficiency when using crank lengths as far apart as 145 and 195 mm. This is because, as crank length decreases, cyclists pedal at a higher cadence to compensate.
Three of the seven papers put forward possible benefits to shorter cranks, while the rest found no difference. One paper found shorter cranks reduced the time taken to increase power output in a sprint; one recommended shorter crank arms for standing cycling to reduce fatigue; another noted that longer cranks increase the range of motion of the hip and knee joints, which they say "could have long-term adverse effects".
One study estimated that the optimal crank length (for sprinting) is about 20.5% of the cyclist's leg length. Leg length is typically around 45% of height, which would imply an optimum crank length of about 9.2% of total height. That rule of thumb would put the ideal crank for someone of average male height (175cm or 5'9") at around 161 mm, or 150 mm for the average female height. But remember these are very rough numbers and the main takeaway is that, within sensible limits, crank length doesn't matter much in terms of pedalling performance.
As mountain bikers, all we need to know is that running 10 mm shorter cranks won't slow you down when pedalling, but that much more clearance is a huge difference in terms of reducing the risk of those heart-stopping and axle-bending stalls when the pedal catches a rock. Of course, you could also increase the BB height to achieve this, but that has its own list of disadvantages which could fill another article.
There are a couple of practical challenges to going to shorter cranks. Most importantly, the requirement to pedal at a higher cadence will require easier gearing (going from 175 to 165 mm cranks should be paired with swapping a 32t to a 30t chainring to compensate for the reduced leverage and allow for higher cadences). This may result in some ribbing from less educated riders. If the gearing isn't adjusted, the shorter crank arm effectively gives a harder gear in terms of the force required at the pedal to create a given propulsive force at the wheel, potentially leading to more strain on the body when you run out of gears.
Also, shortening crank length by 10 mm should ideally be paired with raising the saddle height by 10 mm so the leg extension with the pedal at the bottom of the circle is the same. For a given dropper post length, this means the saddle will be 10 mm higher when descending, so fitting a longer dropper post may be worth considering too. Finally, from experience, when fitting shorter cranks it may take a few rides to get used to the higher cadence.