Crank length: Forget leverage and power, it’s all about the fit
by Matt Wikstrom
It’s almost inevitable that every road cyclist will start
to wonder about the crank length they are using. It’s a topic that is
surrounded by a lot of personal anecdotes and opinion, but how much
formal testing has been carried out?
In this article, Australian tech editor Matt Wikstrom takes a
look at research on the influence of crank length on the performance of
road cyclists, and explains that the results are actually quite clear.
There’s no denying the fascination that surrounds cycling equipment.
For some, it is a matter of style and/or function; for others, it
represents an opportunity for improving their performance. Given that
there is a strong competitive aspect to cycling, it’s not surprising
that the latter has been responsible for an incredible amount of
innovation.
The whole notion of free speed and improved efficiency is compelling
in an endurance-oriented sport like cycling. “Marginal gains” has become
a popular catch-cry for coaches and bike engineers alike but the
compulsion for re-visiting the design of any part of the bike and every
piece of kit with the hope of finding free speed is decades old.
Cranksets have received a fair share of this attention. Originally
made from steel, they have evolved from largely utilitarian creations to
become lightweight and elegant. Aluminium alloy remains the most common
construction material, however the last decade or so has seen the
successful introduction of composites. The same period has also seen
immense proliferation in axle and bottom bracket designs while
chainrings have been getting smaller and using fewer bolts.
One thing has remained constant throughout all of this refinement:
the length of the crank arms. It has hovered around 170mm since the
inception of the safety bike at the turn of the 20th century, and with
good reason: it’s long enough to serve as an effective lever yet short
enough to remain within the range of motion of the human leg.
At one point, during the dominance of the English bike industry prior
to World War II, an attempt was made to standardise crank length
(6.5in/165mm) but road racers started to challenge that notion in their
quest for an edge. Campagnolo’s new cotterless cranks, introduced in
1958, were distinguished by a generous range of sizes, 165-180mm in
2.5mm increments, and remained so for decades to come.
Interestingly, this range of crank lengths pre-dated much of the
formal research on the impact of crank length on a cyclist’s
performance. One early study was published in 1953; otherwise it wasn’t
until the ‘80s and beyond that the issue was examined with any rigour.
While research on this topic may have been slow to start, it has
received a lot of attention over the last 10-15 years and efforts are
ongoing. As with any field of research, there has been some contention,
and some of the results may run counter to conventional wisdom. I will
discuss this in more detail below, but for those hoping for a quick
answer, here it is: there is no evidence that crank length has an effect
on a road cyclist’s power or speed.
The cranks as a lever
Any debate on the influence of crank length normally starts out by
considering the problem in terms of simple physics. When viewed from
this perspective, a bicycle crank is considered a lever, and hence, any
increase in the length of the cranks has the potential to provide the
rider with extra leverage.
While this approach does a lot to simplify the problem, it does not
allow for the influence of biomechanics, which, as it turns out, is
quite considerable. After all, there are three human-powered joints
involved in driving each side of a crankset that require energy in
extension and flexion, so there is more to the problem than simply
calculating leverage.
Nevertheless, the influence of crank length on leverage for the drive
train can be demonstrated under a set of very specific circumstances,
namely from a standing start with a fixed gear over a short distance
(100-200m). Then, longer cranks allow a rider to develop more speed than
shorter cranks,
even when the difference is as little as 2mm.
This kind of scenario is quite removed from road cycling, since
riders spend most of their time seated and have the freedom to change
gear ratios as they please. Under these circumstances, crank length has
no effect on maximum power output, and indeed, near-identical results
have been observed for a substantial range of crank lengths.
For example,
Inbar et al. (1983)
measured the mean and peak power output for 13 subjects during a seated
30s effort using crank lengths 125-225mm. While the authors identified
an optimal crank length of ~165mm for this kind of effort, there was no
significant change in power when cranks were as long as 200mm or as
short as 150mm. Beyond that, there was evidence of a small decline in
power for 125mm and 225mm cranks, however the losses were relatively
small (2-5%).
Martin and Spirduso (2001)
essentially repeated this study with 16 trained cyclists for a 3-4s
effort and five crank lengths (120/145/170/195/220mm) with very similar
results. The researchers noted a small decline (~4%) in power for the
shortest and longest cranks, otherwise there was no difference between
145mm, 170mm and 195mm cranks.
Can a change in crank length save energy?
If crank length has no effect on power output for a road cyclist, can
a rider save energy by changing the length of the cranks? Research on
this question goes back as far as 1953 when
Astrand measured oxygen consumption by cyclists
riding a bike on a treadmill. Changing the crank length from 160mm to
180mm and 200mm had no effect on oxygen consumption whereas a change of
tyres did.
Morris and Londeree re-visited this topic in 1997
with a group of six trained cyclists and found that a small change in
crank length (5-10mm) increased oxygen consumption by up to 11% during a
lengthy submaximal effort. However, it’s worth noting that the subjects
in this study were required to maintain the same cadence (90rpm) for
each crank length tested, which may have influenced the effort required.
Indeed,
a subsequent study by McDaniel et al published in 2002
clearly demonstrated that the metabolic cost of cycling was largely
dependent upon power output, cadence, and pedal speed. A switch between
three crank lengths (145/170/195mm) during the course of this study
actually had no effect on metabolic cost
per se.
Ferrer-Roca et al. (2017) subsequently confirmed these findings
with a smaller range of crank lengths (±5mm preferred crank length)
while considering the effect on biomechanics, noting that longer cranks
increased flexion and the range of movement required at both the hip and
knee. This wasn’t the case for shorter cranks, leading the authors to
recommend that where there is indecision, cyclists should opt for a
shorter crank to reduce the risk of injury.
Wondering what crank length you’re using at the moment? Look on the back of any crank arm to find the length.
Does crank length really need to be optimised?
Based on the evidence presented above and elsewhere in academic
literature, there does not appear to be a strong argument for optimising
crank length for an individual, at least in terms of pure performance.
But there is more to cycling than simply generating power. There is
the demand of maintaining a highly repetitive activity for long periods
in the context of fluctuating loads. The bicycle itself is a highly
symmetrical machine while the human body is typically asymmetrical, so
the potential for uneven loading is enormous and injuries are common.
In fact,
a high proportion of cycling injuries relate to overuse for both
recreational and
professional cyclists.
The legs are commonly affected, especially the knees, and while the
causes are many and varied, the most common prevention strategy is to
modify the rider’s position on the bike.
This is where the optimisation of crank length becomes important.
While the position of the saddle can be adjusted to suit the overall
reach of the legs, the length of the cranks largely dictates the range
of motion. As a result, bike-fitters have come to view crank length as
an important parameter that can be optimised for every individual. While
this optimisation probably won’t improve the performance of the rider
in terms of measurable power, it can add to comfort and prevent
injuries.
A formula for deciding crank length?
The current market offers a pretty generous range of crank lengths,
starting as short as 160mm and extending to 180mm, often in 2.5mm
increments. In addition, there are a few manufacturers offering
custom-built cranks outside this range, so it is possible to fit
significantly shorter (e.g. 130mm) and longer (220mm) cranks to any
given bike. Thus, there are plenty of products available for optimising
crank length, but how does a rider to decide on a specific length in the
first place?
Longer cranks can make a difference, but only for short sprints from a standing start with a fixed gear ratio.
While it is generally acknowledged that crank length should increase
with the height and leg length of the individual, the exact association
remains vague at best.
One early study (1976) experimented with different proportions of crotch height and concluded that ~20% was the most suitable. Decades later,
Martin and Spirduso (2001) arrived at much the same recommendation (20% of leg length).
A variety of other formulae have been proposed over the years ranging from
simple equations to
more complicated approaches.
Each formula is an attempt to describe an association between
measurable parameters (e.g. leg length) and a functional outcome based
on a finite number of subjects, so
a lack of consensus really isn’t surprising. Nevertheless, they have found favour because of the ease they offer, but in strict terms, they do little to settle the matter.
That’s because crank length is part of a system of hinges and levers
that must operate in the larger context of an individual’s biomechanics.
Most of these formulae fail to consider this at all, effectively
isolating the issue from all other considerations, and for this reason,
it is probably best to view any result as theoretical at best.
While any of these formulae might provide a starting point for
further investigation, it makes more sense to get some advice and
direction from an experienced bike-fitter, if only because cranksets
tend to be quite expensive.
It’s all about the many aspects of a rider’s fit
Stewart Morton
has over 10 years experience as a bike-fitter and he still considers
crank length a can of worms. “For the most part, by understanding a
rider’s cycling goals and their riding discipline, and assessing their
body (flexibility, joint range, injury), I can figure which crank length
might be most appropriate.”
“For those in the middle of the bell curve for height then
167.5-175mm cranks will work. The industry has done a pretty good job
using anthropometric studies to create bike models with size-appropriate
crank lengths,” said Morton.
Thus, in some circumstances, there is no need to change the cranks,
and in others, it’s possible to accommodate the rider’s preference for a
specific crank length. “Whether a rider runs longer or shorter cranks, I
will still aim to get the knee extension and saddle placement neutral
to allow injury-free and efficient pedalling.”
Morton also understands that a change in crank length can allow the
rider to safely assume a more aggressive position on the bike without
discomfort or the risk of injury. “Ironman athletes are running shorter
cranks — down to 155mm, in some cases — to help maintain a healthy hip
angle as they rotate their bodies around the bottom bracket,” said
Morton. “They can move forward and lower the front end of the bike and
still make a good transition off the bike for the run.”
Ryan Moody spends his days deciding the final fit for
Baum’s custom-built bikes.
“I try to put anyone on the longest crank possible within their range
of movement to use the muscle mass they have,” he said. “This is
especially important for strength-oriented riders. For those riders with
greater cardiovascular strength, a shorter crank works better because
they tend to ride at higher cadences.”
Deciding on the crank length is not a simple matter, though. The
handlebar and saddle position can influence how well the legs are
moving, as can the cleat position. Moody’s ultimate goal is to achieve a
clean motor pattern for the entire pedal stroke and has found that even
minor changes (1-2mm) to the position of a contact point can have a
profound effect.
And because he is not working on modifying an existing bike, he is
free to customise the geometry of the frame to accommodate each part of
the bike, including the cranks. “The bottom bracket height should suit
the length of the cranks, especially if they are longer than normal.
Putting a long crank (180mm) on a bike with a low bottom bracket will
cause problems with pedal strike when cornering.”
Interestingly, Moody also sees crank length as having an effect on
the way that a rider can balance their weight on the bike. “A typical
male is top-heavy because of their upper-body strength and a longer
crank can help them keep it balanced over the handlebars. It’s the
opposite for females, who don’t have the same kind of upper-body mass,
so a shorter crank is often better.”
Both Morton and Moody are generally happy with the range of crank
lengths available and consider 2.5mm increments adequate for their
needs. Nevertheless, there may be a growing need for shorter cranks
(155-160mm), and not just for Ironman athletes.
“The cycling culture is changing and it’s no longer all about
reasonably tall men with lots of leg strength,” said Moody. “A lot of
women need shorter cranks for a better fit on their bikes, shorter Asian
populations too, but I don’t see crank lengths getting smaller on
mainstream bikes yet. These are the kind of riders that are going to
benefit most from a change in crank length.”
Summary and final thoughts
The biomechanics of cycling is complex and multi-faceted, so
concentrating on a single aspect, like crank length, is bound to suffer
from oversimplification and generalisation. Nevertheless, academic
researchers have managed to examine this issue with growing
sophistication over the last few decades to better understand the
influence of crank length on the performance of cyclists.
On balance, the weight of the available research indicates that crank
length does not influence the speed, power or efficiency of a road
cyclist. What is more important is how the cranks are used, and this is
where the training, experience and intrinsic capabilities of the rider
make all the difference.
For those riders that have been tempted to try a different crank
length based on the promise of extra leverage and perhaps increased
efficiency, there are no such gains to be made. The cost of a new
crankset will be better spent on formal training with a coach.
By contrast, those cyclists that have been suffering with a recurring
overuse injury may find relief with a change in crank length, but this
is not something that should be attempted through self-directed
experimentation. A trained bike-fitter with extensive experience is
likely to take less time and arrive at a more robust solution because
they possess the understanding and objectivity to judge a cyclist’s
position on the bike.
Author’s note: While
links to some of the primary research papers on crank length have been
provided in this post, it is far from an extensive review of the
literature. In addition, it is not always possible to access original
research without a subscription. For those hoping to read more of this
research, I’d recommend visiting a university library that has a strong
commitment to biomechanics.
