At Arm’s Length: The Great Crank Arm Length Debate
“What’s the right crank arm length for me?” The answer to one simple question is far more complicated than it seems. Two bike fit experts weigh in.
The bike industry is slow to change. Tradition is often repeated, sometimes for better and sometimes for worse but every few years or decades, when a shift does happen, it is notable. Over the last two decades, one of these shifts has happened, one that is at odds with industry standards and at odds with what most cyclists, coaches, and bike fitters have been advocating for many years. Crank arm lengths are getting shorter, and we are not turning back.
Crank arms have been built around the ‘norm’ of a 172.5mm crank arm length for as far back as anyone can remember, and no one really knows why. Mountain bikes and tri bikes sometimes got slightly longer cranks for more “leverage,” track bikes went shorter (maybe they were on to something), and road bikes generally got 172.5mm cranks. If you were short, you got 170s (maybe 165 if you were lucky); if you were really tall, you got 175s, and that was about all there was to it. Eventually, however, the experts in the industry have all begun to agree that we’ve had it wrong for a long time, and empirical evidence as well as scientific studies have shown that for the average height—or slightly less than average height cyclist—a shorter length crank arm length has some serious benefits. In 2012 Craig “Crowie” Alexander, who is 5-foot-11-inches, won the Ironman World Championships on a 167.5mm-long crankset, 5mm shorter than he rode previously, proving on the world stage that a “short” crank was anything but slow.
Crank Arm Length: The Problem
The crank arm length is defined as the distance between the pedal axle and the center of the crank rotation axis. Traditionally, crank arms have been widely used in lengths between 165mm and 175mm and the general rule was that crank arm length should simply be a proportion of rider height or leg length. Taking rider height as the driver, consider for a moment two riders, one who is 60 inches tall and the other who is 75 inches tall—a height difference of 21.5%. If we look at a range of crank arms widely available from 165mm to 175mm, we only see a difference of 6% meaning that the range of crank arm lengths is not nearly in proportion with a range of heights of full-grown adults. Some smaller brands have introduced crank arms in lengths that are both longer and shorter than this range. Shimano recently introduced 160mm cranks on their 105 platform, Rotor makes their Aldhu crank as short as 150mm, Speed and Comfort (formerly Cobb Cycling) makes a crank as short as 145mm. Zinn Cycling—of Lennard Zinn fame, author of tons of cycling tomes and writer for VeloNews—the crank length champions, make cranks from 130mm all the way up to 220mm!
Taking rider height as the driver, consider for a moment two riders, one who is 60 inches tall and the other who is 75 inches tall—a height difference of 21.5%. If we look at a range of crank arms widely available from 165mm to 175mm, we only see a difference of 6%.
Crank Arm Length: Why It Matters
So how exactly does crank arm length affect your bike fit? The crank arm length most notably affects the range of motion from the hip joint but it also affects the knee joint and to a lesser extent, the ankle. A shorter crank arm length will open up the hip angle at the top of the pedal stroke which is generally where long cranks create the most problems. A shorter crank also reduces the bend in the knee at the top of the pedal stroke, helpful for riders who suffer from patellofemoral issues. Muscles work most efficiently and powerfully through approximately the center range of the joint’s total range of motion, and since bike fit directly affects range of motion, the goal of bike fit is to optimize the variables at hand (seat position, crank length, handlebar position, etc.) to best position the rider to work comfortably within their specific range of motion.
The question we get most when asked about crank length, is “Will a different crank length affect my power output?” The short answer is “No.” The now fairly famous studies from Jim Martin from the early 2000s determined that maximum power output was no different when using cranks from 145mm to 195mm. During this study they also noted less oxygen consumption with the shorter cranks. The results of this study, as well as the empirical evidence from the thousands of people that have tried a change in crank arm length show that crank arm length should be used as a variable to experiment within your bike fit—no different than playing with different saddle heights or different handlebar positions in order to achieve the optimum balance. We have been advocating for people to not only consider their overall height or leg length when considering crank arm length, but also how they use their bike and any physical limitations or restrictions they might have.
Crank Arm Length: Get Out Of Your Way
Triathletes tend to benefit from a shorter length crank arm more than any other cycling discipline because triathletes spend hours and hours in an aero position that forces their hip into deep flexion at the top of the pedal stroke. Triathletes also tend to have fairly tight hip musculature from all the hours of training and not nearly enough time giving their body the TLC it deserves. All muscles have something called a “resting length,” which is essentially the muscle’s start point prior to contracting for maximum muscular force. Muscles also experience tension due to stretching, not unlike a rubber band, that tries to bring the muscle back to its resting length. When the hip joint goes deep into its range of motion at the top of the pedal stroke, the muscle forces due to stretching (known as passive forces) can become significant and actually oppose the driving forces being produced by the other leg (since the cranks are connected), hence robbing an athlete of power (see graph below). In other words, if you go too deep into the upward motion of your pedal stroke, you can actually get in the way of the other leg’s work. By switching to a shorter length crank arm, the cyclist’s hip angle at the top of the pedal stroke is less restricted, thereby reducing the wasted energy created by essentially “fighting yourself.”
Crank Arm Length: Tri Specific
Triathletes tend to have more than their fair share of hip injuries: imperfect biomechanics and technique, lots of training, and the unfortunate crash or two can really pay a toll on the body. Greg Close, pro triathlete and coach at TriBy3 Performance Coaching in Brooklyn, N.Y. started his triathlon career riding 175mm cranks. After about five years he switched to 170s for some time, and now he’s been on 165mm cranks, thanks to his astute bike fitter’s advice. In 2014 Greg suffered a bad bike crash and has had some lingering injuries to his right hip including a torn labrum and deformed femoral head. Greg explains: “Whenever I rode, I would get a sharp pain at the top of the pedal stroke and some weakness as I started the downstroke. When I ran, I had this constant feeling that my hip was going to pop out of its socket (the labrum helps to keep things together in there).” After finally shortening to 165mm cranks, the majority of Greg’s hip issues are resolved, however he still has to be careful with form and take care of his body. For reference, Greg is 5-foot-11-inches tall, and he is very fast.
Crank Arm Length: Side Effects
Most athletes tend to report an increase in cadence (revolutions per minute) when shortening their crank length and a slowing of cadence when going longer. Some theorize that cyclists tend to pedal with an optimal foot speed, which is not cadence but the actual speed at which the foot moves through the air—this is a big difference, but something that’s tough to get one’s head around. If you assume that this foot speed is the same for an athlete moving their foot around a smaller circle, you can see why cadence might increase when using shorter cranks. Many athletes also note that they have to use slightly different gear combinations when shortening their crank length. That’s not really true either. A bicycle is driven by a series of levers: the crank arm itself is a lever; the front chainring, rear cassette and chain make up a lever; and the rear wheel acting upon the ground is the last lever. The overall system leverage, or put practically, the ability to climb the steepest of hills and power back down them as fast as possible—all at a reasonable cadence—is best described by the gain ratio which accounts for the crank arm length, wheel size, and gear selection.
Studying the equation for gain ratio, you can see that shortening the crank arm length would increase the overall gain ratio if no other factors are changed. This would make it more difficult to climb a hill in the same gear ratio that one was previously using. The good news however, is that today’s drive systems have more gears than most triathletes know what to do with, so in practice this works out to be a non-issue, since there is no excuse to “run out of gears” any more.
Crank Arm Length: Not As Simple As It Looks
While we’d love to give you a simple answer for determining your ideal crank length, like a lot of aspects of bike fit, it’s not that easy! There are some tempting charts and formulae floating around to match a rider with their “ideal” crank length. The issue with many of these calculators is that they overweight the importance of inseam or rider height when calculating the proper length crank. These simplistic calculators fail to include other determining factors including, fit, physical limitations, and discipline. On top of that, depending on the theoretical support for the calculator, the formula may tell you to multiply your height by .95 or your inseam by 2.1 or your inseam by 1.25 plus 65. All this is to say that there is a lack of consistency when it comes to crank length, but there’s a reason! Since there are multiple variables affecting a rider’s ideal crank length, it is nearly impossible to rely on a single calculation. Instead, a more accurate process would look more like a flow chart. What is your height? What is your inseam? Are you riding a triathlon bike, road bike, or mountain bike? What are your goals as a rider? How flexible are your glutes, hamstrings, and hip flexors? Do you have knee issues? With this information we can start to make an informed decision on how to alter crank length to suit the individual needs of the rider. In our very informal test lab (the fit studio) we find that a large number of riders would benefit from a change in crank length. There are also real-world considerations of price, crank length availability, compatibility, and what length crank the rider is used to using. A rider who has spent 10 years on 175mm cranks will most definitely find 170mm cranks awkward at first. And even though they might adapt in a few rides, they have to be willing to take on the experiment.
Crank Arm Length: Get Right
Short of a session with your fitter and a trial of different crank lengths, there is a relatively simple test to identify the effects of your cranks on your efficiency. (We are assuming the correct seat height and a reasonable bar height here.) On a stationary trainer in the drops on a road bike or the aerobars on a tri bike, unclip one foot and focus on the top of the pedal stroke. Do you notice a hitch or a dead spot? Are you forced to rock your hips as your knee comes over top dead center? Is your hip flexor cramping? Are you unable to get through a single pedal stroke smoothly? If you answered yes to any of these questions, an investigation into your crank length may be warranted. In the 12 o’clock position, with only one leg to maintain momentum, inefficiencies are amplified. We can no longer rely on the other leg (at 6 o’clock) to “drag us through” this dead spot. Now picture a shorter crank. Let’s say we are going from a 175mm crank arm to a 165mm arm. The top of the pedal stroke is now 10mm lower, giving us more room between the knee and the torso, thus opening the hip angle. As we shorten the crank, we are also shortening the saddle height as measured at the bottom of the pedal stroke. For this reason, we would want to raise the seat in a magnitude equivalent to the change in crank length (in this case 10mm). Effectively doubling down on opening that hip angle,at the end of the day we now have now moved the top of the crank at the 12 o’clock position 20mm lower from the saddle. Depending on the rider and the various factors at play, there is a chance that we just: A) increased their pedaling efficiency by moving them into a smoother range of motion or B) allowed the rider to “lower” their handlebars without entering into a limited range of motion. And while there are most definitely ways to alleviate the dead spot issue by raising your handlebar height, in the pursuit of optimal comfort, speed, and efficiency the cranks may be a great place to look.
About the authors:
Jon Blyer and Colin Tanner are co-owners of Brooklyn, N.Y.-based ACME Bicycle Company. Blyer is a professional engineer, longtime triathlete and cyclist, and a Retul Certified Master Bike Fitter. He also teaches the art and science of bike fit at the Guru Academy in Bethel, Connecticut. Tanner is an experienced road racer and a former USA Cycling coach who has a background in physical therapy.