Taking a Look at Crank Length

Crank length is an often overlooked aspect of bike fit. A majority of production road-oriented bikes come outfitted with either 170mm or 175mm cranks. You do occasionally see 165mm cranks spec’d on the very smallest frame sizes, and some manufacturers offer 172.5mm cranks on their mid-sized frames, but it’s a rare occasion to encounter a crank under 165mm or over 175mm on a bike primarily intended for road riding.

There are a number of methods for determining proper crank length. Peter White recommends 18.5% of the distance from the top of the femur to the floor in bare feet. Leonard Zinn recommends 21% of inseam measured in bare feet. Bill Boston recommends measuring femur length from the center of the hip joint to the end of the bone in inches, then using this measurement as crank length in centimeters (for example, if your femur measures 17 inches, you would use a 17cm [170mm] crank).

If you accept that muscles and joints work most effectively when operating in a certain range of motion, then it only makes sense that muscles, bones and tendons work that way for everyone.
-Lennard Zinn

Let’s use Peter White’s method as an example. For simplicity, let’s assume a rider with a top of the femur to the floor measurement of 919mm (36 1/16″). Multiply by Peter’s 18.5% (919mm x .185 = 170mm) and you end up with a recommendation for 170mm cranks. That’s convenient for someone with a 36″ floor to femur measurement, but it gets more complicated for individuals outside the “normal” range. Shorten the floor to femur measurement by three inches (33″) and the recommendation quickly drops down to 155mm (838.2mm x .185 = 155mm). Lengthen the floor to femur measurement by three inches (39″) and the recommendation jumps up to 183mm (990.6mm x .185 = 183.25mm). Both fall outside the normal range of sizes typically offered in road cranks.

I believe there’s quite a bit of room for personal preference in these formulas. I also think the human body is amazingly adaptable and we can probably get used to just about any crank length currently being manufactured, regardless of our leg length. That said, it’s only reasonable to assume that crank length should vary based upon an individual’s physiology. As Lennard Zinn said, “If you accept that muscles and joints work most effectively when operating in a certain range of motion, then it only makes sense that muscles, bones and tendons work that way for everyone. Short riders should not be required to force their muscles through a greater range of motion than the person with an 80cm inseam riding a 172.5mm crank. And on the other end, 7-foot basketball players do not bend their legs any less when they jump than shorter players. So why should they use minimal knee bend and operate their muscles only through a tiny part of their range when they ride a bike?”

Assuming Mr. Zinn is correct, taking a closer look at crank length should be particularly advantageous for those who are significantly taller or shorter than average. On the outer edges of the bell curve, it’s very unlikely people are riding cranks that are close to their ideal length.

While it’s not easy to find unusually short or long cranks, they’re out there. The Specialites TA Carmina is a beautiful, if expensive, production crank that has interchangeable arms available from 155mm up to 185mm. Lennard Zinn offers oversized cranks, and da Vinci Designs offers custom cranks in almost any size within reason.

Those who aren’t involved in the recumbent community probably don’t realize that recumbent riders have been experimenting with ultra-short cranks for some time now (see the links below). Some of their findings are quite interesting and may possibly be applicable to upright bikes. Short crank advocates claim reduced knee strain and higher crank RPMs among their advantages.

Ultra-long cranks may introduce more knee strain, and they will definitely reduce cornering clearance, so both of these facts should be taken into account if you’re considering retrofitting a longer crank on your existing bike.

And finally, whether you’re going up or down in length, keep in mind that a major change in crank length will very likely require a change in saddle height, which, of course, will affect overall bike fit.

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20 Responses to “Taking a Look at Crank Length”

  • Sonia says:

    I read your post with interest as I’m 4’11 and am riding a bike with ‘regular’ cranks. Having had a bike fit recently I’m now more comfortable on my bike but couldn’t help wondering if shorter cranks would suit me better. I’ll raise it when I return for my bike fit part 2.

  • Zyzzyx says:

    I’m part of the recumbent community, and also a person of relatively short legs (30″ inseam, 40.5″ exseam). For years I used 170s and 175s on my mt bikes, didn’t really care, they worked just fine. I could keep a nice spin (compared to most other mt bikers) but didn’t have a need (or want) to sustain such a spin. I’ve still get the 175 on the mt bike (heck, got 180s on the biketrials setup), but for the recumbents… 153 or 155mm cranks on all of them. Just feels so much better. And since I find it easier to spin faster on a recumbent (not ending up bouncing around), the short cranks make that even easier.

    Another option for short cranks is to have them shortened by someone like Mark Stonich (www.bikesmithdesign.com)

  • Dolan Halbrook says:

    I ride 170mm on my Xtra, 172.5 on my cross bike, and 175mm on my commuter and notice the difference in each, but am rarely conscious of it. I’ll try applying those formulas above and see what comes if it: thanks for providing the info!

  • kww says:

    My old Novarra had 170 cranks on it and I had knee strain from mashing on the pedals going up hill. My new Surly bike has TA Carmina 185mm cranks and no more knee strain. I don’t buy into the conventional wisdom that shorter cranks are better. Taken to the extreme, you then do a lot of work with a really small range of motion. Try walking or running with the range of motion that bicycles have, it is not that much.

    I think the most important element is crank length in combination with seat position. If not optimized, you are putting a lot of force into your knee joint when they are hyper-extended, leading to strain. Many people get a set back seat post and eliminate strain without going to different crank lengths.

  • Claudio says:

    I thought that a longer crank producing more leverage would reduce the strain on the knees, but it seems that some results show the opposite.
    The higher cadences that the shorter cranks help sustain may have something to do with it, as, regarding torque, the lower cranks need more force on the pedal to produce the same torque.

  • bongobike says:

    This is one of those subjects where opinions vary widely. All I can say from my experience switching from 170mm to 180mm on my city bike, is that I feel it has slowed down my cadence considerably. It is very hard to spin fast. I have no idea how it has affected my average speed, since I don’t have a computer on that bike. I had always heard that tall guys with long legs, like me, benefit from longer cranks. So the minute I came across this used crankset I immediately went for it. After riding it for several months now I’m not sure it was a good idea. I have ridden 175mm cranks and they felt fine. It’s funny how half a centimeter can make such a big difference to some people, yet others say they notice no difference.

    I am also a recumbent rider, but have yet to try shorter cranks. I’ll have to try that experiment sometime.

  • Logan says:

    Hi Alan, :)

    The late, great Sheldon Brown also has a wonderful article on his empirical experiments with crank length and leverage “gain ratios”. http://www.sheldonbrown.com/cranks.html that you may want to add to your related reading list.

    He comes to a similar conclusion about length as you and advises folks to error on the side of shorter cranks for the health of their knees. In fact it was his article that made me order 165mm sugino cranks for Tammy when building up her bike. Thus far it seems to help a bit with hill climbing on the touring bike for her. After we do a few long tours I may follow Sheldon’s advice and try smaller cranks on my bike even though I have relatively long legs. ;)

    Cheers and thanks for the great topic! :)

  • Alan says:

    “The higher cadences that the shorter cranks help sustain may have something to do with it…”

    Yup; the assumption is that you’ll ride higher cadences with shorter cranks. Also, shorter cranks place the knee in a more open position at the top of the power stroke where it’s under the most strain.

  • Alan says:

    @Logan

    Thanks for the Sheldon link! It’s added to the list…

    Best regards,
    Alan

  • Gary Fisher says:

    Shorter cranks do better at accelerating from around 60 rpm to way high rpm, even big track sprinters use 165 – 167.5 mm arms.
    BMX and MTB ers pedal at under 60 rpm at critical times and long arms help the BMX ers out of the gate and MTB ers clean things. Longer cranks give me better power, shorter more speed.
    I have a 36″ inseam I like 177.5 on my MTB 175 on my road bike 172.5 for crits.
    It gets harder every year to get the right length cranks.
    I have had a joker put one arm 2.5mm longer on one side of one of my bikes, I felt it right away.

  • Leon Webster says:

    My wife is short (5’0″ on a good day). We put some shorter (155mm ) crank arms on our recumbent tandems, and now she is much more comfortable spinning at my usual 90-100 rpm cadence. As a result, we climb hills better, even with loaded panniers. Her everyday bike (a Brezzer uptown) has conventional length cranks, and she doesn’t mind them. But she only rides that bike short distances.

    So I am a believer in fitting the crank arm length to the individual, and wish there were more options that were readily available.

  • Helton says:

    I’m using a 160mm on a relatively cheap tandem I bought less than one year ago. The bike fit was terrible, because I needed to rise the saddle too much, interfering with the foot reaching the ground on stops, which is very important on tandems.

    Also, the reduced leverage provided by the pedals (besides the extra saddle heigth) made the stress on my wrists become unbearable on long rides, such that my wrist is still sore after many months after a century club ride.

    Yesterday I rode a friend’s old singlespeed roadster which seat tube had a fair amount of slope, and even though the low handlebar reach made a kind of short and supposedly wrist-busting bike, that extra leverage provided by the forward cranks provided the counter-torque necessary to sustain spine erection reducing stress on the wrists.

    With these mixed reflections I just want to emphasize that these seemingly not-so-important milimeters may have unpredictable side-effects if we forget to consider some subtle interactions between bike and rider.

  • MohjhoRyder says:

    I use 155mm cranks on my recumbent and 170mm on my single speed. Got my 155 cranks at Peregrine Bicycle Works, an online recumbent store out of Chico, Ca.

  • Matt says:

    Alan,

    I built up a 16″ Big Dummy for my wife last summer as a car replacement and used 155mm TA cranks. We had done a two month tour in the British Isles many years back and after a very long century in Ireland, my wife developed a nagging knee injury that made the rest of the trip a labor. From then on, she had a hard time riding, with her knees always feeling very fragile, and pain developing on anything past 5 miles.

    After a year on her longtail, complete with 100lbs of kid and gear, she hasn’t had any pain and swears by her short cranks. She says that she finally understands how i was able to keep a faster cadence on the tour and finds joy in her ability to really spin comfortably. We are now custom crank length evangelicals and encourage all we meet to look into variable crank lengths to care for unusual leg lengths or chronic knee issues.

    My wife is 5’1″ Riding a dutchified Big Dummy on 24″ rims. Its a beautiful bike…and she’s happy…which was the goal.

    Thanks for the great blog…We’re in Sebastopol and appreciate your incredible photos and posts. Thanks for building such a positive site!

    ps…Are you planning any more reviews on the Bryant or the Mundo?

  • Alan says:

    @Matt

    Thanks for sharing your wife’s story… that’s super!

    I’m currently working on a full review of the Mundo. We’re running a little behind, but should have it ready within the next 2-3 weeks.

    Regarding the Bryant, I have tentative plans to purchase one later this year. It’s my understanding that Civia blew through their first batch much quicker than expected (that’s good!), so I probably won’t be able to get my hands on one until late summer.

    Best regards,
    Alan

  • Matt says:

    I’m looking at the Bryant as well, though I hesitate knowing that Shimano’s Alfine 11 will be out next year. What are your thoughts on its ability as a loaded touring cross-over. You discussed it a bit in your earlier posts, but what’s your take on its ability to handle a full load front and rear? I’m 220#, 6’0″ and carry about 50-60lbs on long tours.

  • sbcommute says:

    Great post Alan!

  • Fergie348 says:

    I started off my ‘adult’ cycling life with a mountain bike with 175 mm cranks and then got a road bike that had 180’s on them. I loved the long cranks for awhile for the torque you can apply up hills. It was necessary to spin low RPMs with power because the lowest gear combo on the old Campy 7 speed setup was 42×23 which is pretty high for hills.

    I’m older now and have reduced my road crank size to 175, in fact all my cranks except for my single speed road are 175 with the single being 170. I can spin pretty good with the 175s, but I’m 6’1″ with a 33″ inseam.

    I think it makes a ton of sense for smaller riders under 5’4″ to run 165s or maybe even smaller depending on preference. When you think about it, the standard range of 170 to 175 mm just isn’t enough to accomodate the range of sizes and riding styles out there.

    I like to keep a cadence above 80, usually topping out at about 110 when I’m really spinning. It helps to change it up during a long ride, I feel fresher as if I’ve worked a wider range of muscles than if I kept a steady cadence.

  • Charlie says:

    I’ve got a knee that often gives me trouble. Even though I’m 6’1″, with proportionally long legs relative to my torso, I find that 155 cranks are a big help for my knee, and I like the way they feel.

    So if you’ve got knee trouble, give short cranks a try, even if you are tall.

  • rg says:

    I found this discussion long after it was current, but want to add my two cents. I think any short cyclist who has experienced overall fit problems and/or knee pain should consider running an experiment with short armed cranks. This is really not intended as a direction for anyone of average or nearly average body proportions — those folks are well enough served by the narrow range of 165 – 175 mm products on offer in the mainstream market.

    I became tractable to proportionalist arguments in favor of shorter cranks about 10 years ago when riding induced discomfort was beginning to drain all the joy out of the activity. At that time Andrew Bradley, Kirby Palm, and Peter White all informed my interest, and their respective crank length criteria recommended a range for me spanning 145 – 155 mm cranks for my 72cm inseam. I had my doubts at first, but eventually ordered a set of Thorn 155’s from St John’s Street Cycles in Britain to replace my 170’s. I’m pretty sure the Thorns are re-branded Suginos.

    Adaptation time was practically zero. I put them on for my 11 km morning commute on my old platform pedal MTB commuter (I don’t clip in), and noted only that it felt a little extra ‘spinny’ for that ride. My cadence tends to settle out at a little over 100, so ‘spinny’ means something to me. By the end of the ride home that day I couldn’t have described the feel of the bike as anything but natural. Over the next few rides I noted that the bike seemed to fit me better in a more general sense, particularly after I lowered the seat a bit. Now there was an interesting surprise. For all the years I’d ridden, I’d always kept my seat high for my leg length relative to other cyclists I knew. “I ankle deep!” I’d tell my fellow riders by way of explanation. However, the change in crank length and subsequent seat lowering told me that in fact I had only been running with a high seat position to compensate for the overly long cranks that had long forced me to endure too much compression at the knee and hip angles. A very telling subjective experience was how much better the bike seemed to fit while making short turning manoevers while pedalling forcefully, something any rush hour urban commuter does routinely in the struggle for survival. And more importantly, I managed to end the regular cycling season without significant knee pain for the first time in over 20 years. Not bad.

    I now have several bikes equipped with short crank arms, the 155mm Thorn’s mentioned, some ridiculously over-priced 155mm TA Carmina’s, some absolute P.O.S., over-priced, clunky, 150mm Slocum/Zinn-ish atrocities, some nice and economical 152mm Sugino XD 600’s, and some equally cheap and pleasing 155mm Origin8’s. The latter two sets are readily available from North American mail order houses and allow anyone to run the experiment for themselves at little cost. I’d strongly recommend that any rider of small stature or having particularly short legs consider trying some short crank arms out for themselves. Don’t waste a lot of time theorizing or being swayed by mere opinion from those for whom the narrow average is adequate. Experience will prove that proper fit arises once your crank length matches your leg length.

 
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