By Lennard Zinn
What is your formula for measuring crank arm length? I have a 73cminseam with a size 39 foot. I have been on 170’s for two years and havebeen able to progress with all training except hills. I was becoming frustratedbecause I’m only 130 pounds and should be able to fly up them. I’ve trieddifferent styles of climbing, etc. My husband and I decided the lack ofa 25 was not the issue; I just could not get on top of the gears I wasusing. I read your column and we had a 165mm from my son’s bike and decidedto try.I feel I’m getting on top of the gear, a good feeling because I feelI have somewhere to start now. My legs and intercostals did cramp up duringthe hill training rides but that should subside as my legs become accustomedto the new circle. My question- what formula do you use and where did itoriginate? Does it include the foot measurement? Also, going from 170 to165 would increase saddle height 5 mm but this is too high (for me). Anythoughts on this?Thanks for your input!
Take a few minutes to check out this site: www.nettally.com/palmk/crankset.htmlI think the formula on this site (21.6 percent of inseam) is prettygood. I have been using 21 percent of inseam for the last three years,and it has been working great, but my experience is primarily on the longend with the tall customers I usually deal with as a frame builder. I hadto come up with custom cranks (see www.zinncycles.com/cranks.aspx)as well as higher bottom brackets in order to be able to apply the solutionsthis formula suggests to tall riders, however.Another interesting formula yielding similar results comes from fitguru Bill Boston (www.billbostoncycles.com).He suggests measuring your femur (thighbone) from the center of the hipjoint to the end of the bone in inches. This number will be your cranklength in centimeters. For instance, if you have a 20-inch femur, you wouldhave a 20cm (200mm) crank. He also has proportionality formulas on hissite based on femur length that give a very wide range of acceptable cranklengths.Andy Pruitt, director of the Boulder Center for Sports Medicine andfit expert of many superstars, has a few other things to add. “Crank lengthformulas using femoral length or leg length are fine,” he says. “But ifyour style is mashing, use longer cranks, and if you are a spinner, shortenthem a bit. Mountain bike cranks should be a bit longer for that momentto get you over a rock. Use 2.5mm or 5mm longer for purely time trial usage,and vice versa for the track.” Pruitt also warns that, although a MarshallUniversity study showed that all participants regardless of body size wentfaster over short distances with each increase in crank length, you canhurt yourself if you use cranks too long for your legs. In that case, hesays that the compressive and shear forces in the knee joints “go up exponentially,”due to the sharper knee bend. (Compressive forces in the knee are stagnant,felt behind the knee. Shear forces are the result of fore-aft sliding ofthe condyles – cartilage-covered rounded femur ends – as they are rotatingon the soft meniscus – cartilage pad – atop the knee platform.) So, donot stray on the long side much beyond this proportionality relationship.Cranks that are too short are not dangerous, however. You may forfeit somepower by not using your muscles as effectively, but you put less stresson your knees.Using your 730mm inseam, Kirby Palm’s method (X .216) gives you 158mm,while 0.21 gives 153mm. I think that what is particularly significant isthat you clearly recognize that crank length should be proportional toleg length. Foot size only comes into play if you have relatively smallor large feet for your leg length. With an exceptionally large foot, theeffective leg length and leverage is greater, so the crank should be abit longer, and vice versa. Seems to me that a size 39 foot with a 73cminseam is not out of the ordinary.The 165mm crank is 22.6 percent of your inseam, which is much betterthan the 23.3 percent that the 170s represented for you.And yes, your seat should go up by 5mm when going to 165mm from 170mm.So should your handlebar. I don’t understand why you say that it wouldbe “too high,” since the distance to the pedal is the same.A side note: Since I get so much mail on this subject, I will take thisopportunity to clarify a few things. I published some crank-length testsin VeloNews in 1995 and 1996. Some of you may remember them and will have noted that they certainly did not come out with the 0.21 or 0.216 factorI am espousing here. These tests were either inconclusive or seemed toindicate that all riders, regardless of size, put out more maximum powerwith super-long (220mm) cranks, and that all riders had lower heart ratesat low power outputs with super-short cranks (100 to 130mm). My experimentalmethod in these tests was lacking, and if you click on the Kirby Palm linkabove you can find that pointed out.I was simply not willing to stop there, since I knew from personal experiencethat increasing crank length for a tall rider like myself (6 foot 6 inches)made a difference. When racing in the late 1970s, when I went from 177.5mmto 180mm cranks, the improvement in my results was marked. When I was onthe national team in the early 1980s, Eddie Borysewicz, the coach at thetime, told me that I should be using even longer cranks yet for time trialsand hill climbs. Miguel Indurain also understood this and had the cloutto get longer cranks made for him, though. Good cranks longer than 180mmcranks were not available when I was racing, but the past three years Ihave used 202.3mm and greatly prefer them.Following up on my interest in the subject, I have conducted other cranktests in the last eight years that improve on those early efforts. However,in understanding what went wrong in those 1995 and 1996 tests, I developedhigher standards for what constitutes a publishable test, and my subsequenttests still have not met that standard and thus have not been in VeloNews.Too bad, because I have put a lot of time and effort into a number of them!It is one thing if you are a physiology researcher trained to do thesesorts of studies and who has funding to do it. It is not easy to do a testin which you eliminate all other variables besides crank length. It requireslots of time, planning, subjects and equipment. Hardly the type of thingthat is realistic to undertake with no budget in order to write one articlefor a cycling magazine where another article on a different subject isdue right on its heels.Anyway, I have conducted all of these recent tests on the road and primarilywith tall riders (6 foot 5 and over) because it was simpler and cheaperto use my personal stable of bikes than to always be switching cranks onother people’s bikes. By being willing to take my custom crank length recommendations,my tall custom frame customers have also graciously acted as test subjects.While having data showing tall people going faster and generating morepower with proportional-length cranks on my own personal bikes is great,testimonials from people may be even more valuable. And my customers alwaysrave about how much more comfortable, natural and powerful they feel onextra-long cranks proportional to their leg length. Tall mountain bikecustomers report being able to smoothly power over obstacles they couldnot have before. And the higher bottom bracket makes hitting the chainringson logs and the like almost impossible, yet the rider’s center of gravityis no higher (since the bottom foot is still the same height above theground due to the longer crank).All of this indicates clearly enough to me that crank length must beproportional to rider size in some way. Whether you decide it is proportionalto leg length, thigh length, overall height or something else is a minorpoint relative to that. The same goes for what you think the constant ofproportionality should be. It could be something different from 0.21 or0.216, but whatever it is, it will indicate for a lot of people that theyshould be using a vastly different length than they are. That is the partthat is very hard to accept for a lot of people.No matter our size, all of us are by and large all stuck on cranks ofthe same length. The 3 percent difference between a 170mm and a 175mm hardlyconstitutes a length choice, and the 180mm length available in only high-endcomponents still does not broaden the range much. Accepting that cranksshould be scaled up or down with rider size opens up a whole can of wormsthat an awful lot of riders and component companies would just as soonstayed closed.Obviously, economies of scale of producing cranks go out the windowif you have to supply a range from say, 140mm to 220mm. The same goes forbike frames; if a manufacturer increases the bottom bracket height withevery increase in frame size in order to accommodate crank arms proportionalto the size of the rider, its costs and complexity of frame jigs goes up.There are obvious practical reasons to stick with the status quo. Thosemay have to do with what is best for the rider’s pocketbook but not necessarilywhat is best for the rider’s performance and comfort. No other conclusionmakes sense to me. If you accept that muscles and joints work most effectivelywhen operating in a certain range of motion, then it only makes sense thatmuscles, bones and tendons work that way for everyone. Short riders shouldnot be required to force their muscles through a greater range of motionthan the person with an 80cm inseam riding a 172.5mm crank. And on theother end, 7-foot basketball players do not bend their legs any less whenthey jump than shorter players. So why should they use minimal knee bendand operate their muscles only through a tiny part of their range whenthey ride a bike?Whew! That was a long answer. Sorry.
Technical writer Lennard Zinn is a frame builder, a former U.S. national team rider and author of several books on bikes and bike maintenance including the pair of successful maintenance guides “ Zinn & the Art of Mountain Bike Maintenance” and “Zinn & the Art of Road Bike Maintenance.”Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn. Zinn’s column appears here each Tuesday.