Q & A on crank length with Lennard Zinn.

A Q & A on crank length by Lennard Zinn that was posted in VeloNews.

William

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By Lennard Zinn
VeloNews technical writer
This report filed November 18, 2003
Dear Lennard;
What is your formula for measuring crank arm length? I have a 73cm inseam with a size 39 foot. I have been on 170's for two years and have been able to progress with all training except hills. I was becoming frustrated because I'm only 130 pounds and should be able to fly up them. I've tried different styles of climbing, etc. My husband and I decided the lack of a 25 was not the issue; I just could not get on top of the gears I was using. I read your column and we had a 165mm from my son's bike and decided to try.

I feel I'm getting on top of the gear, a good feeling because I feel I have somewhere to start now. My legs and intercostals did cramp up during the hill training rides but that should subside as my legs become accustomed to the new circle. My question- what formula do you use and where did it originate? Does it include the foot measurement? Also, going from 170 to 165 would increase saddle height 5 mm but this is too high (for me). Any thoughts on this?

Thanks for your input!
Erica

Dear Erica;
Take a few minutes to check out this site: www.nettally.com/palmk/crankset.html

I think the formula on this site (21.6 percent of inseam) is pretty good. 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 long end with the tall customers I usually deal with as a frame builder. I had to 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 solutions this formula suggests to tall riders, however.

Another interesting formula yielding similar results comes from fit guru Bill Boston (www.billbostoncycles.com). He suggests measuring your femur (thighbone) from the center of the hip joint to the end of the bone in inches. This number will be your crank length in centimeters. For instance, if you have a 20-inch femur, you would have a 20cm (200mm) crank. He also has proportionality formulas on his site based on femur length that give a very wide range of acceptable crank lengths.

Andy Pruitt, director of the Boulder Center for Sports Medicine and fit expert of many superstars, has a few other things to add. "Crank length formulas using femoral length or leg length are fine," he says. "But if your style is mashing, use longer cranks, and if you are a spinner, shorten them a bit. Mountain bike cranks should be a bit longer for that moment to 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 Marshall University study showed that all participants regardless of body size went faster over short distances with each increase in crank length, you can hurt yourself if you use cranks too long for your legs. In that case, he says 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 of the condyles - cartilage-covered rounded femur ends - as they are rotating on the soft meniscus - cartilage pad - atop the knee platform.) So, do not stray on the long side much beyond this proportionality relationship. Cranks that are too short are not dangerous, however. You may forfeit some power by not using your muscles as effectively, but you put less stress on 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 is that you clearly recognize that crank length should be proportional to leg length. Foot size only comes into play if you have relatively small or large feet for your leg length. With an exceptionally large foot, the effective leg length and leverage is greater, so the crank should be a bit longer, and vice versa. Seems to me that a size 39 foot with a 73cm inseam is not out of the ordinary.

The 165mm crank is 22.6 percent of your inseam, which is much better than 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 would be "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 this opportunity to clarify a few things. I published some crank-length tests in 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 factor I am espousing here. These tests were either inconclusive or seemed to indicate that all riders, regardless of size, put out more maximum power with super-long (220mm) cranks, and that all riders had lower heart rates at low power outputs with super-short cranks (100 to 130mm). My experimental method in these tests was lacking, and if you click on the Kirby Palm link above you can find that pointed out.

I was simply not willing to stop there, since I knew from personal experience that 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 177.5mm to 180mm cranks, the improvement in my results was marked. When I was on the national team in the early 1980s, Eddie Borysewicz, the coach at the time, told me that I should be using even longer cranks yet for time trials and hill climbs. Miguel Indurain also understood this and had the clout to get longer cranks made for him, though. Good cranks longer than 180mm cranks were not available when I was racing, but the past three years I have used 202.3mm and greatly prefer them.

Following up on my interest in the subject, I have conducted other crank tests in the last eight years that improve on those early efforts. However, in understanding what went wrong in those 1995 and 1996 tests, I developed higher standards for what constitutes a publishable test, and my subsequent tests 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 these sorts of studies and who has funding to do it. It is not easy to do a test in which you eliminate all other variables besides crank length. It requires lots of time, planning, subjects and equipment. Hardly the type of thing that is realistic to undertake with no budget in order to write one article for a cycling magazine where another article on a different subject is due right on its heels.

Anyway, I have conducted all of these recent tests on the road and primarily with tall riders (6 foot 5 and over) because it was simpler and cheaper to use my personal stable of bikes than to always be switching cranks on other 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 more power with proportional-length cranks on my own personal bikes is great, testimonials from people may be even more valuable. And my customers always rave about how much more comfortable, natural and powerful they feel on extra-long cranks proportional to their leg length. Tall mountain bike customers report being able to smoothly power over obstacles they could not have before. And the higher bottom bracket makes hitting the chainrings on logs and the like almost impossible, yet the rider's center of gravity is no higher (since the bottom foot is still the same height above the ground due to the longer crank).

All of this indicates clearly enough to me that crank length must be proportional to rider size in some way. Whether you decide it is proportional to leg length, thigh length, overall height or something else is a minor point relative to that. The same goes for what you think the constant of proportionality should be. It could be something different from 0.21 or 0.216, but whatever it is, it will indicate for a lot of people that they should be using a vastly different length than they are. That is the part that 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 of the same length. The 3 percent difference between a 170mm and a 175mm hardly constitutes a length choice, and the 180mm length available in only high-end components still does not broaden the range much. Accepting that cranks should be scaled up or down with rider size opens up a whole can of worms that an awful lot of riders and component companies would just as soon stayed closed.

Obviously, economies of scale of producing cranks go out the window if you have to supply a range from say, 140mm to 220mm. The same goes for bike frames; if a manufacturer increases the bottom bracket height with every increase in frame size in order to accommodate crank arms proportional to 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. Those may have to do with what is best for the rider's pocketbook but not necessarily what is best for the rider's performance and comfort. No other conclusion makes sense to me. 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?

Whew! That was a long answer. Sorry.
Lennard