By Rick Shultz, MBA, DBA
Over the past 18 months, I have collected crank arm length data from my bike fit clients. During the bike fit interview process, most of my clients volunteer that they have had prior bike fits, but they are still in pain. Many have had refits and even been to chiropractors, but are still experiencing knee pain. Most of the issues I have resolved focus on the following (a) wrong cleat placement, (b) wrong saddle position and (c) wrong crankarm length.
Both (a) and (b) above can be fixed by an experienced bike fitter, (c) can be fixed by bicycle and component manufacturers placing shorter cranksets [crankarms] on most bicycles.
But first, a background story. Two and a half years ago, 3 best friends, all in their 70’s, started cycling. They have known each other for decades and did everything together. After retiring, they started cycling. First were the shiny new bikes followed by what they thought was a good bike fit.
After riding for several months, they all developed severe knee pain. They initially attributed this pain to not having ridden before, but, when their pain got worse, they went back to their local shops for another bike bit. Fast forward another month and the pain in their knees got even worse. They were thinking of giving up on cycling when they decided to try one more bike fitting. Several of the San Diego teams that I do bike fitting for referred the three gentlemen to me.
There were two major problems I needed to immediately address. (a) their cleat placement was way off and (b) each of their crank arms were much too long for them. After fixing their cleats, I placed each of them on my Serotta Size Cycle. Using the 140mm to 185mm Vari-cranks, the resulting change is shown in the table to the right.
Thirty days later I contacted them and they all said that their pain was gone and that they are enjoying cycling with friends and have even gone on several group rides!
Fast forward 6 months.
The fitting was such a success for client #1 that he called asking if he could bring his wife by for a bike fit. He said that 3 years ago, he bought her a new bike and that the bike shop had done a bike fit for her. The next day, they had planned a 10-mile ride. 2 miles into the ride, his wife couldn’t pedal anymore, her legs had locked up. The next morning, she was in so much pain that she couldn’t go to work. Her wrists hurt, her trapezius hurt, her shoulders, lower back AND knees hurt. He told me that he convinced her that her pain was due to this being her first bike ride in 20 years, so they tried it again the following weekend. This time she made it 1-mile before calling it quits. He said that her bike has hung in the garage for 3 years.
After fixing her cleats, I placed her on the Serotta Size Cycle and adjusted the Vari-cranks to 145mm. I saw a HUGE smile! She was spinning and cycling pain free. That was on a Sunday. He called me the following Monday and told me that after the bike fit, they drove to Dana Point Harbor and rode 20 miles! He said he had trouble keeping up with her. She was spinning easily at 100 rpm, and that was on her first ride. The next morning, no pain! The following Sunday they started in La Jolla and did a 25-mile ride including the famous Torrey Pines grade! Both are now enjoying cycling together pain free!
Knee pain is a common problem. The other problem is that most cyclists believe that pain is part of cycling. I partially fault the component manufacturers that aren’t making crank arms short enough AND, I fault the bicycle manufacturers for putting on too long of crank arms.
So, exactly what length crank arm should I use?
Recommended Crankarm Lengths
Before we get to this, let me ask you a question. When is a 56cm frame not a 56cm frame? OK, trick question. As you can see on the table to the right, a given frame size (56cm in this case) comes in many different frame sizes. For example, although my preferred frame size is 56cm, I could ride several ‘larger’ 55cm frames. I could also fit on a 57, 58 and even 59cm. But, looking at my general recommendations on crank arm lengths, should I choose a 165, 170, or 172.5 crankset?
The only way to really tell is via a comprehensive bike fit. Most bike fitters will adjust the saddle to correctly determine and set the max extension of the knee. Then, they move on to the saddle fore-aft followed by cockpit controls. But, what I have experienced fitting clients is that the max flexion of the knee is as (or more) important as max knee extension…and this is adjusted by crank arm length.
I have metrics on over 100 cyclists that I have helped by replacing their long crankarms with shorter ones. In every case, knee pain went away. The table to the right is a list of crank arm lengths that I have ‘generally’ advised cyclists with knee pain to install on their bikes.
Other Studies
Andy Pruitt is one of the most respected bike fitters in the world and helped to develop the Specialized BG Fit system. Andy also states that cranks that are too long can cause injuries. This is because “the compressive and shear forces in the knee joints ‘go up exponentially’ due to the sharper knee bend. Cranks that are too short are not dangerous, however.” In his book Bicycle Design, Mike Burrows warned against using cranks that are too long to avoid knee problems and Sheldon Brown has written about crank length and how riding with cranks that were too long for him has caused him knee pain.[1]
There are 5 methods listed in [1] and I’ve added my own formula as well. I am 6’0” or 183cm and currently use 172.5mm crank arms on my 56cm bikes, and although I am a spinner, I could/should go to 170mm crankarms. My inseam is 89cm and 48.5cm femur length. Also, the manufacturers sizing charts say I should be on a 57cm or 58cm frame, but I prefer 56cm. Let’s see how close I am to their results…
NOTE: for the formulas below, use cm for height and inseam…
I am still testing the [1.2 * inseam(cm) + 65] formula, go ahead and see if it works for you. I have tested on a dozen cyclists and this seems to closely match the new shorter paradigm. For an accurate determination of crank arm length, you will still need to do a comprehensive bike fit where the bike fitter looks at BOTH max flexion AND max extension.
Summary
Cycling is a culmination of micro-injuries and my paradigm errs on the side of caution. Think of your knees during a typical 2-hour ride and spinning at 85rpm. You have pedaled 10,200 circles. Four rides a week and you have pedaled 2,121,600 circles in a year. If anything is out of adjustment, some part of your body is going to absorb it, and that part is usually the knees.
You might feel fine at 25 years old, at 35, even 45, but, eventually, this will all catch up to you. It’s better to be safe now than sorry later.
So, go ahead and check out a shorter crank. Your knees will thank you!
[1] https://ridefar.info/2017/02/crank-length-and-comfort-for-long-distance-cyclists/
[2] http://www.kneeclinic.info/knee_sports_injuries_cycling.php
[3] https://www.ncbi.nlm.nih.gov/m/pubmed/23898683/
[4] https://www.cervelo.com/en-us/engineering-field-notes/a-new-spin-on-crank-length
[5] http://www.powercranks.com/cld.html?gclid=EAIaIQobChMI3ZmiuMah2QIVTZN-Ch0WAwGrEAMYASAAEgKp3_D_BwE
[6] https://kbaracing.wordpress.com/2012/01/31/a-word-from-cervelo-about-crank-arm-length/
shortening the cranks, though, raises the gear ratio – leading to knee problems
wle
Please explain this statement. Some would argue that it reduces leverage, but that is not the same as changing the gear ratio.
Your pedal circumference change. Loss of leverage in practice is actully over exertion.
Since your normal foot speed will result in higher cadence with smaller circumference. You got more knee flexion in the same hour. And longer crank actually restrict you force abit. Shorter crank opens your hip right up. You may end up hurting your Patellar tendon. I did.
Was doing 65-70 cadence with 170 crank. 75-80 with 160 crank…. And boom…. Injury.
No, the gear-ratio remains the same, regardless. What changes is the torque: a shorter lever arm requires more force to produce the same torque (value).
Changing crank-arm length necessitates raising the saddle height; so, the knee and hip angles tend to decrease. This makes the sheer-stress less on those joints, at least.
Shorter cranks will probably be easier to spin; so, one’s (average) cadence may increase, comfortably. However, lower (ratio) gears may be needed to compensate for the difference; but, it is scarcely significant.
(Longer cranks should be better for heavy hill climbing.)
Much of this seems like psychological since the differences we are talking about are generally small fractions of an inch.
For those that don’t remember from high school, an inch is over 25 mm.
Good stuff, thanks.
When you say inseam, do you mean pants inseam, or pubic bone height?
Thanks again.
Interesting article which does not resonate with me at all. Based on opinions of Leonard Zinn and others I’ve always thought my cranks should be longer, but the longest I’ve ever had was 177.5mm. Not that I’m very tall, just 185 cms. But I never noticed the difference between 172.5mm and 177.5mm and I’ve never experienced knee pain, so I guess no action is required.
Thank you Mr. Shultz for this article. I particularly appreciate your comparison chart and accompanying formulas.
Personally, I haven’t been experiencing knee pain, but according to the author, my cranks are too long.
If I’m correct, shortening the crank-arm length would also require raising the seat’s height by half the difference between the older (longer) length, and the shorter (newer) length – i.e. a 10 cm shorter crank-arm would result in a 5 cm higher seat height.
Correct me if I’m wrong on this.
What other consequences (intended or unintended) would be experienced by shorter crank-arms.
I will have to give the matter some thought before purchasing my next bicycle.
All my best to everyone.
Richard
Interesting, but your science is not very good. In your first “studies” you begin by stating that you first adjust the cleats. There are simply too many variables to your “studies” to draw any scientific conclusion. Factors as, cleats, seat adjustments, gear ratios used, physical attributes of subjects, etc. etc.
Your observations, while interesting, are no better than my observation that most cyclists with knee issues use too high a gear too often, especially on climbs.
Here is my final non scientific observation, I am 73 inches and use 177.5 cranks. I am 64 years old, and I run. I rode over 7000 miles last year. No knee issues.
Where in the world do you find cranks with arms less than 165.5?
Here is one source: http://zinncycles.com/Zinn/custom-cranks/
My experience is different from the main stream here. I am 76 and most of my life I rode with 170 cranks. I am at the point where I need knee replacement. About three years ago I put a 175 on the bike I ride most f the time and I seem faster and more comfortable. Prior to that I put a 172.5 on another bike and that was better than 170,also. I have shrunk to about six feet
Cobb Cycling, Rotor
Interesting article. I wouldn’t discount longer cranks for folks who are taller or who have disproportionately long legs. I am 6’6″ with a 35″ inseam, and for years had some minor knee discomfort with cranks (mostly 175, but two sets of 180 cranks that were available from Shimano at the time). I eventually stumbled on two separate sources for crank length calculations. The first was a retired engineer named Kirby Palm who did a pretty comprehensive piece on calculating crank length. The second Lennard Zinn, a past USA team member who now makes custom frames and cranks for tall and short cyclists. I pored over the information available on both sites, and the upshot was that I moved from 180 cranks to 190. Voila! Knee pain gone, power and endurance up, and overall just a much better cycling experience.
The one thing I would add is this: If the optimum bend angles at knee and hip are “X” and “Y” for a pro cyclist to maximize power and minimize injury, why wouldn’t those bend angles be the same for a taller or shorter cyclist? Using that logic, and excepting any biomechanical problem or injury that would require a shorter or longer crank, the proper crank length would be the one that causes the knee and hip to attain those optimum X and Y angles, the same as for any other cyclist. Just a thought.
http://www.nettally.com/palmk/crankset.html
L(mm) = 5.48 x I(in)
Now, multiply the inseam measurement (in inches) by 5.48. This provides a good estimate of proper crank length, in millimeters, for general road cycling or racing.
In case it help anyone, here are 2 sources for shorter crankarms:
https://cobbcycling.com/collections/cranks/products/alloy-short-cranks
https://www.lightningbikes.com/cranks/index.html
Happy rides,
Jim Langley
I have a 2011 Specialized Roubaix with a BB30 bottom bracket, Specialized crank with 172.5 crank arms. I’d like to switch to 165. How can I determine what manufacturers crank kits are compatible with the Specialized BB30 bottom bracket?
While this article is interesting, I have to agree that you simply cannot make multiple changes to someone’s position and equipment, then attribute improvements to only one of them. What you have proven is that making several changes worked. We’ll never know whether any single change was the key or if all of them in concert were required for the improvement the riders experienced.
That said, I’m glad the riders in question are pain free now.
Thank You, I’m using 172.5 cranks. Had a fit by a renowned fitter in the Bay Area and he had calculated I should be using 170 which is what your formula calculates. However, we ride in the Santa Cruz Mountains and shorter cranks would mean more torque required to climb.
The difference in leverage is less than 1.5%, meaning that it should be imperceptible to you. For perceptible differences, a rider could swap out their cassette for one with more teeth.
Another data point: I have a 34″ inseam ( measured it!), and I calculate I should have a 168.6 mm crank using the “plus 65” measurement. I ride a 172.5 mm crank and have been for at least 22 years. I am 69 and have not experienced knee pain. In fact riding with this setup helps my knees.
No doubt poor fit can cause knee pain, but crank length is only one part of a complex puzzle. Shortening crank length will decrease overall range of knee motion during the pedal stroke (smaller radius means smaller circumference), however at the same power & cadence it will also require increased peak pedal force (shorter crank = shorter lever arm). Increased peak force will mean increased stress on the knee. Which side of this trade-off is best will vary between cyclists.
That said, I see riders arguing that a 2.5mm difference in crank length is critical to their fit while changing shoes/pedals with no thought at all to how those change their effective pedal stack height.
Placebo effect is strong among cyclists.
Does anyone know of any good fit books/articles on fitting recumbent tandems? My wife and I own two and have knee problems and bike shops in our area are at a loss for fitting recumbent tandems.
I like this. I went from 170 to 165 crankset several years ago and found a big difference in comfort…especiallt on the rides longer than 100km
I read an article that said it would be best for everyone to use cranks that are only 150mm, even tall folks. (Forget the exact explanation, but one reason was that a smaller leg bend would allow a much lower bar to be used without the usual loss in power output.) Any thoughts on that?
Thanks for the article Rick. I’m still wondering for the calculations if you mean Pants Inseam or Pubic Bone Height? I am 6’2″ but have a 96cm Pubic bone height. If the table you give is Pubic Bone Height, then my 180mm Crank arms should be perfect.
Thanks,
I did my racing in the mid-1970s when ALL bicycles were equipped with 170s. During 1975 I flew bike to Miami for US/Soviet-orbital-docking Apollo launch and stripped pedal-threads. Parental-Supplied-EMERGENCY-Travelers-Checks were used in bike shop where EVEN THEN they recommended 177.5 or 180mm for my 3-ft inseam (36.75″ to coccyx-base). 177.5 Campy Nuovo Records (144-ring-Circle) now on my track bike where I won last 2006 practice-day-sprint with trivial ease at age 50. Fixed-Gear club rides had 175max-155rpm-16tCog — 177.5-needing-15t-152maxRPM — 180MAX160rpm. Never tested my 185mm TA Carminas (both 130 and 110mm spiders) that way.
https://specialites-ta.com/pedalier.html now shows only ultra-wide 185’s with their assortment down to 155. Try Peter White (my first set Carmina) and Lennard Zinn (second two) for quicker-from-US-shipping and with eBay searches.
My last knee-pain occurred BEFORE I learned to shave 3mm/1/8″ off outer-shoe-interface-edge of my cleats (spreading knees AWAY from Top-Tube-contact-as-well.)
At 5′ 7″ with a 29″ inseam the above calculations suggest that i should be using a crank length between 152 and 161 cm, last week I ordered the smallest I could find to fit my Felt 60. 165 cm was the smallest I could find to replace my fsa 170 cm 110/80 crankset, Frustrating!
Great reading all the comments. I’m 73 and been a cyclist ‘all my life’ and 6 years ago had a ‘Total Knee Replacement’ , and was distraught for many months as I couldn’t ride my beloved bike ( or bikes! ) I was able to ride my stationary ‘exercise bike’ after a couple of months , but was baffled why I couldn’t ride my ‘real bike’ , then it ‘dawned on me’……..crank length !! Exercise bike had 140mm cranks , while my real bike had 170mm cranks . I took my bike to my ‘LBS’ (local bike shop) and asked them to drill another hole for the pedal 25mm ‘further-up’ the crank , and I was then able to cycle again !! I WAS IN HEAVEN !! I only needed to do the one side ( chainset side ) , It meant that the angle my knee had to bend was significantly reduced and allowed me to ride . I’ve since put the pedal back in the original hole now my knee is a lot better .
Most saddles on factory bikes are set way too low. Have you forgot to factor that in?
I saw no mention of saddle height.
Are you for real Prince? Bikes come deconstructed in boxes to your local bike shop. A mechanic puts them together. She or he sets the saddle to a random height because they don’t who’s going to buy the bike. They’re not clairvoyant ya know… It’s up to the customer to get a bike fit, or try to dial it in on their own.
This article isn’t on saddle height. There’s thousands of articles on saddle height. Go look for another one.
Just came across this article and wanted to share my story. I started riding a bike daily at age 35, and I was riding 172.5mm cranks. I just thought i was tired because i was out of shape etc. I bought a different steel beater with 165mm cranks and it was life changing. Didn’t know anything about crank arm length when i bought both bikes and learned fast… I’m 5’9 riding a 53cm old steel Panasonic bike with 165mm cranks, and it feels about perfect. I still experience a tiny bit of knee discomfort, so I’m considering trying 162.5 or 160mm. Long story SHORT (you like that? lol), if you are reading this page and think it doesn’t matter, it does! I still have 170mm cranks on my foldy and I just deal with the discomfort when i ride that bike, for the main bike, you gotta get this measurement right before you end up in surgery or quitting biking forever. – John
New to these calculations. If your crank is to long and causing your knee to bend to much why not just raise the seat or vice versa? Also the formula taking .95 x height in cm to get mm does one just take the height figure and put mm after it?
You could raise your saddle assuming you don’t overstretch the hamstrings leading to injury. As he mentioned, bike fits start with a saddle height adjustment to achieve a desired knee extension angle. That angle SHOULD depend on the rider’s hamstring flexibility.
I have speed play zero pedals. The foot is 8.6 mm lower than a flat pedal system. I have an adaptor for the cleat. It is 3 mm. I am still 5.6 mm lower than a flat platform pedal. My inseam is 34. 9375. The 1.2 inseam + 64 gives me a 170.4895 crank. My knee, being 5.6 mm lower than a standard flat pedal system should put me in the green range using 172.5 cranks.
People saying that shorter cranks cause more stress one the knee because you have to put more force. Well, the force one exerts on the pedal is what he has in his legs and cannot change with the crank length. Otherwise it would make the knee explode if we go for a very short crank. That’s just not the case. At some point, ahortening the crank further will simply reduce the leverage effect to the point that the pedal stroke impossible. But the knee won’t explode though.
Riding a bike at a certain speed requires a given amount of power. This power is delivered at the crank as torque*cadence. The torque at the crank is force*crank length. The force is whatever the rider can produce, thus a shorter crank requires just a higher cadence for the same power. The argument is that crank lenght is striclty related to the capacity of a rider of spinning the pedals at a certain cadence. And this is a fisiological subjective aspect. I used some formulas to support my reasoning but we should actually stop thinking in a solely mechanical way. We are humans and not machines. With that said I’m using now a 172,5mm crank and have just ordered a 165mm. Really curious to see how the shorter crank feels. Only the road test will tell me the truth (well, my truth).
I have had knee pain when cycling for decades. I am short anyway (160cm) and my legs are proportionately short on top of that ( 72cm inseam). When the seat height is set to have the proper leg extension at the bottom of the pedal arc, my knees are flexed far to much at the top of the arc. It is only now that I have realised the solution is the crank length – setting the seat height for extension says nothing about the knee angle at maximum flexion. By the formulas I should be using a 150mm crank; my bike has 175’s. It has always been a mystery to me why my knees work fine for hiking, walking, stairs, etc but hurt on the bike; now I am in search of replacement cranks, or perhaps a new threaded hole for my pedals 25mm up my existing cranks.