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Tech & Wearables

Technical FAQ: Crank length and bike fit

Examining the effects of crank length on saddle position, and changes to gearing.

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Have a question for Lennard? Please email him at veloqna@comcast.net to be included in Technical FAQ.

Dear Lennard,
I S=still have and read Bicycle Road Racing by Ed Borysewicz. Eddy (who was “the man” back in the day) recommended longer cranks for leverage, climbing, and time trials. My inseam is 29 inches, I was not a spinner and used 170mm cranks for many decades.

Currently, many of my cycling friends who are taller are turning 165mm cranks, which they tell me are kinder on the knees.

I have been considering going 165mm and understand I will need to raise the saddle 5 centimeters to maintain the same leg length but what about the fore-aft position over the peddles? Would I be moving the saddle 5cm backward, to keep the same position over the spindle, or does the seat tube angle make it a zero-sum game?
— Dan

Dear Dan,
In general, I do not necessarily agree with your friends’ contention about a 5mm shorter crank being easier on the knees.

Indeed, Eddie B. was the man back in the day, and I also have had his books for many decades. When I was on the national team, Eddie B. told me I should be using longer cranks when he did a fitting with me, upon my entry into the Olympic Training Center in Colorado Springs. I was riding 180mm Campy Super Record cranks, and those were the longest available that I knew of, at the time. He told me to use 185mm cranks for time trials and hill climbs and 182.5mm cranks for everything else (my inseam is 38 inches). When I told him that nobody makes those lengths, he replied, “Oh sure, they make, they make.” Well, “they” didn’t make those at the time (1980). However, it did put a bug in my ear, and, 20 years later, I started offering custom cranks in lengths that “they” don’t make, whether shorter (we offer 130mm in some models and have done as short as 100mm) or longer (we offer up to 220mm in some models and have done as long as 250mm).

Suffice to say: I think about crank length a lot. Assuming your saddle height to the bottom pedal is the same with either crank, shortening your crank will put slightly less stress on the knee at the top of the stroke because the knee bend is less; the crank circle diameter is decreased by double the difference in crank length. But people generally apply minimal pressure on the pedal at the top of the stroke. By the 2 o’clock position, where most people begin applying significant force, that vertical distance that the foot is less high above the bottom is only slightly more than the crank length difference. So, in your case going from 170mm to 165mm cranks, your knee is maybe 6mm vertically lower relative to its bottom position than it would have been on your 170s at the 2 o’clock position, and by the bottom of the stroke, the knee bend is exactly the same. That will result in a tiny reduction in pressure of the back of the patella against its grooved path in the femur end and tibial plateau; I doubt it will be noticeable.

Unless you change your gearing, however, the strain on your knees will be greater with the shorter crank. That’s because, if you pedal the same gear at the same speed (and hence the same cadence) as you did with your old crank, you have to provide more force due to the decrease in leverage. It’s analogous to pulling a stuck nail out of a hard board with a claw hammer or with a crowbar; your muscles and joints will strain more with the (shorter) claw hammer than with the crowbar. Now perhaps you can see why I am not willing to accept the argument of your buddies at face value. If you actually were to ride differently, namely gear down more and spin faster with the shorter crank, then I will be willing to concede the point. Of course, you could also try that with your current crank…

The big advantage that I do see of shortening one’s crank is for events where aerodynamics is of the essence (time trials and triathlons). There, the shorter crank allows you to get lower on the bike and significantly reduce aerodynamic drag. When adhering to the UCI regulations of a 5cm setback of the nose of the saddle behind the bottom bracket, very few people can pedal with their torso horizontal, because the hip angle has become so tight that the hamstrings and low back muscles won’t allow it, and the knees may also hit the chest or the “lunch muscle.”

Shortening the crank allows the rider to lower their elbow pads (and hence their shoulders) by the crank-length difference (see crank circle diameter discussion above—pedal at the bottom is higher by the length difference but at the top is lower by that same amount). Doing this provides such a dramatic reduction in aerodynamic drag that even if there is a loss in power output due to the shorter crank, it will be overshadowed by the reduction in power required to go the same speed. And the higher the speed, the more this aerodynamic effect overwhelms any power output difference.

To answer your question: Yes, it is advisable to also move your saddle back when raising it to maintain the same fore-aft position of the knee relative to the pedal at the 3 o’clock position. Obviously, these changes also change your drop and reach from saddle to handlebar and bring up other questions like, “Do I also raise my stem the same 5mm?” and, “Do I also shorten my stem 5mm?” The answers to those are, “Maybe. It depends.” You would need to try it and see what you think. You may feel good leaving the bar where it is (see above about a lower upper body position with a shorter crank).
― Lennard

Dear Lennard,
I just read your response to a question regarding fit for a rider with particular objectives for a gravel bike. It spotlighted the absence of the obvious, to me.

The majority of the options are carbon fiber, monocoque balloon-molded frames with production geometry dimension ratios locked to the mold of that size.

Riders optimizing their fit are reduced to stems, bars, and seat posts of varying sizes and setbacks. These bikes are not inexpensive and fitting by means of peripherals to the real sizing issue, the frame, is bodging.

I rode for years on a production frame that was a 90 percent fit, built to cover the last 10 percent for my riding. My legs are “too long for my height” according to production size geometry ratios.

I often read your articles and advice providing solutions from production products, but my years and miles of riding have given me the confidence to turn my back on production bikes; precious or less, they are just OEM models.

I am sad about the idealized status that production-sizing and geometry of the escalating-cost superbikes and hyperbikes receive, given their lack of ability to offer geometry options but also the advice to skip them and get a bike made by a custom builder for what has become a comparatively affordable price. Fit is more valuable than “I’ll fit unobtanium.”
— Asc Pgh

Dear Asc,
Being a custom framebuilder, that’s music to my ears. I certainly agree with you that the best way to get the ideal fit is to have someone you trust to build it to fit your body, and riding style.
― Lennard


Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder (www.zinncycles.com) and purveyor of non-custom huge bikes (bikeclydesdale.com), a former U.S. national team rider, co-author of The Haywire Heart,” and author of many bicycle books including Zinn and the Art of Road Bike Maintenance,”DVD,” as well as Zinn and the Art of Triathlon Bikesand Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.” He holds a bachelor’s in physics from Colorado College.

Follow @lennardzinn on Twitter.