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Technical FAQ with Lennard Zinn: When it come to crankarm length – no easy answers

A reader asks for a rule of thumb for crank arm length, and LZ explains that there isn't one.

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Q. Dear Lennard,
First, thank you for the articles on frame flex and high-speed wobble. I read through the brief ones but printed out the lengthy one in order to read it a couple of times. I felt like I was back in grad school!

Coincidentally, I had been building up a Lynskey frame and have since completed it. What a world of difference in terms of stability while descending. No flex whatsoever. Mind you I weigh 155 pounds. It makes me really wonder why the issue of frame flex on these ultra light and ultra expensive carbon frames is never discussed critically in reviews. Influence of the manufacturer’s advertising dollar or perhaps the current lust for all that is light, flashy and carbon?

However, my question has to do with crankarm length. I was speaking with a builder at the NAHBS show in Austin about this. I’m 5-foot-9 and average in inseam, torso, arm length. I’ve always ridden 172.5 cranks. (My first high-end bike back in the 70’s was an all-Campy French-made bike that came with 175’s. That seemed to be the standard then.) He suggested I go with a set of 165’s. He stated it would allow me to spin easier and more efficiently, plus at my age (58) my back after a couple of lower back surgeries isn’t very flexible. Two level fusion. He stated the 165’s would put less strain on the hamstrings going up my back actually allowing me to tuck down more comfortably and aero. His overall opinion, and strong opinion, was the trend was going to be for shorter cranks in general. Intriguing idea but an expensive idea if it doesn’t work.

Your thoughts?
— Peter

A Dear Peter,
Well, I’m not sure I want to open this particular can of worms, as my email inbox fills up far to quickly as it is, and there are few subjects cycling tech geeks, particular roadie tech geeks, have stronger opinions on than crank length. However, you’ve hit upon the cycling tech subject that I think about probably more than any other, so I might as well wade into this one.

A sample of a few riders does not a trend make. You won’t get me signing onto some one-size-fits-all and this-is-what-everybody’s-riding-now theory of crankarm length. To me, the only solution that makes any sense is to have the cranks be proportional to the leg length of the rider, just as the frame size and handlebar width are. There are plenty of reasons for the status quo of 170-175mm being the “range” on almost all road bikes, and 175mm being the standard stock length on all sizes of mountain bikes, but I don’t think any of them are defendable from a performance and comfort perspective. They are entirely based on financial realities and traditional thinking with a lot of inertia behind it.

It makes sense to all of us that bikes for small children have very small wheels and short cranks, as well as small frames, saddles, stems, and handlebars, and that, as the children grow, the bikes sized to them have increasingly larger wheels and longer cranks (as well as larger frames, saddles, stems and handlebars). But, for “adult” road bikes, there are lots of frame, handlebar and stem size choices, but the wheel size and crank length options basically disappear. Okay, if you really look, you can maybe find a bike with 26-inch or 24-inch road tires, but there ain’t much choice there; 700C is what you get, and you can’t do much about it. And 5mm – from 170 to 175mm, or even 15mm – from 165-180mm, is a laughable crank-length “range” when you consider the leg length difference between a 7-footer and 4-footer. How can two so diametrically opposed perspectives on a similar set of parameters both make sense to us? It’s nonsensical.

The only thing that makes any sense is to have the cranks, frames, stems, bars, and wheels all be proportional to the size of the rider. I can’t crack the wheel-size-proportionality nut, although I can tell you a lot about how the lack of it screws up road bikes. (Mountain bikes have transcended this; riders of a wide range of sizes can get good mountain bikes with 24-inch, 26-inch, 260B (27.5-inch), and 29-inch wheels.)

Without proportionality of the wheel size, proportionality of the crank length results in non-workable gear ratios on either rider-size extreme that render true crank proportionality on road bikes impractical for pro racers. Tall sprinters cannot get a high enough gear to work with a long crank, as 80kph on a 53 X 11 gear is 135RPM, a cadence that is unrealistic when the crank circle gets much bigger than 180mm in radius. And short riders spinning a crank a lot shorter than 165mm will be held back by gears that are not low enough to offer the high cadences called for by short cranks on steep climbs. And then there’s the problem that pro riders no longer ride custom frames; they ride cookie-cutter carbon frames coming out of molds, so their bottom bracket heights and seat angles cannot be adjusted for extremely short or extremely long cranks.

However, crank proportionality is something that is attainable and useful for most cyclists. We can argue about what the constant of proportionality would be, how it changes for the particular cycling discipline, or even if it might have a logarithmic component to it. But, having once sponsored a women’s racing team for over a decade and seen what gyrations a woman under five feet tall — particularly one with a long torso and disproportionately short legs — has to go through to turn a 165mm or 170mm crank, I know that is not efficient.

And I know what it’s like as a tall rider to work like a sewing machine with a little 175mm crank — and my legs are short compared to the legs of my 7-foot customers. I also know how poorly the frame geometry works of extra-tall or extra-small stock frames designed to fit a standard crank, one that would be disproportionately short or long for the rider who fits the frame. I have whole books I could write on this subject, and I’ll spare you that here.

Here’s a great theoretical blog discussion on the subject that you may find enlightening. Don’t peruse it for test results; you won’t find them, since it is a purely a theoretical argument, and a convincing one at that.

I could go on for hours about how no decent crank length test has ever been done by anyone and why it is actually virtually impossible to do a definitive one. Rider adaptation to the crank over time is required for each crank length to ensure optimal performance, but, if adaptation is allowed, it then becomes impossible to ensure that the subjects have exactly the same fitness level for the testing of each crank length. And a proper scientific test is double-blind, so the subject doesn’t know what they are testing. But with crank length, the subjects can feel the difference, and the bike has to be set up differently and the cadences used have to be adjusted for the different crank lengths. So the test riders are tipped off, which can also skew the results and make it not stand up to scientific scrutiny.

So a clearly-explained logical examination of crank length and its relation to rider size may actually be about as good as can be done. There simply will never be a definitive test that will tell riders exactly what crank length will make them fastest. After doing lots of crank-length testing over a few decades, in and out of labs, the best thing I’ve settled on is simply to run numerous full-out climbing test on the same approximately 30-minute climb, year after year.

Interestingly, BMX riders, doing a discipline where one might think that short cranks that you could spin around quickly and not hit on the dirt would be an advantage, embrace proportionality of cranks much more than road riders. We’ve had elite BMX pros only six feet tall wanting 225mm cranks from us. Here is a BMX crank-length selection chart and here is one manufacturer offering a full 145mm-190mm length range ( as well as custom lengths outside of that range.

As for your tight back, it depends on what kind of riding you will be doing as to what solution makes sense to try. If you climb a lot, you can raise your bars to relieve hamstring tension. If you ride flats in the wind, you can move your saddle forward (and slightly up) to open your hip angle with a lower handlebar and longer stem in the same way that time trial and triathlon bikes rotate a rider’s road position clockwise (from the drive side) around the bottom bracket.

A 165mm crank may make your back feel better, and if you do it and it works, then you should stick with it, as life’s too short to be in pain while riding. But know that switching to a 165mm crank is not the only solution for you, and many of the things I’m suggesting you try cost a lot less — in most cases, they cost nothing other than your time, and I think they will result in you riding faster. From my own decades of experience and testing with different crank lengths, if you venture a lot shorter with the crank than 21-22 percent of your (measured) inseam length, you won’t climb as fast.
— Lennard