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I have been doing a lot of research into crank length, I have a 172.5mm crank which I find my knees come up too high, and sometimes I get a bit of back pain. I have a Scott Speedstar 2017.

I am 175cm tall, and an internal leg length of 78cm. I looked at the following video:

which was invaluable but, I find I am around 164mm for a crank when using the 9.5% of my height.

should I go down from 172.5 to 165? or just move down to 170mm? as was said at the end of the video, "Modern bikes are designed around 170 to 175mm long cranks"

  • should I goto 165mm or 170mm long cranks? I know I need a shorter crank, just how much shorter is the question :)

Thanks! :)

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  • Back pain is a complex phenomenon, and you might consider consulting a bike fitter and/or a physical therapist if able. It can be hard for a layperson to figure out if their knees are coming up too high as well.
    – Weiwen Ng
    Jul 13, 2020 at 18:08
  • How aggressive is your position on the bike? Do you have the stem slammed all the way down and ride in a very aerodynamic, aggressive position? And when's the last time you did any core or back strengthening exercises?
    – Andrew Henle
    Jul 13, 2020 at 19:57
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    I'm 177 cm tall and love 165 mm cranks. That's my personal preference, but that's all I can really offer.
    – Paul H
    Jul 13, 2020 at 21:05
  • There are no 'rules' around crank length, try it, if you like it its the right thing to do. Bolt on crank shorteners are a cheap way to 'try before you buy' - Thorn UK do a nice one that is infinitely adjustable but cheaper ones are around.
    – mattnz
    Jul 14, 2020 at 5:22

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Percentile based formulas for crank length are trash. Femur length, health or special needs of all the joints in your leg, and the desired relationship between your saddle, knee, and pedal are all factors, as are the clearance needs of the bike, the type of riding being done, and Q. There is zero reason why a simple percent of your height would suffice.

Fitters who are serious about delivering a good result are mostly going to use some kind of system (or haphazard rule of thumb) to come up with an ideal saddle setback and knee to pedal spindle relationship, then choose a crank length based on that, then test it with either different cranks or a sizing crank.

As for your situation, if going to a fitter is off the table, you are going to be much better off making your own estimation of whether 2.5mm or 7.5mm is going to be the right amount of reduction. I would make sure you're feeling very confident about your saddle height being on point first, because that's a far more common source of issues with knees being put in an uncomfortable position.

Some shops do have sizing cranks and might be willing to set you up to spin on a trainer in a corner for a while if you're going to buy cranks from them afterwards.

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