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What crank arm length for Tri Bike?

I am looking for some guidance with crank length, I cuttently run a 170mm crank arm on my triathlon bike although on my road race/summer bike I have a 175mm. The rationale behind this is, I am sure I read once that by having a shorter crank arm length it creates a better hip flexion angle which benefits you on the run?
Can anyone clarify or expand on this?



  • ZacniciZacnici Posts: 1,385
    Dan Empfield wrote an article about this.

    If you have say a 175 crank and swap to a 170, because your leg doesn't travel as far down you need to raise your seat by 5mm to get the correct leg extension which also means at the top of the stroke the knee is further away from the torso and doesn't compromise the hip angle. This also means that the arm rest height can drop by 5mm, result a more aerodynamic position.

    Looking at the reverse, if you swap a 170 for a 175 you need to drop your seat by 5mm otherwise your leg extension will be overstretched, your knees will then be 10mm closer to your torso (5mm lower seat height and 5mm higher foot position on the crank) will liklely bang into your torso or elbows and hip angle will compress. The only way to alleviate it will be to raise the arm rests by 10mm, result less aerodynamic position.

    A shorter crank also allows higher cadence and less stress on the knee.

    Hope that helps
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