Crank Length: Does Shorter Really Help?
Almost every bike ships with 170 to 175 mm cranks regardless of who rides it. Shorter cranks have gone from a niche tweak to a mainstream fitting tool, and for good reason, but not the reason most people assume.
Within the lengths you can buy, shorter cranks do not reduce power. They reduce knee flexion at the top of the stroke and open the hip angle, which helps riders with front-of-knee pain, tight hips or a short inseam. After fitting shorter cranks, raise the saddle by about the same amount.
Rather than guessing, measure your actual position with your own camera.
Measure my knee and hip angles€5 for one fit, or 10 fits for €25. See pricing →
What crank length actually changes
Crank length is the distance from the centre of the bottom bracket to the centre of the pedal axle. It is printed on the inside of most crank arms: 170, 172.5 or 175 mm.
What it determines is the size of the circle your feet trace. A longer crank means your knee bends further at the top of the stroke and your hip closes more; a shorter crank does the opposite. That is the entire effect, and it matters more than the lever-arm argument everyone reaches for first.
The power myth
The intuitive claim is that a longer crank gives more leverage and therefore more power. The research does not support this within the range you can actually buy. Across commercially available lengths, differences in power output are negligible, because you simply choose a slightly lighter gear with a shorter crank and the leverage advantage disappears.
So the decision is not about power. It is about how your joints move.
Why shorter often helps
A shorter crank opens the hip angle and reduces knee flexion at the top of the stroke. Three groups benefit clearly:
- Riders with front-of-knee pain. Less flexion at the top means less pressure behind the kneecap on every revolution.
- Riders with tight hips or a low, aggressive position. A more open hip angle makes it possible to hold a lower position without the hip closing past what you can sustain.
- Shorter riders. A 160 cm rider on 172.5 mm cranks is turning a proportionally far larger circle than a 190 cm rider on the same cranks, which is why standard lengths suit tall riders better than short ones.
The trade-off is a higher cadence to produce the same speed, which most riders adapt to within a few rides.
Wondering where your own position sits? You can measure it in twelve seconds.
Measure my knee and hip angles →The rules of thumb, and why they fall short
Older guidelines tied crank length to inseam, around 20 to 22 percent of inseam (for example 0.216 times inseam in millimetres). These get you into a sensible range, but they ignore flexibility, riding style and what the rest of your position looks like.
A more useful question than "what does the formula say" is "do I have a complaint that a shorter crank addresses". If your knees and hips are fine, your current cranks are probably fine too.
The change nobody tells you about
If you switch to a shorter crank, your saddle height changes. A 5 mm shorter crank puts the pedal 5 mm higher at the bottom of the stroke, so to keep the same knee angle you raise the saddle by roughly the same amount. Skip that step and you have effectively lowered your saddle by 5 mm and reintroduced the front-of-knee load you were trying to remove.
Re-measure after any crank change. This is exactly the kind of knock-on effect that a knee-angle measurement catches and guesswork does not.
Should you change yours?
Probably not, unless you have a reason: knee pain at the front, a hip angle that limits your position, or a body that is small relative to the cranks you were sold. If you do have one of those, going one step shorter (172.5 to 170, or 170 to 165) is a low-risk change with a realistic chance of helping.
A BikeFit/LAB analysis measures your knee flexion at the top of the stroke and your hip angle, the two numbers a crank change actually affects, so you can see whether your cranks are closing your joints more than they should before you spend money on new ones.
FAQ
Do shorter cranks reduce power?
Not within the lengths you can buy. Studies across commercially available cranks show negligible differences in power output, because a shorter crank simply pairs with a slightly lighter gear. The decision is about joint angles, not leverage.
What crank length do I need?
A rough starting point is 20 to 22 percent of your inseam, but the better question is whether you have a complaint a shorter crank addresses: front-of-knee pain, a hip angle that limits your position, or being short relative to standard cranks. If not, your current cranks are probably fine.
Do I need to change my saddle height after changing cranks?
Yes. A crank that is 5 mm shorter puts the pedal 5 mm higher at the bottom of the stroke, so raise the saddle by roughly the same amount to keep the same knee angle. Forgetting this effectively lowers your saddle.
References
- Martin JC, Spirduso WW (2001). Determinants of maximal cycling power: crank length, pedaling rate and pedal speed. European Journal of Applied Physiology, 84(5):413–418. doi:10.1007/s004210100400
- Johnston TE, Baskins TA, Koppel RV, Oliver SA, Stieber DJ, Hoglund LT (2017). The influence of extrinsic factors on knee biomechanics during cycling: a systematic review of the literature. International Journal of Sports Physical Therapy, 12(7):1023–1033. doi:10.26603/ijspt20171023
General position guidance, not medical advice. Individual anatomy varies; consult a physician or professional fitter for persistent pain. How BikeFit/LAB measures, and where its reference ranges come from · How accurate is an AI bike fit?
Rather than guessing, measure your actual position with your own camera.
Measure my knee and hip angles€5 for one fit, or 10 fits for €25. See pricing →