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Longer crank = shorter seat height. Keep that in mind. Too long cranks = greater knee flex = might hurt unless you are a tall person.

In general (with a gazillion exceptions), the taller you are the longer your cranks.

I switch between bikes with 170, 172.5, and 175mm cranks. I can't even tell difference apart from the over all feeling of a completely different style of bike. 5mm is nothing, it's only 1/5 of an inch. If you're tall get 175mm just because, if you're short get 170mm just because, If you can't decide, get 172.5mm, but there's really not much difference in my opinion.

It depends on your leg length, with variation for the kind of riding you are doing. The longer your leg, the longer the crankarm(And I cannot remember the numbers off hand). However if you need a high cadence you want a shorter arm(smaller circle, lower foot/leg speed) But if you want more oomph you want a longer arm(off road torque) For a putz around bike it does not matter to much.

Depends on how your knees feel about the crank length. It's different for many people and some prefer longer cranks because they feel they can get more leverage/torque while other prefer spinning/higher cadence with shorter cranks. It's a matter of preference and/or what your body tells you is the right comfort length for your riding style/conditions/distance.

Sometimes 5MM makes a world of difference, as it did for me!

https://www.bikeforums.net/fifty-plus-50/361045-your-cranks-too-long.html

I have 172.5 and 175 on my bikes. I can't feel a darn difference. If it affected my stroke that much, I think buying a new pair of shoes or shorts might end my cycling career!

If you assume that 170mm is normal for an average male 5'10" rider then you can scale the crank length to some proportion of leg length (often femur length) to get a normal crank for any other size rider.
The reasons for departing from "normal" size are several:
You might use a shorter crank if:
You want to spin faster.
Ride a banked track
Your bike lacks sufficient ground clearance.
You want to avoid toe-clip overlap with the front wheel.

You might use a longer than normal cranks for:
slow, hilly off roading.
If you prefer to apply lots of force to the pedals.

Crank length affects the torque at the bottomk bracket but has no effect on the speed (at constant power). Longer cranks rotate through a longer distance so require more work per rev but each rev takes longer.

A great explanation. I would like to add two things:

You might use a shorter crank if:

You have knee problems
You have ankle flex problems

Short version:
longer cranks allow more low-speed torque, but can't be spun as fast and is considerably harder on your knees on long rides.
shorter cranks is the other way around.

You don't see a whole lot of variation among upright bikes, but it's not unusual for people with recumbents to run shorter-than-usual cranks: 150mm cranks won't raise any eyebrows in that crowd. Even fairly-tall people use them.

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It would seem to make sense to me that crank length should be somewhat proportional to leg length, but many studies have found not much difference in outputs. Overly-long cranks do tend to cause knee problems, and I wonder how many of these studies used durations long enough to detect that.
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There'e are multiple schools of thought on this. Shorter cranks = less stress on joints and easier spinning. Longer cranks = more muscle use so larger gears can be used, shorter cranks = better cornering clearance, longer cranks = better for taller riders, shorter cra.....etc.

Everything from homemade 100mm cranks on recumbents to 200mm+ cranks for triathletes

https://www.google.com/search?hl=en&q...e+crank+length

Powercranks and a few other crank companies make adjustable cranks if you really want to spend the time working out what's the optimal length for you.

https://www.google.com/search?num=100...length&spell=1

Just read of a study conducted by Leonard Zinn on this exact subject. Supposedly the findings suggested strongly that most people would do better with 165mm cranks. I have 180mm cranks on both of my bikes, road and mountain, and after having reconsidered my purpose in riding I will go to shorter cranks, possibly 170mm.

I"m 5'8" and went from 175mm to 170mm cranks on the roadie. I liked the change. I noticed I could spin easier, cadence went up 10-15rpm immediately. My Fixed gear Bianchi has 165mm, as does my mtb. Pedal strike is bad, but pedal strike on a fixed gear bike can be catastrophic.