Crank length rule of thumb? (Toe?)
 

I’m 5’8”, sometimes 5’9” depending on which convenience store I walk out of.

Most my bikes have the default 170 campy cranks

I Was going though the crank drawer and noticed some 172.5 sets and a single pair of 175

do we base this on:
height?
inseam?
riding style?
flats vs hills?

I’ve read the longer cranks give more leverage but are harder on the knees.

shorter is better for those who prefer to spin more?

what’s the real rule of toe?

For you 170 is probably great. A longer crank can increase leverage, some. It can be helpful to have a longer or shorter largely depending inseams. Some people cannot handle longer, it increases range of motion. The longer cranks may feel loopy, some may feel stronger.

I have the inseam of almost 33” the 172.5 is really good, 175not bad. I ride 170, I feel confined.

This
 

I ride all three. I can't tell the difference. The 5mm is less than a 1/4". I also ride several types of shoes (one even has insoles) and cleates and I'd bet that there is more than a 1/4" difference between the meat and the pedal axis among all those. I'm not even sure that the saddle height is +/- 1/4" among all bikes. And then there's that fore/aft saddle thing relative to the pedal that's who knows where.
But then I'm a pretty insensitive clot all around. Not to mention old, fat, and stoopid. I still enjoy the scenery.

I'm about the same height and I found 170mm to be the sweet spot. When I was using 175mm cranks I was huffing and chuffing before I even got to the end of the block.
The other benefit of shorter cranks is that you can get lower on the drops without your knee or thigh hitting your body.

@Robvolz

This seems like a simple equation but is not, the 2.5, 5mm or what ever can be a pretty big deal.

The extra or less arm length is multiplied around the arm travel of the stroke making it have a far bigger impact than it would seem.

2.5mm crank arm increase = 15.7mm stroke travel increase X rpm = potentially a lot. ;)

Whether or not you can optimize it depends on fit, fitness, maladies, gearing, etc.

The 177.5's on the Strawberry make a huge difference going up Mt. Tabor which is a miracle for me as I have little to nothing for hills with a 38in. inseam.

I like Matt Appleman's approch to this. Matt Appleman on Crank length This is a webpage with a discussion and different measurement guidelines.

It is a bit of a read and it can come down to individule preferences. I am 5'10 but with shorter legs and I tried 165mm cranks and find that they work well for me. And I want to follow his guidence for my wfe who is (Is she on here? Does she know my screen name?) 150cm depepeniding on stocking feet or with shoes. There coding it in metric should be safe.

I also like Dylan Johnson on this topic. A video from a performance standpoit for racers.

172.5 all

Yup. Same here, at least old fat and stupid part. I have all three, and 165s, can feel a slight difference but after a few miles I'm used to whatever under me. I ain't no princess.

I think there's a level of complexity to choices of crank length that defies making rules. 'Flexion' is the amount that a knee can bend and the average person has about 125 degrees. I have 30" inseams but I've been inflexible from birth and could never sit cross legged, even as a child. I've always gravitated to 165 cranks because of that inflexibility. Later in life, a substandard knee replacement limited my bend to ~105 degrees and forces me to use cranks between 135 and 150 mm. I know I'm an outlier but it simply demonstrates that body measurements aren't the only factors in choosing crank length.

Long cranks require more bend and flexibility.

I certainly noticed the difference today when I rode my new-to-me Batavus. All my other bikes have (or had) 175mm or 177.5mm. The Strada cranks on the Batavus are 170 and it just feels "odd'.

I always used 170 on my road and MTB bikes until I put 175 on my MTB some years ago because I wanted to try it. Later I built my hybrid city bike and put 165 on it because that's what I had at hand at that time. Like some above, I can't feel the difference - but I am also old and slow, and ride for endurance rather than speed. BTW, my cycling inseam is 83cm.

My default length = 170mm. (5'8" tall, 30" trouser inseam, 55cm C-T road frames). I switched the UO-8 to 165s because I had them and to reduce toe-to-tire overlap with my aftermarket fork.

I don't notice a big difference, but I think I do slightly prefer the 170s.

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back in the late seventies was given a manuscript to evaluate by the owner of Tenspeed Press in Berkeley, California. this gentleman is Mr. Phil Wood and bears no relation to the producer of sealed bearing cycle fittings. Tenspeed Press had been the publisher of Anybody's Bike Book and the historical volumes done by Andrew Ritchie such as King Of The Road.

the manuscript was entitled The Other End Of The Crank and came from a man who had developed the idea that crank length should be one half of femur length. to give some idea of what this means a tall person might need a length somewhere in the mid two hundreds. text needed a good deal of work and book would need an editor knowledgeable regarding cycling. never heard if it ever got published.


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I started on 170s never thinking about it. Bought a race bike with 175s mid season. Everything about that bike felt right. All my ride times dropped radically and continued to do so the rest of the season. Had a major crash that fall. Started my next season on my beater fix gear with its short cranks. 3 days before my first late winter race I rode a long ride on the race bike. Mistake! My knees hurt after. Started a now 45 year journey of chrondomalcia patellae. And now had knees that really didn't like crank length change. On shorter cranks I feel the higher loads and my knees not liking it at all. 172.5 are just OK but not as good as 175s. 170s are playing games with keeping my knees happy enough to function. (Like "how many rides before I gotta get those replacements?")

So I keep it really simple. All my bikes including my 3 fix gears that get over half my miles are 175s. It works.

Bicycle cranks are available in lengths from 140mm to 220mm… if you know where to shop.

Part of the problem with experimenting with different crank lengths is that radical changes in length require radical changes in bicycle frame design with regard to bottom bracket height.

I suspect that 170mm and 175mm cranks became “standard” because they work OK for most people.

Why would cranks that come in 2.5mm increments require "radical" BB height changes?

Or have you seen road bikes made specifically for 140mm cranks?

High bottom brackets all? Pedal strike is dangerously fun in your 20's and frightening for the rest of us.

Junior racing cranks in Europe are more commonly available with suitably smaller frames and geometry.diffs.

I am about the same size now that gravity has struck back.

50 years ago, 165 on the track, 170’s for the road bikes. I bought a set of 167.5’s and thought they were grand, remember this was in the age of Junior gear limits, last year of 44 x14 on the road.

15 years later bought a mtb. 17” frame with 175 cranks, I adapted and those worked better than I expected.

my comment is translate the saddle fore/aft to zero out the crank length change.

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slightly OT sidenote -

wonder how the makers of lever drive machines determine their crank lengths...


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And someone with that size bike is likely to try 175s?

By force and stroke of the piston. Lever drive machines are never driven by anything as complex as legs.

Do you think 175mm cranks are odd or rare on bikes with normal BB heights?

Leg length, your flexibility+age, how you like to ride, and what feels right to you.Oh, and what your bike came with.That would be how I decide. Obviously someone with a longer leg in both femur and tibia could like longer cranks. However this person could have been inactive, have flexibility issues, old injuries, etc that make it harder to use say a 175mm crank. Same with how people ride, if someone likes to spin to win then a smaller crank could be better. Or maybe their aero position means that their thigh hits their stomach/chest with 175mm cranks so 170 or 165's are better.
Then there is what you are used to. Many shorter legged people can and have ridden on 170 or 175mm cranks for ages and they adapt or it's a non-issue. Then 20+years later they come back to cycling after time off and find they have issues with the longer crank.

My rule of thumb is just try it out. Try a different size for a couple weeks and see how your body adapts. Then make the decision. Of course if you are on the far side of short leg or long leg spectrum it may just feel bad running really short or really long cranks.

175 is my preferred size, and I have a 34.25in inseam with the book method and no shoes on. 170's are fine for me, 165's are kinda funky but I could get used to it. Want to try 177.5 or 180's just to see what maybe too long is like.

There is no rule.
Experiment.
As a short guy, I've settled on 165.
I ride two fixed, one ss, and one multi-geared.
I picked the 165 because I can corner sharply without catching a pedal.
The 170 feels weird to me, but more likely because it has a greater q-factor.
You have to discover what works best for you.