Crank Length - formula?

[Mick F]

Just in an idle moment outside looking at my bike, I realised that I've been using 170mm cranks for as long as I can remember.
No interest in changing to a different length as I'm happy as they are, but it got me wondering that 170mm can't be a "one size fits all" sort of thing ................. as everyone is different even though 170mm seems ubiquitous and universal.

What formula would be used to calculate the optimum crank length?

Inside leg?
Thigh length?
Lower leg to ankle length?
Shoe size?
Combination of them all, or more?

[tatanab]

even though 170mm seems ubiquitous and universal.

In the MTB world 175 seems to be the norm.

[hayers]

Yes there are formulae - interesting (and rigorous) discussion here:

https://bikedynamics.co.uk/FitGuidecranks.htm

[Samuel D]

170 mm is common but not universal. However, nearly universal are cranks between about 165 mm and 180 mm, which is a smaller range than that of human height or leg length.

With a few exceptions, people seem to get along fine with that just as they get along fine with one size of step height on stairs and ladders.

I don’t believe in formulas for bike fitting but Graham Obree proposed 9.5% of rider height for crank length.

[Brucey]

the little rigorous research I have seen in this area suggests that there is a correlation with leg length (as you might expect) but personal preference, habituation etc are at least as important. This means if you plot leg length against best crank length (either using 'preference' or 'optimised power output' as a proxy for 'best') then the results are not so much a neat line as a broad splodge that you can (without very great confidence) draw a trend line through.

FWIW folk either do or don't

a) go faster on different length cranks
b) get different amounts of leg cramp etc on different length cranks
c) notice different length cranks at all.


Its one of those things you need to try for yourself; it makes a difference to me, e.g. if I use an MTB on the road, I like it to have 170mm cranks not 175mm. Offroad I'm more likely to be mashing not spinning so 175mm cranks seem more tolerable. For many years my training bike had an old crankset which had ~168.3mm (6-5/8") cranks on it. However I thought they were 170mm, and persisted in this belief until I ran out of possible reasons why my road bike (with 170mm cranks) felt so different. Careful measurements showed the truth of the matter. 165mm is too short for me but if I could buy modern cranks in the 6-5/8" length I would have them in a heartbeat.

cheers

[bazzo]

170 mm is common but not universal. However, nearly universal are cranks between about 165 mm and 180 mm, which is a smaller range than that of human height or leg length.

With a few exceptions, people seem to get along fine with that just as they get along fine with one size of step height on stairs and ladders.

I don’t believe in formulas for bike fitting but Graham Obree proposed 9.5% of rider height for crank length.

I tried your formula, I am 1.8 m tall ride 22" or 56cm frame two bikes have 172.5mm the 22" 170mm. The formula says 171mm my legs are a little short for my height. I have to say I don't notice any difference in the pedalling dynamics.

[Cyril Haearn]

170 mm cranks cannot be right for riders 155 cm tall AND for riders 195 cm tall
One wonders whether the majority of us are using cranks of the wrong length

[Brucey]

170 mm cranks cannot be right for riders 155 cm tall AND for riders 195 cm tall
One wonders whether the majority of us are using cranks of the wrong length

read my post; there are other things that are at least as important as leg length.

cheer

[bgnukem]

Having long legs and pedalling fairly slowly I find I prefer 180mm cranks and would try 185s if I could find them, but the choice isn't there.

9.5% of my height would be 186mm according to the Obree formula.

20% of inside leg length (the other formula I've seen used) would be ~183mm.

Does anyone still make triple cranksets with crank lengths 180mm or more??

Ben

[NUKe]

Isn’t this another case, where the theories are all based around racing,, that said I did notice a positive difference when I dropped the Road bike to 170mm after years of riding 175, The grasshopper is currently running the 155mm it came with and these are very comfortable.

[mig]

why would a crank be 6 and 5/8ths inches in the first place? seems an odd amount.

[Brucey]

why would a crank be 6 and 5/8ths inches in the first place? seems an odd amount.

old english crank, with cotter pins etc. Came on my first 'racing' bike so got demoted to the training bike in time.

cheers

[The utility cyclist]

170 mm cranks cannot be right for riders 155 cm tall AND for riders 195 cm tall
One wonders whether the majority of us are using cranks of the wrong length

why not?
Why do you need absolute efficiency 100% of the time in every single aspect? Crank length is so tiny in terms of importance even at the top end.

[mig]

why would a crank be 6 and 5/8ths inches in the first place? seems an odd amount.

old english crank, with cotter pins etc. Came on my first 'racing' bike so got demoted to the training bike in time.

cheers

i just wonder why someone would choose *that* length and not, say, six and a half etc. looks maybe to be even a conversion from some other unit of length previously. 8% of a fathom, two and a hlaf pyramid inches etc

[mig]

170 mm cranks cannot be right for riders 155 cm tall AND for riders 195 cm tall
One wonders whether the majority of us are using cranks of the wrong length

why not?
Why do you need absolute efficiency 100% of the time in every single aspect? Crank length is so tiny in terms of importance even at the top end.

potential of injury at the extremes?