Crank length cogitations

[moonlite]

I've been looking for a new (old) set of cranks for a project I'm working on. I've always had 175mm cranks on my bikes -- I'm not sure why, other than that I'm 6'2" so the larger sizes of bike are fitted with those.

But now I'm finding 175mm versions in short supply -- time and again I click on an ad to find that the cranks are 172.5mm or even 170mm. So my question is -- how much does 2.5mm really matter? Would it even be noticeable to replace 175mm with 172.5mm?

 

[7Speed]

I swap between 172.5 on the road (typically the size fitted on M or 54cm frame) and 175mm / 172.5mm on the MTBs depending on age. I really don’t notice much difference TBH and typically put down more torque as well as use an higher cadence off-road where I guess you would see it more. I have a shot right knee which is super sensitive to set-up - as long as I have the right saddle set-up to get the right amount of knee flex I am fine with a little change in crank length.

+/- 2.5mm each side isn’t much. But bike fit is a very personal thing…

 

[DrDaveTampa]

I75 makes my knees hurt, 170 doesn't so I can definitely feel that difference. Haven't tried 172.5. Between 170 and 165, I dunno. It's not like I'm trying to squeeze every single Joule out of the thing, ymmv

Back when I had a single-speed 29er I did notice the difference between 175 and 165 was about 1 tooth's worth, so in other words I might want a 16t cog with one and a 17t with the other. I guess really a half-tooth, if you run it through Sheldon's gear calculator, but it was noticeable. If your lowest gear is marginal for your needs you might want to take that into consideration.

I see such a wide range of recommendations, suspect it's a matter of personal preference

Interesting, I have a *really* hard time finding vintage cranks in anything other than 175, granted I'm mostly shopping in the 90's

 

[7Speed]

Thought this might happen!

Just checked my Brompton which is 170mm so I am running a 1cm difference in pedal circle diameter (there must be a better term for this?) between bikes with no bother - but then I do set up my saddle carefully (height, fore/aft etc).

 

[moonlite]

Interesting, I have a *really* hard time finding vintage cranks in anything other than 175, granted I'm mostly shopping in the 90's

Haha maybe it's some kind of confirmation bias on my part

 

[moonlite]

Thought this might happen!

Just checked my Brompton which is 170mm so I am running a 1cm difference in pedal circle diameter (there must be a better term for this?) between bikes with no bother - but then I do set up my saddle carefully (height, fore/aft etc).

I think that means I can go ahead with at least 172.5mm without too much hassle. In fact, it might be a little easier on the knees . . .

 

[minor_LEGEND]

It matters little tbh, there is a slow-building trend & theory that shorter is better. I commute on a bike I've just built with 175mm XT M750 cranks & after my 172's & 170's on my road bikes, these 175's feel so long & uncomfortable. I rode all through my youth on 175's but now really dislike them. I am not esp flexible, so the shorter length is appreciated as my knee rolls over the top the pedal stroke & enters the more difficult phase of the pedalling circle. I am fairly tall/long in leg & love the 170mm cranks I now run.

By having a shorter crank, your knee will not rise up quite as far, sure only 2.5mm (or 5mm) but this actually results in less of an angle at the knee joint. Much more comfort if you have issues or indeed just ageing a little. Likewise the pedal is easier to reach when at the bottom. You can either leave seat height as it is & gain more stability & better reach to the lowest pedal, or raise the saddle by the change in crank length (mm) & still benefit the new found comfort in pedalling over Top dead Centre.

I am actually on the look out for some 167.5 or 165mm cranks to try.

 

[7Speed]

Have heard the same trend - especially a thing with sprinters. My bike fitter who is also a physio says it’s all just a trend . Interestingly he said it’s more about optimizing kneee angle through the pedal stroke - like @minor_LEGEND mentioned. He also works with the Dutch junior team so he usually deals with much younger, more flexible and more talented folks than me!

 

[minor_LEGEND]

A trend worth getting onboard tbh.
If 175mm cranks were a trend in the 90's, the thinking was more leverage through a longer crank arm, but obviously puts the feet & legs through a larger pedaling circle. Logic makes you think it's only of benefit up to a point. Mike Burrows was quite outspoken on the subject as well as compact frame design.
Each to their own regards the findings & feelings regarding crank lengths, theres no need to sell off all your cranks & invest in newer, shorter parts but I definitely notice the extra comfort of 170's & I guess it's more apparent on a road bike due to the relentless pedaling & the typically lower/longer position one adopts on a road bike, typically.

 

[FluffyChicken]

It matters little tbh, there is a slow-building trend & theory that shorter is better. I commute on a bike I've just built with 175mm XT M750 cranks & after my 172's & 170's on my road bikes, these 175's feel so long & uncomfortable. I rode all through my youth on 175's but now really dislike them. I am not esp flexible, so the shorter length is appreciated as my knee rolls over the top the pedal stroke & enters the more difficult phase of the pedalling circle. I am fairly tall/long in leg & love the 170mm cranks I now run.

By having a shorter crank, your knee will not rise up quite as far, sure only 2.5mm (or 5mm) but this actually results in less of an angle at the knee joint. Much more comfort if you have issues or indeed just ageing a little. Likewise the pedal is easier to reach when at the bottom. You can either leave seat height as it is & gain more stability & better reach to the lowest pedal, or raise the saddle by the change in crank length (mm) & still benefit the new found comfort in pedalling over Top dead Centre.

I am actually on the look out for some 167.5 or 165mm cranks to try.

Just go for it and get 154mm.
Why do it in steps, when shorter is better...