Crank lengths

by earbyphil   January 18, 2011  

Crank lengths. Any thoughts? Is it primarily leg length or does hills/flat come into the equation?

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It's an interesting one.

First up, you won't change your power output from changing your crank length, BUT, if a longer or shorter crank results in more or less comfort then your power output might change as a result (subtle but important difference).

I have a 91cm inseam and felt that 172.5mm was probably too short for my leg length. I switched to 175mm and was no faster, more powerful etc. My pedal stroke was smoother though.

Consequently I did also get some subtle knee discomfort. I've since switched back to 172.5mm (after a little over a year on the 175s) and my knees love me for it. My pedal stroke is less smooth again, but I can work on that. It matters more to me that my knees are happier.

Basically crank length is horses for courses. Cav runs 170s, most bikes seem to come with 172.5s, you might need 177.5s, or you might need to visit Zinn for something even longer. You need to try them all and see though.

posted by RichTheRoadie [86 posts]
18th January 2011 - 12:53

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Well leg length certainly comes in to it - Sheldon Brown has plenty to say on the subject here http://www.sheldonbrown.com/cranks.html and if you google crank lengths you'll find plenty of online crank length calculators and plenty more theories. To add a further wrinkle some people will point out that it's your femur (thigh bone) that does most of the levering so really it's the length of that you should focus on - not everyone of the same leg length has the same length of femur.

There's loads of interesting stuff here on crank length from Leonard Zinn, who also quotes cycling ergonics experts like Andy Pruitt http://barbarianbikeblog.blogspot.com/2005/02/q-on-crank-length-with-len...

The bottom line seems to be that you can't really do any harm by having a crank that's too short but you can by having one that's too long.

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posted by Tony Farrelly [4131 posts]
18th January 2011 - 13:04

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i had 172.5mm for a few years thinking they were 170mm only realized when i got some new cranks which were 170mm, cant say i noticed any difference.

i may stick the 172.5's on again and see what happens..

Fringe's picture

posted by Fringe [1081 posts]
18th January 2011 - 21:05

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I hop between the mtb (175's) and either road bike (both
172.5's) and like fringe can't realy tell the differenec!

still on the 3rd switch-back of Bwlch !

posted by therevokid [664 posts]
19th January 2011 - 8:39

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I crashed my bike and had the LBS replace a bent crank. Only after several months did I realise they'd replaced a 170 with a 172.5. I hadn't noticed a difference - which perhaps says more about me than it does about differences between crank lengths Worried

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posted by simonmb [360 posts]
19th January 2011 - 9:29

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I've made an interesting discovery recently. I've used 172.5 cranks for years; my choice was based on the fact that I used to use 170 but felt that I climbed better on the 175's on my MTB, although they tweaked the top of my hamstring. So 172.5 was the compromise.
I'm now suffering the after effects of a DVT in one leg (swollen ankle/behind of knee) so I decided to try 165's to reduce the flexion at ankle and knee. That's certainly improved but, guess what - they actually feel much better to ride and climbing is no worse (some would say it couldn't get worse anyway). It is also much easier to ride at a higher cadence.
Having said all that it could also be age-related. I'm 66 and, although I've ridden since I was a kid, there's no doubt that we are less flexible as we age.
So here's another take; maybe crank length choice is also partly age-related.

Follow me, I'm right behind you.

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posted by kenem [3 posts]
19th January 2011 - 12:02

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In theory .... and please correct me if I'm wrong cos I'd really like a definative answer on this subject too.

Longer cranks are longer levers and therefore allow you to generate more leverage (force) over a shorter lever.

However, longer cranks have a larger rotational circle and therefore they require an additional energy input to maintain a given number of revs (over shorter cranks).

As power = force x time one kinda negates the other ..... I think.

I'm 6' 1" and have used everything between 170's and 177.5's before settling on 175's.

I noticed a big leverage / slower rev scenario with the 177.5's and, in my case, as the circle subscribed by my foot, knee and hip was bigger I got groin strain issues - but this could have been the use of big gears in TT's.

175's just feel right for me.

Ain't it odd how the baseline standard here (as with a lot of bike standards over the years) is probably the one some French geezer came up with in a factory cos that was the length of metal rod, tube, etc he had to hand - BB sheel widths (why 68mm?), old school steel frame tubes, etc.

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posted by Rather Be Cycling [13 posts]
21st January 2011 - 15:39

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Power isn't force x time; it's force x distance /time

posted by Mat Brett [1808 posts]
21st January 2011 - 17:22

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Some useful info here, and as a result I have changed to 175.
Many thanks.

Never Look Down On Anyone Unless You're Helping Them Up

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posted by Kenny533 [1 posts]
10th February 2011 - 20:27

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