Crank lengths !

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hondated
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Location: Eastbourne

Crank lengths !

Postby hondated » 5 Aug 2020, 6:31pm

Browsing through Youtube videos about crank length I found one that recommends a formula to use to determine the correct length and i would be interested in learning of forum members view on it.
You use your inside length measurement ie 81.28mm
x by 1.25
+ 65
which gives me a measurement of 166.6 mm

Shortest one I have ever used is 172.5 and my Roberts was built for me with a 175mm crank set. If you cannot trust a professional bike builder who can you trust.

Appreciate all and every comment on this issue.

Jdsk
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Joined: 5 Mar 2019, 5:42pm

Re: Crank lengths !

Postby Jdsk » 5 Aug 2020, 6:35pm

I think of two main issues:

1 What's comfortable?

2 How does it affect power output etc? The current answer seems to be "Not as much as previously thought".

Jonathan

PS: Agree about the exclamation mark. ; - )

rotavator
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Joined: 6 Jun 2016, 9:50pm
Location: North Wales

Re: Crank lengths !

Postby rotavator » 5 Aug 2020, 6:44pm

hondated wrote:You use your inside length measurement ie 81.28mm


I would have thought that a bigger problem would be to get a frame that fits such a short person :D

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hondated
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Joined: 27 Mar 2008, 7:59am
Location: Eastbourne

Re: Crank lengths !

Postby hondated » 5 Aug 2020, 9:06pm

rotavator wrote:
hondated wrote:You use your inside length measurement ie 81.28mm


I would have thought that a bigger problem would be to get a frame that fits such a short person :D

:lol: Oh come on 5' 10 " isn't that short :wink:

Valbrona
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Joined: 7 Feb 2011, 4:49pm

Re: Crank lengths !

Postby Valbrona » 5 Aug 2020, 10:23pm

The 'trend' is towards shorter crank lengh in order to open the hip angle a little.

But the best way is trial and eror and see which you prefer.
I should coco.

Debs
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Location: Powys

Re: Crank lengths !

Postby Debs » 5 Aug 2020, 11:07pm

There is a trend for shorter crank lengths when getting older [ and wiser ] shorter cranks are kinder to the knees.

Another advantage of shorter cranks is improved aerodynamics - feet not churning up such a wide cycle of air.

Marcus Aurelius
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Joined: 1 Feb 2018, 10:20am

Re: Crank lengths !

Postby Marcus Aurelius » 6 Aug 2020, 1:00am

Debs wrote:There is a trend for shorter crank lengths when getting older [ and wiser ] shorter cranks are kinder to the knees.

Another advantage of shorter cranks is improved aerodynamics - feet not churning up such a wide cycle of air.

It’s mostly the aero advantage that’s utilised by the pro’s. The comfort thing is coincidental. That said, I can produce more power when it’s comfortable, so it’s a win win, as I find 170mm cranks more comfortable due to the smaller pedal circle.

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Tigerbiten
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Joined: 29 Jun 2009, 6:49am

Re: Crank lengths !

Postby Tigerbiten » 6 Aug 2020, 3:04am

Everybody can cope with a fair range of crank lengths.
Longer cranks gives you a plain mechanical advantage which makes it easier to push down on each pedal in turn when you've run out of gears down and are out of the saddle grinding your way uphill.
Shorter cranks gives you a biomechanical advantage in your feet don't travel so far per pedal rev so it easier to up your basic cadence.
Old school thinking was because you are going to run out of gears down then it's probably better to have slightly longer cranks for hill climbing.
More modern think is because it's now easy to have silly low gear then shorter cranks may be the way to go.
So it all depends on your riding style.
If you like to get out of the saddle and power your way uphill then stick to longer cranks.
But if you like to stay in the saddle and keep spinning uphill then shorter cranks may be worth an experiment.

Luck .......... :D

De Sisti
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Joined: 17 Jun 2007, 6:03pm

Re: Crank lengths !

Postby De Sisti » 6 Aug 2020, 8:02am

I've only ever ridden 170mm cranks (they're comfortable enough). I don't want to go to the expense of
changing the cranks on my three bikes to find out that a different crank size isn't for me.

alexnharvey
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Joined: 10 Jan 2014, 8:39am

Re: Crank lengths !

Postby alexnharvey » 6 Aug 2020, 8:44am

De Sisti wrote:I've only ever ridden 170mm cranks (they're comfortable enough). I don't want to go to the expense of
changing the cranks on my three bikes to find out that a different crank size isn't for me.


You only need to change it on one bike to test the idea out. I have 165mm on one bike and 170 on another. I think overall I prefer the 165s but it is no problem to ride the 170s on other days.

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hondated
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Location: Eastbourne

Re: Crank lengths !

Postby hondated » 6 Aug 2020, 1:02pm

Thanks every one for responding to my question There's some great advice been given and interesting experiences as well.
I just assumed , and it appears wrongly ' that the longer the crank the bigger the lever.
Just to experiment last night I ordered a 170mm Crank Set which I will fit to my Trek Madone and see how I get on.
What has prompted all of this is that I have developed a pain in my left knee and I have seen a video that mentions crank length could be causing it.
It of cause couldn't be because of old age , :wink: could it . :lol:

Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: Crank lengths !

Postby Brucey » 6 Aug 2020, 1:31pm

trying it out is the only way to be sure.

FWIW these 'formulas' for crank length vs leg length are a load of cobblers; to generate the 'formula' they have drawn a straight line through an amorphous blob of data. This means that the 'exact length predicted' is likely to be 'accurate' +/- 15mm or so, and that is probably limited by the availability of cranks rather than anything else.

I've used 165mm to 175mm cranks on different bikes and it does make a difference (for me) to the way it feels. However most of the time I ride 170mm these days and I don't think it is a limiting factor in any aspect of my bike riding.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

bgnukem
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Joined: 20 Dec 2010, 5:21pm

Re: Crank lengths !

Postby bgnukem » 6 Aug 2020, 1:37pm

The formula in the original post is gobbledigook.

E.g. My inside leg is 36" or 91.44cm.

Multiple by 1.25 gives 114.57cm. Add '65' (I assume mm) gives 116.85cm.

So my crank length should be 1.16m then, or longer than my actual legs.....

Jdsk
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Joined: 5 Mar 2019, 5:42pm

Re: Crank lengths !

Postby Jdsk » 6 Aug 2020, 1:39pm

hondated wrote:I just assumed , and it appears wrongly ' that the longer the crank the bigger the lever.

That bit's correct: it's the implications and the other effects that make this complicated.

Jonathan

Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: Crank lengths !

Postby Brucey » 6 Aug 2020, 1:44pm

I think the idea is that you measure your leg length in cm and then use 1/10th that value (i.e. the same number but with mm suffix) to input into the formula.

Anyway bgnukem's predicted crank length is

114.57 + 65 = 179.57mm

plus or minus ~15mm..... :roll:

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~