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Crank lengths !

Posted: 5 Aug 2020, 6:31pm
by hondated
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.

Re: Crank lengths !

Posted: 5 Aug 2020, 6:35pm
by Jdsk
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. ; - )

Re: Crank lengths !

Posted: 5 Aug 2020, 6:44pm
by rotavator
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

Re: Crank lengths !

Posted: 5 Aug 2020, 9:06pm
by hondated
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:

Re: Crank lengths !

Posted: 5 Aug 2020, 10:23pm
by Valbrona
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.

Re: Crank lengths !

Posted: 5 Aug 2020, 11:07pm
by Debs
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.

Re: Crank lengths !

Posted: 6 Aug 2020, 1:00am
by Marcus Aurelius
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.

Re: Crank lengths !

Posted: 6 Aug 2020, 3:04am
by Tigerbiten
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

Re: Crank lengths !

Posted: 6 Aug 2020, 8:02am
by De Sisti
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.

Re: Crank lengths !

Posted: 6 Aug 2020, 8:44am
by alexnharvey
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.

Re: Crank lengths !

Posted: 6 Aug 2020, 1:02pm
by hondated
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:

Re: Crank lengths !

Posted: 6 Aug 2020, 1:31pm
by Brucey
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

Re: Crank lengths !

Posted: 6 Aug 2020, 1:37pm
by bgnukem
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.....

Re: Crank lengths !

Posted: 6 Aug 2020, 1:39pm
by Jdsk
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

Re: Crank lengths !

Posted: 6 Aug 2020, 1:44pm
by Brucey
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