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 Post subject: Crank length confusion
PostPosted: Thu Jul 13, 2017 2:19 pm 
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https://youtu.be/bjrNuPjACnU

So I'm trying to figure out what crank length to use on my next bike. After much research it seems there's no good answer. I'm sure I'll get the "have a pro fit you" , but everything I've read says that doesn't necessarily work either. Or really does anything?

I'm on 170mm now. Sometimes I feel I'd like to go longer. Not really sure why. I think it dates back to bmx days when a short crank wasn't good. Of course there were no gears.

So I'd like some opinions as I've read many different ones from pro riders. For instance Sagan used 172.5 and so does Quintana, despite their height and I presume leg differences. Many others don't fit the traditional mold and many ametuers are the same.

Different articles mention different terrain as a factor as well.
I'm 5"10" and ride flats and climbs about equally.


Last edited by Greatestalltime on Thu Jul 13, 2017 11:26 pm, edited 1 time in total.

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 Post subject: Crank length confusion
Posted: Thu Jul 13, 2017 2:19 pm 


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PostPosted: Thu Jul 13, 2017 2:27 pm 
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The Zinn formula for length has worked well for me. 21% – 21.6% of your inseam. I ended up trying much shorter cranks than I had ridden previously (170-172.5) going down to 155-160 and have a much smoother pedal stroke and it has taken a lot of pressure off my knees. If you have average leg length and mobility at 5'10 170-172.5 is likely close to where you want to be and not going to make a huge difference going up 2.5mm.


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PostPosted: Thu Jul 13, 2017 2:46 pm 
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Thank you


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PostPosted: Thu Jul 13, 2017 4:29 pm 
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That would put me on 185-190mm cranks and i am on 172.5mm.
I could use 170mm, but 175mm i don't like.

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PostPosted: Thu Jul 13, 2017 4:31 pm 
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wheelsONfire wrote:
That would put me on 185-190mm cranks and i am on 172.5mm.
I could use 170mm, but 175mm i don't like.

What is your saddle height at 172.5?


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PostPosted: Thu Jul 13, 2017 4:41 pm 
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I have different saddle height with different saddles.
I ride Cobb Randee (generation 1 = not the one available now)
This saddle has quite a bit of hull flex and all padding you need, is in the saddle.

I am at about 78cm.
I am also not using traditional bib shorts (with fat chamois that is)
Pedals are Mavic CrossMax SL Pro and Giro VR-90 shoes.

You could say, i am almost exactly on, inseam length * 1.09 from top of saddle to center hole of crankarm (where you insert pedal).

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PostPosted: Thu Jul 13, 2017 5:02 pm 
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wheelsONfire wrote:
I have different saddle height with different saddles.
I ride Cobb Randee (generation 1 = not the one available now)
This saddle has quite a bit of hull flex and all padding you need, is in the saddle.

I am at about 78cm.
I am also not using traditional bib shorts (with fat chamois that is)
Pedals are Mavic CrossMax SL Pro and Giro VR-90 shoes.

You could say, i am almost exactly on, inseam length * 1.09 from top of saddle to center hole of crankarm (where you insert pedal).

Something seems wrong with you arriving at 185-190. My BB to top of saddle with a Toupe and 155mm is 71.8cm. Possibly the way you measure inseam compared to the way Zinn recommends. I know he's a proponent of long cranks for very tall riders (himself included) but I don't think you would fall into that category.


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PostPosted: Thu Jul 13, 2017 6:25 pm 
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Location: United Kingdom of Great Britain and Northern Ireland
Crank length is about what level you ride at.

If you are like a noob and your bike is poorly set up and you only ride recreationally, crank length will not matter that much because other things are wrong.

If you are an aspiring amateur rider and race then the best way to find out what suits is to try different lengths - trying just two lengths might sort it. There is a general move towards shorter cranks, which are said to open up the hip angle.


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PostPosted: Thu Jul 13, 2017 6:53 pm 
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Don't over think it. There is such a small difference between 170 and 172.5.

I'm 6 foot tall and have come down from 175s. I like that I can achieve a higher spin with 170s or 172.5s. That means I'm usually riding one gear easier on climbs and using a higher cadence. The nice thing about going from 175s to 170s is the decreased hip angle which means I can be lower and more aero.

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PostPosted: Thu Jul 13, 2017 7:19 pm 
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viewtopic.php?f=3&t=143296

viewtopic.php?f=3&t=141159

viewtopic.php?f=3&t=128234


Some Science in there.


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PostPosted: Thu Jul 13, 2017 7:30 pm 
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Did you read the link I posted to over in BF? There's a lengthy and thorough discussion on the affects of crank arm length on cadence and power.


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PostPosted: Thu Jul 13, 2017 10:13 pm 
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This is weightweenies, right?
shorter crank weight less. Then it allow you to use higher cadence (like sprint at 120RPM with same linear foot speed as 112RPM on longer crank). That allow you to drop down chainring size. 52/36 to 50/34 -> smaller chainring also weight less. win-win. :P

/on serious note, 2.5mm doesn't really feel like much. Maybe try 5mm difference to feel it more noticable.


Last edited by Hexsense on Thu Jul 13, 2017 10:43 pm, edited 1 time in total.

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PostPosted: Thu Jul 13, 2017 10:34 pm 
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If you race crits or love to carve corners, shorter crank is preferred for clearance.


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PostPosted: Fri Jul 14, 2017 6:50 am 
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Location: Loveland, CO
I'm 170cm tall and use 170mm cranks. But my MTB comes with 175mm cranks. Who do MTBs come with longer cranks?


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Posted: Fri Jul 14, 2017 6:50 am 


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PostPosted: Fri Jul 14, 2017 7:00 am 
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Posts: 170
Is anyone here with longer legs (38+ inches inseam) riding longer cranks than 175mm? If so, what's your experience? I'm currently on 180mm's but I'm kinda considering the 175mm as well, mainly because the 180mm model I'm interested in is not available ;)

When I had a bike fit 2 months ago I've asked to try both the 175mm and 180mm to check if I'll see the difference but honestly, I didn't. Maybe I could've get 1-2 RPMs more on the shorter crank, maybe not, but that's it; no discomfort, no different power transfer feeling. Think I might be forcing myself to using longer cranks for no real reason, guess I'll never know.


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