Crank arm length??????

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kbbpll
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Joined: Wed Oct 20, 2010 5:56 am

by kbbpll

Ha, my cycling inseam is 96.5cm and saddle height is 105.5. I've always thought that 5 different crank lengths within the span of 1 cm was a bit silly, especially given traditional frame sizes have a span of what, 16 cm or more? For most people, one of their feet is probably longer than the other one, by more than the 2.5mm increment of crank length that they fret over. But I guess there must be science or something behind it, other than "that's just the way we've always done it". I rode 170s for 30 years before finally switching to 180s about 4 years ago, and frankly I don't think I noticed much difference. Leonard Zinn wanted to put me on 210s, but I wasn't quite willing to make that kind of leap of faith. The article cited by @username says "Riders with an inseam over 33 inches use 175 millimeter cranks" - seriously?? 31" use 170, 31-32" use 172.5, every other cyclist on the planet taller than about 178 cm just use 175? It's hard for me to take such advice seriously. But as mentioned earlier, this subject has been discussed to death.

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kauphy
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by kauphy

I'm 6ft and use 170s on my road bike. I have relatively poor hip flexibility and they help with that. I previously ran 172.5s and I find that the 170s give me a smoother stroke. What I lost is a bit on my max power but that's alright for me. I most certainly can notice the 2.5mm. While I like the 170s I want to give the 175s a try once I improve my hip flexibility.

tinozee
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by tinozee

Agreed, longer crank closes off the hip angle. 170mm user here 6'3" long legs.

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kbbpll
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by kbbpll

I don't intend to be argumentative, but I really can't see how anybody notices a pedal stroke diameter change of .5 cm. You could wear thicker socks and shorts and get a similar difference. Perhaps there is something more "scientific" to go on so if somebody has a link I'd love to educate myself.

tinozee
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by tinozee

What about stack height then? Or the length of upper and lower leg and foot length and how that ratio plays into it?

The ideal is for op to test himself on the same/bike same trainer with a power meter with 170 and 175, 180, whatever and look at the data. It's going to be hard to fit one answer to this for all bodies.

dogg
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by dogg

my bikes have 165s and 170s and id run 165s on all of em if i could find a decent looking road double in my price range. not sure if its made me faster or smoother or "better" but my bad hip and knee certainly give me less trouble now. I'm 5'11 but I'm pretty convinced height has no bearing on preferred crank length.

this is some interesting reading for those interested in short cranks though: http://www.cobbcycling.com/articles/cra ... ull-circle

research, experiment, and ride what you find works best for you.

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Rick
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Joined: Sat Aug 29, 2009 4:30 pm

by Rick

kbbpll wrote:I don't intend to be argumentative, but I really can't see how anybody notices a pedal stroke diameter change of .5 cm. You could wear thicker socks and shorts and get a similar difference. Perhaps there is something more "scientific" to go on so if somebody has a link I'd love to educate myself.

That's a reasonable question.
But if you set your seat height so that it is the same at the bottom of the stroke, then you actually have to raise your leg twice the difference in crank length. Then, you also have to reach out further on the front stroke and back further on the upstroke. Combined with the 3.14 (pi) times the difference in total circle diameter.
The difference in feel is still small (to me), but I can see how it is really a greater effect overal than just the small increment in crank length.

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j0oftheworld
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Location: Phoenix, AZ

by j0oftheworld

I think going shorter could benefit people w knee issues. I've read most of the studies online and shorter would in my mind limit the range your leg has to travel in all directions this putting less leverage on it.
Taking this from a performance to a health discussion.
JasonM. Tempe, AZ
'14 SW Roubaix / '16 Allez Sprint

KLabs
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Joined: Wed Dec 01, 2010 4:29 am

by KLabs

j0oftheworld wrote:I think going shorter could benefit people w knee issues. I've read most of the studies online and shorter would in my mind limit the range your leg has to travel in all directions this putting less leverage on it.
Taking this from a performance to a health discussion.

... riders w/ hip and knee issues, riders who prefer higher cadence, perhaps older riders, weaker/heavier climbers, and probably other situations, such as a little more pedal cornering clearance, etc...

kauphy
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Joined: Mon Jul 08, 2013 3:38 am

by kauphy

Wanted to put one more point in: shorter cranks means less leverage and therefore you're effectively riding a slightly bigger gear. So it has a bearing on your gearing as well.

@kbbpll, the thicker socks argument works for when the crank is at 12'o clock and 6'o clock. It doesn't work at 3'o clock for instance, where the largest chunk of your power comes from.

KLabs
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by KLabs

Perhaps riding a heavier rear rim (450/550g) with a shorter crank arm (160/165cm) is similar to riding a light rear rim (300/400g) with a longer crank arm (175/180cm) ...

CedrickG
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Joined: Sun May 11, 2014 12:47 am

by CedrickG

KLabs wrote:... riders w/ hip and knee issues, riders who prefer higher cadence, perhaps older riders, weaker/heavier climbers, and probably other situations, such as a little more pedal cornering clearance, etc...


Especially this...shorter crank length is a godsend for those with unusual limits in joint mobility. If you're not coming up against those you have room to experiment, but lots of folks are effectively 'locked out' of some of the longer options by their anatomy.

fallzboater
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Joined: Wed Feb 03, 2010 2:26 am

by fallzboater

I'm "only" 6' 3" but have a 98cm cycling inseam. I've used 180s (Dura Ace, Record, XT, XTR) on all my bikes for many years. If I borrow or rent a bike with 175s it feels odd to me for about an hour and I feel like I'm not climbing as well, but I don't have data to back that up. My range of hip and knee rotation is less with 180s than most riders have with 170s. I'd try longer, but prefer to stock with commonly available parts and frames.

JN2Wheels
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Location: Maryland

by JN2Wheels

One consideration not mentioned yet ... Cornering clearance. Imagine that feeling of just brushing a pedal leaned over red-lined in a crit using 170mm. Scary, but part of pushing the limits. Now imagine that same scenario, but feel the pavement tearing through your jersey. That is certainly a difference that 2.5-5 mm can make.

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kbbpll
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by kbbpll

If anybody has the skills to corner so accurately that 2.5mm is the difference between winning and crashing, I certainly bow to their superhuman status. :beerchug: Yes I'm trying to be funny and sarcastic. And yet, seriously, we can all hold out our thumb and index finger 2.5mm apart and realize how big that is, right?

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