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Crank-arm length for a tall guy

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Crank-arm length for a tall guy

Old 07-02-10, 08:19 PM
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cooleric1234
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Crank-arm length for a tall guy

I'm 6'5" and the bike fit showed a 36.2" inseam (I think that's without shoes). I've been reading lately (spurred by the recent post here about the Zinn interview) about proper crank-arm length. Well, the various calculators, forum posts, and articles are all over the map. I've seen and heard suggestions ranging from the standard 175 mm to 190 mm (yeah right). The issue is that sometime in the next couple years I'll probably be upgrading my entry level bike to something nicer. If 175 is optimal then that's great, bikes my size will come with that. But if I need something longer that will mean swapping out the crank and selling it and getting a (possibly) more expensive longer crank. However, SRAM has a few options that aren't too pricey (both Rival and Apex have 180 mm cranks). Anybody my size move to longer cranks and feel like it made you a better rider (or, as Sheldon implies, effectively gives you lower gearing)?
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Old 07-02-10, 08:27 PM
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I like longer cranks for steep hills, but hate them on the flats. Unfortunately the only way you can find out if you like longer cranks is by getting them.

I think the whole fitting cranks to various bodily dimensions is hooey, personally. Get what feels comfortable to you and gets you up the hills that you want to and keeps your knees out of your chest.

I'm much shorter than you, but after much trial and error, I find I like 175 on MTB and 172.5 on the road. I actually prefer 170 on the road, but prefer not having such a big difference between bikes.

If I had your inseam, I'd probably consider 180s if I climbed a lot of steep hills.
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Old 07-02-10, 09:27 PM
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Originally Posted by LesterOfPuppets View Post
I'm much shorter than you, but after much trial and error, I find I like 175 on MTB and 172.5 on the road. I actually prefer 170 on the road, but prefer not having such a big difference between bikes.

If I had your inseam, I'd probably consider 180s if I climbed a lot of steep hills.
I'm closer to you in height (and pretty close to Les in distance) : 6-foot-4, 36-inch inseam. I had a couple bikes with 180mm cranks and didn't feel comfortable riding them. A switch to 175mm helped my spin without losing speed.

I agree that "x crank length works for y leg length" is hooey. Muscle type and muscluloskelatal flexibility has as much to do with optimal crank length. Since flexibility changes with time and training, the sizing "formulas" don't do what they purport to do.

A friend of mine is 5-foot-10 and is experimenting with 130mm cranks. That's a very special case, though:
https://www.recumbents.com/wisil/lem/.../scimitar4.htm
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Old 07-03-10, 04:52 AM
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I have 220mm cranks on my Zinn Road bike and all of my other bikes have 175.5 (or whatever)mm cranks because they are "off the shelf" bikes (if you can say that about a 67cm tourer). Anyways, I used the Zinn for training and racing in triathlons and I found them to be an improvement on the 175 mm cranks. I think I should have gotten shorter cranks though so that I could keep my cadence up higher because the 220 mils make it pretty hard to achieve an average of 90 rpm.

For non-racing use though I highly recommend the 175mm cranks though for the simple reason that they are more practical. Perhaps it is because I am a bike tourist that I place ability to find replacement parts so high on the list of priorities but even if you never venture more than 20 klicks away from home on your bike you'll appreciate not being sidelined for weeks because you had a crank issue and needed repairs or a replacement. Also, longer cranks are very flexible and they make for a bit of a nightmare when it comes to bike fitting. Even though my Zinn is custom it still had issues with the cranks hitting the chain stays. I try to imagine how frustrating it would be putting the longer cranks on a pre-fabricated bike especially for someone with precise bike fit needs.

It's just an opinion but I'll never go bigger than 175mm again. I am unusual though because even when I ride to work I like to take it easy and smell th' roses.

Last edited by Eternal_Tourist; 07-03-10 at 05:09 AM. Reason: punctuation error
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Old 07-04-10, 08:53 AM
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I don't know that I care about immediate availability. I rarely buy from bike shops, and if something happens to the crankarms I'm much more likely to get a ride home and order a new pair online.

I do hope, however, that 175 is a near optimal length for me. It means no switching of components on new bikes and a wider range of options. I guess the only way to tell is get longer crankarms and test them. I'm not sure if I'll do that, maybe if I find some screaming deals on longer crankarms, which is highly unlikely.
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Old 07-04-10, 03:47 PM
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Hey Cooleric,

I'm 6'5" w/ a 37" inseam and ride 180's on my road bike. Over the years, I've talked to a number of top builders, trainers and physiologists across the country. What I've found is that very few bike builders and even fewer shops have any proven knowledge or experience fitting tall riders (Leonard Zinn is the exception). The standard, "A longer seat post and stem should do the trick"; response is why I do my own research and buy online.

My 2 cents worth: From an engineering perspective (leverage and efficiency), there is a relative relationship between a rider's physical size and crank arm length. Is there an absolute perfect formula to figure crank arm length out? I doubt it. A lot of people like to defend the statement that a shorter crank arm allows one to spin faster which I agree with, as long as it's relative to your size. There is no absolute best size. As others have pointed out, fitness and flexibility are relevant variables as well.

Bottom line: the ONLY way to find the "optimal" size for ANYONE is to test on a power meter. Assuming that you define "optimal" by speed. Other ways to define "optimal" could include: Comfort, Aerodynamics, Power, Availability, Cost or a combination of the above. Instead of guessing what size you need, just search for a higher end shop that has a fitting system with adjustable crank arms.

Sizes offered by bike mfg's are simply based upon size demand and sales (just like any other mfg). The majority of the population is considerably smaller than us. At 6'5" we're in the top 5% minority on the planet. The same reason you don't find "tall" clothes or "big" shoes readily available.

I'm not sure what size crank arms "Tourist" thinks are "flexible" and "impractical" but you will have no problems with any major brand up to 180's. Flexibility is another conversation but I would bet that it's his low spoke count wheels that are flexing (not his crank arms or BB like everyone likes to suggest).

As far as putting anything longer than a 180 on a non custom frame, you will most likely have issues with BB height. One last thing, if you do the math you will see that the difference between a 175 and 190 ("yeah right") is only about 5/8", pretty insignificant in the big picture of things.
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Old 07-04-10, 04:45 PM
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Originally Posted by pjosi View Post
One last thing, if you do the math you will see that the difference between a 175 and 190 ("yeah right") is only about 5/8", pretty insignificant in the big picture of things.
Thanks for the reply. The "yeah right" was also partially due to cost. I've been biking without problems for years now on 175's, I'm not spending more than the cost of my bike for crankarms when at best it would be just an improvement on my current setup. 180's on the other hand, are becoming available and economical with SRAMs Apex and Rival cranks.
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