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Crank length...HELP!

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Crank length...HELP!

Old 05-04-10, 09:29 PM
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BiggSlic
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Crank length...HELP!

I'm getting ready to upgrade from my first road bike. I purchased it about a year ago and at the time I had no clue what I was needing size wise, etc. I am 6'4" tall and have a 35" inseam. My LBS put me in a frame that was too small (59cm). At the time I upgraded the cranks with some nice Dura Ace. They gave me a "good deal". Like I said, I was totally new and had no clue so I bought from my LBS so they could put me in the proper setup.

Fast forward to today...I'm spec'ing out components for my new frameset that is coming in on Thursday. I'm shopping around at other LBS for my new groupset. This different shop notices that my cranks are 172.5! I never noticed or realized. Shame on me I know but I knew nothing when these were acquired. Like I said I'm 6'4"!

So now instead of just swapping my DA cranks over to the new bike I need to purchase a new crankset. How long should I go? Everything I have read online puts me in a 180mm or longer crank arms.

Oh, BTW...I'll have 172.5 DA 7800 cranks with about 1,000 miles on them for sale very soon.
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Old 05-04-10, 09:36 PM
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Do you like them?

If so, then why change?
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Old 05-04-10, 09:39 PM
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Sheldon may be able to help.
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Old 05-04-10, 09:41 PM
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Like them? They are all I know. I have nothing to compare to. I would sure at least like to try something longer then I could answer that question.
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Old 05-04-10, 09:43 PM
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or zinn... who's more acquainted with taller riders.

https://zinncycles.com/Zinn/?page_id=171
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Old 05-04-10, 09:44 PM
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There isn't a "right" crank arm length for someone of a given height, just like you can't size a bike based off someone's height. It's all about what your comfortable with, so if you are comfortable with these then I see no reason to change.
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Old 05-04-10, 09:45 PM
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I'm 6'1" and went from 170 to 175 because I was supposed to, according to a chart somewhere. They say the longer crank arms will make climbing easier, but I really don't notice a difference. I am more of a spinner anyway. 175 may be for you, but frankly 2.5mm seems awful miniscule to me.
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Old 05-04-10, 09:51 PM
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I'm more comfortable turning big gears then spinning at a higher cadence. My preferred cadence always seems to fall in the 80-85 rpm zone. I'm really thinking a 175 or maybe even a 177 crank length might be in order.
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Old 05-05-10, 01:32 AM
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I'm an advocate of proportional length cranks. I recommend crank length being in the range of 19% to 20.5% of inseam. Assuming that the 35" inseam figure is accurate then I would recommend a crank length for you from 170mm to 180mm. Sure you could try something like 175mm or 177.5mm but I think 172.5mm is still in an OK range for you so don't sweat it. A guy called "Zinn" is out there recommending that cranks should be 21.6% of inseam and while its OK for some I think its insanely long for most mortals.

If its not going to cost you an arm and a leg I would nominally recommend you try 175mm cranks but I wouldn't sweat it. You could try some 177.5mm cranks if they weren't too expensive but if they are don't be thinking that you have to have them.

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Old 05-05-10, 02:11 AM
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I am 6'0" but with a 35" inseam, my legs are similar to yours in length. I ride 175mm cranks.
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Old 05-05-10, 03:53 AM
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IMHO it's question of what you like and feel comfortable with. It kind of seems intuitively likely than longer legs should suit longer cranks, but I'm pretty sure there's no conclusive evidence that you're guaranteed more power or speed or anything because of crank length.

Me, I have a 34" inseam and prefer 170mm even though in theory I should be on longer.
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Old 05-05-10, 06:02 AM
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I don't want to be rude but I should add that it would be a good idea to do a search on this subject. We discuss crank length often and if you want more information on the reason for various peoples opinions including mine its all there if you search for it.

Anthony
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Old 05-05-10, 11:39 AM
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I'm your size(6'4). My geared bike has 175mm cranks(which I am replacing with 172.5mm cranks), and my single speed commuter has 170mm cranks. I can't honestly tell any diffrence between them riding around.
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Old 05-05-10, 01:31 PM
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Crank legnth is determined by your cadence style, not by your height unless you are way off the scale/tall or short, which you are not.

In general, the more you spin, the shorter the crank legnth. Track racers usually use 165mm cranks because they tend to spin at very high rpms, 140+ for most sprinters. If you mash, then longer cranks give you a better mechanical advantage.

So figure out whether you mash or spin and get cranks accordingly, but 172.5 is really a crank size that is near the middle of what most people use, just a little weighted towards mashers, you might want to just consider keeping them. If you move radically, say 5-10mm, it will affect your spin and you'll have to get used to the new pedalling style.
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Old 05-05-10, 01:37 PM
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track needs 165mm, because you're less likely to pedal strike on the embankments.
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Old 05-05-10, 02:34 PM
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Whip out a ruler and have a look at how long 2.5mm is and then 5mm. It aint much. I doubt you'll feel much of a difference. Use what you have.
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Old 05-05-10, 02:34 PM
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If I were 6'4" with a 35" inseam, I'd use 180s. My two best friends, both former strong Cat 3s (and now just strong, although one started racing again after a 10+ year break) are 6'2" - 6'4" and both ride 180s. They have been for 15 years or so. Both may be considered all rounders but one, at his best, could absolutely annihilate pretty much anyone in a sprint (including a 7-Up pro "sprinter" - granted, this is on Gimbles, but they were fighting it out).

I switch between 175s and 170s and my inseam is 29" or so. I use 175s for the winter/spring. I used 170s for the track, found that I really needed to be on 170s all the time to be effective the one day I rode on the track, so switched my road bike to 170s. I went out and won a few track races, and for the first time that summer almost finished a hard A training race (guy took me out with about 1/2 lap to go so that put an end to my 3 day streak).

I'm debating what to do now, since I won't be going to the track much this summer, if at all. 175s or 170s. I feel 175s helped me make up for some of my lack of aerobic fitness. 170s are fast but require a lot of fitness to turn them over (for me). I'll probably stick with 175s for a while.

I'm pretty vocal about using long cranks. Others are equally vocal and successful using shorter ones. But at your height, your inseam... 180s.

cdr
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Old 05-05-10, 02:39 PM
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Originally Posted by Hunt-man View Post
Whip out a ruler and have a look at how long 2.5mm is and then 5mm. It aint much. I doubt you'll feel much of a difference. Use what you have.
I can feel the difference between 170mm and 172.5mm.
then again, for me, 165mm is ideal and 170mm is on the longer side, but okay.
I'm also not that flexible, which makes longer cranks a, literal, stretch to use.
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Old 05-05-10, 04:45 PM
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2.5 mm seems so minuscule. Adjusting your cleat position on your shoes seems like it would have a bigger effect.

Also, how much do you want for that 172.5mm DA crank? I'm in the market...
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Old 05-05-10, 06:24 PM
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I'm 5'11", 33" inseam. I used to ride on 170's, this year I switched to 175's, and honestly, I can't tell a difference.
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Old 05-05-10, 06:26 PM
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In a different crank length thread, someone pointed out that 2.5 or 5 or whatever mm isn't significant. This means that, at any time, I could sneak over to your bike and adjust the saddle up or down 2.5 or 5 mm without affecting your pedal stroke (for a total delta in height of up to 1 cm).

If this holds true for you, then a 2.5 or 5 mm crank arm length really is insignificant. If it does not, then it's something to consider.

cdr
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Old 05-05-10, 07:45 PM
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Hi

I did some reading on this - I'm 6'4" with a 37" inseam and ride 175's. An online proportional calculator put me down as needing 195's - and I think it would make a difference (if I got a Zinn crankset, I would probably go for one of their bigger gearings) - I like to ride with a high gear ratio (I only use the little ring when I hit hills over 5%).

I can't justify the cost of it however - YMMV. I would like to try out 180's (maybe put on cheap SRAM rival's that go up that big to try out).

If you moved up to 180's from 172.5's, your feet would be travelling about an extra 5cm every rotation - and about a 5% increase in torque.

As said above though, it is not an exact science and your own muscles and cycling style would determine how useful the change would be.

If I had the funds, I'd definitely go with 180's, and would probably at least talk to people over at Zinn about going longer.

Cheers
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Old 05-05-10, 07:59 PM
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Originally Posted by Toddorado View Post
They say the longer crank arms will make climbing easier, but I really don't notice a difference.
Going from 170 to 175 is equivalent to shifting from a 17T sprocket in the rear to one that is 17.5T. (And there ain't no such animal.) Or between a 35 and a 34 in front.
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Old 05-08-10, 05:17 PM
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Originally Posted by jimmuller View Post
Going from 170 to 175 is equivalent to shifting from a 17T sprocket in the rear to one that is 17.5T. (And there ain't no such animal.) Or between a 35 and a 34 in front.
I ran that through Sheldon's gearing calculator, and you are surprisingly close.

I have been thinking about this today, because i am trying to help someone improve their bike for an upcoming charity ride. She suffered mightily last year,and one improvement I'm considering is shorter crank arms. One thing i have seen mentioned in the fitting guides, is not to have the knees bend more than 90 at the top of the stroke. in her case, at 5' 2" with short legs for her body, 170mm makes for quite a bend at the top.

I am thinking it is more about the way your legs move in the pedaling stroke than gearing.
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Old 05-08-10, 06:31 PM
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OP, did you decide what to do?

FWIW I have a 38" CYCLING inseam and use "only" 175s, even though most experts say I should use 180s or longer.

And my commuter bike has 170s, I can't really tell much of a difference.

My advice is to use what you have for now....maybe next time you get a new crank consider 175 or 177.5 if you want. Probably won't make a big difference anyway.
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