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Discussion Starter · #1 ·
Need suggestions on crank arm length for the wife. My wife currently rides an Orbea Mitis Dama (52.7 ETT) and has FSA 170/50/34 Compact Cranks. She likes the gearing of the compact for the climbs here in Colorado but has commented that she feels like she spins too fast on flats. She is 5'5", has a short torso, long legs (longer than mine and I am 5'10.5" but I am all torso) and avg arms. Do you think she would be better off with a 172.5 compact as opposed to her current 170 compacts? Her femur is definitely longer than mine and I like the 172.5 over the 175s that are on my MTBs. TIA!!!
 

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I don't think the change in crank arm length will have much of an affect on her "spinning out" on the flats. As a matter of fact, the shorter cranks are usually better suited to high RPM's, and the longer arms might make her feel even MORE "spun-out".

I would look more towards her cassette. Is/are you looking for one set of gears to do everything? Or are you handy enough to change out a cassette depending on your planned ride?

For a does everything mountainous and flat set up, you cant beat a triple, say a 12-25 with a 30-39-53. That would be a set-up that covers every possibility. I have a bike with that set up and it serves as both a mountain goat and a flats crusher.

The other option would be to get an 11-23 for the flats, and a 12-26 for the hills. Then stick with the compact crank.

I use a compact 50-34 with a 12-25 on my main go-to bike and I don't think I've ever felt spun-out on the flats, down 2-5% pedaling grades, yes, but not the flats. (I think that 50-12 at like 120rpm is over 30mph)

Like I said, the longer cranks will probably make her feel she is spinning even faster on the flats. Concentrate on the gearing.

Oh, and you can only get a "Gruppo" from one place, Campagnolo. All the others are just groups...
 

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Discussion Starter · #3 ·
Thanks for the suggestions. I am looking for a "do it all" set-up. A factor I didn't mention is that we are moving from mountainous Colorado to semi-mountainous, bunch a little hills Portland and so she prolly doesn't need a 12-27 like I originally planned. A 50/34 with a 11-25 would prolly be sufficient, wouldn't you think? I still don't know about the crank arm length though. When ever I get on a Lifecycle at the gym, I always feel like I am spinning much faster because those machines usually have super short cranks. At least that is what my mind is telling me.
 

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Don't change the crank arm length. She's too short for 172.5. The crank arm length will not help the "spinning out" feeling. That's entirely gearing and technique related. If she's spinning out her big ring, get a slightly bigger one, or smaller in back.

Incidentally, longer crank arms could very well cause knee issues. I'm 5'7" and 172.5 crank arms make my knees hurt really bad.

Either that or she should really learn how to spin a cadence of 130. A set of rollers will do that mighty quick.
 

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Yep, it sounds like she can use the 172.5, although she won't notice the difference. But first, I'd put the money into something she would notice such as a new cassette and wheels. You do put more power to the wheels with a longer crank, but spinning faster makes you work harder. So, if she spins a lot, then getting different gearing would be the way to go.
 

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A change between 170s & 172.5 will be completely unnoticeable. On a metric ruler, measure 2.5 millimeters. See how much different that make. It's about like a pencil lead.
 

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Discussion Starter · #7 ·
Mr. Versatile said:
A change between 170s & 172.5 will be completely unnoticeable. On a metric ruler, measure 2.5 millimeters. See how much different that make. It's about like a pencil lead.
I see your "point" on the pencil lead! :) But consider the sum total of 5 mm in the full revolution and it is a HUGE difference!!! :p (Tongue Planted in Cheek)

But seriously, when I get on my MTB which has 175 mm cranks, I can tell the difference. Maybe it has more to do with gearing- I don't know for sure. But there is an ever so slight difference, like taking just a bit longer stride than my normal stride when running. Maybe the difference won't be as noticeable going from a 170 to 172.5 as it is to going from a 172.5 to 175???
 

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Mr. Versatile said:
A change between 170s & 172.5 will be completely unnoticeable.


not always... some people will feel it, others won't. with that change the difference in the pedal circle radius is 2.5mm but the diameter is 5mm. if you make no other changes, the bottom/6 o'clock position is 2.5mm lower and the top/12 o'clock position is 2.5mm higher. but you generally or often lower your seat a bit when you go to longer cranks (to account for the lower 6 o'clock position), so at 12 o'clock the pedals are even higher, relative to saddle position, compared to where they were using the shoter cranks
 

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Spelling nit-picking

English: Group
Italian: Gruppo
unacceptable: Groupo (maybe he was the sixth Marx brother? ;-)
 

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Dont' change her cranks... 170mm is about right for her.

You need to change the cassette to allow for a higher top gear. A 50/11 is actually a higher top gear than a 53/12... You idea to use a 11/25 cassette might make sense.
 
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