Road Bike, Cycling Forums banner
1 - 20 of 68 Posts

·
Registered
Joined
·
172 Posts
Discussion Starter · #1 ·
I suspect it is height? What are the guidelines?

Are there any other variables to consider?
 

·
Banned forever.....or not
Joined
·
24,554 Posts
If you are under 5 feet tall, use a 172.5.
.
.
.
 

·
Registered
Joined
·
21,725 Posts
I suspect it is height? What are the guidelines?

Are there any other variables to consider?
The "logic" of "crank length should be proportional to leg measurements" has been around for a LONG time, and lots of people have turned that "logic" into a formula for determining crank length. Only one problem: the research doesn't support it. One key feature that is often ignored in these discussions is the duration of muscle contraction that is controlled by cadence. It just may be that there is an optimum here, which is why there is a fairly narrow range of cadence for optimum performance. Longer cranks tend to mean lower cadence, moving you out of that optimum range. Crank length has been a point of debate since the introduction of the "safety" bicycle in the late 1800s, and there have been all sorts of fads in that regard.

There is no reliable formula for predicting crank length. There ARE lots of formulas out there, but they are just figments of the imagination of their purveyors. No one has ever done a study that shows how crank length should relate to anything.

You will find no high quality data to support any particular crank length as being better than any other. This is true whether or not you correct for leg length, femur length, etc. On the other hand, you will find lots of anecdotal or low quality data to support all kinds of conclusions, and more theories than you can shake a stick at. A rider's response to changes in crank length is 1) highly individual, 2) dependent on riding style and the event (TT, climbing, crits, track racing, etc.), and 3) most important, highly adaptive. This is why it is so hard to study the effect of crank length.

A 2008 study by Jim Martin, Ph.D., from the University of Utah shows zero correlation between crank length and any performance factors.
=============================
Fred Matheny Summary: There have been studies of crankarm length, but the results aren't consistent. Some show that longer cranks provide greater leverage for turning big gears. Some show that shorter cranks foster greater speed via a faster cadence. And some show that crank length is completely individual.

So, longer crankarms aren't a panacea for time trialing. In fact, there are dangers associated with them. The added length makes your knees bend more at the top of pedal strokes and extend more at the bottom -- both of which can lead to biomechanical injuries if you jump from 170 mm to, say, 180 mm.

Also, longer cranks reduce cadence -- and a brisk cadence is the key to good time trialing.
 

·
Registered
Escorted from the White House
Joined
·
37,521 Posts
For me, it was feel. I *could* feel the difference, and that's not too odd when you consider that while there's only a 2.5mm difference in crank arm length between 172.5s and 175s, the pedaling circles those cranks create are 16mm different in size.

175s just felt a little slow and cumbersome to me, while 172.5s were 'ahhh, just right'. But it's a very individual choice, aka YMMV.
 

·
Registered
Joined
·
149 Posts
I swapped to 175 from a 172.5mm crank length (5' 11" with 34.75" inseam). I found a noticeable increase in leverage, but as others have said, my average cadence decreased. At first my knees were hurting a little at the end of each ride but I figured my body was just adapting. After around 8 months or the problem was getting worse so ended up going back to 172.5 which fixed the problem.

I think the increased height and tighter angle created at the top of the pedal stroke were to blame.
 

·
25.806975801127
Joined
·
9,790 Posts
What determines the crank length for me is which one is on sale. People who say they can tell the difference (2.5mm) are either smoking crack or experiencing a placebo effect.
 

·
Registered
Joined
·
5,859 Posts
What determines the crank length for me is which one is on sale. People who say they can tell the difference (2.5mm) are either smoking crack or experiencing a placebo effect.
Bingo. I went from 172.5 to 175 and could never tell a difference. Why switch? The cranks I wanted were on sale and all they had in stock was 175. The guy selling them said "you'll never notice the difference". He was right. How could you? 2.5 mm is .25 cm, or around 0.1 inches. Hell, cleats and shoe sole thickness probably differ by several times that.
 

·
Registered
Escorted from the White House
Joined
·
37,521 Posts
What determines the crank length for me is which one is on sale. People who say they can tell the difference (2.5mm) are either smoking crack or experiencing a placebo effect.
What if you've tried 172.5 and 175 side-by-side, did not expect to feel a difference, and, to your surprise, DID feel a difference?

That was my experience.

– signed, a non-crack-smoker :wink5:
 

·
Anphaque II
Joined
·
6,176 Posts
I suspect it is height? What are the guidelines?

Are there any other variables to consider?
I suggest experimenting with crank lengths.


From the time I started riding bikes back in 1970 all the way up until 2010 I used/rode with 175mm cranks.

Then I started experimenting by using a 170mm crankset for about 6 months. It sucked when I was out of the saddle; felt like a kids tricycle cranks.

Then I bought a 180mm BMX crankset and have been using that for the last year or so. It's awesome when I'm out of the saddle! But when I did my first metric century last Summer I discovered at hour 3.25 of the 4.75 hour ride that those cranks now felt like 200mm! I felt every revolution for the rest of the ride.

So my next experiment will be using 172.5mm cranks during a metric century and see if my legs will like that length during 3+ hour rides. I'll also do the same with the 170mm cranks.

I want to be able to do centuries and double centuries so finding out that the crank length I like for rides at or less than 3 hours are not the crank length I like during the 4th hour and onward!


For reference, my legs are 33.5" from floor to sit-bones and I have a 32.5" inseam. YMMV
 

·
BONC founding member
Joined
·
107 Posts
For me, it was feel. I *could* feel the difference, and that's not too odd when you consider that while there's only a 2.5mm difference in crank arm length between 172.5s and 175s, the pedaling circles those cranks create are 16mm different in size.

175s just felt a little slow and cumbersome to me, while 172.5s were 'ahhh, just right'. But it's a very individual choice, aka YMMV.
That's the same unassisted-by-drug experience I had. I was strictly 175 coming from a heavy MTB period so goddam many years ago. My road machines started the same, then I took a couple over to 172.5 and my cadence went up and it so felt much better.
Why not the reverse? Simply not.


.
 
1 - 20 of 68 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top