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165mm vs 170mm cranks for Randonnuering

Old 03-12-19, 08:07 AM
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friday1970
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165mm vs 170mm cranks for Randonnuering

So I have two cranksets lying around in my shed. FSA Omega 170mm with the Megaexo BB, and a Shimano 105 165mm crankset with its BB. The weight differences aren't much. The FSA set is about 150g heavier. I am wondering if the extra mechanical advantage of the extra 5mm negates the extra weight.
Though my legs are shorter than average, I usually don't have any knee problems with 170mm cranks. I originally bought the 165mm cranks based off recommendations because of my shorter stature. For short rides, either set works great. But with a 200k ride, the wrong choice could lead to a longer day near the 100 mile mark.
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Old 03-12-19, 12:02 PM
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I'd vote for the 165's.

If you want more, here's my thinking on this:

Crank arm length does not seem to matter much. I have come to that conclusion after a lot of experimentation. I found it was easy to get used to cranks as short as 140 mm, and hard to get used to long ones once you get used to short ones. In the course of that experimentation I learned that longer cranks offer no advantage at normal pedaling speeds. If you let your cadence get uncomfortably low (like, when it gets hard to turn the pedals over at all) the leverage of longer cranks will help you get another couple yards up the hill before you start to walk, but that's realistic only in rough off-road situations, not what we encounter randonneuring.

Shorter cranks also have their advantages. The main advantage, I think, is that shorter cranks tend to promote a higher pedaling cadence. So I prefer them for that reason.

I use 165's on just about all my bikes, but 160's on my fixie.

Again, I must emphasize that crank arm length does not seem to matter much. Just as people of all different sizes are able to walk up and down the same stairs, just about anyone can ride 170 mm cranks. It stands to reason that there is an ideal size for each person, and that a smaller person will want shorter cranks, and I'm sure that's true; but there has been very little research into what is actually the ideal size. I have no data to support this claim, but I am convinced shorter is better.
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Old 03-14-19, 03:55 PM
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There is plenty of data on this and some controversy. In general, the industry doesn't like to talk about it. But it stands to reason that if frames need to be different sizes probably cranks do too.

Its not so much about power as it is about your knees. Longer cranks mean you have to move your knees more and your knees are the most important part of the mechanism. If you have knee problems you are better off on shorter cranks.

Your inseam says much about what crank length you can handle. I have an inseam of 30" and 170mm is the most I can handle. It seems I'm better off on 165s.

Here's a nice article on the issue:

https://ridefar.info/2017/02/crank-l...ance-cyclists/
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Old 03-14-19, 04:22 PM
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My inseam is 29.5" and both my road bikes are 165 mm (105 and Ultegra).
Since the R7000 has a shorter 160 mm one, I am looking on getting it someday....
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Old 03-14-19, 05:52 PM
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Our legs are strongest the closer they are to full extension. This is why it's easier to lift more weight with a partial squat than a full squat. So as long as your saddle is set properly for full leg extension at the bottom of the pedal stroke, the length of your cranks the rest of the time isn't that critical for power output. This is why most of the industry can get away with only making a couple of crank lengths within a narrow range. That said, a person's build or riding style may be factors in why they might favor a particular crank length when not at full extension (for example, a short rider may be annoyed with long cranks that cause their knees to bounce really high on the top part of the stroke, or a tall rider may get that "shoes tied together" feeling with short cranks that don't make use of their greater range of motion.)

It would seem like cranks that are perfectly proportional to the rider's body would be ideal, but I think there are other factors that end up confounding this.

More relevant to the OP, I've used anywhere from 165mm to 175mm on my bikes, but switched from 170mm to 165mm on my rando bike for two completely non-physiological reasons: they were lighter, and negated 5mm of toe overlap. So far, so good.
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Old 03-15-19, 06:06 AM
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Originally Posted by Salubrious View Post
There is plenty of data on this ...

Here's a nice article on the issue:

https://ridefar.info/2017/02/crank-l...ance-cyclists/
Thanks for the link! That is an interesting article, and pretty well sums up the available literature.

Nonetheless, I do not agree, that "there is plenty of data." There are plenty of anecdotes, and plenty of attempts to apply mathematical formulas to anecdotal evidence, but very little actual data. So, for example, Kirby Palm (Bicycle Crank Length Derivation) writes "After much qualitative observation and some informal testing, I have concluded that: The standard crank length of 170mm is optimum for a cyclist with a 31-inch inseam."

"Much qualitative observation and some informal testing"? Why do I suspect he means most bikes come with 170 mm cranks and most people are fine with that, so that must be the right size for average size people? I doubt he spent much time trying cranks that were a whole lot longer, or a whole lot shorter, than 170 mm.

I would like to see the results from experiments using a range of crank arm lengths so wide that there is an actual decline in performance at the extremes. I mean, if there is little measurable difference between 170's and 165's, they should try 150's... and 135's, and 120's, and so on. I would like to see power output, heart rate data, etc, for the full range of possible rider sizes, all of them using a full range of crank arm lengths-- not just the ones commonly available.

I think we can safely assume there is an ideal size crank arm for each rider, and that power or efficiency or something will drop off dramatically when using cranks that are too short or too long. But I have never seen test data that clearly identified what is "too short." I assume that even for tall riders it will be much shorter than 165 mm; but that assumption, along with all the other assumptions, really should be tested.
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Old 03-15-19, 11:03 AM
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Originally Posted by rhm View Post
Thanks for the link! That is an interesting article, and pretty well sums up the available literature.

I think we can safely assume there is an ideal size crank arm for each rider, and that power or efficiency or something will drop off dramatically when using cranks that are too short or too long. But I have never seen test data that clearly identified what is "too short." I assume that even for tall riders it will be much shorter than 165 mm; but that assumption, along with all the other assumptions, really should be tested.
Its pretty well known that if you have short cranks you can increase your cadence, not unlike what happens in a reciprocating engine.

I don't think power is the big deal here though. There have been some studies that show that the power output only varies by a few percent but the internet being what it is, I can't find the studies that I read ten years ago. The big deal is actually joint injury. This probably won't happen on a shorter ride (600km) but if you go on a longer ride its a common problem which shorter cranks do address. That is how I found out that I can't do 175s- I feel it as soon as I get on a bike using them. If I try to go longer distances (500 miles over several days) I get knee problems. Going twice that distance with 170s no problems at all. A person well-versed in this is Mark Stonich of Bikesmith Design- he's been shortening cranks for many years.

My GF is 5'3" and has the shorted frame Trek makes- but amazingly it is equipped with 170s which seems obvious are too long. She's had knee problems for many years and so doesn't do longer rides anymore. I pointed out the issues with longer cranks and she got a shorter set- no more knee issues! I know its anecdotal, but this idea has been around a long time. However, in doing a search I ran across this:
https://www.stevehoggbikefitting.com...gth-which-one/
Which has some very interesting comments, very nicely explained.
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Old 03-15-19, 12:23 PM
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All in all, @Salubrious, I think we agree. My wife, who is much smaller than I, and does not have disproportionately long legs, rides a 650c bike with 170 mm crank arms. Her bike is tiny, really looks comical next to one of my bikes., but it has longer crank arms. I am reluctant to shorten them, because (a) she likes them, and (b) if I shorten them* then I would have to raise her saddle by 17 mm or so, which she does not want.

(*Shortening them is not a problem; I have a 175 mm crank that is otherwise the same as what's on her bike, and I have taps etc for shortening it. I could safely cut the 175 mm crank down to 153 mm or so (or shorter). I've done this several times.)

Let me rephrase something. I subscribe to the principle that the ideal crank length should be proportional to some bodily measurement. I am not convinced it is inseam; maybe it's femur length, I don' t know, I haven't studied this. The reason I reject the usual recommendations is that they are based on an assumption, or the narrow range of crank arms that is readily available, rather than a full range of theoretically possible crank arms.

By "ideal crank arm length" I mean the crank arm length that gives the best compromise between power, efficiency, comfort, wear and tear on the body, or any other factor that might seem relevant. Given that the choice is between 165's and 170's, I think it important to emphasize that there is not much difference between them. I am convinced the shorter of the two is better; but I am not convinced it is shorter enough.

Ten years ago I read an interesting study on the internet, in which the tester had had several different riders try out cranks ranging from 140 to 180 mm, if I recall correctly. I cannot find this article any more. The result was that all riders strongly disliked the shortest crank arms at first but soon became accustomed to them and stopped complaining; but when they went back to longer crank arms, they strongly disliked the longer ones, and it took longer for them to stop complaining. As for power, speed, comfort, etc, there was not much difference among the crank arms tested. Logic would dictate that they repeated the experiment with shorter and even shorter cranks (as well as longer and longer ones), but i don't know if they ever did that.

Mike Burrows and Mark Stonich --both of whom have an interest in recumbent bikes-- are both proponents of shorter cranks, both for recumbent riders and for the rest of us.
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Old 03-24-19, 09:32 PM
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Originally Posted by rhm View Post
Mike Burrows and Mark Stonich --both of whom have an interest in recumbent bikes-- are both proponents of shorter cranks, both for recumbent riders and for the rest of us.
On my recumbents, having smaller cranks do seem to relieve stress on my knees far more than longer cranks do. I was running 155's, now 165s, and I might go back.

But on my road bikes, my 170's seems to be OK. I can usually find a gear combination to allow me to do 95-100 rpms. I might stick to this size simply because most of the brevet routes around SE MI tend to be rolling terrain, and I could use the mechanical advantage going up hills (especially at my weight).
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Old 03-28-19, 06:42 AM
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Yesterday rode with a friend who had just installed 152mm cranks. Had previously been on 170s. He's 5'6" but with distinctly short legs. It was his first ride of the season. Faster than he has ever been before. Not even close. And more comfortable. Completely sure he wants the short cranks on the other bike too. Not a guy who is even concerned with high speed and riding with him is mostly about socializing. But conversation would often be interrupted by realizing he had just slowed down, again, and was way back there. Not yesterday, he was right there the whole time.

My wife is short, 5'3-1/2". Had been on 165s and 170s and maintained there was no difference at all between them. Got her a pair of 150s, mostly to try and solve toe overlap on a bike that was otherwise great. She had adapted to the overlap and said it was not a concern, annoyed me endlessly. Plus we found antique Campy 150s at give-away. First test ride her response was "get these for the other bike too" after maybe one minute. Hills she had given up on due to advancing age are now done gracefully, and on higher gears. No longer a concern about ever lower gearing.

Purely anecdotal. Not sure there is other than anecdote on this issue. I've tried them myself at 5'11", interesting but no sale.
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Old 03-28-19, 06:56 AM
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Originally Posted by 63rickert View Post
Got her a pair of 150s, mostly to try and solve toe overlap on a bike that was otherwise great. She had adapted to the overlap and said it was not a concern, annoyed me endlessly.
Damned if we do, damned if we don't, I guess. Where "do" = "women complaining about toe overlap". Haha. But more seriously, this is the first thread that's made me want to try shorter cranks; I'm 5'5.5" and short-legged (29" inseam men's jeans are about an inch too long), and have been fine on 170-175mm cranks, but am a poor climber and if I could get a few percent of speed from different cranks I'd take it. Perhaps I'll pick up a cheap set of shorter ones and give them a spin on a ride I don't need for PBP.
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Old 03-28-19, 08:11 AM
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Just make sure to lower the seat 5mm when you put the new cranks on, or it's not really a fair trial.

my experience with crank length is that confirmation bias dominates. I usually ride 170mm cranks. I first tried 172.5 cranks decades ago, and thought they slowed me down. 2.5mm is a tiny difference. About that same time, I got some 165mm cranks for my commuter and couldn't really tell the difference. Years later, I bought a set of 172.5 cranks assuming they were 170mm cranks, and I couldn't tell. Then I started riding 175mm cranks on mtb and my gravel bike and couldn't really feel any difference. 5mm is about the difference in seat height where I can tell if the seat is too high or low, so I feel like that's the big difference with cranks for me.

Toe overlap makes a bike designer's life problematic. You can push the seat tube forward and go slack head tube/long rake, or try to talk the person into smaller wheels. And then people discovered that having cleats further back is better. Whoops. I hit my fenders with my foot all the time. It's always a bit of a surprise, but I'm mostly worried about the fenders.

In a bit of a over-broad generalization, I think randonneurs tend to spin too slow. I know I do. And I need to lower my seat and possibly move it forward a bit.

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Old 03-28-19, 08:19 AM
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Originally Posted by unterhausen View Post
Just make sure to lower the seat 5mm when you put the new cranks on, or it's not really a fair trial.
If you're talking about putting shorter cranks on, shorter cranks would require raising the saddle, not lowering it.

It just occurred to me that crank lengths of around 170 mm for adult bikes were settled on back when gear choices were extremely limited and and that they tended to be higher than the gear ranges people generally choose today. Riders needed all the help that they could get with those high gears, so they used crank arms that were at the extreme of what the average person could handle.

Now that there are no practical limits on gearing ranges on our bikes, it makes sense that people are finding that much shorter cranks can work better for them than cranks in the traditional length range.

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Old 03-28-19, 09:00 AM
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Originally Posted by unterhausen View Post
Just make sure to lower the seat 5mm when you put the new cranks on, or it's not really a fair trial.

my experience with crank length is that confirmation bias dominates. ...
Confirmation bias dominates, yes, that's for sure.

But no, if you shorten your crank arm by 5 mm, you will want to raise your saddle by 5 mm, not lower it.

Before I had my Squarebuilt custom frame made, wanting to try 26 x 2.3 Rat Trap Pass tires, I built up an old MTB frame for long distance riding. The bike was designed for 1.75" tires and 175 mm cranks, so when I adjusted the saddle height to account for my fatter tires and shorter cranks, I was so far off the ground that I almost fell over at stops. I know short women are familiar with this sensation, but I hadn't experienced it since I was a little kid riding my brother's bike.
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Old 03-28-19, 10:16 AM
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I have a friend who's 5'9", has dozens of bikes, and rides constantly. When talking about short people and overlap his rejoinder is he rides normal sized bikes and has never had one without overlap.

Yes, raise the saddle. When we first tried the wife with the 150s we had not even thought of that, she asked for saddle up after about a mile down the road. By seat of pants it was a 3 or 4mm adjustment. Having since read or heard the opinions of many experts the answer is 3 to 5mm up. Yesterday my friend with 152s showed up to ride with his seat drastically low and was already certain the short cranks worked. When we moved his saddle up he was amazed. Saddle adjustments in past for him only resulted in different flavors of wrong. Now he adjusts and considers and can diagnose the feedback.

Guesswork is guesswork. If 150 does not work for any Rider X it could be because 160 was the correct answer. Or it could be because another 5mm was absolutely required and Rider X needed 145. If you guess wrong at least you know what does not work. Keep the cranks around and let someone else give them a try.

The readily available crank with choices of 150, 152, 155, 160, 165, 170 is Sugino XD. Not too expensive and good enough for most all. Works with 8, 9, or 10 in back. Making it work with 11, or with 5, would not be hard. Changes in gear ratios do not seem to be required.
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Old 03-28-19, 03:34 PM
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Mark Stonich, who has shortened cranks for many years, maintains that there is no need to alter the gearing with shortened cranks.
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Old 03-28-19, 06:36 PM
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Originally Posted by Salubrious View Post
Mark Stonich, who has shortened cranks for many years, maintains that there is no need to alter the gearing with shortened cranks.
Stonich recently provided my stoker wife with 151mm tandem cranks. She loves the new cranks. Crank formulas say 148mm for her. Her old cranks were 170mm. We can definitely tell the difference. She spins faster and has more endurance. However, her power on the steep stuff where our cadence drops below about 75 is definitely reduced and we're slower there. We might go faster there with lower gears, hard to say. Overall, we're faster over the road on long rides, largely due to her improved endurance. Reducing crank length reduces power at low cadences simply because one can only push down so hard without wearing ourselves out. Pushing down with the same force at the same cadence on shorter cranks will yield less power, duh.

Generalizing from our experience, use the formula to get crank length and use those cranks. We used Steve Hoggs formula of (leg length in inches)*5.48. Don't use overall height formulas or charts - silliness. Then check your gearing on the steepest pitches you are likely to encounter.

I doubt the OP will notice much difference between 170 and 165 - only 3% difference.
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Old 03-28-19, 09:23 PM
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Originally Posted by rhm View Post

But no, if you shorten your crank arm by 5 mm, you will want to raise your saddle by 5 mm, not lower it.
good point
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Old 03-28-19, 09:59 PM
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Hmm, that (inseam)*5.48 formula would put me on 175+ mm cranks even if I round down every chance I get. (I'm only 5'7".) Pass!
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Old 03-29-19, 05:03 AM
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Originally Posted by Carbonfiberboy View Post
Stonich recently provided my stoker wife with 151mm tandem cranks. She loves the new cranks. Crank formulas say 148mm for her. Her old cranks were 170mm. We can definitely tell the difference. She spins faster and has more endurance. However, her power on the steep stuff where our cadence drops below about 75 is definitely reduced and we're slower there. We might go faster there with lower gears, hard to say. Overall, we're faster over the road on long rides, largely due to her improved endurance. Reducing crank length reduces power at low cadences simply because one can only push down so hard without wearing ourselves out. Pushing down with the same force at the same cadence on shorter cranks will yield less power, duh.

Generalizing from our experience, use the formula to get crank length and use those cranks. We used Steve Hoggs formula of (leg length in inches)*5.48. Don't use overall height formulas or charts - silliness. Then check your gearing on the steepest pitches you are likely to encounter.

I doubt the OP will notice much difference between 170 and 165 - only 3% difference.
I'm curious how Hogg arrived at that 5.48 formula, which would put me on 185 mm cranks (ideally, 186.3) That's not a problem for me, since I recognize it as nonsense, but on Bike Forums occasionally we encounter guys who have found Hogg's advice, or Zinn's, and are looking for ridiculously long cranks.
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Old 03-29-19, 05:44 AM
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I don't think I knew there was an extremely long crank advocate other than Zinn. I know someone that is 6'5" and Zinn says he should be riding on 188 cranks. It seems like it would be difficult to get good data backing up those unusual numbers
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Old 03-29-19, 12:15 PM
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Originally Posted by rhm View Post
I'm curious how Hogg arrived at that 5.48 formula, which would put me on 185 mm cranks (ideally, 186.3) That's not a problem for me, since I recognize it as nonsense, but on Bike Forums occasionally we encounter guys who have found Hogg's advice, or Zinn's, and are looking for ridiculously long cranks.
I guess I don't understand the criticism. You don't think cranks should be proportional to leg length or what? I have a 31.5" inseam, so 172.5 cranks by the 5.48 formula, and ride 170 on my singles and 175 on our tandem, both comfortably. You think my wife should ride shorter than 148 cranks? Of course making crank length proportional to leg length immediately leads to non-industry-standard BB heights for tall riders. But why should there be an industry standard BB height for everyone? What sense does that make, since every frame size has different geometry anyway? I ride with a long-legged 6'5" guy who's on 180 cranks, and looks ridiculous with them, like a clown bike and it's a custom bike, too.

There are some interesting articles on proportional crank length, all worth reading, here:
https://www.stevehoggbikefitting.com...gth-which-one/
https://www.trainingpeaks.com/blog/t...-crank-length/
https://www.velonews.com/2003/11/bik...nk-length_5257

Note that while proportional cranks lengths seem to be universally advocated, the formula is very much in question.

Thus if those interested in commenting would be so kind as to do the following, it would be very helpful: Measure your inseam barefoot by standing against a wall, shoving a clipboard up against your pubic bone, and measuring the distance from its top to the floor. Take that number in inches, divide it into your favored crank length in mm and report that ratio!

Thanks.
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Old 03-29-19, 12:36 PM
  #23  
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I feel like having cranks proportional in some way to a person's leg length is a good starting point, but it's part of a larger formula that has yet to be fully mapped out.

Here's my data: my rando bike has a standover height of 32.5", and I can *just* stand over it barefoot, so I figure that gets pretty close to what I'd get with a book and tape measure against the wall. Dividing my preferred crank length of 165mm by that gets me a ratio of 5.08. Perhaps there's something to my femur-to-tibia ratio, or something with my feet that leads me to prefer shorter cranks, I'm not sure.


P.S. Now that you bring up BB height, I wouldn't be surprised if part of the reason for the industry standardizing on 170mm cranks is to fix that aspect of frame manufacture, rather than needing to offer a zillion more frames with varying BB heights. If a bike company were sympathetic to the need for a wider range of crank lengths, perhaps they could scale the BB height with the frame size. Maybe some makers do already...
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Old 03-29-19, 12:55 PM
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As my TrainingPeaks link points out, shorter cranks can be faster seated, but one runs into issues with OOS efforts on very steep pitches and sprinting. where shorter cranks reduce leverage and it's not efficient or sometimes even possible to just pedal faster.
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Old 03-29-19, 01:03 PM
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Originally Posted by Carbonfiberboy View Post
I guess I don't understand the criticism. You don't think cranks should be proportional to leg length or what? I have a 31.5" inseam, so 172.5 cranks by the 5.48 formula, and ride 170 on my singles and 175 on our tandem, both comfortably. You think my wife should ride shorter than 148 cranks? Of course making crank length proportional to leg length immediately leads to non-industry-standard BB heights for tall riders. But why should there be an industry standard BB height for everyone? What sense does that make, since every frame size has different geometry anyway? I ride with a long-legged 6'5" guy who's on 180 cranks, and looks ridiculous with them, like a clown bike and it's a custom bike, too.

There are some interesting articles on proportional crank length, all worth reading, here:
https://www.stevehoggbikefitting.com...gth-which-one/
https://www.trainingpeaks.com/blog/t...-crank-length/
https://www.velonews.com/2003/11/bik...nk-length_5257

Note that while proportional cranks lengths seem to be universally advocated, the formula is very much in question.

Thus if those interested in commenting would be so kind as to do the following, it would be very helpful: Measure your inseam barefoot by standing against a wall, shoving a clipboard up against your pubic bone, and measuring the distance from its top to the floor. Take that number in inches, divide it into your favored crank length in mm and report that ratio!

Thanks.
Starting with your last question, I measure my inseam at 34 inches. Most of my bikes have 165 mm cranks, so if I'm doing this right, I get 4.85.

I won't say 165 is my favored crank. That's just the shortest widely available crank. I often ride old bikes with old gear on them, and it's rare to find anything shorter than 165 (I do have 160's on three bikes, two of them fixies). I have used 152's, 145's, and 140's in the past, and my only complaint was that they made my other bikes feel funny with their long 165 mm cranks.

I totally agree, in theory, that one could determine a formula by which we could specify a crank arm length that's statistically most likely to be "ideal" for a person based on some bodily measurement. And I agree that inseam is a good place to start.

But I'm not going to tell anyone he or she should use shorter cranks. In most cases I doubt it makes much difference. You ride cranks that are proportionally a lot longer than mine, and you are faster than I, but I don't think your longer cranks are the reason you're faster than I. So should you ride something other than what you like? No, I don't think so. If you were a novice, I'd recommend shorter cranks for you, but you have found something that works for you, and I don't see any reason to change that. On the other hand, I'm pretty confident you could switch your wife's crank onto your bike and you'd get used to them in a short time and get to like them just fine. But it's a tandem stoker crank, and the pedal threads will be backward, so never mind that.

I do believe that riders would in general be a little better off using shorter cranks, and I know it's not going to happen. It doesn't make a big enough difference. It wouldn't make economic sense for factories to make a full range of possible crank arm lengths, frames with a variety of bottom bracket heights, and all the variables.

A few posts back I pointed out that people of all sizes are able to go up and down the same flight of stairs (typical stair rise around 175 mm) without complaining. There's a certain socialism in that -- stairs have to be designed to work for all of society. So the same applies to CitiBike and other bike share programs. There's good reason to stick with 170 mm cranks by default. I just don't consider them ideal.

Originally Posted by Carbonfiberboy View Post
As my TrainingPeaks link points out, shorter cranks can be faster seated, but one runs into issues with OOS efforts on very steep pitches and sprinting. where shorter cranks reduce leverage and it's not efficient or sometimes even possible to just pedal faster.
Yes, all true. You often mention that it's important to spend some time out of the saddle on all rides (I hope I'm not confusing you with someone else), and it's good advice. Shorter cranks do tend to discourage the rider from getting out of the saddle, and that's not a good thing. Anyone investigating 'ideal crank arm length' would need to keep this in mind. Whatever the ideal crank arm length is, it's going to be a compromise.
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