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Scary Teddy Bear
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Okay, this may be a dumb question, but here goes, I recently purchased a cyclocross bike as I would like to do some races this coming fall. I went a size down with the cross bike, and as a result, I have 172.5mm cranks on the cross bike, and 175mm cranks on my road bike. My road bike has Sora/Tiagra which I would like to upgrade...it would be better to go to 172.5mm on the road bike too, correct? I can't imagine that pedaling two different crank lengths would be good. I ride a 58cm road bike, and I bought a 56cm Cross bike. What do you guys think?....Kerry, any thoughts?
 

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What is your riding style and inseam...

physasst said:
Okay, this may be a dumb question, but here goes, I recently purchased a cyclocross bike as I would like to do some races this coming fall. I went a size down with the cross bike, and as a result, I have 172.5mm cranks on the cross bike, and 175mm cranks on my road bike. My road bike has Sora/Tiagra which I would like to upgrade...it would be better to go to 172.5mm on the road bike too, correct? I can't imagine that pedaling two different crank lengths would be good. I ride a 58cm road bike, and I bought a 56cm Cross bike. What do you guys think?....Kerry, any thoughts?
I agree, your postion should be the same on both bikes. Your 'cross may have more seatpost showing and a longer stem, but the seat height, your reach and crank length should be the same.

I recently purchased a new road bike and have some knee issues, so I did a little research on crank lengths. A couple of things that my help you are:

1. The general rule of thumb is that your optimum crank length should be 21% of your inseam (or as close as possible). For instance, my inseam is 31", which equates to 790mm and 21% of that is 165.9. My crank length is 170.

2. The other thing to consider is the way YOU ride. Do you spin at a high cadence or low.
If you spin at a high cadence a shorter crank is a better choice. If you grind it out at a lower cadence say 60 to 80 (or climb a lot) a longer may suit you better.

B/c of my research, I switched from a standard 53/39 172.5 crank to a compact 170 and my knees are already very happy. The shorter crank forces me to spin more, which is improving my form.
 

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physasst said:
I can't imagine that pedaling two different crank lengths would be good. I ride a 58cm road bike, and I bought a 56cm Cross bike. What do you guys think?....Kerry, any thoughts?
I think it doesn't matter one bit. When people have looked at the issue of crank length they have been unable to find any meaningful difference in power output, and typically they compare like 165s to 180s. What's nice about short cranks in cross is they give you more clearance to ride off-camber stuff and pedal thru turns.
 

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Dwayne's right

chrispf007 said:
IThe general rule of thumb is that your optimum crank length should be 21% of your inseam (or as close as possible). For instance, my inseam is 31", which equates to 790mm and 21% of that is 165.9. My crank length is 170.

B/c of my research, I switched from a standard 53/39 172.5 crank to a compact 170 and my knees are already very happy. The shorter crank forces me to spin more, which is improving my form.
I'm afraid you didn't do very good research. Dwayne Barry's summary is spot on. Here is an accurate summary of the research on crank length:

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. Don't you just hate it when the data doesn't support the theory? 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 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. Do you think that we have standardized on this narrow range because of some sort of global conspiracy, or because well over 100 years of experience (and testing the limits) have repeated shown that the 170-180 mm is really what works for human beings?

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. Probably the best work done on this VERY difficult to research topic was by Lennard Zinn. He unintentionally showed how our adaptability was more important than our size or riding style. He's sure that the results of his study are wrong, but he just can't seem to find any data to support his pre-conceptions.

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.

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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.

All this said, many time trialists use crankarms 2.5 mm longer than those on their normal road bike. Because 2.5 mm (one-tenth of an inch) isn't much, it rarely causes an injury. But the jury is still out on whether that bit of extra length actually improves performance.
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Scary Teddy Bear
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Okay...

Kerry Irons said:
I'm afraid you didn't do very good research. Dwayne Barry's summary is spot on. Here is an accurate summary of the research on crank length:

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. Don't you just hate it when the data doesn't support the theory? 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 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. Do you think that we have standardized on this narrow range because of some sort of global conspiracy, or because well over 100 years of experience (and testing the limits) have repeated shown that the 170-180 mm is really what works for human beings?

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. Probably the best work done on this VERY difficult to research topic was by Lennard Zinn. He unintentionally showed how our adaptability was more important than our size or riding style. He's sure that the results of his study are wrong, but he just can't seem to find any data to support his pre-conceptions.

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.

=============================
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.

All this said, many time trialists use crankarms 2.5 mm longer than those on their normal road bike. Because 2.5 mm (one-tenth of an inch) isn't much, it rarely causes an injury. But the jury is still out on whether that bit of extra length actually improves performance.
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So I should try to match Crankarm length? I noticed that the cross bike pedals much easier, but I didn't know if that was the result of a compact chainring setup, crankarm length, or better components....I think I'll change to 172.5 on my road bike....
 

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2.5 mm isn't much. Historically, some riders have changed as much as 5mm depending on the event they are riding. I'd say save your money, but if you really want to replicate the same crank length go ahead. As stated before, it's highly adaptive. I used to ride all my time trials in 165mm so I could turn a bigger gear, all my road stuff in 170mm, but these days I have 170mm on everything since I don't do tt's anymore.
 

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Kenny, Thanks for the summaries. When doing my research, I focused on knee issues and found more than a few references to the "21%" measurement being recommended. It seemed like a logical theory, so I decided to test it for myself as I was already having my shop swap the standard size cranks for compacts so making the switch to 170's seemed no big deal. I then only posted b/c whether there was any real or simply preceived benefit is unknown. However, I was able for feel the difference in my spin - and it wasn't simply moving to the compact crank as the gear ratio was the same (or VERY close).

You do seem to have more information, but in this case the 21% formula seems to have worked for me.
 

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A concept

chrispf007 said:
the 21% formula seems to have worked for me.
Even if your clock is not working, it is right twice a day :) The problem with these formulas is not for "normal height" people but that they end up recommending 150 mm cranks for short-legged folks and 200 mm cranks for tall people. It's pretty shakey to to assume that you could use a 2 significant figure formula, multiply it by a number that is also probably 2 significant figures (3 at best), and come up with a 4 significant figure answer (172.5 vs 175.0).
 

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Kerry Irons said:
Even if your clock is not working, it is right twice a day :) The problem with these formulas is not for "normal height" people but that they end up recommending 150 mm cranks for short-legged folks and 200 mm cranks for tall people. It's pretty shakey to to assume that you could use a 2 significant figure formula, multiply it by a number that is also probably 2 significant figures (3 at best), and come up with a 4 significant figure answer (172.5 vs 175.0).
These formulas have quite simple and, likely, reasonable background - crank length proportional to leg length allow the same angles while pedaling for peoples with different leg length.

It have been shown many times for different sport activities (if not for this specfic one) that angle ranges are of some (sometimes, utmost) importancy. So I tend to assume that pedaling not differs so much of other activities till it not clearly proved (not stated!) otherwise.

I myself have not experimented with crank length but my son (who is 6'5") has gained a lot from switching to 180mm cranks - so much that I even have not belived him at first.
 
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