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Discussion Starter · #1 ·
Ive been training and racing for about 4-5 years now and its been a roller coaster of trying to find my optimal position (like lots of people I guess),

anyway one thing that has stayed constant in all of that time is my crank length 172.5. Now Ive decided to go over to compact cranks (no....nobody try to talk me out of that I have my reasons :cool: , namely 5000m climbing in one day race on a gruelling tour stage).

But over those years Ive heard alot about changing crank length in larger steps ie, 5-7.5mm like form a 170-175 or 177.5 but not so much about the 2.5mil difference of changing from one to another. I want to change over from 172.5 to 175's as its worth an extra 5watts (everything else being equal) and more leverage on these long steep mountainous climbs I have in mind.

Ok so its a bigger deal changing from say a 170-177.5........but

Bearing in mind that Im finally close to 'dilaing in' my position on the bike right now close to optimum :) , should I try and make a change to a 2.5mm longer cranks?? considering a 5 watt increase and slight decrease in cadence is usefull to me???




 

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5 watt increase???

Who told you that?

Power = torque times cadence. The extra length gives you an increase in leverage of only 1.4% (175/172.5) so it's pretty trivial, if you consider that a typical 1-cog change is in the 8-11% range.

Whether the longer crank will produce more power is questionable.

All that said, I;ve been running 172.5's for about 7 years and I just bought a new 175 crank to try, just for the heck of it, but have yet to install them. I don't expect any change that I can measure.
 

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Discussion Starter · #3 ·
Who told you that? C-40

Well I looked at a table at the analyticcycling.com site heres the link: http://www.analyticcycling.com/PedalOpCrankLength_Page.html

if you work it out its 543 watts @172.5mm
.............................555 watts @175mm

That = 2% increase in power everything else being equal except the crank length change

then I looked at my steady state climbing power at least 240watts

2% of 240watts = 4.8watts ...:)
 

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Leverage and torque

I have switched between 170 and 172.5's without much difference. Of course once you get to 175 there might be a noticeable change. I don't know about the wattage, but do know that the increased leverage you get with the longer crank comes at the expense of increased torque on the knee joint. So with all that climbing I would change them back at the first sign of any knee problems if you do decide to go with the 175's.
 

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questionable...

That table only applies to one hypothetical person. The measuremets required to calculate "optimal crank length" are very difficult to take with enough accuracy to produce a valid result.

If it was true that longer cranks always produce more power, then why aren't all the pros using 180mm or longer? Surely they all want the most power.

In reality, the longer crank may or may not be an improvement and the the change is likely to occur over a period of time, as you get used to the longer length.

There's also a debate over how to adjust the saddle for longer cranks. Do you move the saddle forward to maintain the same KOP, do you lower the saddle to maintain the same maximum leg extension, or do you leave it at the same height to split the difference between the top and bottom of the stroke? There are no definite answwers.
 

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2.5 mm probably won't be noticable. I made a bigger change: 175 to 185. THAT was noticable. But then I'm 6'4" and mostly leg.

I don't think that it is as simple as a linear relationship between crank length and watts. Did you see all of the joint fit point options on that analticalcycling page? The real reason to change crank length is to get the correct range of motion for your legs.

I switched because when I tried to sprint or jump with 175s it felt like I was taking little bitty baby steps, and I was constantly getting dropped anytime the pack surged because I was so inefficient during these efforts. The 185's feel much better anytime I'm out of the saddle, including steep climbs. I don't think they changed my power at all when sitting and spinning. In fact, it took a good couple of weeks of focused training to be able to spin at the same cadence as with the 175s.

Would increasing crank lenght help you? It depends. It could hurt (your knees or your power) if you get outside of your natural range of motion.

So, if you're really serious about switching your crank length, go get a good bike fit and tell the fitter about your plans. Most of the good fit bikes have adjustable cranks. Probably the cheapest, fastest and safest way to figure it out.
 

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From experience..

I can't tell that much difference between a 172.5 and a 175. A 170 between a 175 and I can tell...I like 175's cause that just what I'm used to using....no rational explanation. I just go by what works..I read that Jan Ulrich uses a 177...
 

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M__E said:
Who told you that? C-40

Well I looked at a table at the analyticcycling.com site heres the link: http://www.analyticcycling.com/PedalOpCrankLength_Page.html

if you work it out its 543 watts @172.5mm
.............................555 watts @175mm

That = 2% increase in power everything else being equal except the crank length change

then I looked at my steady state climbing power at least 240watts

2% of 240watts = 4.8watts ...:)
Where does this 5 watts come from? Do you think it's magic? The power comes from your legs and you only have so much. If you are going to turn a bigger circle with the longer crank, then you have to turn it slower. Power to the road is the same. - TF
 

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TurboTurtle said:
Where does this 5 watts come from? Do you think it's magic? The power comes from your legs and you only have so much. If you are going to turn a bigger circle with the longer crank, then you have to turn it slower. Power to the road is the same. - TF

Mostly. The exception would be if the rider's efficiency is decreased because of his crank size, but I don't know that optimizing efficiency via crank length change would net such a large power increase. No, such an increase would be make me pretty damned skeptical.

FWIW, I noticed a difference when I switched from 172.5 to 175, BUT there were prolly several factors that caused this:

--leg length discrpency
--long legs.

I felt "confined" on 172.5's, but on 175's I just feel comfortable. It wasn't a huge change....more like something that wasn't quite right stopped being not quite right. You know kinda like when you realize the coconuts are hangin' on the wrong side of the branch.
 

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alienator said:
Mostly. The exception would be if the rider's efficiency is decreased because of his crank size, but I don't know that optimizing efficiency via crank length change would net such a large power increase. No, such an increase would be make me pretty damned skeptical.

FWIW, I noticed a difference when I switched from 172.5 to 175, BUT there were prolly several factors that caused this:

--leg length discrpency
--long legs.

I felt "confined" on 172.5's, but on 175's I just feel comfortable. It wasn't a huge change....more like something that wasn't quite right stopped being not quite right. You know kinda like when you realize the coconuts are hangin' on the wrong side of the branch.
I'm the non-perceiving type that cannot tell the difference between my 165s and 175s except in corners (and that probably isn't real). - TF
 

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Discussion Starter · #11 ·
Here's some of my reasoning...on some points raised

C-40 said:
That table only applies to one hypothetical person. The measuremets required to calculate "optimal crank length" are very difficult to take with enough accuracy to produce a valid result.
I agree with you there...but I was trying to make the question easy by using that hypothetical rider to illustrate a point, Like a lab test..ie cut the variables and measure one thing the crank length. I never took these measurements from myself as you say too DAMN HARD to get acurately, so I did the next best thing just looked at their base case table on that page, and saw the watts at 172.5mm -> 543 watts at 175mm 555 and worket out the rest from there...then I said to myself " If I was that rider what would a 2% increase mean to me when I used my power values in place and the answer was 4.8watts rounded to 5 watts!

C-40 said:
If it was true that longer cranks always produce more power, then why aren't all the pros using 180mm or longer? Surely they all want the most power.
....Longer cranks within the limits of their range of motion for that specific rider, so obviously a really short guy wont have huge cranks cause he'd be outside his limits of whats right for him so...

....also I know Lance uses 175's for road and 172.5's for Time Trial stages, he's 2 inches shorter than I am and he's on 175's?? true I dont know his leg length but I wouldn't imagine it'd be proportionatly longer than mine given Im 2 inches shorter than him and my legs are not exactly short....

Turbo Turtle said:
Where does this 5 watts come from? Do you think it's magic? The power comes from your legs and you only have so much. If you are going to turn a bigger circle with the longer crank, then you have to turn it slower. Power to the road is the same
...yeah turn it slower for the SAME POWER!!! .....well if you turn it the same speed (cadence) the leverage is greater with the longer crank so therefore the potential for more power is available ie the 5 watts....more power for same effort at same cadence is the theory...;)

You have looked at the link ye? nothing here will make much sense without that!

and Thanks for everyones input so far!
 

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Discussion Starter · #12 ·
Heres some of my reasoning on some points already raised....

C-40 said:
That table only applies to one hypothetical person. The measuremets required to calculate "optimal crank length" are very difficult to take with enough accuracy to produce a valid result.
I agree with you there...but I was trying to make the question easy by using that hypothetical rider to illustrate a point, Like a lab test..ie cut the variables and measure one thing the crank length. I never took these measurements from myself as you say too DAMN HARD to get acurately, so I did the next best thing just looked at their base case table on that page, and saw the watts at 172.5mm -> 543 watts at 175mm 555 and worket out the rest from there...then I said to myself " If I was that rider what would a 2% increase mean to me when I used my power values in place and the answer was 4.8watts rounded to 5 watts!

C-40 said:
If it was true that longer cranks always produce more power, then why aren't all the pros using 180mm or longer? Surely they all want the most power.
....Longer cranks within the limits of their range of motion for that specific rider, so obviously a really short guy wont have huge cranks cause he'd be outside his limits of whats right for him so...

....also I know Lance uses 175's for road and 172.5's for Time Trial stages, he's 2 inches shorter than I am and he's on 175's?? true I dont know his leg length but I wouldn't imagine it'd be proportionatly longer than mine given Im 2 inches shorter than him and my legs are not exactly short....

Turbo Turtle said:
Where does this 5 watts come from? Do you think it's magic? The power comes from your legs and you only have so much. If you are going to turn a bigger circle with the longer crank, then you have to turn it slower. Power to the road is the same
...yeah turn it slower for the SAME POWER!!! .....well if you turn it the same speed (cadence) the leverage is greater with the longer crank so therefore the potential for more power is available ie the 5 watts....more power for same effort at same cadence is the theory...;)

You have looked at the link ye? nothing here will make much sense without that!

and Thanks for everyones input so far!
 

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M__E said:
....Longer cranks within the limits of their range of motion for that specific rider, so obviously a really short guy wont have huge cranks cause he'd be outside his limits of whats right for him so...

....also I know Lance uses 175's for road and 172.5's for Time Trial stages, he's 2 inches shorter than I am and he's on 175's?? true I dont know his leg length but I wouldn't imagine it'd be proportionatly longer than mine given Im 2 inches shorter than him and my legs are not exactly short....
You can't determine the suitability of a given crank size by measuring someone's body. If that were the case, there'd be zero contention about which crank sizing method is correct. Given that, the size crank that LA uses tells us nothing, other than he uses shorter cranks for TT's so he can spin more easily (prolly).

M__E said:
...yeah turn it slower for the SAME POWER!!! .....well if you turn it the same speed (cadence) the leverage is greater with the longer crank so therefore the potential for more power is available ie the 5 watts....more power for same effort at same cadence is the theory...;)

You have looked at the link ye? nothing here will make much sense without that!

and Thanks for everyones input so far!
There's a flaw in your reasoning. If you are putting out X watts with a Y length crank at Z rpms, if you want to turn (Y + delta Y) length cranks at Z rpms, you have to increase your power output. The crank cannot and does not increase your power output. If X is already your max power output, then you will NOT be able to spin the (Y+ delta Y) length cranks at Z rpms. It's that simple. The ONLY exception to this would be if you've some physiological circumstance which limits your power output w/ a given crank size because you cannot assume the most efficient biomechanical position. Again, in this case, the crank is not creating power. You just changed the geometry of the machine so that the machine could operate in an optimal way.
 
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