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I have recently perchased a new carbon bike(Kuota Khan),it is a lightweight bike about 7kg-15.5Lbs with 170mm record carbon cranks ,my other bike is a Specialized Allez with 172.5mm cranks aprox 9kg-20lbs.I have noticed that the Specialized seems easier to hold in the large chain ring,does anyone know if this could be caused by the shorter cranks or by an instillation problem(bb chain lenth)etc any help would be great.:
 

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NeoRetroGrouch
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markrider said:
I have recently perchased a new carbon bike(Kuota Khan),it is a lightweight bike about 7kg-15.5Lbs with 170mm record carbon cranks ,my other bike is a Specialized Allez with 172.5mm cranks aprox 9kg-20lbs.I have noticed that the Specialized seems easier to hold in the large chain ring,does anyone know if this could be caused by the shorter cranks or by an instillation problem(bb chain lenth)etc any help would be great.:
What do you mean by easier to 'hold' in the large ring? Easier to pedal so you don't have to shift down to the small ring? That will depend on your cadence and cog size. - TF
 

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NeoRetroGrouch
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sdnomis88 said:
with longer cranks you'll get more mechanical advantage, leverage, so it should be easier to turn a larger chainring. shorter cranks just mean your legs don't have to travel as far.
"with longer cranks you'll get more mechanical advantage, leverage, so it should be easier to turn a larger chainring."...slower. You won't get faster so the cadence has to be lower. You can get an equal boost in mechanical advantage by shifting. - TF
 

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with a longer crank more force can be exerted on the chainring...if your in the same gear ratio with two diffrent bikes, one with short cranks the other with longer ones, the one with longer cranks is going to be easier to pedal...if your legs can get around the longer rotation. I've heard that there is a "sweet spot" for how long your cranks should be, do a search and you should come up with something. I (6' 155lb) found out that mine should be 175mm.
 

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Once many bikes ago, I didn't notice that it came with 170mm crank arms. I had a hard time "holding" a gear as you try to describe. I know the feeling. Eventually upgraded to 172.5 and noticed an improvement in my ability to keep higher speeds. It also seemed to help me on steep climbs. The extra leverage made a difference.

I've got very short legs and "should" be running short cranks, but my knees aren't complaining after 1000's of miles, so sticking with the longer ones. On my mtb, I run 175 and have no problem there either.

My completely unscientific reasoning here is like that of using a 1/2" breaker bar vs. a small wrench....lots more leverage on the longer bar as long as you can turn it.

Mark
 

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NeoRetroGrouch
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Mark16q said:
Once many bikes ago, I didn't notice that it came with 170mm crank arms. I had a hard time "holding" a gear as you try to describe. I know the feeling. Eventually upgraded to 172.5 and noticed an improvement in my ability to keep higher speeds. It also seemed to help me on steep climbs. The extra leverage made a difference.

I've got very short legs and "should" be running short cranks, but my knees aren't complaining after 1000's of miles, so sticking with the longer ones. On my mtb, I run 175 and have no problem there either.

My completely unscientific reasoning here is like that of using a 1/2" breaker bar vs. a small wrench....lots more leverage on the longer bar as long as you can turn it.

Mark
What it may actually be doing is slowing your cadence to where you may (temporarily) be able to put out more power. - TF
 

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TurboTurtle said:
What it may actually be doing is slowing your cadence to where you may (temporarily) be able to put out more power. - TF
I'm no scientist, so can't question that scientifically, but am curious....assuming I can't spin any faster with shorter crank, and don't slow down with a longer crank, wouldn't the increased leverage of a longer crank give more power??

mg
 

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NeoRetroGrouch
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Mark16q said:
I'm no scientist, so can't question that scientifically, but am curious....assuming I can't spin any faster with shorter crank, and don't slow down with a longer crank, wouldn't the increased leverage of a longer crank give more power??

mg
No, if you apply the same cadence with the same gears you end up with the same speed. The same speed means the power to the rear wheel is the same.

You will need less force on the pedal to get the same cadence, but you just have to push with that force farther because circle it travels through is bigger. They cancel.

TF
 

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markrider said:
I have recently perchased a new carbon bike(Kuota Khan),it is a lightweight bike about 7kg-15.5Lbs with 170mm record carbon cranks ,my other bike is a Specialized Allez with 172.5mm cranks aprox 9kg-20lbs.I have noticed that the Specialized seems easier to hold in the large chain ring,does anyone know if this could be caused by the shorter cranks or by an instillation problem(bb chain lenth)etc any help would be great.:
Longer cranks is like lowering the gearing slightly. Sheldon Brown goes into this in detail as the reason why you should go with his "gain ratio" system for calculating gearing. http://www.sheldonbrown.com/gain.html Run this calculator and you can see the difference in gearing on your two bikes.

But the real answer to your question is that the crank length should be dicated by your physiology, ie, how the length of your legs and joints interact with the pedaling circle. Remember, you've got 20 gears on that bike so if you need a lower or higher gear you can change the gear pretty easily with the flick of your wrist.
 

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Drone 5200 said:
But the real answer to your question is that the crank length should be dicated by your physiology, ie, how the length of your legs and joints interact with the pedaling circle. Remember, you've got 20 gears on that bike so if you need a lower or higher gear you can change the gear pretty easily with the flick of your wrist.
Physiology definately plays a big part. I am very flexible therefore at the top of the pedal stroke I am not limted by my physiology when using longer cranks. As my leg comes up higher or at the top of the pedal stroke I don't hit a limit where my legs might feel like they are bent more than I am capable of doing comfortably. (175 vs 170). So in some ways, phsiologicaly speaking, it doesn't make a difference which crank I use.

I prefer 175's though I have short legs and am only 5'8". Some might see see this as too long for my height/inseam but, being flexible it isn't.

I am also less of a spinner and like the slight more leverage of a 175 vs 172.5 vs 170.
 
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