170mm cranks vs 175mm...

I switched to 165mm cranks from 175 on my medium frame trail bike, mostly for knee comfort. I have shorter legs so the longer crank arms tended to force my knee to go through more extreme angles that caused discomfort, especially when I shifted forward on my saddle. The pedal strike reduction was just a bonus. I have been riding like that for over a year and have no regrets. I find it ridiculous that a bike frame sized for someone who is 5’ 8” can come with the same length crank arms as one meant for someone who is 6’ 2”.
 
Patrick said:
shorter is a disadvantage, but can be made up with the chainring (or any other gear). It is a class 2 lever, ratio of the arm to length to the radius of the chainring.

i've been thinking about this a bit more - probably need to talk to the bike fit gurus - there has to be an optimum high and low position (or bend angle?) for the knee.
Might be different for each person? does that mean some sort of ftp test to optimize?
I'd think i'd opt for more power/efficiency, and better form/technique, than just shortening the crank arms to avoid strikes.
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This actually super false. Hunter Allen did testing with Olympic Track athletes to see of crank length had any effect on wattage output. His test ranged from 100mm to 200mm arm lengths and there was zero watt difference.

The only reason to change lengths is for fit. And that's only on a road bike. The top triathletes are all running 155mm cranks in order to drop the bars lower and have a tighter pedal circle.
 
I'd go 170 over 175 on a modern FS bike for sure. The worst crash I have had to date was due to a pedal strike. I'm not going to render my rear suspension (that I paid a bunch of $$ for) less effective to raise my BB.
 
Patrick said:
shorter is a disadvantage, but can be made up with the chainring (or any other gear). It is a class 2 lever, ratio of the arm to length to the radius of the chainring.

i've been thinking about this a bit more - probably need to talk to the bike fit gurus - there has to be an optimum high and low position (or bend angle?) for the knee.
Might be different for each person? does that mean some sort of ftp test to optimize?
I'd think i'd opt for more power/efficiency, and better form/technique, than just shortening the crank arms to avoid strikes.
Click to expand...
It appears you are looking for something to read:

https://www.stevehoggbikefitting.com/bikefit/2011/06/crank-length-which-one/

Note, I dont endorse anything said in this article as I just skimmed it. Any of us are far from crank length making a major area for potential performance/efficiency improvement. From a math standpoint, it makes no sense that crank arm length doesn't change the mechanical advantage. EVER other situation with a lever, the length of the lever makes a difference. I think the issues comes from the other levers involved, femur, foot, etc, and trying to isolate the slightly different muscles you are using with the different length.
 
Why do people think reducing 5 mm will magically make pedal strikes disappear? Sounds silly. Also out of curiosity I just measured the crank arms on my horsethief vs blur. Both are 175 yet even with significant wear on the horsethief crank ends vs brand new pristine blur, they are still about 6mm longer. ymmv
 
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rick81721 said:
Why do people think reducing 5 mm will magically make pedal strikes disappear? Sounds silly. Also out of curiosity I just measured the crank arms on my horsethief vs blur. Both are 175 yet even with significant wear on the horsethief crank ends vs brand new pristine blur, they are still about 6mm longer. ymmv
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People will look for anything to blame outside of themselves.
 
I don't have to read a study. If it can be measured and there is movement involved, then one measurement is going to produce different results than the other. It might get lost in the noise in a small sample, but it is there.

That doesnt mean optimum, and it doesn't mean maximum. Should it be power, efficiency, or the trade-off between them for the activity? Should it be bio-mechanical efficiency or perhaps confidence?

I reject that it doesn't matter at all.
I would consider that it matters very little for a large part of the population.
I would bet the house that it matters very much for a certain group.
 
jimvreeland said:
This actually super false. Hunter Allen did testing with Olympic Track athletes to see of crank length had any effect on wattage output. His test ranged from 100mm to 200mm arm lengths and there was zero watt difference.
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This is overall avg watts?
Would be curious for the max peak
 
I think the ratio of length to teeth is significant. Shorter length should have less teeth on the chain ring.
 
JerseyPete said:
I think the ratio of length to teeth is significant. Shorter length should have less teeth on the chain ring.
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It can be made up anywhere in the system - shift cassette, diameter of rear wheel, height of tire sidewall. My statement is that just changing the arm length is a disadvantage given everything else is equal. And by equal, I mean climbing in the granny gear!!!

It is more about bio-mechanical efficiency than raw mechanical advantage. Why am I willing to tweak the seat height a couple mm to make a difference but not the crank length?
 
jimvreeland said:
This actually super false. Hunter Allen did testing with Olympic Track athletes to see of crank length had any effect on wattage output. His test ranged from 100mm to 200mm arm lengths and there was zero watt difference.

The only reason to change lengths is for fit. And that's only on a road bike. The top triathletes are all running 155mm cranks in order to drop the bars lower and have a tighter pedal circle.
Click to expand...

I don't know how the produced wattage would be measured in that test but in terms of required force changing the leveraging ratio must make a difference. Think breaker bar. Or Galileo Galilei https://exhibits.museogalileo.it/ar...onstrationLawLeverFormulatedByArchimedes.html
 
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