Being nitpicky with terms of physics, a shorter crank will require more force on the pedal for the same cadence and the same gearing, but torque will be the same. Theoretically, you should be riding at a slightly higher cadence / lower gear with shorter cranks.

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Yep, as Jason says, you may have your terms mixed up.

Power = rotational speed x torque

If you are talking about fixed cadence then by definition you are talking about the same torque to produce the same power.

Torque = force x radius at which the force is applied

So, if you reduce the length of the crank then the force must increase in order to maintain the torque.

As Jason alludes to: it might make sense to raise the cadence with shorter cranks to keep the pedal force lower. However, the pedal speed will not actually increase, because the cadence will increase in proportion to the reduction in the circumference of the pedal stroke.

Not necessarily. I found that with a shorter crank my pedal action had less of a dead spot and you put maybe the same amount or torque but for a greater degree of pedal rotation. With shorter cranks my power didn't increase but it felt like a nicer pedal action with no hip impingement issues and I can maintain power longer over an ironman. I would race and train on the same length as you are training muscle memory. That being said I use 170 on my road and 160 on my tri bike but you are in completely different positions. Below 170 feels too short on my road bike and 160 feels best on the tri bike.

Sorry for my mismatch.
The reason I was principally asking is because I already use short cranks (155) but have broken one, so need to use a new set (150) on one of my bikes, so was thinking of having it on the turbo/training bike with a slight increase in force required for the same cadence, maybe with some small training effect carry over, or no loss at all!

Unless the physic in your world is different than for the rest of us, it is "necessarily" true.

That being said, the differences in moving a lever that is 7.5 mm shorter - a 4% change in overall length - is going to be tough to measure when the human body is providing the force, and the change in fit and mechanical action are going to mask it anyway.

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"...the street finds its own uses for things"

Peak forces could be lower, in theory, if you can maintain a higher average force over a greater arc.

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Swimming Workout of the Day:

Favourite Swim Sets:

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Maybe you should have read more than not necessarily?

Unless you pedal putting exact same power for 360 degrees of your pedal stroke your philosophy would be true yes but I found I have more dead spots in my pedal action with a closed off hip angle so I engage for more degrees of a circle putting out the same force but for a full revolution you average the same power with shorter cranks. The other finding in studies is your leg as it goes through a dead spot and is essentially negative power counteracting positive power on the other pedal and with shorter cranks you get less restriction so average more power for more degrees of your pedal stroke without increasing force on the pedal.

Do some research and study physics...

I guess my degree in Applied Mathematics doesn't count?

The original question is as simple as a teeter-totter. Where very complex mathematics comes into play is if you assume the force is applied evenly through the 360 degree arc. That was my "human" machine comment. Not possible, and I would guess that the vast majority of us put the majority of the force between 1 and 5 o'clock to the point that the teeter-totter example applies, and your "egghead" evaluation fails.

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"...the street finds its own uses for things"

It doesn't count if you're using it wrong!

Don't try playng the authority card quite so easily. There's more than a few engineers, physicists, mathematicians and other well versed forum members around here. Applied maths isn't necessarily going to impress anyone.

That was my response to "learn physics." Someone else drew first blood.

What exactly am I using wrong? It's a simple question with a simple answer - the length of the crank, in the range we use, doesn't make a measurable different in force application in the human machine.

10 years ago I was having the same argument with the "wheel weight is more important" crowd. Are going go there again also?

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"...the street finds its own uses for things"

If you are simple and don’t get it that’s cool. I’m no mathematician but I have done my research and trialed down to 150mm cranks on my tt settling on 160. I’m just sharing what I found.