Crank Length

[cyclespeed]

Would you agree or disagree with the following statements; (I have put my opinion below each one).

1.) Bicycle A with a 180mm crank provides more torque at the BB axle than Bicycle B with a 165mm crank for the same force.

TRUE

2.) Bicycle B can counter this torque deficit (i.e. increase torque) by changing up a gear on the cassette, by dropping to the small chainring, or by using smaller wheels.

TRUE

3.) Bicycle A will move the exact same distance as Bicycle B for 1 full crank revolution. (assuming all else equal).

TRUE

4.) Bicycle B will have to push slightly harder (apply more force) to make that revolution, whereas Bicycle A will scribe a larger pedal circle.

TRUE

5.) Bicycle Rider B may benefit* over Bicycle Rider A, due to

a) less leg compression to chest = less strain
b) lower torso height for better aero
c) better ground clearance
d) easier to maintain high cadences.

TRUE

*depending on height, inseam and morphology of rider

[Raineman]

Whilst all the facts and opinions you have stated are true in the situation defined there are other factors to consider.

Moving to shorter cranks reduces the distance the foot has to travel per pedal revolution so for the same foot speed a higher cadence can be achieved. Power = Torque x Cadence so the higher cadence actually allows for a lower torque to be applied.

Since torque = force x crank length and cadence = foot speed/(2 pi x radius) and the crank length = radius. Therefore Power = force x foot speed/2 pi. So the force applied is independent of crank length for a given power and foot speed.

However with shorter cranks each pedal revolution happens faster so the force (actually total effective force as described above) is applied more often for a shorter duration which may put less stress on the muscular system allowing for better endurance (I'm not a physio though).

For most people their cadence is limited by how fast they can move their foot over any other factor which is why I have used it over cadence. This is shown by most people having a higher cadence on shorter cranks.

[kulivontot]

Summary of literature i have read has concluded that crank length isn't that critical except for the purposes of aerodynamics, crit pedal striking, and for very tall individuals. There are some slow twitch people who claim some advantage for triathlon, but I think it has some relationship with aerobar positioning and pushing the saddle as far forward as possible. somebody with more knowledge on the matter can elaborate further

[RyanH]

What are the aerodynamic considerations for crank length?

[perwjensen]

I switched from Ultra 172.5mm to SRAM 175mm and was able to cut two minutes off my usual ride. I have never been under 54min and now it's 52min. My inseam is 34", so going longer helped me.

[Raineman]

What are the aerodynamic considerations for crank length?

Larger hip angle at the top of the pedal stroke so a lower position can be adopted without power loss theoretically

[Rick]

Already discussed in a prior thread.
But I experimented with 170 through 180 and was actually quite surprised at how it made almost no difference whatsoever to me on ~30 minute steep climbs. I had spent most of my time on 175.
I had assumed that climbing is where longer cranks would help the most, but if it doesn't help there, then why bother with them.
So, I stayed with 170...for now.

[cyclespeed]

What are the aerodynamic considerations for crank length?

The theory goes that you can get your torso lower without the problems of having your knees and thighs squeezed up into your chest at the top of your pedal stroke.

Google it, there's quite a lot of info online which all agrees with each other.

[cyclespeed]

Already discussed in a prior thread.
But I experimented with 170 through 180 and was actually quite surprised at how it made almost no difference whatsoever to me on ~30 minute steep climbs. I had spent most of my time on 175.
I had assumed that climbing is where longer cranks would help the most, but if it doesn't help there, then why bother with them.
So, I stayed with 170...for now.

I know it's been discussed a lot, but there still seems to be little agreement on the 'best' crank length (for a given height).

What I find most interesting is that many out there say that a longer crank gives you more leverage, and therefore must be better. But it's the same extra leverage as changing down a gear, so their point is invalid.

I find I climb best by maintaining a high (100ish) cadence, and smaller cranks help with that.

[wingguy]

Would you agree or disagree with the following statements; (I have put my opinion below each one).

2.) Bicycle B can counter this torque deficit (i.e. increase torque) by changing up a gear on the cassette, by dropping to the small chainring, or by using smaller wheels.

TRUE

3.) Bicycle A will move the exact same distance as Bicycle B for 1 full crank revolution. (assuming all else equal).

TRUE

But all else is not equal if bicycle B has taken the measures stated in step 2. The thought process here is illogical and ambiguous.

Instead of asking a bunch of leading questions, how about you just come out and state your point?

[sawyer]

My view is an individual's physiology including adaptation to whatever they are used to is important here and it makes it hard to draw conclusions.

It's something that is possibly worth trying mainly to reduce the hip/chest angle

Shorter cranks can mean the saddle has to be raised however which isn't a good thing

[helldiver]

It's not that simple, that "longer cranks enable higher torque at BB spindle". While it's certainly true that they enable higher peak-torque (at some point in the pedal circle), for some riders, shorter cranks may enable them to begin their effective stroke sooner, so reducing the duration of dead-point. Peak torque will be lower, but average torque can be the same, or even higher. I heard about some riders which could push harder gear at the same cadence for longer when they moved to shorter cranks - albeit I'm certain that they were fitted with waaay too long cranks before (unfortunately it's really common to see very small riders/women riding 175mm cranks...).

[wingguy]

Shorter cranks can mean the saddle has to be raised however which isn't a good thing

What makes you say that?

[cyclespeed]

Would you agree or disagree with the following statements; (I have put my opinion below each one).

2.) Bicycle B can counter this torque deficit (i.e. increase torque) by changing up a gear on the cassette, by dropping to the small chainring, or by using smaller wheels.

TRUE

3.) Bicycle A will move the exact same distance as Bicycle B for 1 full crank revolution. (assuming all else equal).

TRUE

But all else is not equal if bicycle B has taken the measures stated in step 2. The thought process here is illogical and ambiguous.

Instead of asking a bunch of leading questions, how about you just come out and state your point?

No, 3) is unrelated to 2). As i said, all is equal, i.e. same gear. It's just a series of statements.

I suppose my point is that there appears to be a long list of Advantages to moving to smaller cranks, with very few Disadvantages.

I've already go down to 170's, but I'm thinking I might move to 167.5 or 165's. Maybe I'm missing something? What do I stand to lose? (I see no problem in raising my saddle a touch).

[cyclespeed]

Force diagram for bicycle crank system;

Happy to be corrected if there's a mistake;