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Crank Length?

Old 11-12-23, 02:48 PM
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Originally Posted by PeteHski
The guy with the shorter cranks is effectively riding a harder gear. He has less leverage, but also less foot travel per crank revolution. So he requires a higher pedal force at a lower pedal speed for the same cadence and power. Agreed?

So if we now compensate for the shorter cranks with a slightly lower gear (not a higher gear as you stated), the pedal force is reduced and the pedal speed is increased for the same power. Agreed?

The part of your post I highlighted in bold is misleading. If the rider with shorter cranks uses a faster cadence (not to be confused with pedal speed) or uses a higher gear at the same cadence then he is simply applying more power, which is independent of crank length.

Back to sprinters, one of those studies actually found that shorter cranks allowed the riders to reach their peak power slightly quicker, although I haven’t read that study in detail. But do pro sprinters actually tend to use longer cranks? From what I’ve read they don’t. Phil Burt talks about the GB track cyclists moving to shorter cranks (165) and Mark Cavendish is on 170 cranks, which seems pretty average for his height.

Anyway I think we both agree that crank length in the usual available range is all about fit and comfort, while inherent power is unaffected. Leverage is just a trade-off of force vs travel.
Yes, we agree. And I very much like your final statement. It sums things up quite nicely.
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Old 11-13-23, 04:59 AM
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Originally Posted by Turnin_Wrenches
What I have said and what I will continue to say is that all other factors being equal (i.e. sprockets and chainrings) a longer crank (e.g. a longer lever) will provide more mechanical leverage compared to a shorter one.
That's just common sense. I don't think anyone in the history of the world has ever said otherwise. It's when chainrings and sprockets are involved that people get confused (other people; not us!), not realizing that sprockets and chainrings are just additional levers in the system (along with wheels, as Jack Tone astutely pointed out a few posts ago).
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Old 11-13-23, 06:25 AM
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Originally Posted by Trakhak
That's just common sense. I don't think anyone in the history of the world has ever said otherwise. It's when chainrings and sprockets are involved that people get confused (other people; not us!), not realizing that sprockets and chainrings are just additional levers in the system (along with wheels, as Jack Tone astutely pointed out a few posts ago).
The other common confusion is between torque and power. As a mech eng I see this often in general discussion forums.

The only question for me was whether or not human biomechanics would tend to favour longer or shorter cranks. But repeated independent studies have shown that there is no obvious connection between crank length and power or efficiency or even leg length beyond a very weak correlation in one study.

Once you realise that there is no inherent mechanical power advantage in using longer cranks, bike fit then strongly favours shorter cranks or simply your personal preference. But if you do change to significantly shorter cranks, it is worth reviewing your choice of gears. Especially if you do a lot of climbing in your lowest gear.
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Old 11-13-23, 09:11 AM
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Originally Posted by PeteHski
The other common confusion is between torque and power. As a mech eng I see this often in general discussion forums.
A mentor of mine recommended changing my "power" field on my head unit to "torque". He said it's a much more useful metric than watts. He gave me a brief explanation as to why it's better, but it was quick and I didn't fully absorb it. I need to do some information gathering on my end so I can better understand this topic.

If it's not too much trouble and it won't sidetrack the thread, would you mind elaborating a little on the difference between torque and power and the value of tracking torque vs watts from a training perspective? Thanks!
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Old 11-13-23, 11:20 AM
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Originally Posted by Turnin_Wrenches
A mentor of mine recommended changing my "power" field on my head unit to "torque". He said it's a much more useful metric than watts. He gave me a brief explanation as to why it's better, but it was quick and I didn't fully absorb it. I need to do some information gathering on my end so I can better understand this topic.

If it's not too much trouble and it won't sidetrack the thread, would you mind elaborating a little on the difference between torque and power and the value of tracking torque vs watts from a training perspective? Thanks!
Torque = pedal force x crank length.
Power = torque x cadence.

Since crank length is a constant value on your bike, then torque is directly proportional to your pedal force.

Power however is a function of both torque and cadence. So you can generate the same power with either low torque and high cadence or high torque and low cadence. If you only tracked power as a metric, then it wouldn’t show how the power was generated between torque and cadence.

So from a training perspective it might be useful to track torque and cadence to analyse exactly how your power was generated. But I can’t say I’ve ever done that. I just look at power and cadence, which is what most structured training intervals are based on.

It would be interesting to know how your mentor was using torque. I’m guessing he was trying to keep pedal force within a certain working range. It might help to show how efficiently he was using his gears.
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